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Suction-Monitored Direct Shear Testing of Residual Soils from Landslide-Prone Areas
Abstract
The apparent cohesion due to soil suction plays an important role in maintaining the stability of steep unsaturated soil slopes with deep ground water table. In this paper, a modified direct shear box is used to determine the relationships between the value of this additional cohesion and the associated soil suction. The apparatus incorporates a miniature tensiometer which allows for the simple and direct measurement of suction during shearing. The soil-water characteristic curves and shearing behavior of intact residual soils, being low-to-medium plasticity silts, as well as silty sand, taken from four landslide-prone areas in Thailand, have been investigated. The relatively low air-entry suctions (0-7 kPa) and bimodality of the soil-water characteristic curves gives an indication of the structured pore size distribution of the materials tested. Samples with higher suction tend to display stronger bonding at particle contacts and thus are more brittle. The shear strength is found to increase nonlinearly with suction, though linearization can be reasonably assumed for suction below around 30 kPa. Prediction of shear strength based on soil-water characteristic curves agrees better with ultimate than peak values. A simple equation is proposed for the minimum ultimate strength that can be expected in an unsaturated residual soil with a suction lower than about 30 kPa.
... where is the apparent cohesion in unsaturated soils. Apparent cohesion is controlled by interparticle bonding from capillarity and physiochemical forces and water distribution in the pores of soil (Jotisankasa & Mairaing 2010). ...
... The suction stress is paramount in slope stability analysis of steep unsaturated slopes with a deep groundwater table (Jotisankasa & Mairaing 2010). Suction stress increases with matric suction and vertical stress (Kim, B-S et al. 2010). ...
... It is noted that this relationship is primarily used for coarse-grained material in which the difference between the saturated (sat) and unsaturated (unsat ) shear strength is typically related to the apparent cohesion. This assumption is supported by previous laboratory studies Jotisankasa and Mairaing 2010;Montrasio et al. 2018; which indicated that changes in water content greatly affect the apparent cohesion of soil while the effect on soil's friction angle is rather limited. The validity of Eq. 46 will be examined in this study by testing the strength of three different soil under saturated and unsaturated conditions. ...
Globally, landslides have occurred regularly and caused massive destruction with numerous losses of life. Although they may result from earthquakes, geological factors and human activities, landslides are mostly caused by heavy or prolonged rainfall. There were over 100 landslides occurred along the state road corridors in northern region of NSW, Australia, since 2009 till 2019. Rainfall events triggered about 80% of landslides in northern New South Wales (NSW), Australia. Mountain passes such as Mt Seaview, Dorrigo Mountain, Gibraltar Range, Ramornie – Cangai Bluff, Mallanganee Range and Mt Lindesay are adversely affected by rainfall-induced landslides. The scope of this thesis includes analysis of rainfall-induced shallow landslides occurred in coarse-grained soils, prediction of shear strength of coarse-grained soils using soil suction and prediction of rainfall-induced shallow landslides using different approaches such as rainfall threshold, rainfall index and limit equilibrium method in northern NSW. This study presents the major factors that lead to shallow rainfall-induced landslides in northern NSW and proposes simple tools that can be used to predict this natural disaster. It is believed that these findings will be useful for the relevant industry including decision-makers who manage the slope assets and landslide hazard along the road corridors in northern NSW, Australia.
... A relatively rare method in unsaturated soil testing is suction-monitored direct shear. In this test, soil suction is not controlled; rather it is directly monitored by attaching a tensiometer to the top cap of a shear box (Jotisankasa and Mairaing, 2010). Compared to the first method, the second one is very rarely reported in the literature. ...
... A tensiometer may be placed on the top cap, shear box body, or any part of the shear box. However, the simplest way in this instance was to place the tensiometer(s) on the top cap, as performed by Tarantino and Tombolato (2005) and Jotisankasa and Mairaing (2010). ...
... 10 Schematic diagram of suction-controlled direct shear test (Gan J.K.M. et al., 1988) Figure 2.11 Schematic diagram of suction-monitored direct shear test developed byJotisankasa and Mairaing (2010) ...
The laboratory study on unsaturated soil may comprise the study of devices or apparatus used and the study of the behaviour of soil itself. In both, suction is the main issue of the study’s concern. One of the common devices for unsaturated soil strength-testing is a suction-controlled direct shear box. With this device, suction is generated by controlling water pressure and air pressure during the test. A relatively rare method in unsaturated soil testing is suction-monitored direct shear. In this test, soil suction is not controlled; rather it is directly monitored by attaching the tensiometer to the top cap of the shear box.For flexible pavement design, a very common laboratory test is the California Bearing Ratio (CBR). The CBR has been used as a semi-empirical approach for predicting the bearing capacity of sub-grade soil since the 1920’s. The effect of water content on the CBR is commonly investigated in this type of study. Even though suction is one of the key parameters affecting unsaturated soil behaviour, the effect of soil suction on the CBR is not usually taken into account. This may be due to the difficulty in measuring soil suction while the performing the CBR test.The main objective of this study is to review the behaviour and capability of a suction-monitored direct shear device and to introduce a modified CBR test device in which suction is taken into consideration during the test. A series of laboratory tests was carried out consisting of a saturated direct shear test, an unsaturated direct shear test, and a suction-monitored CBR test on both soaked and unsoaked CBR using the artificial soil of sand and sand-kaolin clay mixtures. During the tests, suction was generated naturally by controlling the specimen in different water content values. For this, the soil water characteristic curve (SWCC) was a very useful tool for predicting the desired specimen water content and/or matric suction.Suction-monitored direct shear and CBR devices were successfully used for the specimens where matric suction was less than 80 kPa. The tensiometer performed effectively during the test. The results indicated that the presence of kaolin clay in the mixture, to some extent, led to the increase in unsaturated shear strength and the CBR. In general, matric suction versus unsaturated shear strength and matric suction versus CBR curves exhibited bi-linear curves with the inflection points occurring around the air entry value (AEV).The correlation between CBR and unsaturated shear strength was developed by plotting the failure envelopes resulting from unsaturated direct shear and unsaturated CBR tests. The range of R-square was between 0.87 and 0.99. The high R-square value of the equations indicated that the correlations were reasonable. This correlation may be applicable only for these particular specimens in the range of suction between 0 and 80 kPa.
... For the direct shear tests, the specimens were compacted at different moisture contents ranging from 24.2 to 33.1%, in order to obtain different suction values for each specimen, ranging between 16 and 100 kPa. It is noted that these suction values were within a typical in-field range, based on monitoring results of pore water pressure and suction in slopes during rainfall and numerical analyses from previous studies [3,[10][11][12] and thus considered relevant for slope stability application in tropical countries. ...
... The soil sample's diameter was 63 mm and the height was about 25 mm for the SWRC tests. The soil suctions were measured using Kasetsart University (KU) miniature tensiometer [3,12,13] for suction range between 0 and 100 kPa as shown in Fig. 4. The equilibration of suction measurement using tensiometers was always ensured and checked that no air-bubbles were present in the tensiometers' water reservoir during measurement. Two undisturbed samples obtained from the block samples and three statically compacted samples were tested for the SWRCs. ...
... After static compaction, the sample was set in the especially fabricated direct shear box, which facilitated suction measurement using KU tensiometer through the top cap (Fig. 4). The unsaturated test was conducted in a constant-water content condition following the procedure suggested by [12]. After placing the sample in the direct shear box, the sample was carefully covered with plastic wrapping and wet cloths to prevent evaporation and to maintain the constant-water content condition. ...
Unsaturated soil mechanics has become a topic of interest among the Sri Lankan geotechnical community owing to its importance in different applications, especially slope stability. In this study, soil samples collected from a landslide area in Sri Lanka were subjected to soil–water retention curve (SWRC) tests and suction-monitored direct shear tests. SWRC tests were conducted on both undisturbed and statically compacted samples and the results were modelled using Van Genutchen equation. The compacted samples having the same dry density as the undisturbed block samples exhibited a similar SWRC. Consolidated drained direct shear tests and suction-monitored direct shear tests were conducted on compacted samples to determine saturated and unsaturated shear strength respectively. The unsaturated shear strength followed a non-linear envelope with respect to matric suction, as shown by a good correlation for the cases of second-order polynomial correlation (R2 = 0.970) and the prediction using Vanapalli et al. model and SWRC with K = 1.2 (R2 = 0.941). These test results confirm the validity of using SWRC for predicting unsaturated shear strength of a compacted Sri Lankan silty sand.
... For the direct shear tests, the specimens were compacted at different moisture contents ranging from 24.2 to 33.1%, in order to obtain different suction values for each specimen, ranging between 16 and 100 kPa. It is noted that these suction values were within a typical in-field range, based on monitoring results of pore water pressure and suction in slopes during rainfall and numerical analyses from previous studies [3,[10][11][12] and thus considered relevant for slope stability application in tropical countries. ...
... The soil sample's diameter was 63 mm and the height was about 25 mm for the SWRC tests. The soil suctions were measured using Kasetsart University (KU) miniature tensiometer [3,12,13] for suction range between 0 and 100 kPa as shown in Fig. 4. The equilibration of suction measurement using tensiometers was always ensured and checked that no air-bubbles were present in the tensiometers' water reservoir during measurement. Two undisturbed samples obtained from the block samples and three statically compacted samples were tested for the SWRCs. ...
... After static compaction, the sample was set in the especially fabricated direct shear box, which facilitated suction measurement using KU tensiometer through the top cap (Fig. 4). The unsaturated test was conducted in a constant-water content condition following the procedure suggested by [12]. After placing the sample in the direct shear box, the sample was carefully covered with plastic wrapping and wet cloths to prevent evaporation and to maintain the constant-water content condition. ...
Unsaturated soil mechanics has become a topic of interest among the Sri Lankan geotechnical community owing to its importance in different applications, especially slope stability. In this study, soil samples collected from a landslide area in Sri Lanka were subjected to soil–water retention curve (SWRC) tests and suction-monitored direct shear tests. SWRC tests were conducted on both undisturbed and statically compacted samples and the results were modelled using Van Genutchen equation. The compacted samples having the same dry density as the undisturbed block samples exhibited a similar SWRC. Consolidated drained direct shear tests and suction-monitored direct shear tests were conducted on compacted samples to determine saturated and unsaturated shear strength respectively. The unsaturated shear strength followed a non-linear envelope with respect to matric suction, as shown by a good correlation for the cases of second-order polynomial correlation (R² = 0.970) and the prediction using Vanapalli et al. model and SWRC with K = 1.2 (R² = 0.941). These test results confirm the validity of using SWRC for predicting unsaturated shear strength of a compacted Sri Lankan silty sand.
... This is also important as soil permeability, shear strength, slope stability and erodibility are largely influenced by soil suction and soil moisture. [24][25][26] In previous riverbank retreat analyses, most researchers employed indirect estimation of unsaturated soil parameters of the bank soils, particularly, the permeability function based on some empirical relationships in their analyses, rather than directly testing them. [27][28][29][30][31] However, recent studies 32,33 indicated the importance of appropriately characterization of permeability function in accurately predicting the pore-water pressure regime in slopes. ...
... The κ parameter has been found to range between 1 and 1.2, based on test results on tropical soils. 24,69 The sum of inherent effective cohesion and matric suction represents an apparent cohesion (u a ). In fully-saturated conditions, the matric suction totally disappear and the apparent cohesion is equal to the effective cohesion, the pore-water pressure becomes positive, the angle ϕ b becomes equal to ϕ ′ , and therefore Eq. (7) reduces to the classical Mohr-Coulomb failure criterion. ...
The impacts of various hydrological processes such as infiltration, saturated/unsaturated seepage and river drawdown on bank retreat are complex, yet crucial in understanding fluvial erosion processes. The bank retreat generally occurs due to a combination of erosion and mass failure. In this study, coupled analyses of a bank retreat during prolonged high flow in Southern Thailand were performed considering these hydrological and mechanical processes, by means of finite element seepage model (SEEP/W), stability analysis (SLOPE/W) and erosion model (BSTEM). The simulation result was then validated based on field observation. A detailed characterization of unsaturated soil hydraulic properties of the bank soils, namely soil–water retention curve (SWRC) and unsaturated permeability (k-function) was carried out on undisturbed soil samples in the laboratory and used in the seepage analysis. Two modes of mass instability were considered in the analysis, namely sliding failure and cantilever failure. The analysis revealed that a prolonged antecedent rainfall lead to saturation of the bank soil which triggered sliding failure of the bank at an early stage of the flood event. As water level progressively increased, repeated cycles of cantilever failure and erosion of the lower bank were reproduced in the model, yielding the estimated total bank retreat of about 5.3 meter, which agreed well with field observation. Another series of analyses with empirically estimated unsaturated soil parameters however yielded a different bank retreat behaviour, thus highlighting the importance of accurately determining hydraulic functions. The lateral erosion was finally shown to be a prominent process that induced overhanging riverbank shape and cantilever failure, thus being a major contribution of the bank retreat in this case study.
... Figure 2 shows the condition of the monitored slope and the position of the tensiometer. The Kasetsart University (KU) tensiometer, which able to measure the pore water pressure in the range of -100 kPa to 600 kPa, was used to monitor the pore water pressure in the field [9,10]. The tensiometer were installed at the depths of 0.5 m (labeled TA), 1 m (TB), and 1.5 m (TC). ...
... Figure 3 illustrates the schematic cross-section of the SWCC test and tensiometer. The undisturbed soil about 63 mm in diameter and 20 mm thickness were tested for SWCC using the method as explained by Jotisankasa and Mairaing [9]. The method involved gradually wetting soil samples in which at each stage suction of the sample was monitored until equilibrium was reached. ...
Some slopes in Yogyakarta, Indonesia, were covered by volcanic soil of the quaternary deposit of Merapi volcano. Typically, the soil layers of the slope consist of fine sand or silty sand. During the wet season (November – February), the slope is prone to instability due to rainfall infiltration. This paper presents the field suction monitoring of silty-sand slope. A 2.57 m high slope at Universitas Muhammadiyah Yogyakarta campus was instrumented with a mini tensiometer, moisture probe, and rain gauge to study the effect of suction on the slope stability during rainfall in wet season. The investigation shows that the rainfall decreases the suction and increase the moisture of soil. This situation leads to slope instability. The factor of safety fluctuates corresponding to the rainfall intensity. The lowest factor of safety was attained on 30 December, in which the rainfall intensity is the highest, i.e. 48 mm/day.
... Then, it was soaked for 2 weeks so that the soil sample became as close as possible to a fully saturated state. The soil sample was then allowed to dry slowly, while measuring the consequent suction using a miniature tensiometer [17,23] for a suction range of 0-100 kPa. The pressure plate technique was used to measure suction in the range of 100-1500 kPa and for suction higher than 1500 kPa, the isopiestic technique [12] was used. ...
... Geosciences 2019, 9, x FOR PEER REVIEW 4 of 18 sample became as close as possible to a fully saturated state. The soil sample was then allowed to dry slowly, while measuring the consequent suction using a miniature tensiometer [17,23] for a suction range of 0-100 kPa. The pressure plate technique was used to measure suction in the range of 100-1500 kPa and for suction higher than 1500 kPa, the isopiestic technique [12] was used. ...
This research aims to propose the use of spectral analysis of surface wave (SASW) tests along with in-situ suction measurements for non-destructive determination of shrinkage cracks. The underlying principle behind this proposed method is that, while suction and the small-strain shear modulus are positively correlated for intact samples, this is not the case for cracked ground. A series of SASW tests were performed on a clay embankment at different periods, during which the suction, modulus, and shrinkage crack depth varied seasonally. The soil water retention curve (SWRC) of the undisturbed sample collected from the cracked zone was determined, which related the suction-to-moisture content and void ratio of the soil. A free-free resonant frequency (FFR) test in the lab was conducted to determine the small-strain shear modulus (G0) at various moisture contents. The small-strain moduli from the SASW tests on the intact ground were generally higher than those from the FFR tests due to the effect of confining stress. A drop in the small-strain modulus determined using the SASW test was observed as an increase in suction-induced cracks and it relieved the horizontal stress. The crack depth measured in the field was then modelled using a semi-empirical procedure that can be used to predict crack depth relative to suction.
... It has been proved that using API value instead of 3 days accumulated rainfall in the critical rainfall envelope gives a better warning time and accuracy (Fig 3). Jotisankasa & Mairaing (2010) later elaborated the testing technique further in direct shear apparatus by incorporating a miniature tensiometer through the top cap to monitor soil matric suction during shearing as shown in Figure 4. The miniature tensiometer was manufactured at Kasetsart University using MEMs pressure sensor as described by . ...
... ypical results during constant water content shearing in the suction-monitored direct shear apparatus(Jotisankasa and Mairaing, 2010) ...
... The soil-water retention curve (SWRC) was obtained by measuring the suction at different moisture contents using the miniature tensiometer (Jotisankasa et al. 2007(Jotisankasa et al. , 2009Jotisankasa and Mairaing 2010). The test was performed on a statically compacted sample (64 mm in diameter and about 20 mm in height) in a confined ring at zero vertical stress. ...
... Since live staking is used mainly for the stabilization of shallow slips, a shearing test was performed at a relatively low normal stress. The matric suction of about 23 kPa was chosen to represent the typical range of suction occurring at 1 to 2 meters deep in unsaturated slopes in Thailand and the tropics (e.g., Tsaparas et al. 2003;Jotisankasa et al. 2008;Jotisankasa and Mairaing 2010). This suction was achieved by directly adding water to the specimen using a very fine spray to reduce the suction or by exposing the specimen in air to increase the suction. ...
Large direct shear tests were conducted on compacted clayey sand, reinforced with Jatropha live stakes of various ages, under saturated and unsaturated conditions. The relationship between root cohesion and the side root area ratio appeared linear. For samples with stake stabilization but no fibrous roots, a stake installed in a larger drilled hole appeared to contribute lower strength increase and greater shear displacement than a stake in a smaller hole. All the unsaturated samples at a suction of approximately 23 kPa exhibited strain-hardening behavior. The rate of strength increase due to the root was affected by the soil's suction. The additional strength due to live stake and fibrous root could be reduced by a factor of 3.5 if the conditions change from unsaturated (at about 23 kPa suction) to fully saturated. A new model was proposed to account for the influence of suction on root cohesion considering the suction effects on bond stress, root tensile strength, root tensile modulus, and shear zone thickness.
... The determination of the SWRC and the soil unsaturated hydraulic conductivity using combined empirical and statistical models is based on the consideration that both the SWCC and the unsaturated hydraulic conductivity are mainly determined by the pore-size distribution of the given soil. Various empirical models have been used by different researchers such as Brooks and Corey, (1966); van Genuchten, (1980);Fredlund and Xing, (1994) (Jotisankasa andMairaing, 2010, Thakur, Sreedeep andSingh, 2005) to determine the hydraulic conductivity of an unsaturated soil using the SWRCs. ...
Soil water characteristic curve (SWCC) is very important in the study of unsaturated soil because it represents a soil's ability to store and release water as it is subjected to various soil suctions. This storage of moisture in soil is paramount in irrigation engineering, as it determines the irrigation scheduling in dryland. This study reviews some SWCC model's performance to predict the soil from some selected soils in Nigeria from previous work. This previous work contains 7 points between 0-1,500 kPa. Seki, which is bimodal, tends to perform better than the rest with R 2 value of 0.99. Generally, the bimodal models performed better than the unimodal, due to the flexibility of the curve. However, with the removal of the reference point of 1,000,000kPa (leaving 7 points), all the unimodal models had good performance in the topsoil. Fredlund and Xing performed best and van Genuchten performed poorly in the subsoil. Also, it was observed with the increase in both the organic matter contents and electrical conductivity, the performance of the models decreased. From this review, it is observable that in analysis of 8 or more points and 3 to 7 points Seki's model and Fredlund and Xing model can be used respectively for further research in the study of dryland soils of Nigeria.
... These types of soils are common in numerous parts of the world. The geotechnical characteristics of residual granitic soils have been studied in Brazil [1,2], China [3,4], Hong Kong [5], Malaysia [6], Portugal [7], South Korea [8], and Thailand [9], to name a few. A granitic residual soil, locally known as "maicillo" is commonly found in Chile. ...
Granitic residual soils are soils formed by the in situ weathering of intrusive granitic rocks and are present in different parts of the world. Due to their large presence, many civil engineering projects are carried out on and within these soils. Therefore, a correct characterization of the slopes is necessary for slope stability studies. This investigation aims to study the influence of the values of geomechanical parameters (specific weight, cohesion, and friction angle) and the geometry of a slope (height and inclination) on slope stability of residual granitic soils in dry and static conditions. To this end, an automatic system was developed for the numerical study of cases using the finite element method with limit analysis. The system allows modeling, through Monte Carlo simulation and different slope configurations. With this system, the safety factors of 5000 cases were obtained. The results of the models were processed through the SAFE toolbox, performing a Regional Sensitivity Analysis (RSA). The results of this research concluded that the order of influence of the factors were: slope angle > slope height > cohesion > friction angle > unit weight (β > H > c > ϕ > γ).
... It was shown that soil strength tends to decrease with increasing moisture content. The relationships between soil moisture content, the soil-water characteristics curve (SWCC) and suction have been established for different soils and linked to soil's apparent cohesion that contributes to the overall shear strength of unsaturated soil [16][17][18]. As a result, a number of empirical correlations to predict shear strength of soil have been proposed in the literature and summarized in [19,20]. ...
There are many empirical equations published for unsaturated fine-grained soils. However, there is only one empirical equation established for silty sand using the shear-box test and filter-paper-based suction test, but with the suction range of 0 to 200 kPa. It is reported that there is a significant discrepancy between the predicted values and test results of apparent cohesion within the range of 0 to 100 kPa for unsaturated coarse-grained soils. The purpose of this research is to study the effect of water content on apparent cohesion and predict apparent cohesion for coarse-grained soils within the range of 0 to 100 kPa using shear-box test and suction test results without much inconsistency. In this research, soil samples from the rainfall-induced landslide sites were obtained; laboratory tests such as soil-classification tests, shear-box tests and consolidated undrained triaxial tests were carried out. Test results were analyzed, and the findings are presented. When the water content is increased from 0% to 30%, there is a reduction of 89% in apparent cohesion on average. A newly developed prediction model for apparent cohesion based on the low range of matric suction from 0 to 100 kPa for unsaturated coarse-grained soils is introduced in this paper and compared with published models.
... There were five independent parameters: θ r , θ s , α, n, and K s in the above equations. Jotisankasa and Mairaing (2010) estimated the pore connectivity parameter l in the hydraulic conductivity function was about 0.5 as an average for many soils. ...
... In these slopes, shallow soil mass up to a depth of 1-3 m remains unsaturated during most time of the year. In such case, the apparent cohesion induced by negative pore water pressure, or soil suction, ensures the stability of the slope [2][3][4]. During prolonged rainfall events, the excess infiltration could create near-surface saturated zones which diminish the matric suction. ...
Root biomass and plant age have been recognized to influence the hydro-mechanical properties of root-permeated soils, but its direct field observation was scarce. In this study, influences of Chrysopogon zizanioides (vetiver grass) of various plant ages (up to 2 years) on soil–water retention curves (SWRCs), saturated permeability, specific water–retention capacity and soil aggregate stability have been investigated. Undisturbed soil samples were obtained at different locations, representing the root zone, upslope, and downslope from the vetiver hedgerow at three bio-engineered slopes in Western Thailand. As root biomass increased, air–entry suction of soil slightly decreased, and porosity significantly increased due to the formation of macropores and aggregated soil structure in the root zone (0–15 cm depth). Nevertheless, below the root zone (15–25 cm depth), the air–entry suction of soil increased with root biomass due to the occupancy of soil pores by roots. Saturated permeability shows a positive correlation with plant age in the root zone, while a slightly negative correlation was found below the root zone. The upslope and downslope soil samples showed a negative correlation due to the effect of sediment trap. The specific water retention capacity and soil aggregate stability became higher in the root zone and positively correlated with plant age.
... The SWRCs were determined using the miniature tensiometer [54,55] for matric suctions less than 100 kPa, the pressure plate for matric suctions between 200 and 1500 kPa, and the isopiestic technique (salt solution equilibrium) for total suctions greater than 1500 kPa. In each compaction condition shown in Table 2, two samples were used to obtain the two paths, wetting and drying, of SWRCs. ...
A change in soil modulus in response to suction-moisture variation after compaction plays a vital role on modulus-based compaction control during earthwork construction. This study investigated the small-strain modulus-suction-moisture relationship for a silty sand subgrade using the laboratory free-free resonant frequency (FFR) and the in-situ spectral analysis of surface waves (SASW) in a trial section of highway construction project in Thailand. The post-compaction soil water retention curves (SWRCs) at different compaction states were investigated over the entire range of suction. Relationships between the SWRC parameters, i.e., air-entry suction, water-entry suction, residual suction etc., and as-compacted dry density and water content were obtained using the multiple-linear regression analysis. Such relationships are useful for the prediction of post-compaction suction in the field. The variation of SASW and FFR shear moduli with the suction stress, inferred from SWRCs, was found to be linear. A modified model was proposed, considering the combined effect of void ratio and the suction stress on the shear modulus, represented as Parameter A which was found to decrease as suction increased. The inclusion of suction stress and void ratio within the model variables yields better accuracy of prediction as compared to those models that considered either only suction or water content.
... Figure 14 demonstrates that at a given normal stress, shear strength increases with decreasing Sr (i.e., increasing matric suction), and the relationship between shear strength and Sr is nonlinear. Nonlinearity of this relationship was also noted by Escario and Juca (1989) and Jotisankasa and Mairaing (2010). ...
Mechanical behavior of residual soils are studied by performing constant water content direct shear tests on reconstituted specimens using total stress analysis. The testing program involves initial degree of saturation (Sr) and applied normal stress as the control parameters. For three different soil samples, a total of 55 direct shear tests are conducted at Sr = 60%, 80%, and saturated conditions, in the normal stress range of 15–60 kPa. The results showed that shear strength increases with decreasing Sr and the relationship between shear strength and Sr is nonlinear. More dilative response is observed with decreasing Sr and decreasing applied normal stress. Maximum dilatancy and Sr relationship is nonlinear. At large displacements, samples prepared at different Sr levels showed similar ultimate shear resistances under similar applied normal stresses. It is concluded that Sr and applied normal stress dramatically influence the mechanical behavior of compacted residual soils studied.
... During dry seasons these slopes have higher safety margins due to the effect of additional shear strength resulting from negative pore water pressure or the matric suction when these slopes experience prolong rainfalls, infiltration of water results in depletion of this matric suction and subsequent loss of the additional shear strength. This mechanism could diminish the suction to nearly zero and initiate slope failures at shallow depths, even in the absence of positive pore water pressures profiles [5,8,9]. In general, such slope failures are triggered by short duration, high intensity rainfall events [10]. ...
Shallow slope failures triggered by excessive rainfall events is a common geotechnical problem mostly in tropical regions of the world. Threshold rainfall intensities leading to such slope instabilities are associated with number of factors. Behaviour of the unsaturated zone of the slope is vital in establishing rainfall threshold values governing slope failures. In this research, a study has been carried out to develop a conceptual approach to investigate the threshold rainfalls leading to slope instabilities in residual soil slopes of Sri Lanka. Soil water characteristic curve (SWCC) was obtained using undisturbed soil samples collected from a slope area in Central Highlands of Sri Lanka. The other unsaturated soil properties required for infiltration analysis were established using the SWCC data and its relationships with other parameters, proposed in the literature. Stability of a typical slope was assessed by a parametric analysis using the derived soil properties under different rainfall intensities. Infiltration was modelled and potential failure through a shallow translational slide was studied. A critical rainfall threshold envelope has been obtained, considering the exposure of the slope to different rainfall events
... The modified triaxial apparatus is commonly used to perform suction controlled (e.g., Wheeler and Sivakumar 2000;Cunningham et al. 2003) or the constant water content test (e.g., Thu et al. 2006;Marinho et al. 2013). Samples are commonly sheared under constant net normal and matric suction values in the case of a modified direct shear apparatus (e.g., Nam et al. 2011;Schnellmann et al. 2013), although in several studies low and highcapacity tensiometers were successfully utilized to perform suction-monitored direct shear tests as well (e.g., Tarantino and Tombolato 2005;Jotisankasa and Mairaing 2010). The infiltration test is another type of experiment in which matric suction of tested specimen is decreased while water content increases under constant confining net and shear stress conditions. ...
Slope instabilities in the form of shallow and deep-seated landslides in flysch formations throughout Europe typically occur after prolonged periods of heavy rainfall. The Rječina River Valley, Croatia, is characterized by the presence of flysch material in the lower part of the Valley, where numerous historical and recent landslides have occurred. The weathering process and climate conditions result in a complex engineering geological profile of flysch slopes in the Valley, with unsaturated residual soil covering the slopes. To investigate the behavior of residual soil existing on the flysch slope under increasing water content due to the rainfall infiltration process, undisturbed soil samples collected at natural water content were tested in the modified direct shear apparatus. Under imposed stress conditions, samples of low hydraulic conductivity were subjected to a prolonged wetting process simulating the rainfall infiltration process in the field. The obtained results suggest that a gradual decrease of matric suction and an increase of water content resulted in an increase of displacement rates under constant shear stress, which was interpreted as a failure of samples in partially saturated conditions. A unique shear strength envelope expressed in terms of Bishop’s effective stress equation was found to be able to predict stress conditions at the slip surface at the time of failure, while the relationship between measured matric suction and water content closely matched with the main wetting curve. Although the testing results did not point out any special characteristics of residual soil from flysch rock mass behavior, the data about hydro-mechanical behavior of unsaturated residual soil from flysch rock mass, as well as similar fine-grained soils, are very rare and presented results would be valuable for further research. The presented testing procedure and obtained results are useful for studies of rainfall-induced landslides triggered in fine-grained soil materials in zones above the phreatic line, such as shallow landslides occurring in natural flysch slopes or in physical landslide models built in laboratories.
... developed a suction-monitored direct shear device by using a low capacity tensiometer (90 kPa). The device has been utilized in Thailand on residual soil and also on sands in Perth, WA for unsaturated soil studies (Jotisankasa and Mairaing, 2010;Purwana et al., 2011). ...
One of the common devices for unsaturated soil strength testing is suction-controlled direct shear. In this device, suction is adjusted by controlling water pressure and air pressure during the test. Suction was generated indirectly by adjusting the specimen’s water content to the desired value. During the test, suction was monitored using the attached tensiometer. Thus in this method, pore water and pore air pressure control is no longer required. The tests were carried out using sand and sand-kaolin clay mixtures in 95:5 and 90:10 proportions. The results indicates that shear strength with respect to matric suction exhibits a bilinear envelope with an initial value of b is higher than the effective internal friction . Despite the suction capacity of the tensiometer was relatively low, this device was effective for low air entry soils such as sand or sand with relatively small portion of fine grained material.
... The tensiometer is developed by Geotechnical Engineering Research and Development Center, GERD, Kasetsart University, Thailand. Even though the capacity of tensiometer used is low (0-90 kPa), suction-controlled direct shear proposed by Jotisankasa and Mairaing [5] is suitable for material with low air entry value such as sand or residual soil. However the literature about this test is still very few and needs further study. ...
Synopsis: Perth, Western Australia is a semi arid climate area. A soil characteristic at a near surface layer is relatively dry due to the deep water table, low humidity and high rate of evaporation. Subsurface exploration carried out in this study indicated that most soils in Perth are classified as poorly graded sand and remain dry along the year. Therefore, most Perth soils are considered to be in an unsaturated condition. The aim of this study is to evaluate the unsaturated Perth soil properties. Conventional and modified direct shear apparatuses were utilized in the shear strength evaluation of the saturated and unsaturated Perth soils. Modification of the direct shear apparatus was conducted by attaching a low capacity tensiometer to the direct shear top cap to measure the matric suction of the soil samples. The results indicate that during shear, suction is relatively low for a range of saturation degree of 50 to 100% and well within the tensiometer capacity, indicating that the modified direct shear apparatus is suitable for measuring the unsaturated poorly graded Perth sand.
... The range of suctions used was intended to represent the typical values reported for slopes during rainy seasons in Thailand and the tropics (e.g. Jotisankasa et al., 2008;Jotisankasa and Mairaing, 2010;Rahardjo et al., 2014). The normal stress of 15.5 kPa represented an overburden for a shallow slide depth of about 1 m and at the root zone in typical bioengineered slopes. ...
The influences of root concentration and suction were investigated on Chrysopogon zizanioides (vetiver grass) root-reinforcement of clayey sand, using suction-monitored direct shear tests at four different suctions ranging from zero to 50 kPa. The vetiver grass specimens were grown in containers for about a year until they showed various root concentrations prior to testing. Of all suctions, soil samples with greater root concentrations had a higher peak strength and were more ductile than those without roots. Unsaturated root-reinforced soils contracted while the non-reinforced soils dilated during shearing. The rate of increase in root cohesion with root concentrations were highest at the suctions around 20 kPa. The rates of increase in shear strength with suction were similar for non-reinforced soils and reinforced soils up to a threshold in root concentration beyond which the rate became slightly smaller. The obtained relationships between root area ratio and strength were used in the infinite slope stability calculation of an instrumented slope based on field pore-water pressure and mini-rhizotron measurements. The analysis showed that in unsaturated condition, the stabilizing effect of suction was greater than that of roots, while in saturated condition, the stabilizing effect of root reinforcement was more important especially for shallower depth.
... The increase in rainfall intensity creates a risk that affects the environment sustainability. In this region, numerous slope failures commonly occur in steep residual soil slopes with a deep groundwater table during rainfalls (Toll et al., 1999;Rahardjo et al., 2013;Singh et al., 2008;Tohari, 2012;Jotisankasa et al., 2010). ...
... Wang et al. ( , 2003, Igwe et al. (2015) and Igwe (2017) have used the ring shear device to study several rainfall-induced landslides in Japan and Nigeria. Similarly, other devices have also been applied in landslide analysis among which are Stark et al. (2005) and Jotisankasa and Mairaing (2010). ...
Sandy soils were investigated by means of a new ring shear device to understand the combined effect of particle size distribution and relative density on the soils’ deformation characteristics at large strains. The soils were sheared in undrained condition to at least 10 m to observe their responses before and after the attainment of steady state. Two main responses—purely contractive and partially contractive—were identified. Loose specimens were purely contractive and mobilized peak strength at small strains before undergoing rapid loss of shear resistance until steady state strength was achieved. On the contrary, dense soils exhibited partially contractive behavior by first contracting and then dilating as shearing progressed. Analyzed shear resistance-pore pressure relationships indicated that changes in relative density induced corresponding changes in dilation characteristics. The shear behavior of the soils at critical density demarcated the behavior of the loose from that of the dense soils. At densities below the critical, the specimens exhibited purely contractive behavior, and above the critical, the specimens’ behavior was defined by three distinct stages of deformation of phase transformation, peak and steady states. A comparison of the friction angle at phase transformation, steady state and critical state in all the soils revealed only little difference. At a given normal stress, the friction angle at phase transformation was equal to the friction angle at steady state in specimens at critical density. This study shows that the large strain behavior soils can offer valuable insight on the mechanism of rapid, long-travel slope movements that are often catastrophic.
... This study applied a multistage shear test for finding shear strength parameters of the undisturbed soil sample. The multi-stage shear test is not an ASTM standard, but numerous researchers applied this technique for shearing the soil sample [14], [15], [18], [21], [26]. The benefits of the multi-stage technique are: 1 the effect of soil sample variability was completely removed and 2 usages the soil sample is less than the single-stage technique which is suitable for a difficult to access place. ...
Landslide is a natural disaster which occurs very often in mountainous areas. Climate is an important determinant on the amount of moisture in the ground, which is a key to the stability of soil slope. Therefore, climate change due to the global warming may affect the intensity of rainfall and the evaporation in the future and influences situation of slope stability in a long time. The purpose of this study is to propose a method for assessing the effect of climate change on slope stability using general circulation model (GCM). A method for predicting climate change impact on slope stability is to link the antecedent precipitation index (API), hydrological model, obtained through downscaling GCM to critical antecedent precipitation model. The GCM is downscaled using a dynamical technique to derive regional climate models. Then a statistical correlation is used to adjust for the basis of the regional climate model. The GCM used in this study is the ECHAM4/OPYC3 model. The analysis found that the trend of susceptibility to slope failure depends on the pattern of simulated rainfall and the recession constant of the antecedent precipitation index.
... As regards basic research, Kasetsart University has been active in areas aimed at understanding the hydro-mechanical properties of unsaturated residual soils and interaction between the climate factors and slope stability by means of laboratory and field studies as well as appropriate modelling (e.g. Jotisankasa & Mairaing, 2010;Mairaing et al., 2012). This section of the paper highlights some overview of research and practical works carried out as related to slope safety preparedness in Thailand. ...
... More importantly, results of unsaturated tests indicate an increase in both peak and post-peak shear strength with suction even at the high suction range considered in this study. For the lower range of suction up to 100 kPa, Jotisankasa and Mairaing (2010) reported that suction induces stronger bonding at particle contacts and hence specimens become more brittle. Figure 6b illustrates variations of vertical displacement with horizontal displacement for tests carried out under 50 kPa vertical stress. ...
Compression and shear behavior of a loosely compacted loess is investigated via two series of saturated and unsaturated direct shear tests. The vapor transfer mechanism is used to modify a shear box device for control of suction at high range. In spite of significant volumetric strain upon wetting (up to 14%), suction-induced volumetric shrinkage is less than 2% for all suction levels considered because of the as-compacted moisture content at the dry side of the optimum value. During shearing, all unsaturated tests dried to high suctions indicate a strain-softening mode of failure associated with noticeable dilation. There is a continuous increase in peak strength with suction but at a reduced rate, which cannot be captured by the improved Bishop’s effective stress model, as it underestimates the contribution of high suctions by approaching zero degree of saturation. Unsaturated tests at high suctions also show an increased rate of dilation with suction for both values of net stress, which cannot be predicted well by the classical stress–dilatancy models. It is shown that for suction values beyond 8 MPa, dilation angle increases by 2° and 6° per 100 MPa increase of suction under net stress of 50 and 200 kPa, respectively.
Keywords: direct shear tests, high suction range, dilatancy, shear strength, collapse
... The geological setting in each hazardous area is different depending upon certain factors, such as climate conditions, rate of weathering, and slope geometry. As such, this study gathered the soil properties reported from the relevant literature on slope failure, including Dahal et al. (2008), Godt and McKenna (2008), Jotisankasa and Vathananukij (2008), Jotisankasa and Mairaing (2010), Vieira et al. (2010), Bordoni et al. (2015), and Oh and Lu (2015), as summarized in Table 1. It is evident that the parameter a ranges from 0.016 to 0.360 kPa −1 , the desaturation parameter n ranges from 1.290 to 2.780, u sat ranges from 0.286 to 0.480, and u res ranges from 0.0 to 0.250. ...
Assessment of rainfall-induced shallow slope failures is important for reducing damage to infrastructures as well as for the safety of people living close to hazardous areas. The rainfall intensity-duration thresholds for initiation of slope failure (ID thresholds) based on the historical slope failure data are commonly used to assess slope failure. However, in these slope-stability assessments, the critical influence factors triggering shallow slope failures are often disregarded. Three sets of parametric studies were performed through finite-element modeling to investigate the effects of saturated permeability of soil, slope angle, and antecedent rainfall on instability of a shallow slope. It was found that the hydrological mechanism involving the rainfall-induced shallow slope failure is either (1) the rising of water table mode or (2) the rainfall infiltration mode. The hydrological mode during the failure depends on the magnitude of rainfall intensity compared with the infiltration capacity at the soil saturation state. The rate of reduction of safety factor (FS) increases with an increasing intensity of rainfall, only in a range lower than the infiltration capacity at the soil saturated state. As such, the saturated permeability of the soil, which is equal to the infiltration capacity at the soil saturated state, plays an important role in the shallow slope failure. The saturated permeability was also found to govern a range of applicability of the ID thresholds. If the rainfall intensity is not greater than the infiltration capacity at the soil saturated state, the rainfall duration to failure (Trf) can be read from the ID thresholds. Slope angle and antecedent rainfall were found to play significant roles in the instability of shallow slopes, because they control the initial stability of slope, which results in the different linear relationship of ID thresholds. In addition, the slope angle might accelerate the rate of rainwater infiltration; hence, it reflects the slope of the ID thresholds.
... As regards basic research, Kasetsart University has been active in areas aimed at understanding the hydro-mechanical properties of unsaturated residual soils and interaction between the climate factors and slope stability by means of laboratory and field studies as well as appropriate modelling (e.g. Jotisankasa & Mairaing, 2010;Mairaing et al., 2012). This section of the paper highlights some overview of research and practical works carried out as related to slope safety preparedness in Thailand. ...
... It is evident that despite the difference in soil texture and other basic properties, the failure envelopes of the two soils fall closely within the same range. Only one set of shear strength properties (c', ') will therefore be used in the stability analysis, as will be explained in Section 3. Unsaturated shear strength was also determined using the suction-monitored direct shear, of which the test method is described by Jotisankasa & Mairaing (2010) as shown in Figure 9. Figure 10 shows the unsaturated failure envelope at normal stress of 31 kPa and shearing rate 0.1 mm/min. The test results are summarized in Table 1 using Fredlund & Rahardjo (1993) ...
... Soil suction plays a vital role in maintaining the stability of unsaturated slopes [1], [2]. Soil suction is present in all grounds that lie above the water table [3], [4]. ...
Climate change is affecting rainfall patterns which in turn are having a demonstrable effect on slope stability. This is a serious safety concern for EU rail networks. Accurately assessing slope stability is critical in maintaining a safe and efficient railway infrastructure. Soil matric suction is naturally present in all soils which lie above the water table. Therefore, it is a critical component in maintaining embankment stability. Rainfall infiltration causes a decrease in matric suction which subsequently reduces the shear strength of the soil. This can be a triggering effect for many slope failures. This paper describes a tensiometer based apparatus for measuring soil matric suction in a laboratory environment. It also outlines a method for determining the minimum matric suction left during a heavy rainfall or flood event.
... developed a suction-monitored direct shear device by using a low capacity tensiometer (90 kPa). The device has been utilised in Thailand on residual soil and also on sands in Perth, Western Australia for unsaturated soil studies (Jotisankasa and Mairaing, 2010;Purwana et al., 2011). ...
Two very common devices used in unsaturated shear strength tests are suction-controlled direct shear apparatus and triaxial devices. During the tests, matric suction is applied to the specimen by controlling pore air and pore water pressure. Compared to using triaxial device, the suction-controlled direct shear apparatus is considered to be simpler to use due to the shorter drainage path of the specimen. A relatively new and a simpler technique for unsaturated shear strength is the suction-monitored direct shear test. The device is made by modifying the conventional direct shear apparatus with the attachment of low capacity tensiometer on its top cap; this is connected to an electronic readout monitor which assesses negative pore pressure during the test. The aim of the study is to find out the capability of the suction-monitored direct shear apparatus with regard to use in unsaturated soil. Conventional, as well as suctionmonitored direct shear tests have been conducted on various compacted brown sand-kaolin clay mixtures of differing proportions. The first one was performed to obtain the effective shear strength parameters of the saturated specimen, with subsequent testing carried out on unsaturated specimens. The results indicates that in general, shear strength with respect to matric suction, exhibits a bilinear envelope with an initial value of φb higher than the effective internal friction φ. This phenomenon was due to the effect of dilation on the strength development of sand. Matric suction of the specimen was generated indirectly by adjusting the specimen's water content to the desired value, and for this purpose the Soil-Water Characteristic Curve (SWCC) can be a very useful tool for predicting the required water content. Despite the suction capacity of the tensiometer being relatively low, the suction-monitored direct shear apparatus was effective for soil with a low high entry value, such as sand or sand with relatively small portion of fine grained material. Suction-monitored direct shear, tensiometer, matric suction.
Among influence factors of slope stability, rainfall is the most triggering factor. When rainwater flows through the slip surface, it will cause slope movement. This paper studies the slope movement in Kalibawang irrigation channel at km. 15+900, which induced failure of the irrigation channel and other surrounding structures. The slope movement was modelled as coupled stress-pore water pressure analysis using SIGMA/W. Satellite-based rainfall data of Tropical Rainfall Measuring Mission (TRMM) was collected from 1 July to 30 August 2017. The numerical model was validated with the field monitoring for deformation and pore water pressure to obtain a reliable soil parameter. Several attempts have been made to obtain an acceptable of E parameter by determining the smallest root mean square error (RMSE) value of deformation and larger coefficient of relation (R ² ). The numerical model has been extended to evaluate the slope stability after remedial work using piles. Six rainfall scenario were applied to model. The analysis concludes that the piling works increases the slope stability under several rainfall scenarios, which is indicated by increase in factor of safety.
Many geotechnical analyses require the investigation of water flow within partially saturated soil zone to incorporate the effect of climatic conditions. It is widely understood that the hydraulic properties of the partially saturated soil should be included in the transient seepage analyses. However, the characteristics of dual porosity soils with dual-mode water retention curve are normally modelled using single-mode mathematical equation for simplification of the analysis. In reality, the rainwater flow can be affected significantly by the dual-mode hydraulic properties of the soil. This paper presents the variations of safety factor for dual porosity soil slope with dual-mode water retention curve and dual-mode unsaturated permeability. This paper includes the development of the new dual-mode unsaturated permeability to represent the characteristics of soil with the dual-mode water retention curve. The finite element analyses were conducted to examine the role of dual-mode water retention curve and dual-mode unsaturated permeability on the variations of safety factor under rainfall loading. The results indicate that the safety factor variations of dual porosity soil slope modelled using the dual-mode water retention curve and the unsaturated permeability equation are lower than those of dual porosity slope modelled using single-mode water retention curve and unsaturated permeability equations.
Seasonal variations in the soil moisture content can result in significant changes in soil suction. Rainfall infiltration can cause loss of matric suction in the unsaturated lateritic soil. Several failures are reported in lateritic soil slopes in Kerala, associated with rainfall. Geosynthetics are generally used for improving the stability of soil structures constructed with poorly draining soils. Shear strength at the interface of unsaturated lateritic soil with geosynthetics plays a crucial role on the internal stability of geosynthetics reinforced structures. The present study investigates the influence of rainfall-induced wetting on shear strength of lateritic soil and lateritic soil-geosynthetics interface. Lateritic soil was collected from a site in Kerala, which was subjected to rainfall-induced slope failure. Geotechnical characterization of the soil was carried out. Shear testing was conducted on soil samples of size 305 × 305 × 200 mm. When the moisture content was increased by 4% due to wetting, the shear strength of the lateritic soil was reduced by 20%. In contrast, the corresponding reduction in strength at the interface for the soil–geosynthetic system was only 3%. Similarly, an increase in moisture content by 8% due to wetting resulted in a reduction in strength by 30% and 4% for soil–soil and soil–geosynthetic systems, respectively.
With the advent of ever-growing urbanization and industrialization, there exists a requirement for heavy infrastructures that can retain heavy earth masses and are sustainable in its functioning. The conventional earth mass retention methods using rigid retaining walls are not preferred for most of the projects, as they are expensive and are time-consuming for the construction when compared to the recently developed methods of earth mass retention by Mechanically Stabilized Earth (MSE) structures. MSE walls having large height when constructed in a single tier, often require a huge volume of excavation and an effective land area which is impossible to attain every time. Therefore, the most suitable alternative is to construct it in a tiered fashion. The tiered MSE walls can tolerate large differential settlements without distress, give a sound performance, are aesthetically appealing, are cost-effective, convenient, and provide simplicity in construction. However, the configuration of such walls may present several engineering challenges that have not been covered by the conventional design methods and calculations. This study aims to assess the performance and response of a multi-tiered 12 m high (H) MSE wall and compare it with a single-tiered MSE wall through numerical solutions based on finite element modeling. From the outcomes of this study, it is found that the normalized maximum lateral displacement of the facing of the wall (Δ/Η) is 5.4% and 1.71% in the single-tiered and three-tiered wall system respectively. Also, the factor of safety in three-tiered and single-tiered wall systems observe a growth of 9.4% and 8.4% respectively when the reinforcement length is increased, which establishes the improved performance of the tiered MSE walls and justifies its usage in place of single-tiered MSE walls.
The land scarcity has built up the pressure on the engineers to bring a cost-effective and time-saving solution to utilize the ground with poor strength as a foundation bed for various structures. With the recent progress in the area of ground reinforcing techniques using geosynthetics, the extensive usage of geotextile materials as a reinforcing element in the soil to strengthen the load-bearing capacity of the soil mass and reducing the anticipated settlement of the footing pushes the researchers to evolve new methods to maximize the advantages received from the reinforced earth beds. In the above context, the provision of reinforcing layers with wraparound ends has brought additional improvement in the load settlement behavior of a strip footing resting over such reinforced soil mass but this recently developed technique lacks the appropriate guidelines/recommendations for the geometrical configuration parameters of the reinforcing layer to maximize the benefit from the reinforcing layer. Given the above, a comprehensive numerical study has been conducted to propose some recommendations on the geometrical configurations of the reinforcing layers. Furthermore, this study also investigates the influence of the geogrid–soil interface on the load-settlement response of the reinforced bed under vertical footing load. From the findings of the study, it is concluded that the width of the geogrid layers, governs the overall load-bearing capacity of the reinforced soil mass system, besides it, also suggests an optimum width of the geogrid layers, which equals 1.5 times the width of the footing should be used to maximize the effective utilization of the wraparound technique. Furthermore, it was also noted that appropriate assessment of the interface between soil and geogrid may bring an optimized design of the reinforced soil mass as a foundation bed for the footings.
Yazoo clay is highly plastic and has been associated in Mississippi with swelling problems and causes significant maintenance problems to the highway slopes. The infiltration of rainwater may create a perched water condition within the slopes, which may reduce the shear strength of the soil at the vadose zone. However, limited data is available on the wetting depth of highway slopes constructed on Yazoo clay in Mississippi. The objective of the current study is to determine the active moisture variation zone of highway embankment slopes constructed on Yazoo clay soil in Jackson, Mississippi. In this study, three highway slopes in the Jackson metroplex were considered as referenced slopes, which were monitored by using a rain gauge, moisture sensors, water potential probes in each slope. At each slope, sensors are installed at three different depths (1.5 m (5 ft.), 3 m (10 ft.), and 5 m (15 ft.)) at the crest, middle, and toe of the slope. The 3D flow analysis was also conducted using the Finite Element Method (FEM) in Plaxis 3D. The depth of moisture variation of the highway slopes was determined based on simulation of the unsaturated moisture flow and saturation of the slope existing soil test data and field instrumentation results. The field monitoring data indicated that the highest infiltration occurred at the crest of the slope, and the moisture variation zone continues up to 3.5 m (12 ft.). The 3D analysis showed that with the low-intensity long-duration rainfall, the progressive rainfall saturated the near-surface soil.
Rainfall-induced landslides occur more often with climate change, resulting in destruction and loss of lives in many parts of the world. Predicting such landslides is paramount to maintain local infrastructure and the well-being of communities. Several models have been proposed in the past years; however, most of them only work well when being applied to local site conditions for which they were developed. This work describes a new approach to estimate a model parameter related to shear strength conditions of local soils. To validate the theoretical concepts of a model, a series of shear box and undrained triaxial tests were conducted on soil specimens prepared at different values of water content. The refined model was then applied to three landslide sites to estimate the stability of slopes against past rainfall events. The obtained results showed more accurate predictions of landslide occurrence compared to the existing models. This paper presents and discusses field and laboratory data as well as the outcomes of numerical analysis.
Vegetation, particularly roots, serves different functions in relation to increased shear strength under saturated and partially saturated conditions. Quantification of mechanical contribution of roots due to their tensile strength, and relationships of various vegetation parameters and plant-induced suction, as well as shear strength, have been widely studied. Although shear strength is directly related to the volume change characteristics of soil, dilative or contractive behaviour of root-permeated soils has not been of significant interest so far. This study investigates how volume change during shearing is related to the hydrological and mechanical characteristics of vegetated soils relevant to slope stability and shear strength of root-permeated soils under partially saturated conditions. Direct shear tests, on specimens planted with a mixture of species from different plant functional groups, were performed with an Inclinable Large-scale Direct Shear Apparatus (ILDSA). Matric suctions were monitored throughout the test with tensiometers. Vertical and horizontal displacement graphs were plotted to investigate the volume change behaviour. Maximum dilatancy angle was found to be positively correlated with plant-induced suction and net normalised stress, both of which were linked to root biomass and the root:shoot ratio. It was found that maximum dilatancy is controlled by matric suction and net normal stress.
Identification of the influence of hydrological and geological factors on slope deformation characteristics (i.e., the displacement–time relationship) under rainfall plays a crucial role in providing early warning information and enabling the implementation of emergency remedial actions before a landslide occurs. In this study, a series of coupled hydro-mechanical finite element analyses were performed to investigate slope displacement behavior triggered by rainfall infiltration. First, the numerical model was validated by comparing the predicted displacement with those measured from a full-scale landslide flume test. A parametric study was then conducted, considering various hydrological conditions, and soil hydraulic and mechanical parameters which were statistically determined from a large soil database compiled from the relevant literature. Further, the influences of the aforementioned factors on the timing, magnitude, and rate of slope displacement prior to landslide occurrence were quantitatively evaluated in a sensitivity assessment. The numerical results indicated that the slope deformation characteristics could be significantly influenced by various hydrological and geological factors. Nevertheless, the slope displacement over time for all cases generally can be divided into three stages, namely the constant, accelerated, and critical deformation stages, which correspond to various states of slope movement and pore-water pressure development. The relationships of slope displacement magnitude and displacement rate with the factor of safety were established, which provide a valuable information in practice for engineers to interpret the slope stability level from a large quantity monitoring data of slope displacement.
Compacted soils constitute most engineering projects such as earth dams, embankments, pavements, and engineered slopes because of their high shear strength and low compressibility. The shear strength of compacted soils is a key soil parameter in the design of earth structures but it is seldom determined correctly due to their unsaturated state. The shear strength of compacted soils can be better evaluated under the framework of unsaturated soil mechanics. Saturated and unsaturated tests were conducted on compacted specimens using conventional direct shear apparatus under constant water content condition. Tests were conducted at different water contents and net normal stresses. The main objective of this study is to develop a shear strength model for compacted soils. Initial matric suction was measured before the test using the filter paper method. The two-stress state variables together with the extended Mohr-Coulomb failure criterion for unsaturated soils were used to obtain a lower bound model of the shear strength. The model was demonstrated using published data.
The Yazoo clay is dominant in the central Mississippi and neighboring states. This clay consists of very expansive characteristics and undergoes volumetric changes due to wetting or drying under different seasonal variations. Even though the Yazoo clay causes significant problems to the infrastructures in the southern states of the United States, limited research has been conducted to date on the volumetric changes of the Yazoo clay. This study investigates the change in the void ratio of Yazoo clay under a series of wet and dry cycles. Reconstituted Yazoo clay specimens were used for the experiment. The samples were subjected to 3, 5, and 7 number of wetting and drying cycles in an enclosed chamber for 24 hours period. During the drying process, the temperature ranged from 120–125 deg. F to simulate the typical high summer temperature of Mississippi. The axial deformation of the samples, as well as the change in void ratios at each cycle, were closely monitored. The test results indicated that the void ratio increases with the increase of wet and dry cycles and causes an overall reduction in the shear strength.
The bioengineering method using vegetation is an ecological approach for slope stabilisation. However, due to a large variability of vegetation root patterns, a precise quantification of root reinforcement is relatively difficult, leading to a reluctance to use such a technique in practice. This paper presents a probabilistic framework for slope stability analysis considering the spatial variability of root reinforcement. A residual soil slope under a heavy rainfall event was used to model the seepage and stability analysis. The effect of root reinforcement was considered through an additional soil shear strength or root cohesion. Typical characteristics of the root reinforcement of vetiver grass (Chrysopogon zizanioides) in Thailand were assumed in the analysis. A probabilistic analysis was performed considering both stationary and non-stationary random fields of root cohesion. The results indicated that the failure of the vegetated slope could occur when the variance coefficient of the root cohesion was more than a critical value (a critical cov = 0.45 for the uniformly distributed root cohesion case and a critical cov = 0.32 for the case of linear decrease of root cohesion in this particular slope). In practice, the efficiency of the bioengineering method can be improved by controlling the variation of root cohesion within such limits.
Assessment of rainfall-induced shallow slope failures is very important to reduce damages of infrastructures and lives of people living close to hazardous areas. Although the rainfall intensity-duration thresholds for initiation of slope failure (ID thresholds) based on the historical slope failure data is commonly used to assess slope failure, critical influence factors triggering shallow slope failures are often disregarded. Three sets of parametric study were therefore performed through finite element model to investigate the effect of saturated permeability of soil, slope angle and antecedent rainfall on instability of shallow slope. It is found that the hydrological mechanisms involving the rainfall induced shallow slope failure are either (1) the rising of water table mode or (2) the rainfall infiltration mode. The hydrological mode during the failure depends on the magnitude of rainfall intensity comparing with the infiltration capacity at soil saturation state. The rate of reduction in safety factor increases with an increasing the intensity of rainfall, only in a range of lower than the infiltration capacity at soil saturated state. As such the saturated permeability of the soil, which is equal to the infiltration capacity at soil saturated state, plays an important role in the shallow slope failure. The saturated permeability was found also to govern a range of applicability of the ID thresholds. If the rainfall intensity is not greater than the infiltration capacity at soil saturated state, the rainfall duration to failure (Trf) can be read from the ID thresholds. Slope angle and antecedent rainfall were found to play role on instability of the shallow slope. They control the initial stability of slope, which results in the different linear relationship of ID thresholds. In addition, the slope angle might accelerate the rate of rain water infiltration, and hence it reflects the slope of the ID thresholds
The paper reports on the influence of kaolin content on shear strength of a decomposed granitic silty sand. The granitic soil was mixed with kaolin, with the purpose of erosion protection and slope stabilization. Shear strength and water retention behaviour of the decomposed granitic silty sand, mixed with different proportions of kaolin (from zero up to 20% by weight) were tested. A tensiometer was also incorporated in a direct shear box to investigate unsaturated properties. The silty sand mixed with 10% kaolin possesses the highest saturated shear strength. The effective cohesion increases with increasing kaolin content, while the effective friction angle reaches the maximum value at the 10% kaolin. In the unsaturated condition, the angle of shear resistance with respect to suction increases with kaolin content. Based on the test results and field observation, the silty sand mixed with 10% kaolin appears to be most erosion-resistant.
Physical model tests using a large biaxial laminar shear box on the shaking table at the National Center for Research on Earthquake Engineering (NCREE), Taiwan was conducted to study the liquefaction behavior of saturated sand under one-and multi-directional earthquake shakings. Specimens of clean Vietnam silica sand and Mailiao sand with silt were prepared using specially designed pluviators. Model piles made of steel and aluminum pipes were also placed inside the shear box to evaluate the pile performances and soil-pile interaction within saturated Vietnam sand under shakings. The input shakings included sinusoidal and recorded earthquake accelerations. Pore water pressure changes and accelerations within the soil, displacements and accelerations of the shear box frames, bending and accelerations of the piles at various depths, pile top displacements were measured during the shaking table tests. Settlement of the sand surface after each shaking was also measured. Some analyses using the test results on soil liquefaction, settlement, behavior of pile in saturated sand under shaking were presented.
Rainfall-induced landslides are pervasive in hillslope environments around the world and among the most costly and deadly natural hazards. However, capturing their occurrence with scientific instrumentation in a natural setting is extremely rare. The prevailing thinking on landslide initiation, particularly for those landslides that occur under intense precipitation, is that the failure surface is saturated and has positive pore-water pressures acting on it. Most analytic methods used for landslide hazard assessment are based on the above perception and assume that the failure surface is located beneath a water table. By monitoring the pore water and soil suction response to rainfall, we observed shallow landslide occurrence under partially saturated conditions for the first time in a natural setting. We show that the partially saturated shallow landslide at this site is predictable using measured soil suction and water content and a novel unified effective stress concept for partially saturated earth materials.
This paper presents a review of long-term stability of stiff clay and clay shale slopes, and detailed reanalyses of 99 case histories of slope failures in 36 soft clays to stiff clays and clay shales. We analyzed 107 sections using the observed actual slip surface. In a first-time slope failure in clay or shale, part or all of the slip surface is unsheared prior to the occurrence of the landslide. Most stiff clays and clay shales contain stratigraphic discontinuities such as bedding planes and laminations. The fully softened shear strength is shown to be the lower bound for mobilized shear strength in first-time slope failures in homogeneous soft to stiff clays and on the slip surfaces cutting across bedding planes and laminations. For many of the first-time slope failures it appears that part of the slip surface is at the residual condition. For excavated slopes, the residual condition could be present before the final slope is formed, or it may develop in response to excavation by progressive deformation along nearly horizontal surfaces including bedding planes or laminations. In addition to the permeability dependent rise in porewater pressure, and softening, delayed first-time failure of slopes in stiff clays and clay shales is caused by propagation of the residual condition into the slope, on horizontal or subhorizontal surfaces including stratigraphic discontinuities. The residual condition is present on the entire surface of reactivated landslides.
1] We present a generalized framework for the stability of infinite slopes under steady unsaturated seepage conditions. The analytical framework allows the water table to be located at any depth below the ground surface and variation of soil suction and moisture content above the water table under steady infiltration conditions. The framework also explicitly considers the effect of weathering and porosity increase near the ground surface on changes in the friction angle of the soil. The factor of safety is conceptualized as a function of the depth within the vadose zone and can be reduced to the classical analytical solution for subaerial infinite slopes in the saturated zone. Slope stability analyses with hypothetical sandy and silty soils are conducted to illustrate the effectiveness of the framework. These analyses indicate that for hillslopes of both sandy and silty soils, failure can occur above the water table under steady infiltration conditions, which is consistent with some field observations that cannot be predicted by the classical infinite slope theory. A case study of shallow slope failures of sandy colluvium on steep coastal hillslopes near Seattle, Washington, is presented to examine the predictive utility of the proposed framework.
The concept of the suction stress characteristic curve SSCC for unsaturated soil is presented. Particle-scale equilibrium analyses are employed to distinguish three types of interparticle forces: 1 active forces transmitted through the soil grains; 2 active forces at or near interparticle contacts; and 3 passive, or counterbalancing, forces at or near interparticle contacts. It is proposed that the second type of force, which includes physicochemical forces, cementation forces, surface tension forces, and the force arising from negative pore-water pressure, may be conceptually combined into a macroscopic stress called suction stress. Suction stress characteris-tically depends on degree of saturation, water content, or matric suction through the SSCC, thus paralleling well-established concepts of the soil–water characteristic curve and hydraulic conductivity function for unsaturated soils. The existence and behavior of the SSCC are experimentally validated by considering unsaturated shear strength data for a variety of soil types in the literature. Its characteristic nature and a methodology for its determination are demonstrated. The experimental evidence shows that both Mohr–Coulomb failure and critical state failure can be well represented by the SSCC concept. The SSCC provides a potentially simple and practical way to describe the state of stress in unsaturated soil.
History of Shear Strength Failure Envelope for Unsaturated Soils Triaxial Tests on Unsaturated Soils Direct Shear Tests on Unsatured Soils Selection of Strain Rate Multistage Testing Nonlinearity of Failure Envelope Relationships Between ϕb and χ
Movements and failure of cuts and natural slopes constitute an important geotechnical problem that involves a variety of geomaterials in a variety of geological and climatic contexts, and which has a major socio-economic impact in many countries. The paper reviews the state of knowledge in this domain, examining the basic aspects of soil mechanics that are relevant to the context of slopes, the importance of water on slope behaviour, and then the different stages of slope movements: pre-failure, failure, post-failure and reactivation. Finally it is shown how the geotechnical characterisation of slope movements can be used, in particular for assessing risk associated with such movements. Emphasis is put on for the brittleness of soils and its practical implications for the progressive failure developing at the pre-failure stage and on the characteristics of post-failure movements. The influence of other factors such as creep, fatigue, destructuration, partial saturation and infiltration is also considered.
The slope stability issues concerning rainfall induced slope failures are investigated and presented. Specifically, the effect of both negative and positive pore water pressures on the stability of initially unsaturated slopes are carefully explained and coupled with infinite slope analysis methods in order to present a predictive formulation of slope failures that occur as a result of rainfall events. The formulation serves as a baseline analysis method for evaluating potentially unstable soil slopes that are subject to surface infiltration and explains the various triggering mechanisms that may occur based on individual combinations of the slope geometry, soil strength, and infiltration parameters. A procedural method is outlined for utilizing the analytical formulation to predict the change in the factor of safety for a slope subject to infiltration and a detailed analysis of a case study is presented to verify the method. Quantitative statements are made concerning the time and depth of failure in relationship to the soil, slope, and rainfall parameters.
The soil-water characteristic curve ~SWCC! has traditionally been represented using equations whose fitting parameters do not individually correspond to clearly defined soil properties or to features of the curve. As a result, unique sets of parameters are often nonexistent, and sensitivity analyses and statistical assessments of SWCC parameters become difficult. In order to overcome these difficulties, a new class of equations to represent unimodal and bimodal SWCCs is proposed. The chosen fitting parameters are the air-entry value, the residual suction, the residual degree of saturation, and a parameter that controls the sharpness of the curvatures. The physical meaning for the soil parameters is discussed for different soil types. A unique relation between each of the equation parameters and the individual features of SWCCs is assured. The proposed equations are fitted to data corresponding to a variety of soil types and a good fit is observed.
Factors controlling the strength of partly saturated cohesive soils
- A W Bishop
- I Alpan
- G E Blight
- I B Donald