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Performance of an innovative direct-push piston sampler in soft clay
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... The clay is characterized by its high compressibility and low undrained shear strength as measured in triaxial compression tests. Figure 9 presents the readings of a piezocone sounding conducted by Pineda et al. (2014). A characteristic effective friction angle ' = 33.7° is evaluated the NTH method and piezocone data from the site. ...
... CPTu sounding in soft clay at Ballina, Australia: (a) cone tip resistance; (b) sleeve friction; (c) porewater pressure (data fromPineda et al. 2014) ...
The undrained rigidity index of clays is evaluated from piezocone data using a hybrid cavity expansion and critical state formulation
... Initially a series of analysis was carried out for NC clay and KD value calculated from each analysis is equal to 2.03, which is in good agreement with the mean empirical value KD,NC ≈ 2 reported in literature (Marchetti, 1980, Pineda et al., 2014 Here, an attempt is made to correlate the input OCR values with the calculated KD. Results of these analyses are compared against published correlations and field measurements and are shown in Fig. 3a. ...
Horizontal stress index obtained from dilatometer test (DMT) is an important soil parameter and directly related to the coefficient of earth pressure at rest. In this study, commercial finite element software Abaqus is used to construct a numerical model of dilatometer test in overconsolidated (OC) clay. Tresca constitutive model is used to simulate undrained condition. Installation of DMT is simulated using cavity expansion principle. The change in KD value due to the variation of over consolidation ratio (OCR) with depth is computed to study the effect of stress history on KD in saturated clays. Simple expressions to estimate the OCR and at rest earth pressure coefficient for natural clays from flat dilatometer tests are presented and compared with that available in the literature.
Soil classification by piezocone penetration tests (CPTU) is mainly accomplished using empirical soil behavior charts (SBT). While commonly-used SBT methods work well to separate fine-grained soils from granular coarse-grained soils, in many instances, the groupings often fail to properly identify different categories of clays, specifically: (a)-regular‖ clays that are inorganic and insensitive, (b) sensitive and quick clays; and (c) organic soils. Herein, a simple means of screening and sorting these three clay types is shown using three analytical CPTU expressions for evaluating the preconsolidation stress profile from net cone resistance, excess porewater pressure, and effective cone resistance. A number of case studies are utilized to convey the methodology.
Atterberg limits are index parameters that can be determined using simple, inexpensive, and rapid laboratory tests. These limits are commonly used for the classifications of fine-grained soils. They may be also utilized in the preliminary assessment of strength and stiffness parameters of these soils, particularly at the initial phases of geotechnical investigation programs, ahead of conducting more advanced tests on undisturbed samples. Numerous studies in the geotechnical literature have been directed towards enhancing the relationships between Atterberg limits and the different mechanical parameters of fine-grained soils. However, there is still a necessity to improve the accuracy and applicability of such correlations. In this study, the undrained rigidity index is correlated with the plasticity indices using a comprehensive worldwide clay database. Subsequently, the effective friction angle is determined using an expression of the rigidity index derived from the Critical State Soil Mechanics. New formulations are developed for the undrained shear strength under compression, extension, and simple direct shear modes, as well as expressions for the constrained, shear, and Young’s elastic moduli. The deduced formulations are validated using the results of the geotechnical investigations for well-reported sites. A close agreement is demonstrated between the measured and the estimated parameters, implying that the proposed approach is broadly valid and applicable.
Due consideration of nonlinear soil stiffness is one of the essential requisites in many geotechnical applications. A considerable amount of the geotechnical literature, as well as numerous recent studies, have been directed towards enhancing our ability to formulate mathematical relationships expressing the stiffness nonlinearity of fine grained soils and to determine the controlling parameters of such relationships. At present, the parameters of the nonlinear clay stiffness formulations are commonly correlated to Atterberg's plasticity index, even though it is obtained from testing of remoulded samples, whereas the looked-for parameters are distinctly related to the undisturbed clay condition. In the presented study, a new approach is proposed to establish the link between the nonlinear stiffness parameters and the undrained rigidity index, as a more pertinent factor to the undisturbed compressibility of clays than the plasticity index. The proposed approach is utilised to predict the undrained nonlinear behaviour of several well-reported formations, including London clay, San Francisco Bay Mud, Skå-Edeby Clay, and Hachirougata Clay. The results show a good agreement between the predicted and the measured nonlinear stiffness implying that the presented methodology can be broadly applicable to different clay formations.
The fines content (FC) of a soil is needed in order to assess its susceptibility to liquefaction in earthquake engineering, as well as for evaluating its overall soil behavior. In the absence of soil sampling and laboratory testing, reliable in situ testing correlations offer a practical alternative. The cone penetration test (CPT) has been used to estimate FC through trends established with the soil behavior type index (Ic) and this paper presents a summary of published algorithms for this purpose. Several of these appear to give estimated FC with high uncertainty and/or results that are site-dependent. A large laboratory FC-CPT database is compiled from 53 well-documented test sites located in different countries. A global FC-CPT correlation is developed, and a new formulation is derived to improve current FC estimates using a modified CPT material index, IB.
A generalized methodology for evaluating the yield stress (σ 0 p) of soils from cone penetration tests (CPT) is presented in terms of net cone resistance (q net ¼ q t − σ vo) and material index, I c. The initial baseline expression is derived from a hybrid analytical solution combining spherical cavity expansion theory and critical state soils mechanics (SCE-CSSM) that gives a first-order estimate: σ 0 p ¼ 0.33q net for intact clays. A commonality is found with an algorithm derived from statistical regression analysis for CPT data from calibration chamber tests on clean quartz-silica sands that gives the approximation: σ 0 p ¼ 0.33q 0.72 net. A worldwide database of 93 field sites covering a wide diversity of geomaterials including clays, silts, sands, and mixed soil types was compiled to formulate a general relationship of the form: σ 0 p ¼ 0.33q m 0 net. The exponent m 0 decreases with mean grain size (D 50) and increases with both fines content (FC) and CPT material index (I c), specifically for uncemented, insensitive, and inorganic soils. Several case studies are presented to illustrate the application of the method to several well-documented sites.
The interrelationship between undrained shear strength (su) and downhole shear wave velocity (VsVH) of normally consolidated (NC) and lightly overconsolidated (LOC: OCR < 2) to overconsolidated (OC) to highly overconsolidated (HOC: OCR > 10) clays is investigated in the presented study. The main objective of this research program is to develop a worldwide database of high quality in-situ geophysical and laboratory strength data from thirty seven well-documented geotechnical sites from locations in Australia, and Vietnam. The study includes undrained shear strength measurements on undisturbed samples of normally to lightly overconsolidated intact to overconsolidated and fissured clays using anisotropically-consolidated triaxial compression tests (CAUC). Shear wave velocities were measured in the field by downhole tests (DHT), in many cases via seismic piezocones (SCPTu). Analyses of the compiled database found approximate trends between undrained shear strength and shear wave velocity. Tentative correlations are explored by including other various parameters such as Atterberg limits, void ratio, overconsolidation ratio (OCR), and effective vertical stresses. The correlative trends may aid geotechnical engineers in helping to assess su profiles in clay deposits in preliminary investigations and as an independent method in collaboration with sampling, lab testing, and other field data.
In this study, an established effective stress limit plasticity solution for piezocone penetration tests (CPTu) in clay developed by the Norwegian Institute of Technology (NTH) for evaluating the effective stress friction angle (ϕ⁰) is extended to flat plate dilatometer tests (DMT) readings. A nexus between CPTu and DMT is built through spherical cavity expansion solutions for undrained penetration to link the cone tip resistance (qt) and shoulder porewater pressure (u2) from CPTu to the measured contact pressure (p0) and expansion pressure (p1) obtained from DMT in soft to firm intact clays. Data from 49 paired sets of CPTu-DMT soundings in a variety of clays are used to support and validate the links. A variety of soils ranging from lean to plastic clays and clayey silts from marine, alluvial, lacustrine, deltaic, and glaciofluvial origins are subjected to flat DMTs, and the measurements are utilized to evaluate the effective stress friction angle (ϕ⁰). Data from 46 clays are compiled to examine the DMT-interpreted ϕ⁰ values in comparison with laboratory benchmark ϕ⁰ values obtained from CAUC or isotropically consolidated undrained triaxial compression tests on high-quality samples. An approximate inversion of the theoretical solution is derived to allow ϕ⁰ profiles to be determined with depth. Four well-documented DMT examples, including two natural clay sites and two chamber test series (one pressurized and the other unpressurized), are presented to illustrate the NTH procedures.
The paper presents the results from a preliminary study focused on the influence of pore fluid salinity on the shear strength of a soft clay from New South Wales (Australia). Direct shear tests (DST) were carried out on natural specimens using pore fluids with different salt concentrations. Electrical conductivity measurements were performed on the squeezed pore water after shearing to quantify changes in salinity of the soil pore water. Strength parameters showed a clear dependency of the pore fluid. Changes in c' and φ appear to be controlled by two factors: (i) initial salinity, and (ii) extent of the concentration gradient. Results from mercury intrusion porosimetry tests demonstrated the influence of changes in pore water salinity on the evolution of the dominant pore size. It was in agreement with macroscopic observations. Implications for further laboratory testing are highlighted in the paper.
A method of interpreting conventional oedometer test data using work per unit volume as a criterion for determining both in situ effective and yield stresses in clay is presented. This technique was applied to the results of oedometer tests carried out on samples of natural clay deposits and on specimens consolidated anisotropically from a slurry to a known effective stress state. The work per unit volume – effective stress relationship, using arithmetic scales, can be approximated or fitted using linear relationships. The intersections of these fitted lines are demonstrated to provide accurate values for in situ current and yield (preconsolidation) stresses. The yield stress is defined as the intersection of the initial fitted line and the linear relationship observed at higher stresses. The current effective stress is indicated by the first significant divergence of the data from the initial fitted line. These relationships apply to both conventionally (horizontally) trimmed specimens and to vertically trimmed oedometer samples. It is hypothesized that the in situ effective and yield stresses (in both the vertical and horizontal directions) in a natural clay can be determined by the work per unit volume interpretation of oedometer tests carried out on horizontally and vertically trimmed specimens. Key words: in situ, stress, yield, oedometer, interpretation, clays, work, state, K0, preconsolidation pressure.
The Sherbrooke block sampler has been used at several clay sites in Norway. Samples down to 22 m below ground level have successfully been obtained. Standard piston sampling with the 54 mm NGI sampler have been obtained at the same sites. 75 mm and 95 mm piston samples have been obtained in some cases. Piezocone test results are also available. Subsequent laboratory testing (CAUC triaxial and CRS oedometer tests) clearly show the superior quality of the block samples compared to the piston samples; this is at least the conclusion to a depth of 16 m where comparative results are available The effects of sample disturbance are especially dramatic for low plasticity clays. The results are described in detail and the soil parameters that are most effected by sample disturbance are identified. The results are used to develop a new criteria for evaluation of sample disturbance. It is also stressed that results of CAUC and CRS tests on piston samples should be corrected to reduce the effects of sample disturbance. Soil parameters as measured on the block samples are used as reference parameters for a new generation of correlations for interpreting CPTU results.
A method of interpreting conventional oedometer test data using work per unit volume as a criterion for determining both in situ effective and yield stresses in clay is presented. It is hypothesized that the in situ effective and yield stresses (in both the vertical and horizontal directions) in a natural clay can be determined by the work per unit volume interpretation of oedometer tests carried out on horizontally and vertically trimmed specimens. -from Authors
Results of tests on samples of Speswhite kaolin clay show that for analysis of bender element tests the length of the travel path of the shear waves should be taken as the distance between the tips of the elements. Results of numerical analyses of the signals from bender element tests on a reconstituted sample of a boulder clay demonstrate that the travel time should not be taken as the time corresponding to the first deflection of the received signal. For simple analyses the first arrival of the shear wave can be taken as the point of the first inversion of the received signal. From the present results this may lead to overestimation of the shear modulus of up to about 14%. It is important to recognize that a simple analysis based on the first inversion of the received signal has been validated only for tests on overconsolidated boulder clay in a particular apparatus. For different soils, detailed numerical analyses should be carried out to determine the point on the received signal that corresponds to the first arrival of the shear wave. -from Authors
International Manual for the Sampling of Soft Cohesive Soils, ISSMFE subcommittee on soil sampling eds (2013) Australia's first national facility for soft soils testing. Civil Engineers Australia
ISSMFE (1981) International Manual for the Sampling of Soft Cohesive Soils, ISSMFE subcommittee on soil sampling eds,
Tokyo
Kelly, R.B. (2013) Australia's first national facility for soft soils testing. Civil Engineers Australia, June, 76-78.
International Manual for the Sampling of Soft Cohesive Soils, ISSMFE subcommittee on soil sampling eds
- Issmfe
ISSMFE (1981) International Manual for the Sampling of Soft Cohesive Soils, ISSMFE subcommittee on soil sampling eds,
Tokyo
Australia's first national facility for soft soils testing
- R B Kelly
Kelly, R.B. (2013) Australia's first national facility for soft soils testing. Civil Engineers Australia, June, 76-78.