Article

Centrifuge scaling considerations for fluid-particle systems

Emerald
Geotechnique
Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Two simulations are involved when a centrifuge is used to test models. First, the behaviour of the model in a uniform ngfield is assumed to be similar to that of the prototype. Then the centrifuge is assumed to produce an equivalent nggravitational field. For most static problems, the centrifuge does produce an equivalent ng gravitational field, but for some dynamic problems involving saturated soil these assumptions can break down. When the soil particles and fluid are moving relative to one another, the behaviour in the ngfield is not similar to that in the 1gfield unless the Reynolds number in both conditions is less than unity. Since this is a special circumstance, the centrifugal behaviour is not similar to that of the prototype in most cases. To illustrate this, the similarity requirements are examined for a single particle moving in a fluid. If different fluids are used in the model and prototype, then the difference in densities must also be accounted for. Les essais de modèles dans une centrifugeuse impliquent deux simulations. Premièrement, on suppose que le comportement du modèle dans un champ uniforme ngest similaire à celui du prototype. Ensuite, on admet que la centrifugeuse produit un champ gravitationel équivalent à ng.Dans la plupart des cas, la centrifugeuse produit effectivement un champ gravitationel équivalent à ngmais pour certains problèmes dynamiques concernant des sols saturés ces hypothèses peuvent être erronées. Le comportement à l'intérieur de ce champ n'est analogue à celui de lg que si le nombre de Reynolds est inférieur à l'unité. Le comportement à l'intérieur de la centrifugeuse n'est pas semblable à celui du prototype. Si on utilise différents fluides pour le modèle et le prototype, alors il faut tenir compte de la différence de densité.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... In general if a structure (particle, section of failing slope) has a significant velocity relative to the centrifuge platform the dynamics must include Coriolis effects. The influence of Coriolis forces on centrifuge model tests has been pointed out in many papers (e.g., Schofield, 1980;Tan andScott, 1985 andSteedman andZeng, 1995). Tan and Scott (1985) examined the problem of modelling liquefaction in a centrifuge. ...
... In general if a structure (particle, section of failing slope) has a significant velocity relative to the centrifuge platform the dynamics must include Coriolis effects. The influence of Coriolis forces on centrifuge model tests has been pointed out in many papers (e.g., Schofield, 1980;Tan andScott, 1985 andSteedman andZeng, 1995). Tan and Scott (1985) examined the problem of modelling liquefaction in a centrifuge. ...
... The influence of Coriolis forces on centrifuge model tests has been pointed out in many papers (e.g., Schofield, 1980;Tan andScott, 1985 andSteedman andZeng, 1995). Tan and Scott (1985) examined the problem of modelling liquefaction in a centrifuge. An analytical study of a simplified situation was carried out. ...
Article
Full-text available
Rockfall, and its subsequent motion down the slope, often poses a serious hazard for people and infrastructure. This paper describes the development of a rockfall generation system used to investigate the rockfall behaviour and dynamic process of the overall trajectory in centrifuge model tests. Firstly, the experimental results are compared with the theoretical movement of a free fall event under the hypothesis that this occurs fully in a rotational acceleration field. Secondly, a series of centrifuge model tests were performed to observe the movement of falling boulders, which exhibit various falling modes: free falling, bouncing, rolling or sliding, sometimes associated with fracturing during the falling process, which is not taken into account in most numerical simulations. In order to investigate the influence of a boulder fracturing on the falling velocity and bouncing height, tests were carried out using different parameters for boulder material (stiffness and shape), with the trajectory measurements of the falling boulder captured by a high­speed video camera. The test results revealed that the falling velocity, and the bouncing height in the case of fractured boulders, were larger than those for rolling ones (unfractured).
... Singh and Gupta, 2000; Nakajima and Stadler, 2006 ). Incorporating the scaling within the energy gradient that drives flow, as is the approach in this study, is consistent with the fact that if a pressure gradient was applied to a core sample under zero gravity conditions, the hy- 10 draulic conductivity would remain equal to the coefficient of proportionality between the applied gradient and the flow rate (; Taylor's discussion in Tan and Scott, 1987). For physical modelling of processes such as soil liquefaction however, the scaling that is required for accelerated gravity experiments, could be more appropriate within expressions of permeability or viscosity (Tan and Scott, 1987). ...
... Incorporating the scaling within the energy gradient that drives flow, as is the approach in this study, is consistent with the fact that if a pressure gradient was applied to a core sample under zero gravity conditions, the hy- 10 draulic conductivity would remain equal to the coefficient of proportionality between the applied gradient and the flow rate (; Taylor's discussion in Tan and Scott, 1987). For physical modelling of processes such as soil liquefaction however, the scaling that is required for accelerated gravity experiments, could be more appropriate within expressions of permeability or viscosity (Tan and Scott, 1987). ...
... was applied to the core samples at 1g. This increase in flow rate is consistent with scaling laws for physical modelling (Tan and Scott, 1987), and is greater than would occur within aquitards where the vertical hydraulic gradient is typically less than one. An in situ vertical hydraulic gradient determined from measurements in piezometers that are located above, below and within the aquitard sequence can provide hydraulic 25 gradients that are typically higher than hydraulic gradients for lateral flow. ...
Article
Full-text available
Evaluating the possibility of leakage through low permeability geological strata is critically important for sustainable water supplies, extraction of fuels from strata such as coal beds, and confinement of waste within the earth. Characterizing low or negligible flow rates and transport of solutes can require impractically long periods of field or laboratory testing, but is necessary for evaluations over regional areas and over multi-decadal timescales. The current work reports a custom designed centrifuge permeameter (CP) system, which can provide relatively rapid and reliable hydraulic conductivity (K) measurement compared to column permeameter tests at standard gravity (1g). Linear fluid velocity through a low K porous sample is linearly related to g-level during a CP flight unless consolidation or geochemical reactions occur. The CP module is designed to fit within a standard 2 m diameter, geotechnical centrifuge with a capacity for sample dimensions of 30 to 100 mm diameter and 30 to 200 mm in length. At maximum RPM the resultant centrifugal force is equivalent to 550g at base of sample or a total stress of ~2 MPa. K is calculated by measuring influent and effluent volumes. A custom designed mounting system allows minimal disturbance of drill core samples and a centrifugal force that represents realistic in situ stress conditions is applied. Formation fluids were used as influent to limit any shrink-swell phenomena which may alter the resultant K value. Vertical hydraulic conductivity (Kv) results from CP testing of core from the sites in the same clayey silt formation varied (10-7 to 10-9 m s-1, n = 14) but higher than 1g column permeameter tests of adjacent core using deionized water (10-9 to 10-11 m s-1, n = 7). Results at one site were similar to in situ Kv values (3 × 10-9 m s-1) from pore pressure responses within a 30 m clayey sequence in a homogenous area of the formation. Kv sensitivity to sample heterogeneity was observed, and anomalous flow via preferential pathways could be readily identified. Results demonstrate the utility of centrifuge testing for measuring minimum K values that can contribute to assessments of geological formations at large scale. The importance of using realistic stress conditions and influent geochemistry during hydraulic testing is also demonstrated.
... In a different approach the increase in seepage velocity is accounted for the N times increased hydraulic conductivity k and unchanged i [Tan and Scott, 1985]. Thusyanthan and Madabhushi [2003] showed that when substituting the hydraulic conductivity with the intrinsic permeability K and separating the conductivity from the energy gradient driving the flow, this term is independent of the applied acceleration. ...
... At first, if pore water is replaced by an N times more viscous liquid, the seepage velocity is scaled by 1, and seepage time is reduced by 1/N [Rowe and Craig, 1979], [Tan and Scott, 1985]. The substitution with a viscous liquid is desirable in the modelling of dynamic problems, e.g., dissipation of PWP after earthquakes. ...
... Butterfield [32] and Thusyanthan and Madabhushi [30] considered the scaling factor of the hydraulic conductivity to be 1 from the energy gradient (pressure gradient) perspective. However, Tan and Scott [33] and Cargill and Ko [34] suggested that the scaling factors of the hydraulic gradient and the hydraulic conductivity were 1 and n, respectively. Garnier et al. [35] also suggested that the scaling factor of the hydraulic conductivity was n, which has been approximately verified experimentally [36]. ...
Article
Full-text available
Accurate characterization of soil hydraulic conductivity influenced by temperature under a centrifugal environment is important for hydraulic and geotechnical engineering. Therefore, a temperature-influenced scaling law for hydraulic conductivity of soil in centrifuge modeling was deduced, and a temperature-controlled falling-head permeameter apparatus specifically designed for centrifuge modeling was also developed. Subsequently, a series of temperature-controlled falling-head tests were conducted under varying centrifugal accelerations to achieve the following objectives: (1) examine the performance of the apparatus, (2) investigate the influence of temperature and centrifugal acceleration on the hydraulic conductivity of sand and its scaling factor, and (3) validate the proposed scaling law for hydraulic conductivity. The main conclusions of the study are as follows. Firstly, the apparatus demonstrated good sealing and effectively controlled the temperature of both the soil specimen and the fluid. Secondly, the hydraulic conductivity of sand was not constant but varied over time, likely due to the presence of radial seepage in addition to vertical seepage as the test progressed. Thirdly, temperature significantly influenced the hydraulic conductivity of sand and its scaling factor under the same centrifugal acceleration. Therefore, it is essential to closely monitor the temperature of models during centrifugal tests. Finally, the measured and calculated values of the scaling factor index for the hydraulic conductivity of sand showed good agreement, verifying the proposed scaling law.
... Coriolis acceleration (Schofield 1980) should be taken into account in designing in-flight simulation devices in geotechnical centrifuges. The Coriolis effect is not effectively mitigated in most embankment modeling (Beasley and James 1976;Davies and Parry 1985;Scott and Tan 1985;Allersma, Ravenswaay, and Vos 1994;Nguyen, Takeyama, and Kitazume 2016b). As the sand falls freely in a centrifuge, it has radial velocities relative to the rotating model box and will be subject to Coriolis acceleration. ...
Article
This study develops a novel apparatus to simulate the staged, automatic construction of embankments in geotechnical centrifuges with improved measurement accuracy. Modular test equipment was designed to incorporate the functionality of controllable and precise sand release; its structural components include a box-like sand storage module, a push-pull sand release module, and a stepper motor control module. Emphasis was placed on the design of segmented deflectors, the angles of which were derived with rigorous mathematical analysis to mitigate the Coriolis effect on falling sand. Calibration tests demonstrate that the post-construction embankment geometry approximates its target shape with a relative error of 0.72% to 7.37% in different construction phases at an acceleration of 60 g. A trial was conducted on a staged embankment over the deformable ground, and the displacement of this system was monitored and characterized by tracking reflective marker technology. The results indicate that the proposed method is effective and reflects real-life situations.
... The model columns were constructed from plaster, fine sand, and water (Knappett et al. 2011). Traditional methods for manufacturing column models with mortar or plaster (Scott and Tan 1985;Nguyen et al. 2016b) result in small-diameter model columns that are highly susceptible to damage during fabrication or driving. As shown in Fig. 14(a), a 3-mm-diameter carbon rod with flexural strength of 19.2 MPa was embedded in each model column. ...
Article
Full-text available
Earth platforms supported by rigid columns provide an effective solution for embankments founded on thick deposits of soft soils. Nevertheless, the failure mechanisms and associated soil-structure interactions remain inadequately comprehended, particularly for structures such as columns with caps or ground beams. Recent studies have reported sporadic incidents of basal instability in rigid column-supported embankments over soft soil. To gain further insight into this issue, this study investigates the foundation settlement, failure mode of rigid columns, and ground stability under staged embankment construction by centrifuge model tests. Different ground improvement schemes (head conditions) are compared, including columns, capped columns, and columned ground beams, along with natural ground as a base model. Test results show that the potential slip surfaces for embankments on soft ground are almost circular in shape, with or without capped column support. Rigid columns exhibit different failure modes across the improved domain: compression failure under the crest, bending failure under the shoulders, and tensile/bending failures near the toe. Under critical conditions, the columns near the toe collapse first, followed by the adjacent columns beneath the slope and center of the embankment. In contrast, the column failure sequence is unclear for beam scenario. Capped columns were found to reduce the foundation displacement significantly. Given equivalent area coverage ratios, columned beams demonstrate greater efficacy in controlling ground deformation than capped columns. The interconnecting beams form a strong foundation that mitigates overstressing of subsoil, thus reducing differential and final settlements. Finally, the stability of the ground reinforced with rigid columns under embankment loading is discussed, taking into account the fracturing of columns and the function of caps and ground beams.
... Since water was used as a pore fluid, to allow for an accurate replication of the experiment performed at a 50 g centrifugal acceleration, the adopted permeability coefficient in the numerical analysis should be upscaled by 50 times. However, as described by Tan and Scott [52], there is a conflict between the timescales of dynamic pore pressure generation and diffusion during centrifuge experiments. Hence, many researchers (e.g. ...
Article
This paper presents the verification of a new bounding surface plasticity model with reversal surfaces when used in boundary value problems related to liquefaction. The model is implemented in a commercial finite-difference code using an explicit stress integration algorithm. After validating the model against element tests on Nevada sand, the paper employs the same set of values of model parameters for ascertaining the model's simulative potential at the system level, via comparisons with data from 8 dynamic centrifuge tests performed on the same sand. These pertain to: a) the free-field response of a horizontal liquefiable layer, b) the lateral spreading response of a gently sloping liquefiable layer and c) the seismic response of rigid foundations on a single-layered and two-layered horizontal liquefiable ground profiles. Parametric analyses emphasize the effects of non-constant sand permeability coefficient and excitation intensity on the liquefied system response. Similar analyses highlight the importance of post-liquefaction strain accumulation and the avoidance of stress-strain overshooting, both constitutive attributes of the new model.
... Thus, the results are not scaled in this study. Tan and Scott (1985) found that hydraulic conductivity is not scaled under centrifugal acceleration. The distribution of the theoretical hydraulic conductivity (using equation (6)) through the chalk sample is shown in Figure 8. ...
Article
Full-text available
In this study, a geotechnical centrifuge has been used to study the unsaturated hydraulic properties of rock samples from the chalk aquifer, south east England. This method allows rapid measurement of hydraulic properties in a controlled environment, in contrast to previous studies on the chalk unsaturated zone which required either an extended period (years) of data monitoring in the field, or extended experimental periods (weeks) in the laboratory. Three types of specially built sensors were used to monitor water flow through chalk samples: a water pressure transducer to measure matric potential, frequency domain reflectometry probes to measure the volumetric water content, and pressure transducers to measure the volume of water passing through the sample. Chalk samples were tested during wetting and draining processes in order to understand any hysteresis occurring during periodic recharge of the aquifer. Before undertaking physical tests of chalk samples in the centrifuge, a theoretical model of chalk hydraulic behaviour under centrifugal force was developed. This model was used to define and justify the instrumentation plan of the physical model and predict the shape of the soil moisture characteristic (SMC) curve and unsaturated hydraulic conductivity ( K u ) function. The results were then evaluated and compared to the experimental results. These results show that chalk samples can be successfully tested under centrifuge conditions and hydraulic properties can be measured, including soil moisture characteristic curves and hydraulic conductivity. Supplementary material: Calculation spread sheets are available https://doi.org/10.17033/DATA.00000283
... This is the primary advantage in using a centrifuge. Scaling considerations for 1/N scaled models in the Ng centrifuge environment are well established to study geotechnical problems [19][20][21][22][23][24][25]. For a more detailed discussion on scaling this problem in a centrifuge, see Exton et al. [20]. ...
Article
Full-text available
Tsunami hazards have been observed to cause soil instability resulting in substantial damage to coastal infrastructure. Studying this problem is difficult owing to tsunamis’ transient, non-uniform and large loading characteristics. To create realistic tsunami conditions in a laboratory environment, we control the body force using a centrifuge facility. With an apparatus specifically designed to mimic tsunami inundation in a scaled-down model, we examine the effects of an embedded impermeable layer on soil instability: the impermeable layer represents a man-made pavement, a building foundation, a clay layer and alike. The results reveal that the effective vertical soil stress is substantially reduced at the underside of the impermeable layer. During the sudden runup flow, this instability is caused by a combination of temporal dislocation of soil grains and an increase in pore pressure under the impermeable layer. The instability during the drawdown phase is caused by the development of excess pore-pressure gradients, and the presence of the impermeable layer substantially enhances the pressure gradients leading to greater soil instability. The laboratory results demonstrate that the presence of an impermeable layer plays an important role in weakening the soil resistance under tsunami-like rapid runup and drawdown processes.
... Hereby an effective permeability of the soil samplei. e. kf ≈ 1•10 -4 m/sat 100g is targeted, which is representative of typical North Sea sands (Tan and Scott, 1985). The viscosity was increased through the addition of methyl cellulose ether (DOW, 2002). ...
Conference Paper
Offshore wind turbines supported by suction caisson jackets transfer the loading to the foundation primarily through a vertical push-pull mechanism. This leads to the foundation performance under vertical cyclic loading to be a critical design aspect, with recommended design practice usually limiting the tensile capacity in sandy soil to the drained frictional resistance. Recent research of suction caissons under cyclic vertical loading into tension in sand has revealed the complexity of the load transfer mechanisms and the importance of understanding the soil permeability governing the in-service foundation performance. Soil stratification, with sand and clay layers, adds further complexity. Numerical methods only provide rough estimates in the in-situ caisson performance. The current paucity of data of suction caisson in-service performance in layered soil under loading conditions relevant to offshore wind turbines calls for physical evidence to be gathered. This is addressed here through centrifuge tests on vertical cyclic loading, which unravel the actual impact of tensile loads below and above the drained frictional capacity. The vertical cyclic loading test results indicate that not just the development of suction pressure inside the caisson foundation, but also the interplay between tension and compression have a significant effect on the overall response of suction caisson under cyclic vertical loading in layered soils.
... All the centrifuge tests in this study were conducted with a 25 g centrifugal acceleration. The consequence in using the pore fluid with a viscosity ν times higher than that of water in the centrifuge tests at 25 g to model the liquefaction of the water-saturated prototype soil in the field is that the actual prototype permeability being simulated was k prototype = k/ν·25 (Tan and Scott, [21]). ...
Article
Full-text available
Repeated small shaking events due to earthquakes significantly enhance liquefaction resistance of soils. Analyses of liquefaction case histories show that aged soils in seismically active zones tend to be less vulnerable to liquefaction despite having similar index parameters—such as standard penetration test N-values and shear wave velocities—as young soils. Significant efforts have been devoted to better understand the effects of the cyclic pre-shearing on liquefaction resistance and it was found that this effect depends on the number of cycles and cyclic stress ratio. However, none of these parameters quantify the improvement of liquefaction resistance due to pre-shaking. This study investigates the pre-shearing effects on liquefaction resistance through laboratory tests and centrifuge tests. An attempt was made to explain the effects quantitatively with a single index parameter of the volumetric strain caused by pre-shearing. It was confirmed from triaxial tests that the liquefaction resistance of pre-sheared sand uniquely increased with increasing volumetric strain regardless of the cyclic shear stress ratio and the number of cycles during the pre-shearing. To examine the pre-shaking effects on the liquefaction strength of sand under a level ground condition, centrifuge tests were conducted in this study. Sand models were subjected to small shaking events repeatedly, which were weak enough not to cause liquefaction. It was observed that changes in the index parameters of the models, including soil density (volumetric strain), shear wave velocity, and horizontal earth pressure during the pre-shaking events were very small. At the end of the test, the sand was subjected to a strong shaking event because models that had gone through pre-shaking need larger shaking acceleration to liquefy. Liquefaction resistance was derived from acceleration records with the aid of the cumulative damage theory. The relationship between liquefaction resistance ratio and volumetric strain that occurred in the pre-shaking events coincides with the relationship obtained from the triaxial tests. After the extensive liquefaction event, all index parameters except soil density—K0, Vs, liquefaction resistance—tended to return to their original values (before the pre-shaking).
... Hereby an effective permeability of the sand layeri. e. kf ≈ 1•10 -4 m/s at 100g was targeted, which is representative of typical North Sea sands (Tan and Scott, 1985). The viscosity was increased through the addition of methyl cellulose ether (DOW, 2002). ...
Conference Paper
Offshore wind turbines supported by suction caisson jackets transfer the loading to the foundation primarily through a vertical push-pull mechanism. This leads to the foundation performance under vertical cyclic loading being a critical design aspect. Recent research of suction caissons under cyclic vertical loading into tension in uniform soil profiles has revealed the complexity of the load transfer mechanisms and the importance of understanding the soil permeability and shear strength governing the in-service foundation performance. Soil stratification, with clay and sand layers, adds further complexity. Numerical modelling of the suction caisson under vertical cyclic loading is complex, challenging and the consideration of individual aspects may not necessarily be required. Simplified numerical models, once validated, can provide insights into key aspects of the load bearing behaviour. However, the identification of those aspects and the validation of a numerical model requires an experimental database that does not exist to date. The current paucity of data of suction caisson in-service performance in sand over clay under loading conditions relevant to offshore wind turbines calls for physical evidence to be gathered. This is addressed here through centrifuge tests on vertical cyclic loading, which unravel the actual impact of tensile loads below and above the recommended design capacities. The vertical cyclic loading test results indicate that not just the development of suction pressure inside the caisson foundation, but also the interplay between tension and compression have a significant effect on the overall response of suction caissons under cyclic vertical loading in sand over clay.
... In order to satisfy the geotechnical centrifuge scaling laws (Tan and Scott 1985;Kutter 1992;Taylor 1995), the sand layer was saturated with a viscous fluid having a viscosity of either 20 or 60 cp, depending on the g level to be applied to the centrifuge model (Fig. 1). The difference in viscosities was compensated by running the experiments at different g levels (1 atm at 20g and 6 atm at 60g) yielding identical prototype permeabilities. ...
... Thus, the hydraulic conductivity results are not scaled in this study. Tan and Scott (1985) find that the hydraulic conductivity is not scaled under the centrifuge acceleration. The distribution of the theoretical hydraulic conductivity (using eq. ...
Thesis
Full-text available
The Chalk aquifer is one of the main sources of water in South East England. In the city of Brighton and Hove, it provides about 98% of total water consumption. It is also a significant aquifer in North East England, Northern France and Denmark. The Chalk unsaturated zone plays an important role in the water cycle, controlling the timing and magnitude of recharge and the main pathway of contaminant transport. A range of previous work has addressed flow processes in the Chalk unsaturated zone, but our physical understanding is still incomplete. This thesis attempts to understand the flow mechanism in the Chalk unsaturated zone using statistical analysis and novel laboratory methods. New laboratory scale instruments have been used that are suitable for this condition and can measure matric potential and volumetric water content simultaneously. In addition, a statistical analysis has been applied to field recorded rainfall, potential evapotranspiration, groundwater level and groundwater electrical conductivity at three sites in the study area, North Heath Barn, Pyecombe East and Preston Park. With help of a physically based model, flow and recharge in the study area were simulated using parameters obtained from laboratory and field observation analysis. Results show that flow and recharge occur predominantly through the Chalk matrix, which requires months till the pulse of water reaches the water table. The slow matrix flow occurs when the Chalk unsaturated zone is not completely saturated, and it will continue through summer even when the rainfall has ceased. Faster flow occurs during recharge seasons when the Chalk unsaturated zone is saturated or close to saturation. At that point, flow occurs through the matrix by the action of piston flow and the time lag can be counted in days. Relatively modest increases in effective rainfall during winter trigger fracture flow which minimizes the time lag to hours. Delay in response and slower drainage results from the influence of the subsurface geology, mainly the presence of marl seam and fracture discontinuities. Simulating results from the laboratory and statistical analysis using a physically based model (MODFLOW) shows that the model poorly matches the observed data, but it performs well at the scale of seasonal variation.
... The sand was saturated from the base of the sand layer on the laboratory floor with a solution of water and 1.4% cellulose ether, to obtain a viscosity of 100 cSt (at a temperature of 20°C, DOV 2002). Matching the fluid viscosity with the centrifuge acceleration of 100g allowed the drainage properties to be scaled correctly , Taylor 1987, Dewoolkar et al. 1999, Bienen et al. 2018. ...
Article
Full-text available
Suction buckets are becoming established as a viable foundation solution for offshore wind turbines. In sand, suction-induced seepage flow reduces effective stresses at the skirt tips, which decreases penetration resistance. However, layered seabeds are often encountered in areas of offshore wind farm development. The effect of the presence of a clay layer on the suction-induced seepage flow in the sand layer is not well understood. Therefore in this study, the effects of a clay layer on suction bucket installation in dense sand was investigated. This was achieved by analyzing images of a half-bucket installed against a Perspex window. The images were captured during tests performed in a geotechnical centrifuge, such that the stress levels are realistic and relevant to field conditions. Installations in sand-over-clay were unproblematic and characterized by deformation of the sand-clay interface, with no clear interruption of the seepage flow. Installations in clay-over-sand were also successful. Uplift of the clay plug was identified as the mechanism to transfer suction to the underlying sand, creating seepage flow and thus facilitating further skirt penetration rather than terminating the installation.
... The lead shot 146 stratum, with a high dry unit weight of 62.6 (kN/m 3 ), had two roles: (i) to provide, through its 147 static weight, the required 1 atm or 6 atm at the middle of the liquefiable sand stratum before, 148 during and after shaking; while maintaining a free drainage boundary; and (ii) to provide through 149 its heavy mass during shaking, the necessary horizontal inertia forces and corresponding 150 horizontal dynamic shear stresses in the sand layer, which had to be very high during the 6 atm 151 tests. To achieve the target values of σ'v0 = 1 atm and 6 atm at the mid depth of the sand layer, 152 two different combinations of lead shot layer thickness and g-level were used, as shown by In order to satisfy the geotechnical centrifuge scaling laws (Tan andScott 1985, Kutter 1992, 167 Taylor 1995), the sand layer was saturated with a viscous fluid having a viscosity of either 20 cp 168 or 60 cp, depending on the g-level to be applied to the centrifuge model (Fig. 1). The difference 169 in viscosities was compensated by running the experiments at different g-levels (1 atm at 20g 170 and 6 atm at 60g) yielding identical prototype permeabilities. ...
Conference Paper
This paper introduces a series of centrifuge tests—under low and high confining pressures—designed to study the effect of effective overburden pressure on liquefaction potential of clean sand. All the centrifuge tests simulate about 5 m saturated clean sand deposit under effective overburden pressure of 1 and 6 atm with relative density ranging from 45% to 80%. To achieve the targeted overburden pressures, a dry layer of lead shots with different thickness was deposited on top of the clean sand. Viscous fluid was used for saturation to keep constant prototype permeability. All the centrifuge tests were subjected to 10-cycle sinusoidal seismic motions with different prototype peak acceleration to achieve the targeted maximum excess pore pressure buildup. Acceleration, pore pressure build-up and dissipation, and shear wave velocity were monitored, recorded, and analyzed during or after shaking. Acceleration amplification was observed in experiments conducted under low confining pressures, while de-amplification was found in experiments conducted under high confining pressures.
... Figure 1 shows the typical configuration and instrumentation layout used in these experiments. Dimensions are presented in prototype scale, following accepted scaling relations (Tan & Scott 1985, Garnier et al. 2007). ...
Conference Paper
Full-text available
This paper presents preliminary results from four centrifuge tests, performed at the University of Colorado Boulder, to evaluate the relative influence of different mitigation mechanisms provided by dense granular columns (DGC) on the seismic performance of embankments founded on liquefiable soil deposits. The first test was designed as the baseline, unmitigated case, and the soil below the embankment in the subsequent three tests was treated with a grid of DGCs that had an area replacement ratio (Ar) = 10%. The second test evaluated the influence of shear reinforcement and enhanced drainage provided by these columns. The third test isolated the effect of shear reinforcement by preventing drainage. The fourth and last test combined the effect of shear reinforcement with densification (without drainage) provided by DGCs. For the conditions investigated, the experimental results showed that densification was more effective than drainage in limiting embankment deformations. In addition, there was no significant difference between settlements measured in tests with and without drainage, which suggests that shear reinforcement was more important than drainage in improving the overall embankment's seismic performance.
... The samples were saturated on the laboratory floor at 1g with a solution of water and 1·4% or 1·1% cellulose ether, to obtain a viscosity of 100 or 40 cSt, respectively (DOV (Dow, 2002)), while measurement of the free fluid temperature in the PIV strongbox during the tests confirmed a constant value of 20 ± 1°C. Matching the viscosity with the target centrifuge acceleration level was necessary to correctly scale the drainage properties of the soil (Bienen et al., 2018;Dewoolkar et al., 1999;Tan and Scott, 1985;Taylor, 1987). Sample saturation was achieved through the base of the PIV strongbox. ...
Article
Suction buckets represent a viable solution as foundations for offshore wind turbines. Installation in sand is relatively straightforward, albeit with limited understanding of the resulting changes in soil state. This paper describes an experimental methodology that allows for visualisation and quantification of changes in soil state during suction bucket installation, validated in sand. Insights obtained from particle image velocimetry analyses, performed on images of a half-bucket installing against a Perspex window taken in a geotechnical centrifuge are discussed. Compared with the initial self-weight penetration, the deformation mechanism governing the suction-assisted phase shows a preference for the soil below the skirt tips to move inwards and upwards inside the bucket. The installation process is responsible for changes in relative density and permeability within the bucket. In these experiments, the majority of the soil plug heave can be attributed to the soil displaced inwards by the advancing skirts, with a minor contribution caused by dilation. The confidence in the experimental methodology provided through the results of suction bucket installation in sand discussed herein now enables suction bucket installation in more complex seabeds to be investigated.
... The sample was then saturated from the base on the laboratory floor at 1 g with a solution of water and 1.1% cellulose ether content, to obtain a viscosity of 40cSt (DOV 2002). Matching the viscosity with the enhanced gravity level was necessary to correctly scale the drainage properties of the soil , Taylor 1987, Dewoolkar et al. 1999. Throughout the centrifuge testing, a fluid head above the sample of 100 mm was maintained. ...
Conference Paper
Full-text available
Suction buckets are an increasingly considered foundation option for offshore wind turbines. Although the required suction can be predicted well using existing methods, uncertainty remains around some input parameters, because the effects of suction installation on the soil state are not understood in detail. This paper visualises the mechanisms governing both initial self-weight penetration and following suction-assisted installation in dense sand. Pioneering particle image velocimetry measurements in a centrifuge environment underpinned the investigation, with details of the experimental apparatus offered in the paper. Changes in the deformation mechanisms governing the installation process and in the soil properties are revealed. The findings have an impact on the understanding of the formation of internal plug heave – the cause of premature refusal – and the prediction of the installation response. Revealing changes in void ratio and permeability also present implications on the accumulated displacements under the metocean loading, which may conflict with serviceability requirements.
... Detailed plan and elevation-view drawings of the model tests and their instrumentation layouts are presented in Figure 1. Dimensions are presented in both prototype and model scales following accepted scaling relations (Tan andScott 1985, Garnier et al. 2007). ...
Conference Paper
Dense granular columns are commonly used to reduce the risk of liquefaction or its associated ground deformations in gentle slopes through: 1) enhancing drainage; 2) providing shear reinforcement; and 3) densifying and increasing lateral stresses in the surrounding soil during installation. However, the independent influence and contribution of these mitigation mechanisms on excess pore pressures, accelerations, and lateral and vertical deformations is not sufficiently understood to facilitate reliable design. This paper presents the results of a series of dynamic centrifuge tests to evaluate the influence of dense granular columns on the seismic performance of gentle slopes (e.g. <5%). Granular columns with greater area replacement ratios (Ar about 20%) were shown to be highly effective in reducing seismic settlement and lateral slope deformations, owing primarily to expedited dissipation of excess pore water pressures rather than shear reinforcement. The influence of granular columns on accelerations in the surrounding soil depended on the column’s Ar and drainage capacity. The insight from these experiments aims to improve our understanding of the mechanics of liquefaction and lateral spreading mitigation with granular columns.
... Fig. 1 shows the detailed plan and elevationview drawings of the model tests and their instrumentation layouts. The dimensions are presented at both prototype and model scales following the accepted scaling relations (Tan and Scott 1985). ...
Article
Full-text available
Dense granular columns are often used as a liquefaction mitigation measure to: 1) enhance drainage; 2) provide shear reinforcement; and 3) densify and increase lateral stresses in the surrounding soil during installation. However, the independent influence and contribution of these mitigation mechanisms on excess pore pressures, accelerations (or shear stresses), and lateral and vertical deformations is not sufficiently understood to facilitate reliable design. This paper presents the results of a series of dynamic centrifuge tests to fundamentally evaluate the influence of dense granular columns on the seismic performance of level and gently sloped sites, including a liquefiable layer of clean sand. Specific consideration was given to the relative importance of enhanced drainage and shear reinforcement. Granular columns with greater area replacement ratios (e.g., Ar greater than about 20%) were shown to be highly effective in reducing seismic settlement and lateral deformations in gentle slopes, owing primarily to expedited dissipation of excess pore water pressures rather than shear reinforcement. The influence of granular columns on accelerations (and hence, shear stress demand) in the surrounding soil depended on the column’s Ar and drainage capacity. Increasing the Ar from 0 to 10% was shown to reduce accelerations across a range of frequencies in soil due to the shear reinforcement effect alone. However, enhanced drainage simultaneously increased the rate of excess pore pressure dissipation, helping the surrounding soil regain its shear strength and stiffness faster. At short drainage distances or high Ar values (e.g., 20%), this could notably amplify accelerations and shear stress demand, particularly at greater frequencies. The experimental insight aims to improve our understanding of the mechanics of liquefaction and lateral spreading mitigation with granular columns and may be used to validate the future numerical models used in their design.
... By selecting a pore fluid viscosity to match the enhanced gravity level in the centrifuge (Ng), the permeability with this pore fluid should be the same as that in the prototype (1g) with water. This is because the effects on Darcy's coefficient of permeability k are counteracted (Tan & Scott, 1985;Taylor, 1987;Dewoolkar et al., 1999). k = gK/ν, where g is the gravitational acceleration, K is the intrinsic permeability of the soil and ν is the kinematic viscosity of the pore fluid, which for water is 10 À6 m 2 /s or 1 cSt. ...
Article
Full-text available
Suction caissons are a promising foundation concept for supporting offshore wind turbines. Compared to applications in the oil and gas industry, where most practical experience exists, significant differences arise in terms of load paths and magnitudes, soil type and caisson aspect ratio (skirt length to diameter). In a set of two companion papers, this contribution investigates the response of suction caissons in dense sand through a series of centrifuge experiments. The caisson was installed using suction, followed by sequences of cyclic loading and then extraction, all steps completed continuously in-flight. This first paper discusses installation, limiting capacities and drainage, whereas the second paper focuses on vertical cyclic loading into tension. The work demonstrates that suction caisson installation behaviour is well described by existing calculation methods. Tests performed at different installation rates demonstrate that careful assessment of the pumping rate is needed to ensure successful installation, with low pumping rates resulting in premature refusal. In the centrifuge tests, full skirt penetration was achieved without apparent loosening of the soil plug. The limiting capacity in tension, measured during the testing at both fast and slow uplift rates, was also well described by existing calculation methods.
... Detailed plan and elevation-view drawings of the model tests and their instrumentation layouts are presented in Figure 1. Dimensions are presented in both prototype and model scales following accepted scaling relations (Tan andScott 1985, Garnier et al. 2007). ...
... Butterfield [2] & Dean [3] also discussed this issue. Other parameters such as permeability were considered by Pokrovsky & Fyodorov [4]; Cargill & Ko [5]; Tan & Scott [6]. Recently, Singh & Gupta [7] including other researcher's defined permeability (k) to be directly proportional to gravity and hydraulic gradient (i).This implies that gradient is independent of gravity. ...
Article
Full-text available
This paper studies the rate of permeability passage through Tuotorsity of fluid flow at different soil strata. The rate of fluid flow depends on the permeability of the soil. Such condition has other influences. However, this study monitors fluid flow reflection on the rate of soil permeability through Tuotorsity in the formation. The penetration of fluid is influenced by the structural stratification of the formation. Tuotorsity flows are determined by the structural depositional setting of the soil intercedes. Monitoring the fluid flow condition in lacustrine deposition was possible through the application of mathematical modeling method. The study monitored the permeable condition of the formation to phreatic zone through the effect of Tuotorsity of the formation. The developed model was simulated to express the rate of fluid flow through Tuotorsity to the phreatic zone. Theoretical values were generated from the developed model. The study expressed the rate of permeability through Tuotorsity flow to the phreatic zone. It was confirmed that the permeability was increasing with respect to change in depth under plug flow condition. Subject to this condition, the study produced 4.11E-05, which implies that the condition developed flow based on the deposition from Tuotorsity. This expression implies that the stated parameter influences the low yield rate of phreatic aquifers in the study area. Experts can now use the developed model from this study as a tool in exploitation of groundwater by monitoring the rate of Tuotorsity flow in the study location.
... The time scale factors relating centrifuge model to prototype, in terms of excess pore pressure generation and dissipation during dynamic loadings, are not the same (Schofield 1980;Tan & Scott 1985). A possible approach to unify these two time scaling relationships is to decrease the hydraulic conductivity of the soil by increasing kinematic viscosity of the pore fluid used in the model by a factor N, or reducing pore sizes (Take et al. 2004;Askarinejad 2013). ...
Conference Paper
Full-text available
The common practice in centrifuge modelling of dynamic processes is to use high-viscosity pore fluids to unify the time scaling factors for the generation and dissipation of pore pressures. This paper focuses on the effects of the density and viscosity of the pore fluid on the behaviour of an offshore rigid pile subjected to cyclic and monotonic lateral loads. The soil used in these tests is dense sand and it was saturated with three different fluids: 1) water, 2) mixture of glycerine and water, 3) a fibre based viscous fluid with a density very close to that of water. The results of the tests in terms of the changes in the pore pressures at the interface of the pile and soil as well as monotonic lateral bearing capacity of the pile are compared. The tests illustrate that the behaviour of the model with water is very similar to that of the dry model, implying a fully drained behaviour. Moreover, it was observed that the density of the pore viscous fluid plays a major role in the evolution of the pore pressures. Therefore, it affects directly the stress state of the soil in the model.
... Butterfield (2000) and Dean (2001) also discussed this issue. Pokrovsky and Fyodorov (1968), Cargill and Ko (1983) , Tan and Scott (1985) and more recently Singh and Gupta (2000) are among many others who have considered permeability ( k ) to be directly proportional to gravity and hydraulic gradient (i ) to be independent of gravity. While this explains why seepage velocity has a scaling law of N ( m p v = N v ), there is an alternative explanation for the increase of seepage velocity in a centrifuge. ...
Article
Full-text available
The rate of recharge in ground water aquifers and the migration of E.coli in the study location were thoroughly evaluated in the study location, the rate of microbial activities in deltaic environment in this study has been expressed, the rate of recharge in aquitard were found to influences the migration of E.coli in phreatic aquifers, the condition were found to increase the transport of E.coli in soil and water environment, these condition has definitely Develop lots of fast migration of E.coli through high degree of rain intensities in the study location, such environmental influences through climatic condition were found to be the most influential factors in the migration of E.coli pressured by aquitard in port Harcourt. To monitor the rate of E.coli under the influences of aquitard recharge, mathematical equations were found suitable to determine the rate of influences from aquitard recharge in the study location. The model were derived through the application of discrete system to ensure that all the variations are thoroughly captured in the system, the derived solutions considered every condition as expressed mathematically in the study area. The study is imperative because it assist experts to thorough determined the rate of migration under the influences of aquitard recharge in the study area.
... This method may be satisfactory where it is appropriate to consider N as a constant throughout the depth of a model; however, if there is a requirement for the consideration of N as a variable related to radius of rotation (as in the present investigation discussed later), then a more thorough understanding of the parameters to be scaled is necessary. Cargill & Ko (1983), Tan & Scott (1985), Mitchell (1998) and Singh & Gupta (2000) defined the hydraulic gradient i as independent of gravity and considered permeability k to be directly proportional to gravity with a resultant scaling factor of N. The formal definition of hydraulic gradient, as presented in the majority of the literature, is the ratio of change in total head,˜h, and the length, L, over which that change in head occurs. When scaling of dimensions of a centrifuge model both L and˜h will be N times smaller in the model, and as a result the hydraulic gradient can be argued to be the same in the model and prototype (i.e. ...
Article
Full-text available
The commonly used British Standard constant head triaxial permeability test for testing of fine-grained soils is relatively time consuming. A reduction in the required time for soil permeability testing would provide potential cost savings to the construction industry, particularly in the construction quality assurance of landfill clay liners. The purpose of this paper is to evaluate an alternative approach of measuring permeability of fine-grained soils benefiting from accelerated time scaling for seepage flow when testing specimens in elevated gravity conditions provided by a centrifuge. As part of the investigation, an apparatus was designed and produced to measure water flow through soil samples under conditions of elevated gravitational acceleration using a small desktop laboratory centrifuge. A membrane was used to hydrostatically confine the test sample. A miniature data acquisition system was designed and incorporated in the apparatus to monitor and record changes in head and flow throughout the tests. Under enhanced gravity in the centrifuge, the flow through the sample was under 'variable head' conditions as opposed to 'constant head' conditions as in the classic constant head permeability tests conducted at 1g. A mathematical model was developed for analysis of Darcy's coefficient of permeability under conditions of elevated gravitational acceleration and verified using the results obtained. The test data compare well with the results on analogous samples obtained using the classical British Standard constant head permeability tests.
... The flow rate during centrifugation was N times greater than if a hydraulic gradient of 1 was applied to the core samples 25 at 1 g. This increase in flow rate is consistent with scaling laws for physical modelling (Tan and Scott, 1987). ...
Article
Full-text available
Evaluating the possibility of leakage through low permeability geological strata is critically important for sustainable water supplies, the extraction of fuels from strata such as coal beds, and the confinement of waste within the earth. The current work demonstrates that relatively rapid and reliable hydraulic conductivity (K) measurement of aquitard cores using accelerated gravity can inform and constrain larger scale assessments of hydraulic connectivity. Steady state fluid velocity through a low K porous sample is linearly related to accelerated gravity (g-level) in a centrifuge permeameter (CP) unless consolidation or geochemical reactions occur. The CP module was custom designed to fit a standard 2 m diameter geotechnical centrifuge (550 g maximum) with a capacity for sample dimensions of 30 to 100 mm diameter and 30 to 200 mm in length, and a maximum total stress of ~2 MPa at the base of the core. Formation fluids were used as influent to limit any shrink–swell phenomena which may alter the permeability. Vertical hydraulic conductivity (Kv) results from CP testing of cores from three sites within the same regional clayey silt formation varied (10−7 to 10−9 m s−1, n = 14). Results at one of these sites (1.1 × 10−10 to 3.5 × 10−9 m s−1, n = 5) that were obtained in < 24 h were similar to in situ Kv values (3 × 10−9 m s−1) from pore pressure responses over several weeks within a 30 m clayey sequence. Core scale and in situ Kv results were compared with vertical connectivity within a regional flow model, and considered in the context of heterogeneity and preferential flow paths at site and formation scale. More reliable assessments of leakage and solute transport though aquitards over multi-decadal timescales can be achieved by accelerated core testing together with advanced geostatistical and numerical methods.
... This metolose solution pore fluid was prepared by dissolving 2% metolose by weight in water, to achieve a viscosity of about 50 times the viscosity of water (50 cst kinematic viscosity). The consequence of using the viscous fluid in a centrifuge test at 40g to model liquefaction of a water-saturated prototype embankment soil in the field is that the actual prototype permeability being simulated is k prototype 5 3?1 6 10 26 / 50 6 40 5 2?5 6 10 26 m/s (Tan & Scott, 1985). In centrifuge modelling of a seismic event, it is normal practice to change the viscosity of the pore fluid by the same value as the level of g. ...
Article
Well-compacted embankments resting on non-liquefiable foundation soils with a level ground surface are usually considered stable even during strong earthquake ground motions. Some case histories, however, suggest that such embankments may be damaged if the foundation soil is highly compressible, such as thick soft clay and peat deposits. An increase in the thickness of the saturated zone in the embankment and reduction of stresses of embankment soil in the saturated zone, both owing to subsidence of the embankment, are surmised as underlying mechanisms. In this study, centrifuge tests were conducted to verify this hypothesis. Two types of models – embankments on a stiff foundation soil layer and embankments on a soft clay deposit – were prepared, and horizontal base shakings were imparted. Crest settlement and deformation of the embankment during shaking were larger for the embankment on the clay.
... The sand permeability obtained from a constant head laboratory test, was k ¼ 6.6 Â 10 À5 m/s. In view of the scaling laws applicable to centrifuge experiments [19] the prototype permeability should be 50k, i.e. 3.3 Â 10 À3 . The results with the densification model have been obtained by selecting k ¼ 3.3 Â 10 À5 m/s, while the results with the twosurface plasticity model have been obtained by taking k ¼ 13.5 Â 10 À3 m/s both in the prototype scale. ...
Article
Full-text available
Numerical analyses of liquefiable sand are presented in this paper. Liquefaction phenomenon is an undrained response of saturated sandy soils when they are subjected to static or dynamic loads. A fully coupled dynamic computer code is developed to predict the liquefaction potential of a saturated sandy layer. Coupled dynamic field equations of extended Biot's theory with u–P formulation are used to determine the responses of pore fluid and soil skeleton. Generalized Newmark method is employed for integration in time. The soil behavior is modelled by two constitutive models; a critical state two-surface plasticity model, and a densification model. A class 'B' analysis of a centrifuge experiment is performed to simulate the dynamic response of level ground sites. The results of the numerical analyses demonstrate the capability of the critical sate two-surface plasticity model in producing pore pressures that are consistent with observations of the behavior of liquefiable sand in the centrifuge test.
... This Metolose solution pore fluid was prepared by dissolving 2% Metolose by weight in water, so as to achieve a viscosity of 50 times the viscosity of water (50 cst kinematic viscosity). The consequence of using the viscous fluid in the centrifuge tests at 40g to model the liquefaction of the water-saturated prototype embankment soil in the field was that the actual prototype permeability being simulated was k prototype ¼ 3.1 Â 10 À 6 / 50 Â 40¼ 2.5 Â 10 À 6 m/s (Tan and Scott, 1985). In the centrifuge modeling of a seismic event, it is normal practice to change the viscosity of the pore fluid to the same value as the level of g. ...
Article
It has been reported that the major cause of earthquake damage to embankments on level ground surfaces is liquefaction of foundation soil. A few case histories, however, suggest that river levees resting on non-liquefiable foundation soil have been severely damaged if the foundation soil is highly compressible, such as thick soft clay and peat deposits. A large number of such river levees were severely damaged by the 2011 off the Pacific coast of Tohoku earthquake. A detailed inspection of the dissected damaged levees revealed that the base of the levees subsided in a bowl shape due to foundation consolidation. The liquefaction of a saturated zone, formed at the embankment base, is considered the prime cause of the damage. The deformation of the levees, due to the foundation consolidation which may have resulted in a reduction in stress and the degradation of soil density, is surmised to have contributed as an underlying mechanism. In this study, a series of centrifuge tests is conducted to experimentally verify the effects of the thickness of the saturated zone in embankments and of the foundation consolidation on the seismic damage to embankments. It is found that the thickness of the saturated zone in embankments and the drainage boundary conditions of the zone have a significant effect on the deformation of the embankments during shaking. For an embankment on a soft clay deposit, horizontal tensile strain as high as 6% was observed at the zone above the embankment base and horizontal stress was approximately half that of the embankment on stiff foundation soil. Crest settlement and the deformation of the embankment during shaking were larger for the embankment subjected to deformation due to foundation consolidation.
... Results of the numerical simulations are then presented and compared to the experimental response. All computational and experimental data are reported in prototype scale [Tan and Scott, 1985]. [Adalier et al., 1998], PPT is Pore-Pressure Transducer, ACC is Accelerometer, LVDT is Linear Variable Differential Transducer to measure displacement). ...
Article
Full-text available
Computational simulations are presented for a unique series of centrifuge tests conducted to assess the performance of liquefaction countermeasure techniques. In these centrifuge tests, the dynamic response of an embankment supported on a liquefiable foundation (medium sand) is investigated. The experimental series included: (i) a benchmark test without a liquefaction countermeasure, (ii) foundation densiflcation below the embankment toe, and (iii) use of a sheet-pile containment enclosure below the embankment. This series of experiments documents a wide range of practical liquefaction response mechanisms (including countermeasure implementation). In order to numerically simulate the above centrifuge tests, a new calibrated soil stress-strain constitutive model is incorporated into a two-phase (solid-fluid) fully coupled Finite Element formulation. Comparison of the computational and experimental results demonstrates: (i) importance of post-liquefaction dilative soil behavior in dictating the dynamic response and deformation characteristics of the embankment-foundation system, and (ii) capabilities and limitations of the numerical modeling procedure.
Chapter
Description Cyclic and dynamic testing of soils for geotechnical engineering purposes and centrifuge testing has emerged as a new tool for understanding soil behavior and dynamic soil-structure interaction. STP 1213 is an excellent reference document on these latest testing practices, and emerging technologies and provides additional information on measuring soil properties used cyclic and dynamic loading conditions. 25 peer-reviewed papers give you an understanding of field, laboratory, and centrifuge methods used in cyclic and dynamic testing of soils for geotechnical engineering purposes. The papers discuss: • Descriptions of facilities, apparatus, and instrumentation used • Theoretical analysis of apparatus and/or instrumentation • Results and analysis of experimental research • Discussion of testing procedures, improvements, and guidelines
Article
Offshore wind is a key part of the change toward renewable energy sources. Offshore wind farm developments are moving to sites that are characterized by increased water depths and layered soil profiles, e.g., in the North Sea. Suction caisson–based jacked structures are expected to be increasingly used to support wind turbines at such sites. Experience of suction caissons serving as foundations for offshore wind turbines is limited. The mechanisms governing the suction caisson installation in layered soils are poorly understood and no published data of the in-service performance of suction caisson jackets supporting offshore wind turbines in layered soils exists. This gap is addressed here through a series of centrifuge tests investigating the installation and the response of suction caissons in layered soils—sand over clay and clay over sand—subjected to vertical cyclic loading into tension. The results reveal the mechanisms enabling suction caisson installation in layered soils and the complex load transfer mechanisms governing the suction caisson response under vertical cyclic loading. The clay layer plays a key role: in sand over clay, it encapsulates the sand plug, which leads to predominantly undrained behavior. In clay over sand, the uplift of the clay soil plug is critical both in facilitating suction installation and in response to vertical cyclic loading in tension. These findings provide confidence for suction installation and caisson response under vertical cyclic loading into tension in layered soils.
Article
This paper discusses methods for realistic simulation of slope failure owing to heavy precipitation in a centrifuge. A rain simulator was developed to provide precipitation that satisfies the conditions in a centrifuge so that the impact pressure on the ground surface in the centrifuge (pm) was reduced to the same level in the prototype (pp). Pneumatic spray nozzles, producing fine droplets with a mean diameter of 20 mm, yield large precipitation intensity of 1500 mm/h on the model at 50g. Accordingly, the model precipitation intensity (rm) was provided by n times the corresponding prototype precipitation intensity (rp). Several sets of centrifuge tests were conducted to observe the progress of failure in the shallow section of the slope. Heavy precipitation induces an increase in the saturation of the soil from the surface. Flow failure was observed repeatedly in the shallow section through progress of the wetting front. In addition, experiments using different viscosity used in the liquid for precipitations were carried out to compare the types of failure. Larger failure by precipitation of the viscous liquid was observed than that of water even when equivalent permeability conditions are maintained. This paper also introduces a method of measuring shear deformation in the shallow section by bending strains, allowing monitoring of the progress of failure.
Article
The centrifugal force distributes non-uniformly with radius in centrifugal field. If the stress is set in two thirds height of a model according to the prototype, the error between the stresses of prototype and model can become minimum. The influence depth of model foundation is limited. So the error between stresses of prototypes and models should be considered in this range (the maximum influence depth). Therefore the condition which makes the minimum error between prototypes and models is to set the two thirds maximum influence depth rather than two thirds height of the model. The model height is inconsistent with the maximum influence depth for avoiding boundary effects. Its value is greater. The influences of centrifugal acceleration, similarity ratio and stress are discussed. The modified method of the similarity ratio determined before tests according to the model height is also introduced.
Article
Liquefaction and settlement of reclaimed ground with gravelly decomposed granite soil, which had liquefied during the 1995 Hyogo-ken Nambu earthquake, was studied by a centrifuge model test using in situ soil material and the earthquake record. Three ground models used for the centrifuge test were made with different particle size ranges: 1) under 30 mm, 2) under 2 mm, and 3) 2-30 mm water-washed. A reclaimed layer of about 16 m thickness was modeled in 1/40 scale. The following results were obtained from this study: 1)Inferring from maximum acceleration response, independent of the above three grain size distributions, it was considered that shear failure occurred at a depth between K.P. -8 m to -12 m due to strong motion. 2)The liquefaction degree was not uniform in the depth direction. In grounds consisting of particles smaller than 2 mm, destructive liquefaction occurred at a lower depth ; also, volume compression at a corresponding depth was larger than that at the upper portion. 3)In the case of only-gravel content, no cumulative excess pore water pressure was generated because of high permeability. Settlement after vibration was also smaller compared with two other cases. 4)Permeability during the pore pressure dissipation process, which was identified from back analysis, was higher than the value obtained from the laboratory permeability test ; it also gradually approached laboratory test values during dissipation.
Article
Installation oF Stone Column (SC) is a promising ground improvement technique to mitigate liquefaction hazards in sand stratum. In this study, a three-dimensional (3D) Finite Element (FE) analysis was used to simulate a centrifuge experiment on the mitigation of silty sand strata liquefaction using SC approach. The predicted response of the silty sand and SC matched the experimental data well. The overall site-stiffening effects due to the installed SCs as well as the distributions of the shear stress and shear stress reductions were evaluated. A parametric study was conducted to investigate the effect of the SC permeability and the surface load at the SC zone on the effectiveness of liquefaction mitigation. The results showed that the SCs behaved in a combined shear and flexure mode. Furthermore, the SCs with permeability exceeding a threshold value can dramatically decrease the liquefaction hazard. On the other hand, larger surface load did not prevent soil liquefaction and produced negligible benefits in stiffening. The present study further enhances the current understanding of the effectiveness of SC remediation approaches in the silty sand.
Article
Full-text available
During large earthquakes, soil liquefaction has repeatedly damaged many buildings with shallow foundations. Many researchers have continuously worked to develop more reliable countermeasure techniques apposite to the foundation soils of existing buildings. However, most countermeasure techniques available in the current practice are either too expensive or applicable only to new construction sites. Lowering the degree of saturation by artificially injecting air is a newly developed, innovative technique that significantly improves the liquefaction strength of soil. This paper describes a series of centrifuge tests for evaluating the effectiveness of this technique by applying it to the foundation soil underlying shallow foundations of relatively light structures, such as residential buildings. Models of saturated medium dense sand beds with shallow foundations were shaken in a centrifuge. Except for benchmark models, air was injected into the soil in-flight before the shaking event, which lowered the degree of saturation of the soil in the desaturated zone from 100% before air injection to approximately 85%. The test results show that air injection effectively reduced both foundation settlement and pore pressures generated below the foundation.
Article
Based on a centrifuge model test and distinct element method (DEM), this study provides new insights into the uplift response of a shallow-buried structure and the liquefaction mechanism for saturated sand around the structure under seismic action. In the centrifuge test, a high-speed microscopic camera was installed in the structure model, by which the movements of particles around the structure were monitored. Then, a two-dimensional digital image processing technology was used to analyze the microstructure of saturated sand during the shaking event. Herein, a numerical simulation of the centrifuge experiment was conducted using a two-phase (solid and fluid) fully coupled distinct element code. This code incorporates a particle-fluid coupling model by means of a "fixed coarse-grid" fluid scheme in PFC3D (Particle Flow Code in Three Dimensions), with the modeling parameters partially calibrated based on earlier studies. The physical and numerical models both indicate the uplifts of the shallow-buried structure and the sharp rise in excess pore pressure. The corresponding micro-scale responses and explanations are provided. Overall, the uplift response of an underground structure and the occurrence of liquefaction in saturated sand are predicted successfully by DEM modeling. However, the dynamic responses during the shaking cannot be modeled accurately due to the restricted computer power.
Article
The effective design of earthquake-resistant structures and liquefaction mitigation techniques requires an improved understanding of the development and consequences of liquefaction. In this paper, the results from centrifuge experiments of structures with shallow foundations on liquefiable sand were used to evaluate the predictive capabilities of a state-of-the-practice numerical tool. Fully-coupled numerical simulations with the UBCSAND model implemented in FLAC-2D captured building settlements measured in these experiments reasonably well for one scaled input motion, mostly within factors of 0.7 and 1.8. The soil model captured the overall contribution of deviatoric displacement mechanisms and localized volumetric strains during partially drained cyclic loading. The primary limitation of the model became evident for slower rates of earthquake energy buildup, when the extent of soil softening and building displacement was overestimated by up to a factor of 4. The observations from recent case histories, the results of the experiments, and the insights gained from the numerical analyses are combined to provide guidance on the evaluation of building response on liquefiable sand and the performance of liquefaction remediation strategies.
Article
Dynamically-induced liquefaction of a saturated loose sand stratum is studied in centrifuge model simulations. A laminated container subjected to base excitation is employed to replicate one-dimensional shear beam conditions. Level site and mild infinite slope simulations are conducted at Rensselaer Polytechnic Institute (RPI) and California Institute of Technology (CalTech). The observed acceleration, pore pressure and lateral displacement responses show a high level of consistency, and are used herein to estimate the corresponding dynamic shear stress-strain histories. These histories shed light on the mechanisms of: (1) propagation of liquefaction through the stratum, and (2) soil response during liquefaction, and its effect on ground surface acceleration and lateral deformation.
Article
Full-text available
The degree of saturation significantly influences the liquefaction resistance of sandy soils. In preparation of models for dynamic centrifuge tests and 1g shaking table tests, however, the degree of saturation has not been accurately controlled. A technique to prepare fully saturated model ground for liquefaction study is summarised in this paper, focusing on how vacuum pressure and centrifugal acceleration during the saturation process affect the degree of saturation. A method to evaluate degree of saturation of models accurately is developed first. Then, a total of ten experiments were conducted in which water was introduced to dry sand deposits at different atmospheric pressures at different centrifugal accelerations. It is confirmed that increased acceleration contributes to enhance degree of saturation and reduces time for the saturation process. Two dynamic centrifuge tests on models with different degrees of saturation, one almost fully saturated and the other slightly unsaturated, with the difference in degree of saturation between the two models being 1·5%, were conducted. Differences in excess pore pressure responses between the two models became more significant as the depth increased. It is concluded that a precise control of degree of saturation on the order of 0·1% is important in model tests related to liquefaction problems.
Article
This paper focuses on the validation of a centrifuge permeameter and testing methodology developed to investigate transient water flow processes in unsaturated soils. The centrifuge permeameter is capable of controlling the boundary conditions for infiltration and drainage and contains instrumentation to measure profiles of the volumetric water content and matric suction with height in a soil layer during centrifugation. Evaluations of the measured matric suction and volumetric water content profiles from a series of infiltration and drainage tests performed on a fine sand at centrifuge acceleration levels ranging from 20 to 50 times earth gravity indicate that the established equations for calculating the hydraulic conductivity of unsaturated soil at steady-state conditions might not be appropriate, except at higher centrifuge accelerations. The transient profiles were also analyzed using a form of the instantaneous profile method in which the centrifuge acceleration was incorporated into the hydraulic gradient. Although the calculated soil water retention curve and hydraulic conductivity function data show some scatter, the hysteretic behavior of the soil during wetting and drying could be assessed.
Article
This study was aimed to create an analytical model for simulating deep catastrophic failure of hillslope (or deep-seated landslide) to help determining assessment criteria of potential risk based on numerical manifold method (NMM) and coupled multi-physics computation (MPC), in which the failure status is simulated by the NMM while the risk factors are studied by the MPC. The simulation delivers the landslide results that are compared with a laboratory test for approval. The proposed model includes a small-scale hillslope designed by two-dimensional geometry for the plane strain problem. Thus, the porous materials are considered for coupling fluid–structure interactions in hydraulic and geotechnical analyses. Meanwhile, discontinuous joints are assumed along the potential failure surfaces within the deep-seated layer to simulate collapse behaviors of hillslope once the risk factors, such as effective stress and friction angle, reach the thresholds. Furthermore, the model is initially setup as laboratory scale for comparing with a hydraulic experiment that practices the failure condition caused by seepage. The simulation hence explorers the criteria of potential failure risks due to variations of slope, friction angle, and groundwater level. This study performs feasibility of the proposed model that provides a reliable procedure based on both simulation and experiment to estimate the potential risks for deep catastrophic landslides. In the future, the study can be expanded for evaluating full-scale landslide in a variety of hillslope properties.
Article
A centrifuge testing study is conducted to investigate the effect of over-consolidation on liquefaction in clean saturated sand deposits. Thirty-four shaking tests on 11 level-ground models are performed. Soil models with Over-Consolidation Ratios (OCRs) of 1, 2, and 4 at relative densities of 35%, 50%, and 70% are tested. Model response to dynamic base shaking is monitored with accelerometers, pore pressure transducers, and displacement gauges. Test data show that the potential for liquefaction decreases with the increase in OCR, relative density, and prior shaking. The threshold peak acceleration needed to induce excess pore pressure increases as the OCR increases. Over-consolidated sand layers subjected to lower levels of excitation do not experience any excess pore pressure buildup even when shaken for a long duration. For acceleration marginally above the threshold, pore pressure buildup may be mild and liquefaction may be unlikely. As such, preloading can be a practical cost-effective liquefaction remedial technique in sandy soils under earthquake loading scenarios resulting in peak acceleration of up to about 0.15 g.
Article
An analysis of the process of settling, solidification and consolidation of a sand liquefied by vibration in two centrifuge tests at different acceleration levels is presented. It is shown that the relevant material properties can be obtained readily from appropriate measurements, when the solution for consolidation in a soil layer accreting linearly with time is employed.
ResearchGate has not been able to resolve any references for this publication.