Numerical Analysis of One-Dimensional Consolidation in Layered Clay Using Interface Boundary Relations in Terms of Infinitesimal Strain

The interface boundary relations are derived in this study for the numerical analysis of one-dimensional consolidation in multilayered clay profiles. The finite difference solutions are formulated based on Mikasa’s consolidation equation with infinitesimal strains and constant consolidation parameters under the same fundamental assumptions and limitations of the classic Terzaghi equation. Numerical examples are presented for multilayer clay profiles under single and double drainage conditions that validate the predicted excess pore pressures, strains, settlements, and rates of consolidation using interface boundary relations in terms of infinitesimal strains that are equivalent to those expressed in terms of excess pore pressures.     

Equivalent Effective Stress and Compressibility of Unsaturated Kaolinite Clay Subjected to Drying

Three-dimensional compressibility tests performed on unsaturated kaolinite clay subjected to drying showed that the volume change is a function of the equivalent effective stress (EES). The EES in the clay at different water contents was measured by performing direct tensile tests. When the clay has high water content (saturated funicular state), its volume decreases notably as the water content is reduced, i.e., the equivalent effective stress is increased. If the clay has a water content in an intermediate interval (complete pendular state), the volume is almost constant because the equivalent effective stress is almost constant. For the interval of low water contents (partial pendular state), the volume of the clay increases as the water content is reduced. This occurs because the equivalent effective stress is reduced when the moisture content in the clay is reduced, and contrasts with the saturated funicular state. The minimum volume in the clay was reached when the maximum equivalent effective stress was developed. A conceptual framework explains the influence of the different states of water distribution to the EES.     

Embankments over Soft Clay Deposits: Contribution of Basal Reinforcement and Surface Sand Layer to Stability

This paper evaluates the significance of basal reinforcement and the presence of the surface sand layer in the stability. This evaluation is carried out by means of field measurements and stability analyses of three test embankments on soft clay taken to failure. Two of the test embankments were reinforced and one was unreinforced. Stability analyses were carried out taking into account measured values of reinforcement tension forces during construction. The set of analyses have shown that the top sand layer was more important to the stability of the embankments than the basal reinforcement. The cases studied have also shown that the conventional design practice that assumes for the reinforcement a fixed tension contribution may lead to unrealistic higher factor of safety.     

Effect of Microfabric on Mechanical Behavior of Kaolin Clay Using Cubical True Triaxial Testing

Various aspects of the mechanical behavior of kaolin clay are discussed in light of experimental observations from a series of strain controlled true triaxial undrained tests performed on cubical kaolin clay specimens with flocculated and dispersed microfabric, using a fully automated flexible boundary experimental setup with real-time feedback control system. The laboratory procedures used to prepare flocculated and dispersed microfabric specimens are presented. Mercury intrusion porosimetry is used to evaluate the pore structure of these specimens. The influence of microfabric on the consolidation behavior of kaolin clay is evaluated based on the data obtained from K0 consolidation during constant rate of strain tests and the isotropic consolidation during true triaxial tests. Undrained tests on kaolin clay show that the following vary with microfabric of specimen: The shear stiffness, excess pore pressure generated during shear, and strength and strain to failure. For both microfabrics, the observed strength behavior using cubical triaxial testing shows a similar pattern of variation with applied stress anisotropy; hence, only a marginal influence of fabric-induced anisotropy.     

Effect of Pile Driving on Static and Dynamic Properties of Soft Clay

A full-scale closed-ended pile was driven into a deep deposit of soft clay that was instrumented with inclinometers and pore pressure transducers at three radial locations and three depths. This paper presents the results and interpretation of both field measurements of shear-wave velocity and the laboratory testing program performed on pre-pile and post-pile “undisturbed” specimens. A companion paper provides full details of the site investigation, field measurements of excess pore pressure, and the deformation field around the pile. Shear-wave velocity profiles at four radial distances were obtained as a function of time following pile driving using the suspension logging method. Compressibility characteristics for this soil were determined through one-dimensional constant rate of consolidation tests carried to very high stresses. Shear strength testing included anisotropically consolidated undrained triaxial tests performed on specimens at two confinement levels to study the effect of fabric and evolving anisotropy. Direct simple shear testing was performed on specimens in their normal vertical orientation, and rotated 90° to observe changes in structure/fabric orientation after pile installation.     

Centrifuge Permeameter for Unsaturated Soils. II: Measurement of the Hydraulic Characteristics of an Unsaturated Clay

This paper presents the hydraulic characteristics of an unsaturated, compacted clay, including its soil-water retention curve (SWRC) and hydraulic conductivity function (K function), determined using a new centrifuge permeameter developed at the University of Texas at Austin. A companion paper describes the apparatus, its instrumentation layout, and data reduction procedures. Three approaches are evaluated in this study to define the SWRC and K function of the compacted clay under both drying and wetting paths, by varying the inflow rate, the g level, or both. For imposed inflow rates ranging from 20 to 0.1 mL/h and g levels ranging from 10 to 100 g, the measured matric suction ranged from 5 to 70 kPa, the average volumetric water content ranged from 23 to 33%, and the hydraulic conductivity ranged from 2×10?7 to 8×10?11?m/s. The SWRCs and K functions obtained using the three different testing approaches were very consistent, and yielded suitable information for direct determination of the hydraulic characteristics. The approaches differed in the time required to complete a testing stage and in the range of measured hydraulic conductivity values. The g level had a negligible effect on the measured hydraulic characteristics of the compacted clay. The SWRCs and K functions defined using the centrifuge permeameter are consistent with those obtained using pressure chamber and column infiltration tests. The K functions defined using the centrifuge permeameter follow the same shape as those obtained from predictive relationships, although the measured and predicted K functions differ by two orders of magnitude at the lower end of the volumetric water content range.     

Effect of Surface Heave on Response of Partially Embedded Pipelines on Clay

The as-laid embedment of an on-bottom pipeline strongly influences the resulting thermal insulation, and the resistance to subsequent axial and lateral movement of the pipeline. Reliable assessment of these parameters is essential for the design of offshore pipelines. Static vertical penetration of a pipe into a soft clay seabed—which can be modeled as an undrained process—causes heave of soil on each side of the pipeline. The heaved soil contributes to the vertical penetration resistance and the horizontal capacity. This paper describes a series of large deformation finite-element analyses of pipe penetration, supported by a simple analytical assessment of the heave process. The conventional bearing capacity approach to the analysis of pipe penetration is reviewed, and modifications for the effects of soil weight and heave are presented. It is shown that in soft soil conditions—which are typical for deep water—the soil self-weight contributes a significant portion of the vertical penetration resistance and horizontal capacity. If heave is neglected, the soil weight leads to a vertical force due to buoyancy, based on Archimedes’ principle. When heave is considered, the soil weight contributes an additional component of vertical load, exceeding simple buoyancy, due to the distorted geometry of the soil surface. Archimedes’ principle does not apply. The finite-element analyses, benchmarked against rigorous plasticity solutions, are used to calibrate simple expressions for predicting static vertical pipe penetration, and the resulting horizontal capacity. These simple solutions allow the conventional bearing capacity approach to be used in a manner which correctly accounts for the effects of soil self-weight and heave. An approximate solution for predicting the “local” pipe embedment—relative to the raised soil level immediately adjacent to the pipe—is derived. The local embedment significantly exceeds the nominal embedment relative to the original soil surface. This effect counteracts the tendency for heave to reduce the embedment by raising the penetration resistance.     

Threshold Shear Strain for Cyclic Pore-Water Pressure in Cohesive Soils

Threshold shear strain for cyclic pore-water pressure, γt, is a fundamental property of fully saturated soils subjected to undrained cyclic loading. At cyclic shear strain amplitude, γc, larger than γt residual cyclic pore-water pressure changes rapidly with the number of cycles, N, while at γc<γt such changes are negligible even at large N. To augment limited experimental data base of γt in cohesive soils, five values of γt for two elastic silts and a clay were determined in five special cyclic Norwegian Geotechnical Institute (NGI)-type direct simple shear (NGI-DSS), constant volume equivalent undrained tests. Threshold γt was also tested on one sand, with the results comparing favorably to published data. The test results confirm that γt in cohesive soils is larger than in cohesionless soils and that it generally increases with the soil’s plasticity index (PI). For the silts and clay having PI=14–30, γt = 0.024–0.06% was obtained. Limited data suggest that γt in plastic silts and clays practically does not depend on the confining stress. The concept of evaluating pore water pressures from the NGI-DSS constant volume test and related state of stresses are discussed.     

Verification of the Soil-Type Specific Correlation between Liquefaction Resistance and Shear-Wave Velocity of Sand by Dynamic Centrifuge Test

Liquefaction of granular soil deposits is one of the major causes of loss resulting from earthquakes. The accuracy of the liquefaction potential assessment at a site affects the safety and economy of an engineering project. Although shear-wave velocity (Vs)-based methods have become prevailing, very few works have addressed the problem of the reliability of various relationships between liquefaction resistance (CRR) and Vs used in practices. In this paper, both cyclic triaxial and dynamic centrifuge model tests were performed on saturated Silica sand No. 8 with Vs measurements using bender elements to investigate the reliability of the CRR-Vs1 correlation previously proposed by the authors. The test results show that the semiempirical CRR-Vs1 curve derived from laboratory liquefaction test of Silica sand No. 8 can accurately classify the (CRR,Vs1) database produced by dynamic centrifuge test of the same sand, while other existing correlations based on various sandy soils will significantly under or overestimate the cyclic resistance of this sand. This study verifies that CRR-Vs1 curve for liquefaction assessment is strongly soil-type dependent, and it is necessary to develop site-specific liquefaction resistance curves from laboratory cyclic tests for engineering practices.     

Single Hardening Elasto-Plastic Model for Kaolin Clay with Loading-History-Dependent Plastic Potential Function

The effect of principal stress rotation on the mechanical behavior of Kaolin clay is investigated using combined axial-torsional tests on hollow cylindrical specimens. The yielding behavior and failure criteria are found to be strongly dependent on the principal stress rotation angle (β) and plastic work. A unique plastic potential function determined solely by the current stress state is not sufficient to model the plastic flow observed in these experiments. Therefore, a single hardening elasto-plastic model that includes a loading-history-dependent plastic potential function is proposed for normally consolidated Kaolin clay subjected to principal stress rotation. A general methodology for incorporating history dependency in modeling complex elasto-plastic behavior of cohesive soils is presented along with comparisons of model predictions with experimental data.     

Cyclic Softening of Low-Plasticity Clay and Its Effect on Seismic Foundation Performance

During the 1999 Chi-Chi Earthquake (Mw = 7.6), significant incidents of ground failure occurred in Wufeng, Taiwan, which experienced peak accelerations ～ 0.7?g. This paper describes the results of field investigations and analyses of a small region within Wufeng along an E–W trending line 350?m long. The east end of the line has single-story structures for which there was no evidence of ground failure. The west end of the line had three to six-story reinforced concrete structures that underwent differential settlement and foundation bearing failures. No ground failure was observed in the free field. Surficial soils consist of low-plasticity silty clays that extend to 8–12?m depth in the damaged area (west side), and 3–10?m depth in the undamaged area (east side). A significant fraction of the foundation soils at the site are liquefaction susceptible based on several recently proposed criteria, but the site performance cannot be explained by analysis in existing liquefaction frameworks. Accordingly, an alternative approach is used that accounts for the clayey nature of the foundation soils. Field and laboratory tests are used to evaluate the monotonic and cyclic shear resistance of the soil, which is compared to the cyclic demand placed on the soil by ground response and soil–structure interaction. Results of the analysis indicate a potential for cyclic softening and associated strength loss in foundation soils below the six-story buildings, which contributes to bearing capacity failures at the edges of the foundation. Similar analyses indicate high factors of safety in foundation soils below one-story buildings as well in the free field, which is consistent with the observed field performance.     

Comparison of Stratified Sand Filters and Percolation Trenches for On-Site Wastewater Treatment

Two separate on-site wastewater treatment systems were constructed at premises in eastern Ireland, one using a conventional septic tank, the other using a septic tank followed by a naturally aerated peat filter. The respective effluents were then split at each site whereby half was directed into percolation trenches and the other half pumped into intermittently dosed, stratified sand filters for a year. Samples were taken at different depths in the subsoil beneath both the percolation trenches and sand filters and analyzed for chemical and bacteriological determinants. Samples were also taken at different layers within the sand filters, which were tested at various hydraulic loading rates. Although the sand filters require a much smaller surface area, the respective pollutants on each site were attenuated to the same level in the subsoil when compared to the percolation trenches. As a result of the trials, the recommendations for design hydraulic loading rates in Ireland were 30?L/m2?day for filters receiving septic tank effluent and 60?L/m2?day for filters receiving secondary treated effluent.     

Measuring and Modeling Proportion-Dependent Stress-Strain Behavior of EPS-Sand Mixture

A geofoam was produced by blending expanded polystyrene (EPS) beads and sands in proportions. The formed mixtures, known as EPS-sands, were 26–63% lighter than general earth fills (e.g., sand). Consolidated-drained (CD) triaxial compression tests were conducted on EPS-sand mixture specimens to observe their stress-strain characteristics, specifically, the stress-strain responses in relation to the EPS contents (0.5, 1.5, and 2.5% by weight) used in the mixtures and confining pressures (100, 200, 300 to 400 kPa) loaded on the specimens. The EPS content and confining pressure were found to influence the stress-strain and volumetric strain behavior of the mixtures. Increasing EPS content led to decreased shear strength and increased volumetric strain. Increasing confining pressures enhanced the strength of the mixture. EPS-sand mixtures underwent a shear contraction throughout the CD tests. The optimum EPS bead content (i.e., the one reasonably balancing the unit weight, strength, and deformation) was in the order of 0.5% by weight. EPS content dependent strain increment equations were derived by compromising Cam-clay and modified Cam-clay, and used to model the stress-strain characteristics of EPS-sand mixtures. The established equations were verified being able to depict the stress-strain observations of EPS-sand specimens, at least for the ranges of EPS contents and confinements considered in this study.     

Load-Settlement Response of Rectangular and Circular Piles in Multilayered Soil

Traditionally, analyses developed for circular piles have also been used for rectangular piles by replacing in calculations the rectangular pile with a circular pile of equivalent area. In this paper, we present a settlement analysis that applies to piles with either rectangular or circular cross sections installed in multilayered soil deposits. The analysis follows from the solution of the differential equations governing the displacements of the pile-soil system obtained using variational principles. The input parameters needed for the analysis are the pile geometry and the elastic constants of the soil and pile. Pile displacements and vertical soil displacements calculated using this analysis match well those from finite-element analysis. A parametric study highlights some important insights for rectangular and circular piles in multilayered soil. A user-friendly spreadsheet program (ALPAXL) was developed to facilitate the use of the analysis. Examples illustrate the use of the analysis in design.     

Determining Erodibility, Critical Shear Stress, and Allowable Discharge Estimates for Cohesive Channels: Case Study in the Powder River Basin of Wyoming

The continuous discharge of coalbed natural gas-produced (CBNG-produced) water within ephemeral, cohesive channels in the Powder River Basin (PRB) of Wyoming can result in significant erosion. A study was completed to investigate channel stability in an attempt to correlate cohesive soil properties to critical shear stress. An in situ jet device was used to determine critical shear stress (τc) and erodibility (kd); cohesive soil properties were determined following ASTM procedures for 25 reaches. The study sites were comprised of erodible to moderately resistant clays with τc ranging from 0.11?to?15.35?Pa and kd ranging from 0.27?to?2.38?cm3/N?s. A relationship between five cohesive soil characteristics and τc was developed and presented for use in deriving τc for similar sites. Allowable discharges for CBNG-produced water were also derived using τc and the tractive force method. An increase in the allowable discharge was found for channels in which vegetation was maintained. The information from this case study is critical to the development of a conservative methodology to establish allowable discharges while minimizing flow-induced instability.     

Centrifuge Modeling of Seismically Induced Uplift for the BART Transbay Tube

The BART Transbay Tube (TBT) is an immersed cut-and-cover subway tunnel that runs from Oakland to San Francisco, California. The loose sand and gravel backfills placed around the tunnel are considered to be liquefiable, and the clays under the backfill are soft in some zones along the alignment. These conditions could potentially result in uplift of the tunnel during strong earthquake shaking. This paper describes centrifuge model tests performed to verify numerical methods used to assess the stability and to evaluate the potential uplift mechanisms of the TBT. The observed mechanisms of uplift were a ratcheting mechanism (sand migrating under the tunnel with each cycle of relative movement), a pore water migration mechanism (water flowing under the tunnel), and a bottom heave mechanism, involving soft soils below the base of the trench. A fourth potential mechanism, viscous flow of liquefied soil, was not observed. The volume of the tunnel relative to the volume of the trench and the densities and permeabilities of the nonhomogeneous backfill were important parameters affecting the uplift of the tunnel. From the experiments reported here and analyses reported by the designers, it was concluded that the magnitude of uplift is limited and, hence, that an expensive ground improvement project to densify the backfill was unwarranted.     

Shear Strength and Shear-Induced Volume Change Behavior of Unsaturated Soils from a Triaxial Test Program

A series of unsaturated soil triaxial tests were performed on four soils including sand, silt, and a low plasticity clay. Attempts were made to correlate unsaturated soil properties from these tests and data from the literature with soil-water characteristics curve (SWCC), soil gradation, and saturated soil properties. The feasibility of estimating unsaturated soil property functions from saturated soil properties, SWCCs and gradation data, is demonstrated. A hyperbolic model for estimation of the unsaturated soil parameter, ?b, versus matric suction is presented. Shear induced volume change behavior was also studied, and results are included in this paper. Although not correlated with soil index properties, these shear-induced volume change data are important to complete stress-deformation analyses, and represent a significant addition to the existing data base of unsaturated soil properties.     

Small-Strain Behavior of Granular Soils. II: Seismic Response Analyses and Model Evaluation

Using the recorded response at two vertical array sites, the SimSoil model presented in the companion paper is evaluated. The SimSoil model, which describes the small strain nonlinear behavior of granular materials, is implemented as a material model in AMPLE2000, a nonlinear, one-dimensional site response analysis code. Shear wave velocity profiles and laboratory test data available for both the La Cienega site, which was instrumented over 250?m, and the Lotung site, which was instrumented over 47?m, were used to determine SimSoil model parameters. Predictions from AMPLE2000 are compared with the measured response at several elevations for earthquakes that resulted in both nonlinear and nearly linear soil behavior. Using the available laboratory data and known input motions, the predictions of the response at these sites matched the recorded response well for varied magnitudes of shaking with a single set of parameters for each site.     

Quality Assessment and Quality Control of Deep Soil Mixing Construction for Stabilizing Expansive Subsoils

This paper presents the process and results of a quality management program performed during and immediately after the construction of two deep soil mixing (DSM) test sections. The quality management program consisted of laboratory, in situ, and mineralogical tests to address the effectiveness of the treatment during and after construction. In situ investigations including the down-hole seismic and spectral analysis of surface waves (SASW) test methods were performed to evaluate the degree of improvement achieved through the measurement of compression and shear-wave velocities of the columns and surrounding soils. Scanning electron microscopy and electron dispersive x-ray analysis were performed on raw, laboratory treated and field-treated specimens for qualitative understanding of the degree of mixing achieved in the field and the compounds formed at particle level during stabilization, respectively. Laboratory tests results on field cores indicated that both field stiffness and strength are about 20 to 40% less than the corresponding laboratory prepared soil samples. The down-hole seismic and SASW tests showed considerable improvement in stiffness in and around the DSM columns. Mineralogical studies indicated the formation of silica and alumina hydrates along with interwoven structure of lime-cement treated clay particles in both laboratory and field specimens, suggesting adequate mixing of the soil and binder in both environments.     

Postcyclic Degradation of Strength and Stiffness for Low Plasticity Silt

Stress-controlled undrained cyclic triaxial tests followed by strain-controlled monotonic compressive shear tests were carried out on normally consolidated and overconsolidated reconstituted Keuper Marl silt to investigate the strength and stiffness degradation characteristics of a low plasticity silt. Special attention was paid to the changes in undrained strength and deformation modulus after undrained cyclic loading. It was observed that cyclic degradation in stiffness for low plasticity silt is more marked than that of strength, and this tendency increases with increasing overconsolidation ratio. It was found that a previously proposed model for predicting postcyclic degradation in strength and stiffness of normally consolidated fine-grained soils could be applied to that of overconsolidated silt but not however to the postcyclic degradation in Young’s modulus. Thus, an attempt was made to correlate postcyclic degradation of overconsolidated silt to the equivalent cyclic shear strain instead of the normalized excess pore pressure. It was concluded that cyclic shear strain was a better parameter than cyclic-induced excess pore pressure for correlating the postcyclic stiffness degradation not only for normally consolidated but also for overconsolidated silt.