Journal of Geotechnical and Geoenvironmental Engineering

A prototype pier load test program was performed to study the load-deformation and energy-dissipation characteristics of the response of drilled piers under static and dynamic axial loads. The field tests consisted of six fully instrumented drilled concrete piers 61–76 cm (24–30 in.) in diameter and 5.8–9.1 m (19–30 ft) in length. The piers were constructed on a stiff, sandy clay site adjacent to the University of California, Berkeley campus. A dynamic Fundex pile load test (PLT) was performed on each pier, followed by a static-compression or tension test and a second PLT. The field tests revealed that the stiffness and capacity of a soil-pier system depend significantly on the loading rate. For the type of piers and soil considered in the field test, the increase in dynamic stiffness versus static stiffness is approximately 20–40% at almost all displacement levels. The ultimate dynamic capacity increases approximately 30% compared with the static case. The multiple PLTs conducted on the same drilled pier also indicated that a pier may experience up to 50% stiffness and strength degradation when subjected to full load reversal. The test program showed that the PLT is a fast, innovative method to get useful site-specific information for seismic design of the pier foundation.

Realistic time history simulation of drilled pier/pile-soil systems under dynamic and static loading is essential for the development of effective performance-based earthquake designs of deep foundations. This paper presents the results of the numerical simulation of a series of static and dynamic tests on drilled piers performed at the University of California, Berkeley. A nonlinear soil model was implemented based on multiaxial cyclic bounding-surface plasticity within a general finite-element framework, OpenSees. The model requires a small number of parameters that can be easily obtained through conventional site investigations. The results of the simulations show that the model can reasonably simulate nonlinear response of the soil and that it does a good job of capturing the actual load deformation curves obtained from in situ dynamic and static pier load tests. Although the model is suitable for a fully nonlinear total stress analysis of soil-pile systems under multidirectional shaking, further studies are needed to enhance the model capacity by incorporating the cyclic stiffness and strength degradation caused by full stress reversals.

The nonlinear response of shallow foundations has been studied experimentally and analytically. However, the engineering community is not yet convinced of the applicability of such concepts in practice. A key prerequisite is the ability to realistically model such effects. Although several sophisticated constitutive models are readily available in the literature, their use in practice is limited, because (1) they typically require extensive soil testing for calibration; (2) as they are implemented in highly specialized numerical codes, they are usually restricted to simple superstructures; and (3) in most cases, they can only be applied by numerical analysis specialists. Attempting to overcome some of these difficulties, this paper develops a simplified but fairly comprehensive constitutive model for analysis of the cyclic response of shallow foundations. On the basis of a kinematic hardening constitutive model with Von Mises failure criterion (readily available in commercial finite element codes), the model is made pressure sensitive and capable of reproducing both the low-strain stiffness and the ultimate resistance of clays and sands. Encoded in ABAQUS through a simple user subroutine, the model is validated against (a) centrifuge tests of shallow footings on clay under cyclic loading and (b) large-scale tests of a square footing on dense and loose sand under cyclic loading, conducted in the European Laboratory for Structural Analysis for the TRISEE project. The performance of the model is shown to be quite satisfactory, and discrepancies between theory and experiment are discussed and potential culprits are identified. Requiring calibration of only two parameters and being easily implemented in commercial FE codes, the model is believed to provide an easily applicable engineering solution.

Drained triaxial compression tests on crushed coral sand were performed from near-static strain rates to very high strain rates (up to approximately $1,800\%/\mathrm{s}$). Experiments were performed on dry, vacuum-confined axisymmetric specimens at two different confining pressures (98 and 350 kPa) and two different densities (${\mathrm{D}}_{\mathrm{r}}$ approximately 36 and 60%). A gravity drop weight loading system was used to generate high strain rates. High-speed film photographs of the specimen were taken through the flat sides of a square triaxial cell. By using digital image analysis techniques, strains were locally measured near the center of the specimen to obtain the most uniform assessment. Stress-strain relationships are presented. The following effects were observed with increasing strain rates: the elastoplastic stiffness increased significantly; the failure shear strength increased moderately; the axial strain at peak stress decreased significantly; and volumetric strains became more dilatant. Unusual behavior was observed at very high strain rates. Examples include the peak stress was not always associated with the maximum dilatancy rate and shear band inclination angle was noticeably reduced.

Biological processes may provide great and previously unexplored opportunities for cost-effective, in situ improvement of the engineering properties of soil. A laboratory study was conducted to evaluate the changes in geomechanical properties of sand attributable to the formation of calcium precipitates induced through ureolysis catalyzed by Sporosarcina pasteurii (S. pasteurii). Specifically, direct shear and California Bearing Ratio (CBR) tests were conducted on sand specimens subjected to treatment by growing, resting, and dead S. pasteurii cells in completely stirred tank reactors and completely mixed biofilm reactors, respectively. Scanning electron microscopy analyses were also conducted to evaluate microbially induced precipitation. The results of the study show that the bacterial cells effectively improved the geomechanical properties of the sand. Growing cells improved the sand properties owing to microbially induced precipitation and related pore volume changes, whereas dead and resting cells generally caused smaller increases in friction angle and bearing strength. Analysis of the sand from CBR specimens treated with growing cells demonstrated that the microbial and chemical processes both contributed to the clogging of the porous medium.

This paper presents the results of vane shear, laboratory compaction, isotropic consolidation, cyclic triaxial, bender element, and resonant-column tests that were performed to characterize the dynamic properties of an artificial soil called modified glyben. Modified glyben comprises a mixture of glycerin, water, and bentonite that can be used in scaled model tests performed at $1G$ or $nG$ in a centrifuge to study seismic soil–structure interaction. The results described in this paper show that the vane shear strength, coefficient of consolidation, dynamic modulus, and damping ratio are strongly influenced by the viscosity of the pore fluid which can be varied by altering the ratio of glycerin-to-water. In addition, the properties of modified glyben are stable during prolonged exposure to air and multiple largestrain load cycles making it a suitable model soil for scaled model tests involving seismic soil–structure interaction.

Characterizing the strength of weakly cemented and sensitive soils in the laboratory is difficult because of the difficulty in obtaining high-quality replicate samples necessary for defining the failure envelope. Multistage triaxial tests have long been used to reduce the variability caused by testing multiple samples; however, traditional criteria used for transitioning from one loading stage to another often lead to destructuring or failure in sensitive or structured soils. The objective of this paper is to present a methodology for conducting multistage drained triaxial tests on weakly cemented sands and estimating the resulting shear strength parameters. Both multistage and single-stage drained triaxial tests were performed on artificially cemented samples of a silty sand at two levels of densities and cementation. The use of $d{\epsilon }_v/d{\epsilon }_a=0$ as a termination criterion to move on to the next stage of loading and ${\epsilon }_v=0$ as the failure criterion for the final stage of the shear resulted in an average error of 6% and 5% in $c^{\prime }$ and ${\phi }^{\prime }$, respectively, for the stress range considered in this study when compared with parameters obtained from the single-stage drained triaxial tests. Continuous shear wave velocity ($V_s$) measurements during shear showed that destructuring of the cemented samples did not occur by using the proposed termination criterion. The proposed method has the potential to be a cost-effective alternative to the testing of multiple samples for the characterization of the strength of weakly cemented and sensitive soils.

Reliability-based optimization (RBO) intends to minimize the cost involved in a design while satisfying all technical requirements specified by using reliability theory. Although this idea is attractive, its implementation is generally difficult because an accurate evaluation of the reliability constraints is often computationally prohibitive when it is coupled with the minimization of a cost function. This paper suggests an indirect method for RBO of geotechnical systems initially on the basis of the mean first-order reliability method (MFORM). Because reliability analysis with MFORM is very efficient, the computational work involved in RBO is greatly reduced. The major concern about MFORM is that it is not an accurate reliability method. A reliability index mapping function was used to relate the reliability index calculated by MFORM to that calculated by a more accurate method. When the mapping function perfectly reflects the relationship between MFORM reliability index and that from an accurate reliability method, the RBO from MFORM and the original RBO problem are equivalent. When a perfect mapping function cannot be built, a procedure has been suggested to fine-tune the optimal design such that accurate reliability constraints can be maintained in the final optimal design. The proposed method is illustrated with two geotechnical examples: the optimal design of a shallow foundation and a gravity retaining wall.

Soil nailing is an effective and practical means for stabilizing slopes and supporting excavations. It has been used extensively in slope improvement works in Hong Kong since the mid-1990s. However, once a soil nail has been installed, it is difficult to check its quality, such as the length of the steel bar and the integrity of cement grout annulus. To enhance the quality control of soil-nail installation works, the Geotechnical Engineering Office (GEO) of the Civil Engineering and Development Department in Hong Kong has carried out a study to identify and develop nondestructive means for checking the quality of installed soil nails. Among the various techniques tried, time-domain reflectometry (TDR) was found to be the simplest, quickest, and least expensive. Since then, a large number of field measurements have been taken to gain experience on the practicality and reliability of this technique. In mid-2004, the GEO introduced TDR to its soil-nailing works under the Landslip Preventive Measures Programme for pilot use during independent site audits. In 2007, an independent review was carried out that supports the continual use of TDR in the program and suggests further enhancement of the sampling strategy of the assurance program. This paper gives an overview of the use of TDR in assessing the quality of steel soil nails with preinstalled wires. The sources of uncertainty in relation to the test are discussed. It also describes cases in which TDR has identified anomalies and the corresponding follow-up actions.

An optical fiber strain-sensing technique, on the basis of Brillouin optical time domain reflectometry (BOTDR), was used to monitor the performance of a secant pile wall subjected to multiple props during construction of an adjacent basement in London. Details of the installation of sensors and data processing are described. Distributed strain profiles were obtained by deriving strain measurements from optical fibers installed on opposite sides of the pile to allow monitoring of both axial and lateral movements along the pile. Methods for analyzing the thermal strain and temperature compensation are also presented. Measurements obtained from the BOTDR were found to be in good agreement with inclinometer data from the adjacent piles. The relative merits of the two different techniques are discussed.

The loss in anchor embedment during keying, as it rotates to become normal to the cable load, reduces the uplift capacity of anchors in normally consolidated clay. The keying behavior of plate anchors has been studied previously by using centrifuge and field model tests. In this paper, a large deformation finite-element approach incorporating frequent mesh regeneration and allowing for evolution of the anchor-chain profile, was developed to simulate the keying process of rectangular and strip plate anchors. A parametric study was undertaken to quantify the loss in anchor embedment during keying in terms of the anchor geometry, soil properties, loading eccentricity, and inclination. The embedment loss decreased dramatically with increasing loading eccentricity and decreasing chain angle at the mudline to the horizontal. The loss in anchor embedment during keying increased as the local soil strength increased relative to the weight of the anchor, up to a limit determined by the eccentricity of loading. In contrast, the effect of the soil strength gradient was minimal. Findings of this study yield a recommendation that the most important factor in anchor keying, loading eccentricity, should not be less than half the anchor width for an effective design.

Assuming that concrete is a homogeneous porous material in gravity dams, pore pressure intensities at drain wells near the upstream face of dams were studied. In general, the piezometric head of the seepage flow through a gravity dam may be mathematically represented by the well-known Laplace equation. This equation can be solved numerically by the finite-difference equation developed from a block-centered formulation. By using the boundary conditions of the height of the seepage face in drain wells from the concept in hydraulics of wells and the seepage face along the downstream face of dams, piezometric heads, and in turn, pore pressures in the dam body were obtained. This approach of predicting pore pressures has been verified by 40 years of data from the Tennessee Valley Authority’s (TVA’s) Fontana Dam. The pore pressure intensities in dam bodies and at the drain wells are presented. They are expressed in dam heights, size of drains, and locations of drains from the face of the dams for inspection galleries located at the dam base. The results obtained show the location of the inspection galleries for the lowest pore pressures. The inspection galleries are usually used to intercept seeping water from the drain wells in the dam foundation and dam body. The seeping water intercepted by the dam body can be estimated when the values of hydraulic conductivity of concrete are known. The accumulation of calcium carbonate deposited in the drains can then be studied on the basis of the known values of the calcium carbonate concentration of the seeping water. The data presented may be useful for design engineers in the stability and stress analyses of concrete gravity dams. In addition, the effects of the location (i.e., elevation) of the inspection gallery and the tailwater pressures on pore pressures were studied by using the Fontana Dam as a case study.

Fully drained, load-controlled laboratory model tests and their numerical simulations are presented. The tests were performed on adequately instrumented, small scale physical models of floating stone column group foundations placed in slurry deposited clayey soil beds with known effective stress states. Effect of various group foundation parameters, such as area ratio, length of columns, relative density, and moisture content of the column material is evaluated. The numerical analyses consist of three-dimensional, elastoplastic, finite-element analyses of the model foundation. In the analyses, the clayey soil behavior is represented by the modified Cam-clay model, and the stone column and mat are represented by the elastic, fully plastic Mohr-Coulomb constitutive relationship. The finite-element analysis was successful in predicting the model test results with reasonable accuracy. The results are presented in nondimensional form. The major foundation parameters affecting the group response were identified as area ratio, normalized column length, Young’s modulus of column, overconsolidation ratio, initial geostatic stresses, and clayey soil parameters.

The existence of initial shear stress can have a significant effect on the cyclic strength or liquefaction potential of sand. This effect is not yet fully understood because of a lack of convergence and consistency in the existing data and interpretations, leading to great uncertainty in quantifying the effect for practical applications. This paper presents new experimental results on a silica sand to validate the concept known as threshold $\alpha$, below which the cyclic strength of sand increases with $\alpha$ and above which the cyclic strength decreases with $\alpha$ (with $\alpha$ representing the sustained shear stress level). On the basis of a series of monotonic loading tests on the same sand, and in the framework of critical state soil mechanics, it is further confirmed that threshold $\alpha$ can be well related to a state parameter in the void ratio-mean effective stress plane and thereby a unified and consistent interpretation can be established. A new platform is proposed on which the relationship between cyclic strength and state parameter is represented by a linear line, and this line will rotate clockwise as $\alpha$ increases. This platform provides an effective analytical tool for the study of the effect of sustained shear stress on the cyclic strength of sand. Moreover, the study also shows that the cyclic loading path is well linked with the monotonic loading path under different sustained shear stress levels, and this correspondence sheds light on the mechanisms underlying a variety of experimental observations.

This paper presents the results of a temperature-monitoring program in an organic-waste landfill cell under normal operating conditions. The aim of the program was to capture the temporal evolution of temperature at various locations within the cell. Temperatures were measured at two levels within the central body of the cell over a period of 20 months. The results obtained over that period of time indicate that the temperatures increased within the first few weeks, then decreased to a stable value after a few months. The peak temperatures near the base of the landfill cell (2 m above base liner) averaged 60°C at the start of the monitoring program and leveled off to 53°C after 8 months. The peak temperatures in the second lift (4 m above base liner) averaged 57°C and leveled off to 55°C after 13 months. The temperatures taken close to the base of the cell indicate that the lining system is subjected to high temperatures, outside the range of temperature typically considered during testing and design. This observation highlights the importance of considering the possible impact of the changes in temperature on the performance of the different liner system components from both the hydraulic view point and durability.

The jet erosion test (JET) and the hole erosion test (HET) are two tests used to determine soil erodibility classification, and results are commonly interpreted by two distinct methods. A new method based on fluid energy dissipation and on measurement of the eroded mass for interpreting the two tests is proposed. Different fine-grained soils, covering a large range of erodibility, are tested. It is shown that, by using common methods, the erosion coefficient and average critical shear stress are different with the JET and with the HET. Moreover, the relative soils classifications yielded by the two erodimeters are not exactly the same. On the basis of the energy method, an erosion resistance index is determined for both apparatuses, and a classification of surface-erosion resistance is proposed. For both apparatuses, values of the erosion resistance index are roughly the same for each soil, and a single classification of soil erodibility is obtained.

Near-field effects are inherent in continuous surface wave (CSW) tests. Such effects on the dispersion curves can be studied using the finite-element (FE) method. However, near-field effects may be obscured by boundary effects in a FE model. Although the use of local nonreflecting boundary conditions (NRBCs) may alleviate the problem, these need to be placed sufficiently far away to be effective. Quantitative guidelines on domain size with NRBCs are still lacking. This paper provides specific guidelines to determine the domain size in terms of the wavelength-to-domain size ratio, ${\lambda }^{*}/l^{*}$. With local NRBCs, numerical errors were found to be negligible when the domain size ($l^{*}$) was extended to twice the longest wavelength (${\lambda }^{*}$). However, in modeling soft soil deposits in which shear-wave velocity ($V_s$) is less than $100\mathrm{m}/\mathrm{s}$, a smaller domain size (e.g., ${\lambda }^{*}/l^{*}=2$) is sufficient.