Journal of Geotechnical and Geoenvironmental Engineering

An investigation into the geotechnical properties specific to assessing the stability of weakly and moderately cemented sand cliffs is presented. A case study from eroding coastal cliffs located in central California provides both the data and impetus for this study. Herein, weakly cemented sand is defined as having an unconfined compressive strength (UCS) of less than 100 kPa, and moderately cemented sand is defined as having UCS between 100 and 400 kPa. Testing shows that both materials fail in a brittle fashion and can be modeled effectively using linear Mohr-Coulomb strength parameters, although for weakly cemented sands, curvature of the failure envelope is more evident with decreasing friction and increasing cohesion at higher confinement. Triaxial tests performed to simulate the evolving stress state of an eroding cliff, using a reduction in confinement-type stress path, result in an order of magnitude decrease in strain at failure and a more brittle response. Tests aimed at examining the influence of wetting on steep slopes show that a 60% decrease in UCS, a 50% drop in cohesion, and 80% decrease in the tensile strength occurs in moderately cemented sand upon introduction to water. In weakly cemented sands, all compressive, cohesive, and tensile strength is lost upon wetting and saturation. The results indicate that particular attention must be given to the relative level of cementation, the effects of groundwater or surficial seepage, and the small-scale strain response when performing geotechnical slope stability analyses on these materials.

The paper investigates the probability of failure of slopes using both traditional and more advanced probabilistic analysis tools. The advanced method, called the random finite-element method, uses elastoplasticity in a finite-element model combined with random field theory in a Monte-Carlo framework. The traditional method, called the first-order reliability method, computes a reliability index which is the shortest distance (in units of directional equivalent standard deviations) from the equivalent mean-value point to the limit state surface and estimates the probability of failure from the reliability index. Numerical results show that simplified probabilistic analyses in which spatial variability of soil properties is not properly accounted for, can lead to unconservative estimates of the probability of failure if the coefficient of variation of the shear strength parameters exceeds a critical value. The influences of slope inclination, factor of safety (based on mean strength values), and cross correlation between strength parameters on this critical value have been investigated by parametric studies in this paper. The results indicate when probabilistic approaches, which do not model spatial variation, may lead to unconservative estimates of slope failure probability and when more advanced probabilistic methods are warranted.

Mechanistic-empirical pavement design guide for flexible pavements as per the AASHTO design guide requires characterization of subgrade soils using the resilient modulus $\left(M_R\right)$ property. This property, however, does not fully account for the plastic or permanent strain or rutting of subgrade soils, which often distress the overlying pavements. Soils such as silts exhibit moderate to high resilient moduli properties but they still undergo large permanent deformations under repeated loading. This explains the fallacy in the current pavement material characterization practice. A comprehensive research study was performed to measure permanent deformation properties of subgrade soils by subjecting various soils under repeated cycles of deviatoric loads. This paper describes test procedure followed and results obtained on three soils including clay, silt, and sandy soils. The influence of compaction moisture content, confining pressure, and deviatoric stresses applied on the measured permanent deformations of all three soils are addressed. A four-parameter permanent strain model formulation as a function of stress states in soils and the number of loading cycles was used to model and analyze the present test results. The model constants of all three soils were first determined and these results were used to explain the effects of various soil properties on permanent deformations of soils. Validation studies were performed to address the adequacy of the formulated model to predict rutting or permanent strains in soils.

Despite clear evidence, changes in mechanical properties (i.e., stiffness or modulus) of compacted subgrades in response to subgrade moisture regime changes after construction have rarely been investigated in the geotechnical profession. In particular, when in-service assessment of pavement subgrade is made, the modulus-moisture variation should be addressed on the basis of unsaturated soil mechanics. This study presents the unsaturated small-strain modulus behavior of five predominately fine-grained compacted subgrade soils. The small-strain shear modulus $\left(G_o\right)$ of saturated compacted specimens subjected to a desorption soil-water characteristic curve (SWCC) was evaluated using bender elements. A test apparatus was designed to apply two stress state variables, the net confining pressure and matric suction, during the $G_o$ measurements. The relationship between $G_o$ and the SWCC under a constant mean net stress was developed. Additionally, the effect of compaction moisture content, compaction energy, and soil type on the $G_o$ -SWCC relationship was investigated. Finally, a relationship describing the small-strain modulus behavior of unsaturated compacted soils is proposed.

The paper presents a study of the influence of grain-size distribution curve on the small strain shear modulus $G_{\text{max}}$ of quartz sand with subangular grain shape. The results of 163 resonant column tests on 25 different grain-size distribution curves are presented. It is demonstrated for a constant void ratio that while $G_{\text{max}}$ is not influenced by variations in the mean grain-size $d_{50}$ in the investigated range, it significantly decreases with increasing coefficient of uniformity $C_u=d_{60}/d_{10}$ of the grain-size distribution curve. Well-known empirical formulas (e.g., Hardin’s equation with its commonly used constants) may strongly overestimate the stiffness of well-graded soils. Based on the RC test results, correlations of the constants of Hardin’s equation with $C_u$ have been developed. The predictions using Hardin’s equation and these correlations are in good accordance with the test data. Correlations of the frequently used shear modulus coefficient $K_{2,\text{max}}$ with $C_u$ and empirical equations formulated in terms of relative density, are also given in the paper. A comparison of the predictions by the proposed empirical formulas with $G_{\text{max}}$ -data from the literature and a micromechanical explanation of the experimental results are provided. Correction factors for an application of the laboratory data to in situ conditions are also discussed.

This paper examines the consistency between surface wave dispersion curves measured at wavelengths of up to 600 m using active and passive methods at sites in the Mississippi Embayment. Large-diameter (200 m) circular receiver arrays were deployed at five deep soil sites located in Tennessee, Arkansas, and Missouri to record ambient ground vibrations at low frequencies. Measurements were performed at the same locations using linear receiver arrays and actively generated low-frequency energy using the recently developed Network for Earthquake Engineering Simulation (NEES) field vibrator (termed Liquidator). Characteristics of the ambient wavefield measured at the five sites in the Mississippi Embayment are presented along with comparisons between the surface wave dispersion curves obtained from the active and passive measurements at each site. The ambient wavefield measurements exhibited peak levels in the frequency range of 1–5 Hz. Surface wave dispersion curves developed from frequency-wavenumber $\left(f\text{-}k\right)$ processing of the active and passive methods were in good agreement at four of the sites, with phase velocities from the passive measurements within 5–10 % of the active-source measurements out to wavelengths of about 550 m. Improved comparisons were obtained at the fifth site by applying high-resolution $f\text{-}k$ processing.

An integrated electrochemical oxidation process that utilizes electrokinetics (EK) to deliver the oxidant (5–10% hydrogen peroxide, ${\text{H}}_2{\text{O}}_2$ ) and chelant [40 mM of ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA)] or iron chelate (1.4 mM Fe-EDTA or Fe-DTPA) to oxidize polycyclic aromatic hydrocarbons (PAHs) in soils was investigated. Batch and bench-scale EK experiments were conducted using: (a) kaolin, a low permeability clayey soil, spiked with phenanthrene at 500 mg/kg, and (b) former manufactured gas plant (MGP) soil, a high buffering silty soil, contaminated by a variety of PAHs (1493 mg/kg). Batch experiments showed that chelant solutions dissolve native iron minerals to form soluble Fe-chelates that remain available even at higher pH conditions of soil for the Fenton-like oxidation of the PAHs. In EK experiments, a 5–10% ${\text{H}}_2{\text{O}}_2$ solution was delivered from the anode and a chelant solution or iron-chelate was delivered from the cathode. Preflushing of soil with 5% ethanol and ferrous sulfate (1.4 mM) prior to oxidant delivery was also investigated. An electric potential of 2 VDC/cm was applied in all tests to induce electroosmotic flow for 5–8 days for kaolin and 25 days for the MGP field soil. In the absence of any chelating agent, phenanthrene oxidation was catalyzed by native iron present in kaolin soil, and 49.8–82.3% of phenanthrene was oxidized by increasing ${\text{H}}_2{\text{O}}_2$ concentration from 5–10%. At 5% ${\text{H}}_2{\text{O}}_2$ concentration, phenanthrene oxidation was not increased by using 40 mM EDTA, 40 mM DTPA or 1.4 mM Fe-DTPA, but it increased to 70% using 1.4 mM Fe-EDTA. Maximum phenanthrene oxidation (90.5%) was observed by 5% ethanol preflushing and then treating with 5% ${\text{H}}_2{\text{O}}_2$ at the anode and 1.4 mM Fe-EDTA at the cathode. However, preflushing with 1.4 mM ferrous sulfate did not improve phenanthrene oxidation. The results with the MGP field soil indicated that delivery of 5% ${\text{H}}_2{\text{O}}_2$ alone resulted in oxidation of 39.8% of total PAHs (especially 2- and 3-ring PAHs). The use of EDTA and Fe-EDTA did not increase PAHs oxidation in this soil. Overall, the results reveal that an optimized in situ combined technology of EK and Fenton-like process has the potential to oxidize PAHs in low permeability and/or high buffering soils.

Pseudostatic approaches for the seismic analysis of pile foundations are attractive for practicing engineers because they are simple when compared to difficult and more complex dynamic analyses. To evaluate the internal response of piles subjected to earthquake loading, a simplified approach based on the “ $p\text{-}y$ ” subgrade reaction method has been developed. The method involves two main steps: first, a site response analysis is carried out to obtain the free-field ground displacements along the pile. Next, a static load analysis is carried out for the pile, subjected to the computed free-field ground displacements and the static loading at the pile head. A pseudostatic push over analysis is adopted to simulate the behavior of piles subjected to both lateral soil movements and static loadings at the pile head. The single pile or the pile group interact with the surrounding soil by means of hyperbolic $p\text{-}y$ curves. The solution derived first for the single pile, was extended to the case of a pile group by empirical multipliers, which account for reduced resistance and stiffness due to pile-soil-pile interaction. Numerical results obtained by the proposed simplified approach were compared with experimental and numerical results reported in literature. It has been shown that this procedure can be used successfully for determining the response of a pile foundation to “inertial” loading caused by the lateral forces imposed on the superstructure and “kinematic” loading caused by the ground movements developed during an earthquake.

This paper presents the results of vertical vibration tests on two full-scale single piles. The diameter of pile and embedded depth were 0.45 and 22 m, respectively. The soil samples were collected from three boreholes located at the site of investigation and it was explored to a depth up to 30.45 m below the ground level. The vertical vibration tests were conducted for different eccentricities to determine the frequency-amplitude response of the pile. Static load tests were also carried out on two single piles. A simple axisymmetric two-dimensional finite-element model was developed to predict the dynamic pile response. Novak’s plane strain model was also used for the prediction of the dynamic response of single pile. It was observed that the finite-element model predicted the natural frequency and peak displacement amplitude of pile reasonably well. However, prediction of dynamic response of the pile was found unsatisfactory by Novak’s plane strain model. Possible reasons for unsatisfactory performance of Novak’s model were investigated and presented.

A series of centrifuge model tests was conducted to investigate the behavior of pile groups of various sizes and configurations behind a retaining wall in very soft clay. With a 1.2-m excavation in front of the wall, which may simulate the initial stage of an excavation prior to strutting, the test results reveal that the induced bending moment on an individual pile in a free-head pile group is always smaller than that on a corresponding single pile located at the same distance behind the wall. This is attributed to the shadowing and reinforcing effects of other piles within the group. The degree of shadowing experienced by a pile depends on its relative position in the pile group. With a capped-head pile group, the individual piles are forced to interact in unison though subjected to different magnitudes of soil movement. Thus, despite being subjected to a larger soil movement, the induced bending moment on the front piles is moderated by the rear piles through the pile cap. A finite element program developed at the National University of Singapore is employed to back-analyze the centrifuge test data. The program gives a reasonably good prediction of the induced pile bending moments provided an appropriate modification factor is applied for the free-field soil movement and the amount of restraint provided by the pile cap is properly accounted for. The modification factor applied to the free-field soil movement accounts the reinforcing effect of the piles on the soil movement.

Vertically installed plate anchors have been investigated in this paper by numerical analysis and centrifuge modeling. In the numerical analysis, the large deformation finite-element method (remeshing and interpolation technique with small strain) was used to simulate strip plate anchor rotation. In the centrifuge model tests, transparent soils were used to observe square anchor rotation. The loss in anchor embedment during anchor keying was assessed for anchors in uniform and normally consolidated soils with anchor pullout angle varying from 30° to 90° to the horizontal. It was found that the loss in anchor embedment during anchor keying may be expressed in terms of a nondimensional anchor geometry factor, which is a function of the eccentricity of the padeye, angle of loading, and the net moment applied to the anchor at the stage where the applied load balances the anchor weight. However, once the anchor geometry factor reaches a certain value, the loss in anchor embedment stabilizes at 0.25–0.5 times the anchor width. The loss in anchor embedment decreases linearly with decreasing pullout angle. Simple formulae and design procedures have been proposed to estimate the loss in anchor embedment during keying.

A numerical finite-difference method (FLAC) model was used to investigate the influence of constitutive soil model on predicted response of two full-scale reinforced soil walls during construction and surcharge loading. One wall was reinforced with a relatively extensible polymeric geogrid and the other with a relatively stiff welded wire mesh. The backfill sand was modeled using three different constitutive soil models varying as follows with respect to increasing complexity: linear elastic-plastic Mohr-Coulomb, modified Duncan-Chang hyperbolic model, and Lade’s single hardening model. Calculated results were compared against toe footing loads, foundation pressures, facing displacements, connection loads, and reinforcement strains. In general, predictions were within measurement accuracy for the end-of-construction and surcharge load levels corresponding to working stress conditions. However, the modified Duncan-Chang model which explicitly considers plane strain boundary conditions is a good compromise between prediction accuracy and availability of parameters from conventional triaxial compression testing. The results of this investigation give confidence that numerical FLAC models using this simple soil constitutive model are adequate to predict the performance of reinforced soil walls under typical operational conditions provided that the soil reinforcement, interfaces, boundaries, construction sequence, and soil compaction are modeled correctly. Further improvement of predictions using more sophisticated soil models is not guaranteed.

This paper presents the practical elastic settlement formulae for the eccentrically loaded surface footings resting upon an elastic mass. The presented closed-form solutions can be readily implemented into the practice allowing efficient and accurate prediction of elastic settlement under the rectangular footing which is subjected to the biaxial bending. The presented solutions are determined by evaluating the integration of strain expressions based on the Boussinesq stress equations. The common assumption of linear contact pressure in footing-soil interface is adopted for the solutions. The presented formulae are validated to be used for the settlement under any point of linear full-contact loading and their applicability to the current design process is demonstrated. The solutions are also developed for the theoretical cases where the location of incompressible soil layer is infinitely deep. The simplified influence factors are presented graphically and the numerical examples are provided for their practical use. In this respect, the paper represents a significant step forward in understanding of elastic settlement, rotation, and differential elastic settlement under eccentrically loaded footings.

An experimental and numerical study of the behavior of an eccentrically loaded strip footing resting on geosynthetic-reinforced sand is presented. Particular attention was given to simulate footings constructed on unsymmetrical geogrid layers with eccentricity either direction of the footing. Several configurations of geogrid layers with different number, length, layer eccentricity along with the effect of the sand relative density, and the load eccentricity were investigated. A numerical study on a plane strain prototype footing was performed using finite element analysis. Test results indicate that the footing performance could be appreciably improved by the inclusion of layers of geogrid leading to an economic design of the footing. However, the efficiency of the sand-geogrid system is dependent on the load eccentricity ratio and reinforcement parameters. A close agreement between the experimental and numerical trend lines is observed. Based on the numerical and experimental results, critical values of the geogrid parameters for maximum reinforcing effect are established.

For predicting the settlement rate of a ground area that incorporates prefabricated vertical drains, the hyperbolic method is herein developed based on Barron’s solution and validated by using three documented case studies. This method is suitable within the degrees of consolidation of 60–90%. Results indicate that the estimated coefficients of radial consolidation $\left(C_r\right)$ are slightly larger. In two case studies, the estimated $C_r$ values are closer to the coefficients of vertical consolidation obtained from the standard oedometer tests but differ from those values derived from the cone penetration test (CPT) tests. In another case study, where not laboratory results are available, the estimated $C_r$ values fall within a lower bound determined by CPT results. The reason of these differences is due to various factors such as smear and well resistance, the vertical drain type, and its finite drainage capacity. It also appears that, in contrast to the solution obtained by Barron, the settlement versus time curves from the proposed method concur with the monitored curves. Finally, the applicability of the proposed method is discussed based on the case studies.

Grout piles are often used to reinforce the base soil against base heave when carrying out deep excavations in soft clay. However, there is still a lack of an adequate criterion to describe the shear strength of clay reinforced with grout piles. In general, the anisotropic strength characteristic of clay reinforced with grout piles is more significant than that of clay. The objective of this work is to develop an anisotropic strength criterion for the reinforced soil mass. Only four parameters are needed in this anisotropic strength criterion: two are the strength properties of the in situ clay, namely, the axial compressive and axial extensive undrained shear strengths; another is the undrained shear strength of treated soil; and the final is the improvement ratio which is related to the spacing and layout pattern of the grout piles. To be used in two-dimensional undrained stability analysis, the suitability of this anisotropic strength criterion under plane strain conditions is verified by comparing the results with true triaxial test. The maximum difference between the calculated and laboratory measured shear strengths is less than 8%. The results of this study indicate that the anisotropic undrained shear strength of clay reinforced with grout piles under plane strain condition decreases with an increase in the angle between the vertical direction and the major principal stress and decreases with a decrease in the strength anisotropy ratio of clay reinforced with grout piles. However, there will be a greater improvement in the effect if the grout piles are installed in the active zone rather than in the passive zone. This is because the shear strength of a grout pile mobilized in the active zone is close to its maximum level.

The performance of rigid surface structures such as concrete pavements and slabs-on-grade supported by a deteriorated subgrade and experiencing local contact loss is investigated experimentally and numerically in this study. A laboratory setup has been designed to facilitate the simulation of subsurface erosion and measure the changes in contact pressure at selected locations under a slab-on-grade supported on granular material. The presence of erosion voids under a slab-on-grade can lead to rapid increase in the contact pressure in the immediate vicinity of the void in addition to an increase in tensile stresses at the outermost fibers of the slab. This preliminary study suggests that efforts to detect and arrest the growth of erosion voids under slabs-on-grade should be made before the voids reach the size where significant loss of support develops and the tensile strength of the slab material is exceeded.

Soil compaction quality control plays an important role in earthwork construction. Compacted dry density is only loosely related to the actual deformation of the compacted soil. Rather than using dry density as the controlling factor for compacted fills, it would be better to measure properties more closely related to soil compressibility. The Briaud compaction device (BCD) is a simple, small-strain, nondestructive testing apparatus that can be used to evaluate the modulus of compacted soils. The use of the BCD as a field testing device for compacted soil quality control may be more beneficial than the current practice of measuring in situ dry density. In this study, the laboratory procedures of the BCD were evaluated for compacted silt. The modulus determined by the BCD was compared to the dynamic elastic moduli (Young’s and shear moduli) determined from ultrasonic pulse velocity testing on the same compacted silt samples. The BCD modulus correlated well with the ultrasonic pulse velocity results with R2 value of 0.8 or better. Finally, a repeatability and reproducibility study conducted on the BCD showed a variation of 4% from the mean when only the soil properties were altered.