Journal of Geotechnical and Geoenvironmental Engineering

This paper presents an experimental investigation of the dynamic internal shear strength of a hydrated woven/nonwoven needle-punched geosynthetic clay liner (GCL) for monotonic (i.e., single direction) loading conditions. Displacement-controlled shear tests were conducted using a large direct shear machine for four normal stress levels ranging from 141 to 1,382 kPa and seven shear displacement rates $R$ ranging from 0.1 to $\mathrm{30,000}\mathrm{mm}/\min$. For each normal stress, peak shear strength first increased and then decreased with increasing displacement rate. Maximum values of peak strength occurred for $R=100–\mathrm{10,000}\mathrm{mm}/\min$ and were 16–23% higher than corresponding static values measured at $R=0.1\mathrm{mm}/\min$. For each normal stress, residual shear strength first decreased and then increased with increasing displacement rate, with minimum values occurring at $R=1\mathrm{mm}/\min$. On a relative basis, residual strengths show greater dependence on displacement rate than peak strengths. The standard displacement rate for static shear tests of hydrated GCLs ($0.1\mathrm{mm}/\min$) generally yielded conservative values of peak shear strength but unconservative values of residual shear strength, especially for higher normal stress levels. The GCL experienced large post-peak strength reduction for all test conditions.

This paper presents an experimental investigation of the dynamic shear strength of a composite liner consisting of a high-density polyethylene (HDPE) textured geomembrane (GMX) over a hydrated nonwoven/nonwoven needle-punched geosynthetic clay liner (GCL) for monotonic (i.e., single direction) loading conditions. Displacement-controlled shear tests were conducted using a large direct shear machine for five normal stress levels ranging from 13 to 2071 kPa and five shear displacement rates ranging from 0.1 to $\mathrm{30,000}\mathrm{mm}/\min$. GCL internal failures occurred at high normal stress and low displacement rate. As normal stress decreased or displacement rate increased, failure mode transitioned to the GMX/GCL interface. Peak strength envelopes are slightly nonlinear (concave-down) and show dependence on displacement rate at higher normal stress. Large-displacement strength envelopes show greater dependence on displacement rate at higher normal stress due to the effect of changing failure mode. The standard displacement rate for static shear tests of GMX/GCL composite liners ($1\mathrm{mm}/\min$) yielded conservative values of peak shear strength but unconservative values of large-displacement shear strength for some normal stress conditions. The GMX/GCL composite liner experienced significant post-peak strength reduction for all test conditions.

Frost heave and thaw settlement are the cause of substantial damage to infrastructure in regions of seasonal freezing as well as seasonal thawing (permafrost). While frost heaving of soils has been researched for decades, less attention has been paid to quantitative estimates of settlement and loss of strength of soils due to thawing. A constitutive model is developed, capable of simulating freezing and thawing of soils, and associated changes in the soil strength. While the porosity growth parameters describing frost heaving have been calibrated and validated for a specific soil, the description of the thawing phase has not been validated due to a lack of experimental data. The yielding of the frozen soil is described using the critical state concept, with the pore ice content being an important parameter affecting the yield function. The model was calibrated using available laboratory test data and used in the simulation of a freezing–thawing cycle in the soil with embedded footing.

Unconfined compressive strength (UCS) and tensile strength are important geomaterial parameters that are used for performance prediction and mix design. Both UCS and tensile strength are affected by gravimetric water content and thus the characterization of their relationship with gravimetric water content is of paramount importance. The unified effective stress theory based on the suction stress enables the characterization of soils under both saturated and unsaturated conditions with one closed-form relationship. Using two types of soils, this paper provides experimental verification of this effective stress theory through indirect tensile (IDT) and UCS tests. The verification is extended to low gravimetric water contents and high matric suction levels. A new method is proposed to predict a soil’s UCS and tensile strength at specified gravimetric water contents by characterizing the soil water characteristic curve (SWCC) and conducting UCS and IDT tests at different unsaturated gravimetric water content levels, which is verified experimentally in this paper. The mechanical properties of soils are related to the material parameters $\alpha$ (inverse of the air entry pressure for water-saturated soil) and $n$ (pore size distribution) of the SWCC.

This paper reports two $3\times 3$ pile group tests in saturated silt laterally loaded at eccentric distances of $6D$ and $11D$. The model piles were 0.114 m in diameter and 5.95 m in length. The objectives of the tests are to investigate (1) the response of the pile group subjected to eccentric lateral loading and the influence of load eccentricity; and (2) the internal forces mobilized in the piles and their contributions to resisting the applied lateral loads and torques. The results of the model tests indicate that the lateral resistance of the pile group decreased significantly with eccentricity. The lateral resistance of the pile group with an eccentric distance of $6D$ was 1.5 times that with an eccentric distance of $11D$ at a horizontal displacement of 50 mm at the center of the pile cap. The mobilized pile-head shear forces as well as the bending moments and horizontal displacements in the piles differ significantly in both magnitude and direction. The shear resistances of the piles contributed to resisting both the applied lateral loads and torques. Different twist centers were found in the $3\times 3$ pile group for different load eccentric distances. The distances of the twist center to the pile cap center were 9 and $3.7D$ in the tests with eccentric distances of 6 and $11D$, respectively.

$1\text{-}g$ model experiments were carried out in the laboratory to investigate the behavior of a single pile in sand under combined uplift and lateral load. Dimensions of the model pile were ascertained using physical scaling laws, based on adopted material properties of model and prototype pile foundations. The model pile is made up of aluminum and has outer and inner diameters of 25.4 and 19.0 mm, respectively. Different length-to-diameter ratios of 18, 28, and 38 are considered by varying the pile length to simulate behavior of both stiff and flexible piles. The test tank dimensions were chosen such that boundary effects are minimized. The size of the model steel tank is $1.0\times 1.0\times 1.2\mathrm{m}$ (depth). Experiments were performed on single piles embedded in sandy soil under independent uplift and lateral loading, and combined lateral and uplift loading. Results indicate that the load-deflection behavior is nonlinear for independent uplift and lateral load tests, as well as in the case of combined loading. It is also observed that the behavior of piles under independent loading is substantially different when compared to combined loading. It is found that the ultimate lateral/uplift load capacity under combined loading increased considerably; however, the pile head deflection/displacement at safe lateral/uplift load is found to increase under combined loading. This indicates that although the capacity is increasing under combined load, considering this increased pile capacity may not be possible in a practical design. Rather the safe lateral/uplift loads that are being used in the current design may need to be verified for satisfying the limiting deflection/displacement criteria of the piles considering the effect of combined loading.

During earthquake ground shaking earth pressures on retaining structures can cyclically increase and decrease as a result of inertial forces applied to the walls and kinematic interactions between the stiff wall elements and surrounding soil. The application, based on limit equilibrium analysis, of a pseudostatic inertial force to a soil wedge behind the wall [the mechanism behind the widely-used Mononobe–Okabe (M–O) method] is a poor analogy for either inertial or kinematic wall–soil interaction. This paper demonstrates that the kinematic component of interaction varies strongly with the ratio of wavelength to wall height ($\lambda /H$), asymptotically approaching zero for large $\lambda /H$, and oscillating between the peak value and zero for $\lambda /H<2.3$. Base compliance, represented in the form of translational and rotational stiffness, reduces seismic earth pressure by permitting the walls to conform more closely to the free-field soil displacement profile. This framework can explain both relatively low seismic pressures relative to M–O predictions observed in recent experiments with $\lambda /H>\sim 10$, and relatively high seismic earth pressures relative to M–O from numerical analyses in the literature with $\lambda /H=4$.

The basement of the Stata Center building on the MIT campus required 12.8 m deep excavations covering a large open-plan site and underlain by more than 25 m of lightly overconsolidated Boston Blue Clay. The excavations were supported by a floating, perimeter diaphragm and braced with a system of internal corner struts, rakers, and tieback anchors. The project involved a complex sequence of berms, access ramps, and phased construction of the concrete mat foundation. One of the key goals of the design was to limit ground movements in order to prevent damage to adjacent structures, including the Alumni Pool building, which was founded on shallow caissons and located less than 1.5 m from the south wall. Lateral wall movements and building settlements were closely monitored throughout construction, while photos from a network of webcams located around the open-plan site provide a detailed time history of the construction processes. This paper describes the development of a comprehensive three-dimensional (3D) finite-element (FE) model for the Stata Center basement excavation, which has been enabled by recent advances made available in a FE software package, including efficient multicore iterative solving capabilities, importing of geometric data from computer-aided design files, and use of embedded pile elements to represent tieback anchors. The analyses highlight the effects of the 3D excavation and support geometry on wall and ground movements. The base case results using a simple elasto-plastic Mohr–Coulomb soil model with undrained conditions in the clay are generally in very good agreement with measured performance. The effects of refined constitutive modeling and partial drainage within the clay have a secondary role in numerical predictions for this project.

The widely differing factors of safety obtained by strength reduction and load increase in the ultimate bearing capacity of foundations have been reexamined analytically. In most cases, factors of safety based on load increase need to be higher than those based on strength reduction to achieve the same level of safety in design. The greater sensitivity of foundation systems to strength reduction over load increase may impact the choice of factors in ultimate limit state design.