Journal of Geotechnical and Geoenvironmental Engineering

River embankments and small earth dams are linear retaining structures commonly used to protect densely populated areas from flood phenomena and to provide water reservoirs for human or agricultural use. Their continuity and uniformity are fundamental to their structural efficiency. Due to their significant length and the localized nature of potential weakness points, their characterization cannot rely only on local geotechnical investigations: it requires the application of efficient and affordable investigation methods. The need for new screening tools is becoming increasingly important worldwide because most river embankments and small earth dams are reaching their design life limit due to aging. This study used a new electric streamer and a seismic streamer for the combined measurement of resistivity and shear wave velocity to investigate the Arignano earth dam (Piedmont Region, northwestern Italy), a historical reservoir used for agricultural purposes. A procedure is also proposed to assess hydraulic conductivity from the measured geophysical parameters. The results of this assessment were compared with available geotechnical investigations and also used for calibrating the proposed procedure. Results are in good agreement when compared with local geotechnical investigations. The proposed procedure can therefore provide engineers and local authorities with information to plan maintenance or urgent measures for reducing flood risk.

A series of centrifuge experiments were conducted to identify different internal failure modes of a group of soil-cement (SC) columns (e.g., shearing and tension due to bending) under combined effects of embankment loading and liquefaction-induced lateral spreading. The results were also used to investigate how an increase in the flexural/shear strength of SC columns (using high-strength steel reinforcement bars) can affect their ability to limit settlements or deformations. The centrifuge experiments included two centrifuge tests of liquefiable foundation reinforced by SC columns with and without high-strength steel reinforcement cores during strong earthquake loading. It was found that during and after cracking of the SC columns, shear and tilting failure is the prominent failure mechanism. The test results showed that increasing the flexural capacity of SC columns can reduce the potential for earthquake-induced liquefaction and associated damage. Compared with conventional SC columns, reinforced soil-cement (RSC) columns are more effective in providing confinement for the soil under the embankment during strong shaking events, resulting in lower shaking-induced pore-water pressure and a reduction in the lateral deformation of the soil and embankment.

A multiscale approach that couples the finite-element method (FEM) and the discrete-element method (DEM) is used to model and analyze the earthquake fault rupture–soil–foundation interaction (FR–SFI) problem. The approach obtains the soil constitutive responses from the DEM solutions of representative volume elements (RVEs) embedded at the FEM integration points, effectively bypassing the phenomenological hypotheses in conventional FEM simulations. The fault rupture surfaces and shear localization patterns under normal and reverse faults with or without foundation atop were adequately captured using the multiscale approach and verified with available centrifuge experimental and numerical results. An examination of the stress-strain responses and the microstructural evolutions of local RVEs revealed that the RVEs located in or immediately outside the shear bands behaved distinctly and might change their stress states from initially at rest to active under normal fault or passive under reverse fault. The micromechanics study also sheds light on the capability of heavy foundations to protect the superstructure due to rupture surface diversion under reverse fault and their possible detriment under normal fault.

Recently, a ground-borne vibration mitigation solution using rigid wave impeding blocks embedded in the foundation bed of buildings/embankments has emerged as a passive vibration isolation mechanism, wherein energy dissipation due to material damping was often ignored. The present study aims to examine the performance of a flexible foundation bed composed of a sand-rubber mixture (SRM) for ground-borne vibration isolation, referred to as geotechnical seismic isolation (GSI) bed, an emerging trend in low-cost earthquake mitigation studies. Field investigation consisting of vibration measurement on a model footing resting on a GSI bed with SRM and SRM-geogrid composite subjected to horizontal sinusoidal vibration of frequency 10 to 30 Hz with acceleration amplitude of 0.18 g to 0.33 g were undertaken. Further, a three-dimensional finite-element analysis of a model footing resting on a GSI bed made of geogrid reinforced SRM composite is presented. The strain-dependent nonlinear response of soil and SRM were accounted for using suitable hypoelastic and hyperelastic constitutive formulations, respectively. For the GSI bed, SRM with 10% to 50% rubber content and biaxial geogrids were studied. The numerical simulation and field study results were found to be comparable with a nominal difference that verifies the adequacy of the developed numerical modeling procedure. It was observed that GSI (30% rubber content) effectively reduced the vibration acceleration amplitude of the model footing above 40% and 55% for without and with geogrid reinforcement, respectively. Overall, the field experimental and numerical results demonstrate the proposed geogrid reinforced GSI bed as an effective ground vibration mitigation solution for typical industrial environments.

Many regions of the world have witnessed repeated earthquakes that have caused repeated liquefaction of soil. Therefore, the effect of repeated earthquakes on the dynamic behavior of soil is an important aspect that must be considered. In this study, a series of cyclic shear loading events and consolidation phases was repeated for loose sand to investigate the cyclic stress–strain response and postcyclic reconsolidation volumetric strain that are usually induced by repeated earthquakes. After an initial consolidation, samples were subjected to repeated cyclic shear loading events and subsequent reconsolidations in a cyclic direct simple shear (CDSS) device with various cyclic stress ratios ($CSRs$) under undrained conditions for nonliquefied and liquefied stages. Based on the test results, the number of cyclic shear events ($N_{ev}$), built-up excess pore water pressure ratio ($r_u$), $CSR$, and maximum double amplitude shear strain (${\gamma }_{\max }^{DA}$) were shown to be the factors influencing the behavior of samples. For the samples that were not subjected to any prior liquefaction stage, the reliquefaction resistance continuously increased as $N_{ev}$ increased. For those subjected to repeated liquefaction, the samples presented cyclic liquefaction accompanied by high flow shear strain in the first event. The reliquefaction resistance drastically decreased in the second cyclic shear loading event as a result of the critical anisotropy induced by the first cyclic shear loading event when the contraction tendency prevailed. The samples showed cyclic mobility behavior from the fourth event onward, corresponding to the predominance of the dilation tendency. The obtained ${\gamma }_{\max }^{DA}$ and ${\epsilon }_{rc}^V$ increased as $CSR$ and/or $r_u$ increased and decreased as $N_{ev}$ increased. The relative density ($D_r$) was not a major factor affecting the reliquefaction resistance; however, it might have promoted a lower induced anisotropy level, lower induced ${\gamma }_{\max }^{DA}$, lower ${\epsilon }_{rc}^V$, and the change in behavior of samples from cyclic liquefaction (contraction tendency) to cyclic mobility (dilation tendency). A higher contraction tendency in samples resulted in a higher ${\gamma }_{\max }^{DA}$ and ${\epsilon }_{rc}^V$. The shear strain induced in the immediate-past cyclic shear loading event was found to be the main factor affecting ${\epsilon }_{rc}^V$. From the ${\gamma }_{\max }^{DA}-{\epsilon }_{rc}^V$ relationship, two separate zones were observed, namely (1) noninduced flow shear strain; and (2) induced flow shear strain, regardless of $CSR$ and $N_{ev}$. Additionally, a relationship between ${\gamma }_{\max }^{DA}$ and ${\epsilon }_{rc}^V$ was proposed for practical applications.

Deep foundation applications, such as foundations for jacket structures and reaction piles, may benefit from larger shaft resistances during tensile compared with compressive loading. A suitable analog for designing surfaces whose load transfer depends on the direction of loading is the skin of snakes. Snake scales reduce friction when they move forward (i.e., caudal direction) and increase friction when they move backward (i.e., cranial direction). A series of centrifuge load tests were conducted on instrumented piles in medium-dense sand to investigate the load transfer behavior of piles with snakeskin-inspired surfaces. Load tests were performed on three bioinspired piles, a reference rough pile, and a reference smooth pile at two embedment depths each. The results present distributions of axial load and shear stresses along the pile length and head load–displacement and local shear stress–displacement relationships. During both installation and pullout, the cranial shaft friction mobilized similar shear stress magnitudes and shear resistance distribution with depth to that of the rough pile, while similarities were observed between the caudal shaft friction and that of the smooth pile. The results show that the bioinspired piles mobilize directionally-dependent shaft capacities, where the ratio of capacities during cranial pullout to caudal installation was measured to be 1.6 to 2.1, as evidenced by the computed $\beta$ coefficients. Variations in relative density in the centrifuge models were quantified by means of cone penetration test (CPT) soundings, and the pile test results were corrected for potential scaling effects to obtain $\beta$ coefficients that are more representative of field conditions.

Earthquake-induced soil liquefaction can cause soil settlement around piles, resulting in drag load and pile settlement after shaking stops. Estimating the axial load distribution and pile settlement is important for designing and evaluating the performance of axially loaded piles in liquefiable soils. Commonly used neutral plane solution methods model the liquefiable layer as an equivalent consolidating clay layer without considering the sequencing and pattern of excess pore pressure dissipation and soil settlement. Moreover, changes in the pile shaft and the tip resistance due to excess pore pressures are ignored. A TzQzLiq numerical model was developed using the existing TzLiq material and the new QzLiq material for modeling liquefaction-induced downdrag on piles. The model accounts for the change in the pile’s shaft and tip capacity as free-field excess pore pressures develop or dissipate in soil. The developed numerical model was validated against data from a series of large centrifuge model tests, and the procedure for obtaining the necessary information and data from those is described. Additionally, a sensitivity study on TzLiq and QzLiq material properties was performed to study their effect on the developed drag load and pile settlement. Analysis results show that the proposed numerical model can reasonably predict the time histories of axial load distribution and settlement of axially loaded piles in liquefiable soils both during and postshaking.

The undrained response of sand under complex loading conditions involving principal stress rotation (PSR) is of particular interest in practical engineering. Although this subject has been studied extensively through laboratory experiments, more in-depth investigations of the microscopic mechanism underlying the macroscale observations under PSR have not been reported adequately. The discrete element method (DEM) plays an important role in the investigation of the elementary behavior of sand subjected to various complex loading conditions. It could enable us to comprehend the evolution of particle-scale quantities. Therefore, an advanced discrete element approach that can apply an arbitrary undrained loading path is implemented in this study. Based on this approach, numerical algorithms that implement undrained rotational shear are elucidated, and undrained pure PSR tests are conducted on anisotropic specimens with varying stress ratios, densities, and intermediate principal stress ratios. The evolution of the fabric anisotropy of specimens under PSR is quantified by a contact normal fabric tensor. The macroscopic mechanical results are found to be consistent with the experimental results. The interplay between fabric evolution with stress and strain increments is examined. The findings provide effective microscopic insights into the anisotropic responses of granular materials under rotational shear.

Magnitude scaling factors (MSFs) account for the influence of ground motion duration on liquefaction triggering in simplified stress-based models in which the duration of the motion is quantified in terms of number of equivalent stress cycles ($N_{eq}$). Central to computing $N_{eq}$ and MSF is the relationship relating the amplitude of applied loading and the corresponding number of cycles to trigger liquefaction, that is, cyclic stress ratio $(\mathrm{CSR})\text{-}{N}_L$ curves. Based on empirical evidence (and mathematical convenience), $\mathrm{CSR}\text{-}{N}_L$ curves are commonly assumed to plot as straight lines on log-log scales, with the line having a slope of $-b$. As such, the $b$-value is central to computing $N_{eq}$ and MSF and has a significant influence on computed normalized seismic demand in simplified liquefaction evaluations. It is widely assumed that the $b$-value varies significantly as a function of soil density. However, in this study a review of published laboratory data and analysis of constant-volume cyclic direct simple shear tests performed as part of this study were used to assess the dependency of the $b$-value on soil density and other factors. We show that the criterion used to define liquefaction triggering in laboratory tests and the nonlinearity of the $\mathrm{CSR}\text{-}{N}_L$ curves can result in the apparent dependency of the $b$-value on soil density. However, using a consistent liquefaction criterion based on the cumulative dissipated energy in a unit volume of soil yields $b$-values that are relatively insensitive to changes in soil density. Published modulus reduction and damping (MRD) curves can be used to compute $b$-values using an energy-based framework; this yields more generalized and less test- and soil-specific $b$-values. As a result of these efforts, a $b$-value of 0.28 is recommended for computing $N_{eq}$ and MSF, independent of soil density.

This paper presents an experimental study on the hydromechanical behavior of unsaturated sedimented soil to understand the impacts of suction on the apparent yield stress and gain insight into the differences in behavior from compacted soils. A large-strain oedometer was developed for use in a triaxial cell that permits initial sedimentation of soils from a slurry under backpressure, suction control using the axis translation technique, and mechanical loading to characterize the compression curve. A flow pump was used to control the pore water pressure at the base of the soil specimen and to track water flow during suction application and mechanical loading. After initial consolidation of saturated soil specimens from a slurry, the specimens were unloaded, different suction values were applied, and then the axial stress was increased to 11 MPa at a constant strain rate. An increase in apparent yield stress with suction was observed, and the compression curves for higher suctions diverged without reaching pressurized saturation in the applied stress range. When compared with compression curves for the same soil compacted dry of optimum presented in previous studies, the sedimented soil had a greater yield stress at saturated conditions but a similar increase in yield stress with suction. Sedimented soils also experienced smaller changes in void ratio with applied net stress and a higher air entry suction value compared to compacted soils, reflecting a more compact soil structure. Suction was found to have a greater impact on yield stress than suction stress for both sedimented and compacted soils.

Evaluation of ground failure potential in geotechnical practice is typically based on demand parameters that solely consider vertically propagating shear waves in the free-field. Soil–structure interaction (SSI) modifies demands beneath foundations, and observations from recent earthquakes, physical modeling studies, and numerical modeling simulations indicate that SSI contributes significantly to ground failure. We present a methodology that utilizes elastic solutions to define SSI-induced stresses imposed on the soil beneath shallow foundations during earthquake shaking. Input parameters include a free-field ground surface motion as well as static and dynamic base shear, moment, and axial stresses imposed on the soil by a shallow foundation. The resulting stresses in the soil are analyzed in terms of the deviatoric stress invariant and the mean effective stress, which represents the states of stress leading to shear failure more accurately than the traditional use of stresses on horizontal planes. The invariant-based cyclic stress ratio (${\mathit{CSR}}_q$) is introduced to quantify demands, which is equivalent to the conventional CSR in the free-field. The ratio of the corresponding cyclic resistance parameter, ${\mathit{CRR}}_q$, to ${\mathit{CSR}}_q$ is the factor of safety against ground failure at a point. Application of this methodology to results of centrifuge modeling of shallow foundations resting on low-plasticity fine-grained soils shows that the factor of safety computed from the proposed methodology at a location in the soil below the edge of the foundation correlates strongly to measured permanent settlements and rotations, whereas the free-field factor of safety underpredicts ground failure potential.

The composite nature of geosynthetic clay liners and the contrasting water retention behavior of its bentonite and geotextile components has presented a unique challenge that current water retention models do not fully address. This paper proposes a new water retention model that can accurately describe the bimodal behavior of geosynthetic clay liners across the entire suction range (${10}^{-2}–{10}^6\mathrm{kPa}$) on the adsorption path. The model was formulated based on the pore structures and dominant suction regimes present in geosynthetic clay liners. In addition to the soil adsorptive and capillary water, it incorporates the geotextile capillary regime, which encompasses the pore water fraction in the geotextile, bentonite extrusion into the geotextile, and additionally, any volume changes due to bentonite swelling (including polymer effects). The parameters defined in this conceptual model describe the physical characteristics of bentonite and the geotextile fraction in the geosynthetic clay liner (GCL). The proposed model’s performance was assessed and validated using extensive experimental water retention data sets. The statistical analysis indicated that the proposed model provides a better fit than other models, especially in the low-suction range, and is adept at predicting the water retention behavior of the geosynthetic clay liners on the wetting path.

There is a high risk of ground collapse during the construction of closely spaced overlapping metro tunnels in sensitive environments. When overlapping tunnels pass beneath important structures such as high-speed railway (HSR) bridges whose deformation controlling standard is extremely strict, special ground reinforcement measures should be implemented. This paper investigated stability control measures for closely spaced and overlapping metro tunnels passing beneath the HSR bridge based on the engineering project of the Jinan R1 Metro line in China. The risk sources and engineering difficulties were analyzed. Further, special reinforcement measures including isolation piles and a postgrouting method were proposed. A three-dimensional model was built by a finite element program, and the deformation characteristics of ground and bridge piles were calculated. Subsequently, their deformation characteristics were analyzed and compared with monitoring results. The deformation of an HSR bridge was successfully controlled within permissible value during the shield tunneling process, indicating the effect of ground reinforcement measures is satisfactory.

Abstract

In a full-scale case study, the performance of two methods—the vacuum-surcharge preloading method (S-VP) and the air-booster vacuum preloading method (A-VP)—were compared in terms of the dredged soft soil ground of the Xiang’an International Airport pavement reinforcement project in Xiamen City, China. The pore water pressure dissipation, settlement, lateral displacement, and consolidation degree were measured in the zones reinforced using the two methods. The direct shear test and field vane shear test were conducted before and after improvement. The S-VP and A-VP methods were loaded for 308 and 228 days, respectively. The results show that both reinforcement methods significantly improved the shear strength and foundation bearing capacity of the soft soil ground. The S-VP method has about 30% higher final vane shear strength and more uniform settlement compared to the A-VP method. However, the dissipation value of the average pore water pressure and the maximum foundation settlement were 5.6% and 11% higher, respectively, in A-VP than in S-VP. Compared to the S-VP method, the A-VP method decreased the maximum lateral displacements by 15%–20%, and increased the maximum reinforcement depth by 11%. Finally, compared with the use of sand cushion and vacuum pump, the A-VP method was more economical in terms of cost and energy use.

Practical Applications

Land reclamation projects will produce a large area of ground composed of ultrasoft soil, which has very low strength and cannot be used for construction activities directly on such ground. For such ultrasoft soil ground, the vacuum preloading method is generally used to strengthen the ultrasoft soil. In this study, two different vacuum preloading methods [the vacuum-surcharge preloading method (S-VP) and the air-booster vacuum preloading method (A-VP)] were used to strengthen the ultrasoft soil, and the final reinforcement effect and cost of the two methods were compared. The results show that the final reinforcement effect of the A-VP method was close to that of the S-VP method. Furthermore, the A-VP method saved more than 3 million Chinese currency (CNY) and reduced the power consumption by 80% in this project. In addition, the implementation of the A-VP method does not require a large amount of sand that is difficult to obtain in coastal areas, which is good protection for the natural resources in coastal areas. Therefore, in the ultrasoft soil reinforcement project, the A-VP method is a method worth considering.

In geotechnical engineering, mixed-type boundary problems cannot be solved directly by the integral transform method due to the diversity of boundary conditions. In view of this, the relationships between different boundaries are figured out from the perspectives of physical meaning and mathematical derivation. A novel boundary transform method is then proposed to transform a complex mixed-type boundary into a simple one, and the transform rules for different types of boundaries are graphically presented. Subsequently, the procedures to solve geotechnical problems with mixed-type boundaries are comprehensively elaborated. The application of the proposed boundary transform method for a two-dimensional steady flow problem with mixed-type boundaries is systemically considered. The results show that the boundary transform method can effectively deal with mixed-type boundary problems, rendering accurate solutions when compared with numerical analyses.