Level Ground Conditions Research Articles

Understanding fabric evolution and multiple liquefaction resistance of sands in the presence of initial static shear stress

Recent field observations have shown that mainshock and subsequent aftershock events can induce multiple-liquefaction events, in which soil liquefies several times at the same site. However, the fundamental mechanism of multiple liquefaction in most previous studies is under level ground conditions, and the cyclic loading condition is symmetric under such conditions. In this study, we conduct 3D DEM simulations of multiple-liquefaction processes considering initial static shear stress. 14-sphere clumped models are used to reconstruct the realistic grain shape of Toyoura sand particles. Cyclic liquefaction and reconsolidation tests are carried out alternately five times. The soil samples with various initial static shear stresses are sheared up to different maximum shear strains in each liquefaction stage. We investigate the liquefaction behavior and volumetric compression during multiple-liquefaction stages considering initial static shear stress. In addition, the evolution of contact-based fabrics, including coordination number (Z) and degree of fabric anisotropy (ac), are analyzed during the whole liquefaction and reconsolidation tests. Furthermore, we establish the relations among contact-based fabrics (Z and ac), volumetric compression and liquefaction resistance. The results reveal that in stress reversal cases, the maximum shear strain is the governing factor that influences the multiple-liquefaction resistance. Conversely, the static shear stress becomes the dominant factor affecting the multiple-liquefaction resistance in non-stress reversal cases. Moreover, changes in multiple-liquefaction resistance are attributed to the microstructure of soils represented by Z after reconsolidation.

Experimental Study of Axially Loaded Pile Group Near a Sloping Ground

Understanding the axial loading response of pile group located near the crest of the slope is of practical value to structural design. In this paper, a large-scale test setup (1.8 m × 0.90 m × 0.90 m) has been developed to investigate the response of pile group (2 × 2) located near the crest of the slope under axial loading. This paper presents a series of physical modelling tests performed to investigate the effects of slope angles, distance of the pile group from the slope crest, embedment length of the pile group and pile diameter on the axial loading response of pile groups. The results show that the response of pile group located near the crest of the slope approaches to the level ground condition with increasing edge distance from slope crest. In addition, the horizontal displacement of pile group towards the slope face increases with decreasing distance of pile group from the slope crest or embedded length of piles or with increasing slope angle. Furthermore, increasing pile diameter results in a low increase in the horizontal displacement for edge distances less than 3 dp (dp = pile diameter). However, the horizontal displacement is almost independent of pile diameter at edge distances greater than 3 dp. Higher embedded length results in the response of pile group approaches to the response of pile group located on the level ground at a shorter edge distance from the slope crest.

Mechanisms of shear strain accumulation in laboratory experiments on sands exhibiting cyclic mobility behavior

The factors and mechanisms controlling the accumulation of shear strains of clean uniform sands exhibiting cyclic mobility behavior under level-ground conditions are examined. This phenomenon is investigated through a series of constant-volume cyclic direct simple shear (DSS) tests subjected to uniform and irregular loading conditions, and undrained cyclic element tests collected from the literature. Experimental data show that the rate of shear strain accumulation per loading cycle depends on the relative density, cyclic stress amplitude, and effective overburden stress. Mechanisms of shear strain accumulation are investigated by decoupling the shear strain developed in each loading cycle in two components: γ0, developed at near-zero effective stress, and γd, developed during dilation. Results show that γ0 mostly depends on the shear strain history, while γd depends on the cyclic stress amplitude and relative density. These dependencies of γd and γ0 are used to provide an explanation for the gradual decrease of the rate of shear strain accumulation that is observed while increasing the number of post-triggering loading cycles in tests performed on dense specimens.

Lateral force–displacement response of buried pipes in slopes

A series of full-scale experiments was conducted to estimate lateral soil constraints on pipes buried in dense sandy slopes at different burial depths. The experimental data indicated that the soil force on the pipe increases with increase in the slope grade and burial depth ratio. The lateral soil force against relative pipe displacement response observed from the experiments is presented and compared to those arising from level ground conditions. The study was extended to larger burial depth ratios by simulating pipes under sloping ground conditions using a numerical (finite-element) model that was initially calibrated using the results from physical modelling. The findings from the study in terms of the variation of peak lateral soil restraint as a function of the slope grade and burial depth ratio are presented for consideration in pipeline design.

Determination of Initial-Shear-Stress Impact on Ramsar-Sand Liquefaction Susceptibility through Monotonic Triaxial Testing

Liquefaction risk assessment is critical for the safety and economics of structures. As the soil strata of Ramsar area in north Iran is mostly composed of poorly graded clean sand and the ground water table is found at shallow depths, it is highly susceptible to liquefaction. In this study, a series of isotropic and anisotropic consolidated undrained triaxial tests were performed on reconstituted specimens of Ramsar sand to identify the liquefaction potential of the area. The specimens are consolidated isotropically to simulate the level ground condition, and anisotropically to simulate the soil condition on a slope and/or under a structure. The various states of soil behavior are studied by preparing specimens at different initial relative densities and applying different levels of effective stress. The critical state soil mechanics approach for identifying the liquefaction susceptibility is adopted and the observed phenomena are further explained in relation to the micro-mechanical behavior. As only four among the 27 conducted tests did not exhibit liquefactive behavior, Ramsar sand can be qualified as strongly susceptible to liquefaction. Furthermore, it is observed that the pore pressure ratio is a good indication of the liquefaction susceptibility.

Shaking Table Experiments to Evaluate the Boundary Effects on Seismic Response of Saturated and Dry Sands in Level Ground Condition

The laminar shear boxes (LSBs) are commonly preferred for geotechnical physical modeling over the fixed wall boxes; the latter may suffer from problems associated with wave reflection. This paper mainly describes the seismic response of a uniform layer of loose saturated and dry sands in rigid and flexible boundary conditions. A lightweight LSB was recently designed and fabricated by the authors for physical modeling of geotechnical earthquake engineering problems. To investigate the boundary effect on the seismic response of level ground, a series of shaking table tests were conducted with both LSB and rigid wall containers in identical conditions. The seismic performance of the free-field ground in the experiments is evaluated in terms of time histories, shear stress–strain hysteresis loop, and dynamic soil properties. The dynamic movements of all layers in the LSB, captured by image processing techniques, are compared with that of the rigid end-wall condition. The results demonstrate that acceleration and settlement of the ground surface are highly affected by artificial boundaries in both dry and saturated sands. It is shown that the hysteresis loops estimated from the LSB tests are more compatible with the cyclic behavior of sands, compared with those of the rigid box tests.

Performance of piles with different configurations subjected to slope deformation induced by seismic liquefaction

Liquefaction-induced lateral ground deformation has been a common observation during many past earthquakes. Piles are widely used in liquefaction-prone areas, and their performance under such circumstances has become an important issue for researchers and engineers. Previous studies on this issue have focused primarily on piles located behind quay walls or in level ground conditions, which have considerably different boundary conditions compared to those of an inclined sloping ground condition that is more common for waterfront structures. This study presents a series of shaking table tests modeling the performance of piles subjected to liquefaction-induced slope deformation. Different pile configurations, including single piles, piles aligned in one row parallel to the direction of the slope, piles aligned in one row perpendicular to the direction of the slope, and piles in a 3×3 configuration, were tested. Several aspects regarding the behaviors of excess generation of pore water pressure, pile bending moment and lateral slope deformation under different pile configurations were revealed and discussed. The results obtained in this study can be used for validation of related numerical approaches, or as benchmarks that can further facilitate performance-based pile designs in liquefaction-prone areas.

Pre-shaking effects on volumetric strain and cyclic strength of sand and comparison to unsaturated soils

Repeated small shaking events due to earthquakes significantly enhance liquefaction resistance of soils. Analyses of liquefaction case histories show that aged soils in seismically active zones tend to be less vulnerable to liquefaction despite having similar index parameters—such as standard penetration test N-values and shear wave velocities—as young soils. Significant efforts have been devoted to better understand the effects of the cyclic pre-shearing on liquefaction resistance and it was found that this effect depends on the number of cycles and cyclic stress ratio. However, none of these parameters quantify the improvement of liquefaction resistance due to pre-shaking. This study investigates the pre-shearing effects on liquefaction resistance through laboratory tests and centrifuge tests. An attempt was made to explain the effects quantitatively with a single index parameter of the volumetric strain caused by pre-shearing. It was confirmed from triaxial tests that the liquefaction resistance of pre-sheared sand uniquely increased with increasing volumetric strain regardless of the cyclic shear stress ratio and the number of cycles during the pre-shearing. To examine the pre-shaking effects on the liquefaction strength of sand under a level ground condition, centrifuge tests were conducted in this study. Sand models were subjected to small shaking events repeatedly, which were weak enough not to cause liquefaction. It was observed that changes in the index parameters of the models, including soil density (volumetric strain), shear wave velocity, and horizontal earth pressure during the pre-shaking events were very small. At the end of the test, the sand was subjected to a strong shaking event because models that had gone through pre-shaking need larger shaking acceleration to liquefy. Liquefaction resistance was derived from acceleration records with the aid of the cumulative damage theory. The relationship between liquefaction resistance ratio and volumetric strain that occurred in the pre-shaking events coincides with the relationship obtained from the triaxial tests. After the extensive liquefaction event, all index parameters except soil density—K0, Vs, liquefaction resistance—tended to return to their original values (before the pre-shaking).

Liquefaction assessment of cohesionless soils in the vicinity of large embankments

In a typical seismic dam safety evaluation, standard penetration, cone penetration, Becker penetration, or shear wave velocity (Vs) tests are often first conducted near the toe of an earth dam to infer if any liquefiable soil exists in the foundation of the dam footprint. In current practice, a level-ground condition is commonly assumed when normalizing penetration resistance and Vs, and may be assumed (particularly in preliminary assessments) in applying the cyclic stress method (with or without the Kα correction) to evaluate liquefaction. However, the presence of an earth dam, or any other large embankment or structure, significantly alters the normal and shear stresses in the foundation. This paper identifies and quantifies potential errors in ignoring altered stresses near heavy structures, and presents a methodology to incorporate these effects within the framework of the simplified procedure. Specifically, the effects of these altered stresses (in comparison to the level-ground assumption with and without Kα correction) on the: (1) normalization of field measurements such as penetration resistance and Vs; (2) cyclic stress ratio (CSR); (3) cyclic resistance ratio (CRR); and (4) factor of safety against liquefaction triggering (FSliq), are evaluated by considering static and dynamic analyses of a generic earthen embankment (60m high) resting on a saturated, cohesionless foundation (30m deep). Our analyses indicated that ignoring the presence of induced static shear stresses can result in potentially unconservative errors in overburden correction factors of 30% to 60% at shallow depth (although this error is greatly muted at depths exceeding about 15m), while errors in CSR potentially can range from about 20% too conservative to 40% unconservative. Potential errors in CRR can approach 50% unconservative at shallow depths, but again, this error is muted at depths exceeding about 15m. Combining these factors, potentially unconservative errors in computing FSliq could exceed 100% at shallow depths (less than 15m to 20m) while at greater depth (exceeding 20m) errors approach 20% on the conservative side.

Analysis of pile behaviour in liquefying sloping ground

Abstract In this paper, a numerical procedure based on the finite element method is outlined to investigate pile behaviour in sloping ground, which involves two main steps. First a free-field ground response analysis is carried out using an effective stress based stress path model to obtain the ground displacements, and the degraded soil stiffness and strength over the depth of the soil deposit. Next a dynamic analysis is carried out for the pile. The interaction coefficients and ultimate lateral pressure of soil at the pile–soil interface are calculated using degraded soil stiffness and strength due to build-up of pore pressures, and the soil in the far field is represented by the displacements calculated from the free-field ground response analysis. Pore pressure generation and liquefaction strength of the soil predicted by the stress path model used in the free-field ground response analysis are compared with a series of simple shear tests performed on loose sand with and without an initial static shear stress simulating sloping and level ground conditions, respectively. Also the numerical procedure utilised for the analysis of pile behaviour has been verified using centrifuge data, where soil liquefaction has been observed in laterally spreading sloping ground. It is demonstrated that the new method gives good estimate of pile behaviour, despite its relative simplicity.

Initial Static Shear Effect on Cyclic Liquefaction Behaviour of Sand

Evaluation of soil liquefaction potential plays a key role in geotechnical earthquake engineering design since liquefaction is a major cause of earthquake-induced damages. While the evaluation procedure works well for free-field level ground conditions, the existence of initial static driving shear stress under earth dams or buildings complicates the issue. To address the insufficiency in current understanding, this paper presents an experimental study of the initial shear effect on the liquefaction resistance of sand, focusing mainly on the dependence of the effect on the initial state and sand type. A large number of cyclic triaxial tests have been conducted on two standard silica sands under a wide range of initial shear stress, density and confining pressure. Three distinct cyclic responses are identified to be dependent on the initial state and degree of stress reversal. The test results indicate that while the initial shear effect on loose sand can be beneficial or detrimental, that on dense sand is...

Influence of Varying Level Terrain on Wheelchair Propulsion Biomechanics

To evaluate manual wheelchair propulsion across level ground conditions that are encountered during everyday life. Subjects included 14 individuals (13 with spinal cord injury [SCI], 1 with spina bifida) who were experienced manual wheelchair users and had no current upper extremity injury or pain complaints. Subjects propelled their wheelchairs at a self-selected speed across four different level ground conditions, including smooth and aggregate concrete and tile and carpet flooring. Temporal and kinetic measurements were obtained bilaterally from instrumented wheelchair rims during the steady-state phase of propulsion. There were no side-to-side differences for any of the temporal or kinetic variables. Propulsion velocity and pushrim contact time were consistent across ground conditions. Propulsion frequency was significantly greater during both concrete conditions than either tile or carpet ground conditions. Forces and moments were greatest during the aggregate concrete ground condition and lowest during propulsion across tile flooring. The rolling resistance of level surface terrain significantly impacts wheelchair propulsion biomechanics. Identification of environmental conditions that may contribute to upper extremity pathology is a step toward injury prevention and maintenance of functional abilities for the manual wheelchair user. These results may be used to assist with home and community terrain design to minimize the demands associated with wheelchair propulsion.

Case study of strong ground motion variations across cut slope

We evaluate the influence of topography on motions recorded at the base and crest of an approximate 3H:1V, 20 m single-faced slope. The motions were recorded during the 1983 Coalinga earthquake mainshock and two aftershocks. Mainshock peak accelerations at the crest and base transverse to the slope face were 0.59 and 0.38 g, respectively. The spectral amplification of crest motion occurred across T≈0–2 s. Differences between the crest/base motions are postulated to result principally from soil-structure interaction (base instrument is in a structure), variations in local ground response, and topography. Transfer functions quantifying soil-structure interaction (SSI) effects are evaluated and the base motion is modified at short periods to correct it to an equivalent free-field motion. The different levels of ground response at the crest and base are identified based on location-specific measurements of soil shear wave velocities. Differences between crest/base motions not accounted for by SSI or differential ground response are attributed to topographic effects. By these means, topographic spectral amplification (i.e. amplification relative to level ground conditions) is estimated to be about 1.2 at the crest and about 0.85–0.9 at the base across the period range T≈0.4–1.0 s.

State of the art for the evaluation of seismic liquefaction potential

The state of the art for evaluating the potential for the occurrence of liquefaction in saturated sands and silts under earthquake shaking is presented for level ground conditions which is the most common situation in engineering practice. The latest liquefaction assessment charts based on data from various penetration tests and on in situ shear wave velocity are presented. All these standard charts are for level ground, earthquake moment magnitude MW=7.5 and effective confining stress of 100 kPa. The latest correction factors required to make the findings from the charts applicable for other magnitudes and confining pressures are presented and critically reviewed.

Liquefaction during the 1990 Manjil, Iran, earthquake, II: Case history analyses

Abstract Liquefaction and liquefaction-induced permanent ground deformations and building settlements were widespread during the 1990 Manjil, Iran, earthquake ( M S = 7.7). In the first of these companion articles, geologic, seismologic, and geotechnical data of liquefaction case histories were presented. This article presents results of the analysis of (1) the ground-motion data; (2) liquefaction strength corresponding to level ground conditions; (3) permanent lateral ground deformations; (4) residual shear strength of the liquefied sands; and (5) settlement of buildings on liquefied sands.