Stability of Steep Slopes in Cemented Sands

The analysis of steep slope and cliff stability in variably cemented sands poses a significant practical challenge as routine analyses tend to underestimate the actually observed stability of existing slopes. The presented research evaluates how the degree of cementation controls the evolution of steep sand slopes and shows that the detailed slope geometry is important in determining the characteristics of the failure mode, which in turn, guide the selection of an appropriate stability analysis method. Detailed slope-profile cross sections derived from terrestrial lidar surveying of otherwise inaccessible cemented sand cliffs are used to investigate failure modes in weakly cemented [unconfined compressive strength (UCS)<30?kPa] and moderately cemented (30相似文献   

Laboratory Analysis of Carbon Dioxide Hydrate-Bearing Sands

Gas hydrates are ice-like compounds composed of water molecules that encapsulate natural gas molecules in a very compact form. Three groups of CO2 hydrate-bearing sand specimens were formed using a specialized triaxial test apparatus at different temperature and different pressure to examine the physical effects, such as stress and deformation changes, of hydrate dissociation on the surrounding soil. The triaxial test data indicate: (1) the higher the pressure, the higher hydrate content; (2) the lower the temperature, the higher hydrate content; (3) the greater the amount of gas hydrate initially present in the specimen, the greater the reduction of effective stress upon dissociation and the lower strength; and (4) dissociation of even a small percentage of gas hydrates will cause a dramatic reduction in the soil strength and could lead to failure.     

Instability in Granular Materials: Experimental Evidence of Diffuse Mode of Failure for Loose Sands

The influence of three loading paths on the collapse of loose sand is analyzed with a particular attention paid to the onset of collapse and the mode of failure exhibited. Experimental results on conventional undrained triaxial compression tests, constant shear drained tests, as well as quasi-constant shear undrained path are presented, compared, and analyzed. It is now recognized that some collapses can occur before the Mohr-Coulomb plastic limit criterion is reached, and our recent results obtained with the new arrangement built up highlight that these collapses occur under a diffuse mode of failure. An extensive experimental series of tests shows that the first negative value of the second-order work computed using experimental data corresponds to the loss of controllability. Moreover, it is shown that the stress ratios at collapse and the corresponding mobilized angles of friction are very close for all types of tests. For similar void ratios, the onset of collapse is thus largely independent of the loading path under drained and undrained conditions but depends on a stress state to bring the material inside the unstable domain and also on the current direction of the stress increment. Indeed, it appears that the orientations of the stress increments at collapse for all tests are the same, what explains, according to the second-order work criterion, that collapse occurs at the same stress ratio. A potentially unstable domain, depending on the stress increment direction, can thus be defined.     

Keyblock Stability in Seismically Active Rock Slopes—Snake Path Cliff, Masada

Keyblock stability in the “Snake Path” cliff of the Masada monument, situated on the western margin of the seismically active Dead Sea transform, is studied using field mapping, mechanical analysis, and monitoring of displacement, pressure, temperature, and relative humidity, over a period of 11 months. A linear nonreversible displacement trend is interpreted as the block response to regional microseismicity. A more pronounced cyclic displacement trend however is shown to be a response to climatic changes on the cliff face. This finding introduces a new, time-dependent, failure process in jointed rock slopes—the degradation of shear and/or cohesive strength of joints due climatic effects. Using two-dimensional (2-D) and three-dimensional (3-D) limit equilibrium analyses it is demonstrated that the 2-D solution overestimates the factor of safety against sliding by as much as 15% if water pressures in the boundary joints are considered. Application of a 2-D solution for a truly 3-D case where prismatic blocks are considered proves therefore to not be conservative.     

Effect of Wetting on Unconfined Compressive Strength of Cemented Sands

A weakly cemented sand and gravel has been partly or entirely used in the construction of earth structures such as dams and retaining walls. Such cemented soils that are usually highly permeable can undergo repetitive wetting and drying during curing due to temporary rainfall or a change in the groundwater table. In this study, weakly cemented sand specimens with four different cement ratios were compacted at optimum water content and cured for 28 days. When the cemented sand specimens were exposed to repetitive wetting and drying during curing, their 28-day unconfined compressive strength was evaluated. Wetting for one day on the last day was found to decrease the unconfined compressive strength of cemented sand, whereas wetting for one day in the middle of curing resulted in an increase in strength. The strength reduction due to wetting on the last day decreases as the cement ratio increases. For a specimen under repetitive wetting and drying over 28-day curing, the strength increases as the number of wetting increases up to three cycles. After three cycles of wetting and drying, the strength either becomes constant or slightly decreases due to insufficient water for hydration and/or washing cementitious materials.     

Behavior of Hydrocarbon Reservoir Sands and Sandstones

Most new oil and gas reservoirs are discovered in sands and weak sandstones, and the petroleum industry has therefore generated an extensive amount of data from laboratory tests on these materials. Some of these data are reexamined in this paper within the context of recent soil mechanics research. Of key importance to petroleum engineers is the compressibility of the sands, and this is shown to be related both to the current density and stress level of the soil and to the location of its normal compression line. When present, the cement bonds between the particles appear to be weaker than the particles themselves, so that the major yield point during compression is still associated with breakage of the particles rather than of the cement bonds. The paper also examines the behavior of the sands and sandstones in shearing, allowing a more general understanding of the mechanics of the materials to be developed.     

Engineering Properties of Grouted Sands

A comparison of the behavior of uncemented and grouted sands is presented. Four sands (Fontainebleau sand and three types of alluvial deposits of the Seine River) were tested. Specimens of grouted sands were prepared in the laboratory by injection of very fine cement or mineral grouts. An initial series of unconfined uniaxial compression tests and tensile tests was performed to highlight the effect of some key factors (mainly the cement-to-water ratio of the grout and the relative density of the granular skeleton) on the strength of the grouted sands. Subsequent triaxial tests showed that when a soil is impregnated by either a very fine cement grout or a mineral grout, both stiffness (secant stiffness or small-strain stiffness) and strength of the soil improve. Similar trends were observed for the behavior of both uncemented and grouted sands. The behavior of grouted sands can be roughly reproduced by applying a linear elastic, perfectly plastic model with a nonassociated Mohr–Coulomb yield criterion whose parameters can be easily determined. Finally, preliminary recommendations are proposed relative to improvements ratios of the parameters of this simple constitutive model that is still commonly used in geotechnical engineering.     

Undrained Fragility of Clean Sands, Silty Sands, and Sandy Silts

In this paper, intergranular (ec) and interfine (ef) void ratios and confining stress are used as indices to characterize the stress–strain response of gap graded granular mixes. It was found that at the same global void ratio (e) and confining stress, the collapse potential (fragility) of silty sand increases with an increase in fines content (FC) due to a reduction in intergranular contact between the coarse grains. Beyond a certain threshold fines content (FCth), with further addition of fines, the interfine contact friction becomes significant. The fragility decreases and the soil becomes stronger. The value of FCth depends on e and the characteristics of fines and coarse grains. At FCFCth), fine grain friction plays a primary role and dispersed coarse grains provide a beneficial, secondary reinforcement effect. At the same ef, the collapse potential decreases with an increase in sand content. Beyond a certain limiting fines content, the soil behavior is controlled by ef only. An intergranular matrix diagram is presented that delineates zones of different behaviors of granular mixes as a guideline to determine the anticipated behavior of gap-graded granular mixes. New equivalent intergranular contact void ratios, (ec)eq and (ef)eq, are introduced to characterize the behavior of such soils, at FCFCth, respectively.     

Factors Controlling Instability of Homogeneous Soil Slopes under Rainfall

Rainfall-induced slope failure is a common geotechnical problem in the tropics where residual soils are abundant. Although the significance of rainwater infiltration in causing landslides is widely recognized, there have been different conclusions as to the relative roles of antecedent rainfall to landslides. The relative importance of soil properties, rainfall intensity, initial water table location and slope geometry in inducing instability of a homogenous soil slope under different rainfall was investigated through a series of parametric studies. Soil properties and rainfall intensity were found to be the primary factors controlling the instability of slopes due to rainfall, while the initial water table location and slope geometry only played a secondary role. The results from the parametric studies also indicated that for a given rainfall duration, there was a threshold rainfall intensity which would produce the global minimum factor of safety. Attempts have also been made to relate the findings from this study to those observed in the field by other researchers. Results of this parametric study clearly indicated that the significance of antecedent rainfall depends on soil permeability.     

Liquefaction Resistance of Clean and Nonplastic Silty Sands Based on Cone Penetration Resistance

Liquefaction of granular soil deposits is one of the major causes of loss resulting from earthquakes. The accuracy in the assessment of the likelihood of liquefaction at a site affects the safety and economy of the design. In this paper, curves of cyclic resistance ratio (CRR) versus cone penetration test (CPT) stress-normalized cone resistance qc1 are developed from a combination of analysis and laboratory testing. The approach consists of two steps: (1) determination of the CRR as a function of relative density from cyclic triaxial tests performed on samples isotropically consolidated to 100 kPa; and (2) estimation of the stress-normalized cone resistance qc1 for the relative densities at which the soil liquefaction tests were performed. A well-tested penetration resistance analysis based on cavity expansion analysis was used to calculate qc1 for the various soil densities. A set of 64 cyclic triaxial tests were performed on specimens of Ottawa sand with nonplastic silt content in the range of 0–15% by weight, and relative densities from loose to dense for each gradation, to establish the relationship of the CRR to the soil state and fines content. The resulting (CRR)7.5-qc1 relationship for clean sand is consistent with widely accepted empirical relationships. The (CRR)7.5-qc1 relationships for the silty sands depend on the relative effect of silt content on the CRR and qc1. It is shown that the cone resistance increases at a higher rate with increasing silt content than does liquefaction resistance, shifting the (CRR)7.5-qc1 curves to the right. The (CRR)7.5-qc1 curves proposed for both clean and silty sands are consistent with field observations.     

Evaluation of Different Seed Mixtures for Grass Establishment to Mitigate Soil Erosion on Steep Slopes of Railway Batters

Unpredictable rainstorms can pose greater risks of soil erosion on newly formed unprotected railway batters. A bioengineering approach consisting of revegetation can be an effective tool for soil-erosion control. Perennial species, such as buffel grass, are preferred for durability of vegetation cover in the arid and semiarid tropics. However, it takes a longer time to establish buffel than annual species such as Japanese millet (JM). A replicated plot-scale field trial consisting of different proportions of grass-seed mixtures of buffel and JM was conducted on railway-batters slope in the semiarid tropics of central Queensland, Australia. A supplementary automated drip irrigation system was set up to aid seed germination and early establishment of the grass cover. JM alone or JM-dominated seed mixtures significantly reduced soil erosion during the earlier stages. For example, during the second sampling interval (SI), which occurred 63?days after seeding, JM alone reduced soil erosion by 50% (20.2?versus 10.21??t/ha) in comparison to the sole buffel. This trend began to shift as the growth of buffel commenced at the latter stages. By the end of the 11 months, erosion from all seeded plots was not significantly different among the plots with more than 90% soil-loss reduction in comparison to the bare scenario, resulting in more than 60% grass cover. Results suggested that a monoculture of JM or JM-dominated seed mixture with buffel grass could be an effective bioengineering approach for revegetation of railway batters for protection against soil erosion. Quick establishment owing to faster seed germination, seedling establishments, and growth-producing sizable canopy cover developed by JM in the early stage of batter could reduce soil erosion in comparison to a sole crop of slow-growing perennial species buffel. The established vegetation self-regenerated and responded dynamically and naturally to changing conditions of the railway batter within a year, after which JM started to die back.     

Fundamental Parameters for the Stiffness and Strength Control of Artificially Cemented Sand

The treatment of soils with cement is an attractive technique when the project requires improvement of the local soil for the construction of subgrades for rail tracks, as a support layer for shallow foundations and to prevent sand liquefaction. As reported by Consoli et al. in 2007, a unique dosage methodology has been established based on rational criteria where the voids/cement ratio plays a fundamental role in the assessment of the target unconfined compressive strength. The present study broadened the research carried out by Consoli et al. in 2007 through quantifying quantifies the influence of voids/cement ratio on the initial shear modulus (G0) and Mohr-Coulomb effective strength parameters (c′,?′) of an artificially cemented sand. A number of unconfined compression and triaxial compression tests with bender elements measurements were carried out. It was shown that the void/cement ratio defined as the ratio between the volume of voids of the compacted mixture and the volume of cement is an appropriate parameter to assess both initial stiffness and effective strength of the sand-cement mixture studied.     

Stability Charts for 3D Failures of Steep Slopes Subjected to Seismic Excitation

Design of slopes and analysis of existing slopes subjected to seismic shaking are carried out routinely using approximations of plane strain and substitution of a quasi-static load for the seismic excitation. A three-dimensional (3D) analysis of slopes is carried out, based on the kinematic theorem of limit analysis. A rotational failure mechanism is used with the failure surface in the shape of a curvilinear cone sector passing through the slope toe, typical of steep slopes. A quasi-static approach is used to develop stability charts allowing assessment of the factor of safety of slopes without the need for an iterative procedure. The charts are of practical importance in cases of excavation slopes and whenever a slope is physically constrained, preventing a plane failure.     

Contraction, Dilation, and Failure of Sand in Triaxial, Torsional, and Rotational Shear Tests

The response of a saturated fine sand (Nevada sand No. 120) with relative density Dr ≈ 70% in drained and undrained conventional triaxial compression and extension tests and undrained cyclic shear tests in a hollow cylinder apparatus with rotation of the stress directions was studied. It was observed that the peak mobilized friction angle for this dilatant material was different in undrained and drained tests; the difference is attributed to the fact that the rate of dilation is smaller in an undrained test than it is in a drained test. Consistent with the findings of others, the material is more resistant to undrained cyclic loading for triaxial compression than for triaxial extension. In rotational shear tests in which the second invariant of the deviatoric stress tensor is held constant, the shear stress path (after being normalized by the mean normal effective stress) approached an envelope that is comparable but not identical in shape to a Mohr-Coulomb failure surface. As the stress path approached the envelope, the shear end deviatoric strains continued to increase in an unsymmetrical smooth spiral path. During the rotational shear tests, the direction of the deviatoric strain-rate vector (deviatoric strain increment divided by the magnitude of change in Lode angle) was observed to be about midway between the deviatoric stress increment vector and the normal to a Mohr-Coulomb failure surface in the deviatoric plane. The stress ratio at the transition from contractive to dilative behavior (i.e., “phase transformation”) was also observed to depend on the direction of the stress path; therefore this stress ratio is not a fundamental property. Results from torsional hollow cylinder tests with rotation of stress directions are presented in new graphical formats to help understand and interpret the fundamental soil behavior.     

Effects of Particle Crushing in Stress Drop-Relaxation Experiments on Crushed Coral Sand

Stress relaxation and stress drop-relaxation tests have been performed to complement a test series performed to study strain rate, creep, and stress drop-creep effects on crushed coral sand. Drained experiments with constant effective confining pressure of 200 kPa were performed in which triaxial specimens of crushed coral sand were loaded to initial stress differences of 500, 700, and 900 kPa, followed by stress drops of 0, 100, 200, 300, and 400 kPa at which points the axial strains were kept constant while the axial stress relaxation and the volumetric strains were observed. The stress drops produced delays in initiation of stress relaxation that were proportional with the magnitudes of the stress drops. The experiments show that sands do not exhibit classic viscous effects, and their behavior is indicated as “nonisotach,” while the typical viscous behavior of clay is termed “isotach.” Thus, there are significant differences in the time-dependent behavior patterns of sands and clay. A mechanistic picture of time effects in sands is proposed.     

Key Parameters for Strength Control of Artificially Cemented Soils

Often, the use of traditional techniques in geotechnical engineering faces obstacles of economical and environmental nature. The addition of cement becomes an attractive technique when the project requires improvement of the local soil. The treatment of soils with cement finds application, for instance, in the construction of pavement base layers, in slope protection of earth dams, and as a support layer for shallow foundations. However, there are no dosage methodologies based on rational criteria as exist in the case of the concrete technology, where the water/cement ratio plays a fundamental role in the assessment of the target strength. This study therefore aims to quantify the influence of the amount of cement, the porosity and the moisture content on the strength of a sandy soil artificially cemented, as well as to evaluate the use of a water/cement ratio and a voids/cement ratio to assess its unconfined compression strength. A number of unconfined compression tests, triaxial compression tests, and measurements of matric suction were carried out. The results show that the unconfined compression strength increased linearly with the increase in the cement content and exponentially with the reduction in porosity of the compacted mixture. The change in moisture content also has a marked effect on the unconfined compression strength of mixtures compacted at the same dry density. It was shown that, for the soil-cement mixture in an unsaturated state (which is usual for compacted fills), the water/cement ratio is not a good parameter for the assessment of unconfined compression strength. In contrast, the voids/cement ratio, defined as the ratio between the porosity of the compacted mixture and the volumetric cement content, is demonstrated to be the most appropriate parameter to assess the unconfined compression strength of the soil-cement mixture studied.     

Evaluation of a Predictive Constitutive Model for Sands

In this technical note, an evaluation of the robustness and predictive ability of a constitutive model for sands is performed. The model is shown to capture the main features of sand behavior under both drained and undrained monotonic loadings for a wide range of relative densities and stress paths. The main contribution of this technical note is to evaluate a robust, yet simple, constitutive framework based on a solid theoretical basis that fulfils the most fundamental requirement of any useful constitutive law: accurate predictions.     

Strain Rate, Creep, and Stress Drop-Creep Experiments on Crushed Coral Sand

The part of sand behavior that is affected by time, such as creep, relaxation, and loading rate effects are not similar to those observed for clay. To throw more light on the time effects in sand, many series of drained triaxial compression experiments have been performed on crushed coral sand. These tests were all performed with a constant effective confining pressure of 200?kPa. The test series included experiments with specimens loaded at five different strain rates with a 256-fold ratio between the extreme rates, tests with sudden changes in strain rate from slow to fast and vice versa, and tests in which axial and volumetric creep strains were observed at stress differences of 500, 700, and 900?kPa. Creep creates structuration and this has to be overcome to produce further plastic straining. Experiments were also performed in which the stress difference was dropped quickly from three different values of 500, 700, and 900?kPa followed by creep. In these stress drop-creep tests five different magnitudes of stress drops were employed: 0, 100, 200, 300, and 400?kPa. The results involving conventional creep effects and stress drop-creep effects are presented and analyzed.     

Centrifuge Model Studies of the Seismic Response of Reinforced Soil Slopes

Centrifuge tests were used to study the dynamic behavior of soil slopes reinforced with geosynthetics and metal grids. The main objectives were to determine the failure mechanism and amount of deformations under seismic loading and to identify the main parameters controlling seismically induced deformations. Geosynthetically reinforced soil slopes (2V:1H) and vertical walls reinforced with metallic mesh strips were subjected to earthquake motions with maximum foundation accelerations of up to 1.08g. The experimental results show that slope movement can occur under relatively small base accelerations, and significant lateral and vertical deformations can occur within the reinforced soil mass under strong shaking. However, no distinct failure surfaces were observed, and the magnitude of deformations is related to the backfill density, reinforcement stiffness and spacing, and slope inclination.