Interpretation of Secant Shear Modulus Degradation Characteristics from Pressuremeter Tests

Nonlinear shear modulus degradation characteristics are of interest in many geotechnical engineering applications, such as ground deformation caused by seismic shaking and deep excavations in clay, weathered rock, and stabilized soil. This paper presents an approach to derive the secant shear modulus degradation characteristics from in situ pressuremeter tests, which is based on a digital filter algorithm. The algorithm is described, and data preparation procedures are presented. Use of the algorithm is illustrated by means of pressuremeter data for soils stabilized with deep mixing methods on the Boston central artery/tunnel (CA/T). The nonlinear secant shear modulus degradation characteristics from the digital filter approach are shown to be in good agreement with those from the curve fitting and transformed-strain approaches. They also compare favorably with the results of other in situ and laboratory tests performed in conjunction with the CA/T stabilized soils. The algorithm is implemented by a 26-line MATLAB code in an appendix of the paper.     

Unsaturated Constitutive Surfaces from Pressuremeter Tests

A methodology to identify the collapse potential of unsaturated soils is proposed in this paper on the basis of pressuremeter test results associated with independent measurements of the in situ matric suction. A solution combining the expansion of a cylindrical cavity to a modified Cam clay critical state model has been introduced and accommodated to the framework of unsaturated soil behavior. This accounts for changes in soil properties induced by suction changes. Interpretation of pressuremeter tests performed under unsaturated and soaked conditions links the amount of collapse to strength and stiffness changes and provides assessment to the constitutive soil parameters that are necessary to define the yield envelopes of the soil. A comprehensive site investigation program comprising field and laboratory tests carried out in two residual soil sites is discussed in order to validate the proposed methodology. Values of shear strength, in situ stress, and yield pressure derived from both field and laboratory data are used as input parameters of a constitutive model adopted for describing the yield envelopes of these unsaturated residual soil sites.     

Undrained Anisotropic Monotonic Behavior of Sand from In Situ Tests

A procedure for estimating the undrained stress-strain behavior of sand from drained self-boring pressuremeter and seismic piezocone penetration tests is proposed in this paper. The procedure offers an inexpensive alternative to laboratory testing and avoids the uncertainty of the empirical methods based on index measurements such as the Standard Penetration Test blow count and the tip resistance in a Piezocone Penetration Test (CPTU). To check its validity, the proposed procedure was used to infer the undrained triaxial stress-strain curves and the results were compared with laboratory triaxial tests on undisturbed samples. The undrained limit equilibrium stability of a dike was also assessed using the inferred stress-strain behavior to illustrate the usefulness of the procedure. The result of the stability analysis was found to be in qualitative agreement with the observed performance of the dike during a recent field experiment attempting to trigger static liquefaction.     

Plate Load Tests on Cemented Soil Layers Overlaying Weaker Soil

This paper addresses the interpretation of plate load tests bearing on double-layered systems formed by an artificially cemented compacted top soil layer (three different top layers have been studied) overlaying a compressible residual soil stratum. Applied pressure-settlement behavior is observed for tests carried out using circular steel plates ranging from 0.30 to 0.60 m diameter on top of 0.15 to 0.60-m-thick artificially cemented layers. The paper also stresses the need to express test results in terms of normalized pressure and settlement—i.e., as pressure normalized by pressure at 3% settlement (p/p3%) versus settlement-to-diameter (δ/D) ratio. In the range of H/D (where H = thickness of the treated layer and D = diameter of the foundation) studied, up to 2.0, the final failure modes observed in the field tests always involved punching through the top layer. In addition, the progressive failure processes in the compacted top layer always initiated by tensile fissures in the bottom of the layer. However, depending on the H/D ratio, the tensile cracking started in different positions. The footing bearing capacity analytical solution for layered cohesive-frictional soils appears to be quite adequate up to a H/D value of about 1.0. Finally, for a given project, combining Vésic’s solution with results from one plate-loading test, it is possible (knowing of the demonstrated normalization of p/p3%-δ/D, where the pressure-relative settlement curves for different H/D ratios produce a single curve for all values of H/D) to estimate the pressure-settlement curves for footings of different sizes on different thicknesses of a cemented upper layer.     

Numerical Validation of an Elastoplastic Formulation of the Conventional Limit Pressure Measured with the Pressuremeter Test in Cohesive Soil

An elastoplastic pressuremeter theory for cohesive soil has been used in the design of construction, such as retaining walls, slope stability, or foundation engineering. This theory takes into account the plasticity along the vertical and horizontal planes and allows for the determination of the conventional limit pressure. We compute here the conventional limit pressure using the Plaxis program to check the validity of the theoretical results. First, we present the theory used for the interpretation of the pressuremeter test in cohesive soil and its extension to the conventional limit pressure, which is defined as the pressure at the borehole wall for a volume increase ΔV equal to the initial volume of the borehole. One of the main results is the theoretical expression of the conventional limit pressure. This volume variation is linked to a radial strain of ?1. This conventional limit pressure can be directly measured with the pressuremeter, whereas the theoretical limit pressure is expressed as an infinite expansion and cannot be directly measured. Then, we validate this theory by using finite elements, and determine the conventional limit pressure with the Tresca standard model of Plaxis, which is compared to the theoretical expression. Conclusions are drawn on the validity of this new theory which allows the measurement and the control of the shearing modulus and shear strength of the natural soil.     

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.     

Interference of Two Closely Spaced Strip Footings on Sand Using Model Tests

By using small scale model tests, the interference effect on the ultimate bearing capacity of two closely spaced strip footings, placed on the surface of dry sand, was investigated. At any time, the footings were assumed to (1) carry exactly the same magnitude of load; and (2) settle to the same extent. No tilt of the footing was allowed. The effect of clear spacing (s) between two footings was explicitly studied. An interference of footings leads to a significant increase in their bearing capacity; the interference effect becomes even more substantial with an increase in the relative density of sand. The bearing capacity attains a peak magnitude at a certain (critical) spacing between two footings. The experimental observations presented in this technical note were similar to those given by different available theories. However, in a quantitative sense, the difference between the experiments and theories was seen to be still significant and it emphasizes the need of doing a further rigorous analysis in which the effect of stress level on the shear strength parameters of soil mass can be incorporated properly.     

Behavior of Plate Load Tests on Soil Layers Improved with Cement and Fiber

The load-settlement response from three plate load tests (300 mm diameter, 25.4 mm thick) carried out directly on a homogeneous residual soil stratum, as well as on a layered system formed by two different top layers (300 mm thick)—sand-cement and sand-cement fiber—overlaying the residual soil stratum, is discussed in this technical note. The utilization of a cemented top layer increased bearing capacity, reduced displacement at failure, and changed soil behavior to a noticeable brittle behavior. After maximum load, the bearing capacity dropped towards approximately the same value found for the plate test carried out directly on the residual soil. The addition of fiber to the cemented top layer maintained roughly the same bearing capacity but changed the postfailure behavior to a ductile behavior. A punching failure mechanism was observed in the field for the load test bearing on the sand-cement top layer, with tension cracks being formed from the bottom to the top of the layer. A completely distinct mechanism was observed in the case of the sand-cement-fiber top layer, the failure occurring through the formation of a thick shear band around the border of the plate, which allowed the stresses to spread through a larger area over the residual soil stratum.     

Double Strain Softening and Diagonally Crossing Shear Bands of Sand in Drained Triaxial Tests

A comprehensive understanding of the shear behavior of sand in the context of shear band development has not been achieved yet in spite of many detailed research works on each specified subject. In order to observe the entire drained shear behavior of Toyoura sand from the macromechanical point of view, conventional triaxial tests were performed and analyzed up to an axial strain of 30% for various void ratios, initial confining stresses, and stress paths, paying particular attention to volume changes. The strong correlation was found between “double strain softening” and “diagonally crossing shear bands” as a remarkable result. Finally, a qualitative explanation of relations among the stress–strain curve, the failure shape, the dilatancy index–strain curve and the strain localization, could be clearly made. Also, it is concluded that the dilatancy index is an indicator not only of the ratio of the volumetric strain increment to the axial strain increment but also the condition of the strain localization.     

Theoretical Interpretation of Impulse Response Tests of Embedded Concrete Structures

For a concrete beam resting on a bed of sand, an analytical solution technique is derived by which the mobility can be identified. To achieve realistic predictions, significant damping in the bed needs to be introduced. The modest damping of the concrete has little effect on the mobility for small frequencies whereas it has a significant effect for higher frequencies. An imperfection in the bed in terms of a void increases the mobility dramatically for low frequencies whereas the mobility for higher frequencies is almost unchanged. An imperfection in the beam in terms of honeycombing of the concrete, on the other hand, manifests itself by increasing the mobility for high frequencies while leaving the mobility for small frequencies less influenced. These latter conclusions are in good agreement with field experience for concrete slabs resting on soil.     

Behavior of Model Rafts Resting on Pile-Reinforced Sand

Piles in a pile raft are sometimes considered as settlement reducers, not load-carrying members. In design, one often tries to minimize the number of piles. This often results in a high axial stress in the piles that may deter their use due to the limits on pile stress in practice. An alternative is to consider the pile as reinforcement in the base soil, and not as a structural member. Serving as a soil stiffener, the pile can tolerate a lower safety margin against structural failure without violating building codes. Previous numerical studies on the use of disconnected piles as settlement reducers have shown the effectiveness of such piles. This study aims to verify experimentally the effectiveness of such piles through load tests of model rafts resting on pile-reinforced sand. By varying factors such as raft stiffness, pile length, pile arrangement, and pile number, results of the investigation indicate that structurally disconnected piles are effective in reducing the settlement and bending moments in the model rafts.     

In Situ Pore-Pressure Generation Behavior of Liquefiable Sand

To overcome current limitations in predicting in situ pore-pressure generation, a new field testing technique is used to measure directly the coupled, local response between the induced shear strains and the generated excess pore pressure. The pore-pressure generation characteristics from two in situ liquefaction tests performed on field reconstituted specimens are presented, including the pore- pressure generation patterns at various strain levels, the observed stages of pore-pressure generation, and pore-pressure generation curves. Comparisons of the in situ pore-pressure generation curves with data in the literature and from laboratory strain-controlled, cyclic direct simple shear tests support the in situ testing results. In addition, the effects of effective confining stress on threshold shear strain and pore- pressure generation curves are discussed. Comparisons of the rate of pore-pressure generation among the in situ tests, laboratory strain-controlled tests, and a model based on stress-controlled tests reveal that in situ pore pressures generated in reconstituted soil specimens during dynamic loading develop more similarly to those from cyclic strain-controlled laboratory testing. This observation implies that the evaluation of induced strains rather than induced shear stresses may be more appropriate for the simulation of pore-pressure generation.     

Effect of Loading Mode on Strain Softening and Instability Behavior of Sand in Plane-Strain Tests

Experimental data to study the effect of loading mode on the strain softening and instability behavior of sand under plane-strain conditions are presented in this paper. A new plane-strain apparatus was adopted to conduct K0 consolidated drained and undrained tests under both deformation-controlled and load-controlled loading modes. The drained behavior of very loose and medium dense sand and the undrained behavior of very loose sand under plane-strain conditions were characterized. The test results show that the loading mode affects the postpeak behavior and controls whether strain softening or instability will occur in the postpeak region. Shear bands occurred in tests conducted on medium dense sand, but not in tests for very loose sand. The failure line and critical state line are not affected by the loading mode. The study also shows that the concept of a unique “ultimate state” for both dense and loose sand as previously established based on conventional drained triaxial tests is not supported by the plane-strain data.     

Uplift Behavior of Blade-Underreamed Anchors in Silty Sand

To evaluate the uplift behavior of anchors installed by the blade underreaming system, a numerical model for anchors in silty sand has been developed in this study and the calculated results are compared to the results of full scale anchor pullout tests. Although the blade-underreamed anchor tends to be irregular in shape due to possible collapse of the borehole, the excavated anchor showed an underreamed body of approximately multiple-stepped shape. Despite the difference in shape, the numerical results indicate that the difference between the load–displacement curve of the multiple-stepped anchor and that of the conical shaped anchor is small. In addition, the anchorage behavior of conical shaped anchors calculated from this numerical model was in good agreement with those of full scale anchor tests. No sign of progressive soil yielding along the underreamed body was found from the numerical analysis. So, the pull-out capacity of this underreamed anchor increases more than linearly with the length of the underream. Since only a small underream angle is needed to generate a substantial increase in anchor pull-out resistance, the ultimate pull-out capacity of the blade-underreamed anchor is found to be higher than that of straight shaft anchor in silty sand.     

Shear Strength Behavior of Fiber-Reinforced Sand Considering Triaxial Tests under Distinct Stress Paths

The results of drained triaxial tests on fiber reinforced and nonreinforced sand (Osorio sand) specimens are presented in this work, considering effective stresses varying from 20 to 680?kPa and a variety of stress paths. The tests on nonreinforced samples yielded effective strength envelopes that were approximately linear and defined by a friction angle of 32.5° for the Osorio sand, with a cohesion intercept of zero. The failure envelope for sand when reinforced with fibers was distinctly nonlinear, with a well-defined kink point, so that it could be approximated by a bilinear envelope. The failure envelope of the fiber-reinforced sand was found to be independent of the stress path followed by the triaxial tests. The strength parameters for the lower-pressure part of the failure envelope, where failure is governed by both fiber stretching and slippage, were, respectively, a cohesion intercept of about 15?kPa and friction angle of 48.6?deg. The higher-pressure part of the failure envelope, governed by tensile yielding or stretching of the fibers, had a cohesion intercept of 124?kPa, and friction angle of 34.6?deg. No fiber breakage was measured and only fiber extension was observed. It is, therefore, believed that the fibers did not break because they are highly extensible, with a fiber strain at failure of 80%, and the necessary strain to cause fiber breakage was not reached under triaxial conditions at these stress and strain levels.     

Postshaking Shear Strain Localization in a Centrifuge Model of a Saturated Sand Slope

This paper presents analyses of a test conducted on a 9-m-radius centrifuge to study the redistribution of pore water during diffusion of earthquake-induced excess pore pressures in a sand slope with embedded silt layers. The centrifuge model developed large postshaking deformations associated with shear strain localization at the interface between the sand and silt layers. Dense arrays of pore pressure transducers provided detailed measurements of pore pressure variations in time and space within the slope. A new data analysis approach is presented in which measured pore-pressures are used to compute flow rates and volumetric strains as a function of time and position throughout the slope. Hydraulic gradients were calculated by numerical differentiation of measured pore-pressure distributions with respect to position. Flow rates that were based on Darcy’s law were then integrated with respect to time to obtain flow quantities, from which volumetric strains were computed. A second data analysis approach that computes volumetric strains on the basis of soil compressibility and changes in pore pressure provided an independent computation of strains in consolidating zones. Results using these data analysis procedures confirm that a dilating (loosening) zone of significant thickness developed in the sand immediately beneath an embedded silt layer that had impeded the drainage of high pore pressures. These results support the hypothesis that the dilating zone corresponds to regions where the mobilized friction angle exceeds the critical state friction angle and that the dilating zone can be initially relatively thick before its size diminishes to the thickness of a thin shear band after the peak friction angle is mobilized. Quantification of the evolution of the size of the dilating zone is a key to understanding the magnitude of deformations associated with void redistribution.     

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.     

Computation of Cavity Expansion Pressure and Penetration Resistance in Sands

A cavity expansion-based theory for calculation of cone penetration resistance qc in sand is presented. The theory includes a completely new analysis to obtain cone resistance from cavity limit pressure. In order to more clearly link the proposed theory with the classical cavity expansion theories, which were based on linear elastic, perfectly plastic soil response, linear equivalent values of Young's modulus, Poisson’s ratio and friction and dilatancy angles are given in charts as a function of relative density, stress state, and critical-state friction angle. These linear-equivalent values may be used in the classical theories to obtain very good estimates of cavity pressure. A much simpler way to estimate qc—based on direct reading from charts in terms of relative density, stress state, and critical-state friction angle—is also proposed. Finally, a single equation obtained by regression of qc on relative density and stress state for a range of values of critical-state friction angle is also proposed. Examples illustrate the different ways of calculating cone resistance and interpreting cone penetration test results.     

Transport and Deposition of Suspended Soil Colloids in Saturated Sand Columns

Understanding colloid mobilization, transport, and deposition in the subsurface is a prerequisite for predicting colloid-facilitated transport of strongly adsorbing contaminants and further developing remedial activities. This study investigated the transport behavior of soil-colloids extracted from a red-yellow soil from Okinawa, Japan. Different concentrations of suspended-soil colloids (with diameter <1??μm) were applied, at different flow velocities and pH conditions, to 10-cm long water-saturated columns repacked with either Narita (mean diameter D50 = 0.64??mm) or Toyoura (mean diameter D50 = 0.21??mm) sands. The transport and retention of colloids were studied by analyzing colloid effluent breakthrough curves (BTCs), particle size distribution in the effluent, and colloid deposition profiles within the column. The results showed a significant influence of flow velocity: Low flow velocity caused tailing of colloid BTCs with higher reversible entrapment and release of colloids than high flow velocity. The finer Toyoura sand retained more colloids than the coarser Narita sand at low pH conditions. The deposition profile and particle size distribution of colloids in the Toyoura sand clearly indicated a depth-dependent straining mechanism. By fitting colloid transport models (one-site and two-site models) to the colloid effluent breakthrough curves, transport and deposition of colloids in Narita sand at low pH were best described by a one-site attachment-detachment model, whereas colloid transport and deposition in Toyoura sand at low pH were better captured by a two-site attachment, detachment, and straining model. The coupled effects of solution chemistry, colloid sizes, and medium surface properties have a dominating role in particle-particle and particle-collector interactions in colloid transport and deposition.