Pore Pressure Response Following Undrained uCPT Sounding in a Dilating Soil

The generation and dissipation of pore fluid pressures following standard piezocone sounding (uCPT) sounding in silty sands are observed to exhibit many of the characteristics of undrained penetration in dilatant materials; steady excess pore pressures may be subhydrostatic, or may become subhydrostatic during dissipation, and are slow to decay. Enigmatic pore pressure dissipation histories which transit from sub- to supra- and again to subhydrostatic before equilibrating at hydrostatic are consistent with a response where undrained pressures are maximally negative remote from the penetrometer tip. This surprising distribution of induced pore fluid pressures is accommodated in cavity expansion models for a dilating soil. A Mohr-Coulomb constitutive model is established for undrained loading of a soil with pore pressure response defined by Skempton pore pressure parameters. Defined in terms of effective stresses, this allows undrained stresses and pore pressures to be determined following cavity expansion in a c–? soil. Pore pressures are conditioned by the shear modulus, Skempton A parameter, and the “undrained shear strength.” The undrained shear strength is additionally modulated by the magnitudes of c, ?, A, and of the initial in situ effective stress, σ0′. Cavity expansion stresses, and pore pressures may be backcalculated. Undrained pore pressures are shown to decay loglinearly with radius from the cavity wall; they may be either supra- or subhydrostatic at the cavity wall, and where suprahydrostatic may become subhydrostatic close to the transition to the elastic region. This initial pressure distribution contributes to the observed switching between supra- and subhydrostatic pore pressures recorded during dissipation. “Type curves” that reflect the dissipation response enable the consolidation coefficient, undrained strength, and shear modulus to be computed from observed pore pressure data, and confirmed against independent measurements. In addition to representing the dilatory response of cohesionless silts, the method applies equally to recovering the pressure generation and dissipation response of overconsolidated clays.     

Shear Strength in Preexisting Landslides

Drained residual shear strength is used for the analysis of slopes containing preexisting shear surfaces. Some recent research suggests that preexisting shear surfaces in prior landslides can gain strength with time. Torsional ring and direct shear tests performed during this study show that the recovered shear strength measured in the laboratory is only noticeably greater than the drained residual strength at effective normal stress of 100 kPa or less. The test results also show that the recovered strength even at effective normal stresses of 100 kPa or less is lost after a small shear displacement, i.e., slope movement. An effective normal stress of 100 kPa corresponds to a shallow depth so the observed strength gain has little, if any, impact on the analysis of deep landslides. This paper describes the laboratory strength recovery testing and the results for soils with different plasticities at various rest periods and effective normal stresses.     

Spatial Distribution of Excess Pore-Water Pressure due to Piezocone Penetration in Overconsolidated Clay

This paper presents the results of an analysis of the spatial distribution of the excess pore-water pressure induced by piezocone penetration into overconsolidated clays. From the experimental results obtained for moderately and heavily overconsolidated clays, it was observed that the excess pore-water pressure increases monotonically from the piezocone surface to the outer boundary of the shear zone and then decreases logarithmically, approaching zero at the outer boundary of the plastic zone. It was also found that the size of the shear zone decreases from approximately 2.2 to 1.5 times the cone radius with increasing overconsolidation ratio (OCR), whereas the plastic radius is about 11 times the piezocone radius, regardless of the OCR. The expressions developed in this study based on the modified Cam clay model and the cylindrical cavity expansion theory, which take into consideration the effects of the strain rate and stress anisotropy, provide a good prediction of the initial pore-water pressure at the piezocone location. The method of predicting the spatial distribution of excess pore-water pressure proposed in this study is based on a linearly increasing Δushear in the shear zone and a logarithmically decreasing Δuoct, and was verified by comparing the pore-water pressure measured in overconsolidated specimens in the calibration chamber.     

Undrained Shear Strength of Pleistocene Clay in Osaka Bay

This study presents the undrained shear characteristics of Holocene and Pleistocene clay samples from depths of 20–200 m under the seabed in Osaka Bay. Automated triaxial K0 consolidation tests and anisotropically consolidated-undrained triaxial compression and extension tests are conducted using the recompression method. The average undrained strength ratio (su/σv0′) is 0.33 (SD = 0.03) when the extension strength is defined as the peak strength or the strength at an axial strain of 15%, while su/σv0′ is 0.29 (SD = 0.04) when the extension strength is defined as the shear stress at the axial strain corresponding to the peak compression strength. Circular arc stability analyses are carried out with the modified Fellenius and Bishop methods for the design cross section of the seawall structure of the Kansai International Airport to study the effects of different definitions of shear strength. The seawall is founded on 19 m of soft Holocene clay and 10 m of Pleistocene sand overlying the Pleistocene clay. The stability analyses show that the factor of safety and depth of the critical circle (i.e., above versus below the sand layer) are sharply affected by both the value of su/σv0′ (0.33 versus 0.29) and the method of slices (Fellenius versus Bishop). The marginal stability calls for careful monitoring of construction with field instrumentation.     

Influence of Shear Rate on Undrained Vane Shear Strength of Organic Harbor Mud

Dredging operations in European harbors for maintenance of navigable water depth produce vast amounts of harbor mud. Between 2005 and 2007, the second largest harbor construction project in Germany was designed as a pilot study to use dredged harbor mud as backfill material to avoid expensive disposal or ex situ treatment. During this project, a partial collapse of the backfill highlighted the need for an improved assessment of undrained shear strength of naturally occurring liquid harbor mud. Using vane shear testing, this study evaluates the effect of shear rate on the undrained shear strength of harbor mud. It is shown that measured values for both peak and residual shear strength are significantly influenced by shear rate effects. Furthermore, the influence of shear rate on the peak shear strength is found to be independent of water content while the influence of the shear rate on the residual shear strength strongly depends on water content. New shear rate dependent correction factors μ are proposed using the test results and the observed time to failure in the harbor basin. The proposed correction leads to significant lower design undrained shear strengths than the classical Bjerrum correction and would have predicted the failure during the construction.     

Relationship between the Undrained Shear Strength, Water Content, and Mineralogical Properties of Fine-Grained Soils

The relationship between the undrained shear strength of fine-grained soils and the water content can be described with a nonlinear function in which the type of soil is determined by two parameters. It is well known that these parameters depend mainly on the mineral compositions of soils; these relationships, however, have not yet been investigated. The findings described in this paper define those mineralogical properties of soils which determine the values of both parameters. Experimentally obtained results suggest that the parameters primarily depend on the size of the clay minerals, their quantity in soil composition, and the interlayer water quantity in the expanding clay minerals. As this dependence is well defined, the parameters, and thus the undrained shear strength at different water content, can be defined from knowledge of these mineralogical soil properties.     

Postcyclic Degradation of Strength and Stiffness for Low Plasticity Silt

Stress-controlled undrained cyclic triaxial tests followed by strain-controlled monotonic compressive shear tests were carried out on normally consolidated and overconsolidated reconstituted Keuper Marl silt to investigate the strength and stiffness degradation characteristics of a low plasticity silt. Special attention was paid to the changes in undrained strength and deformation modulus after undrained cyclic loading. It was observed that cyclic degradation in stiffness for low plasticity silt is more marked than that of strength, and this tendency increases with increasing overconsolidation ratio. It was found that a previously proposed model for predicting postcyclic degradation in strength and stiffness of normally consolidated fine-grained soils could be applied to that of overconsolidated silt but not however to the postcyclic degradation in Young’s modulus. Thus, an attempt was made to correlate postcyclic degradation of overconsolidated silt to the equivalent cyclic shear strain instead of the normalized excess pore pressure. It was concluded that cyclic shear strain was a better parameter than cyclic-induced excess pore pressure for correlating the postcyclic stiffness degradation not only for normally consolidated but also for overconsolidated silt.     

Dynamic Centrifuge Testing of Slickensided Shear Surfaces

Movement along preexisting slickensided rupture surfaces in overconsolidated clay and clay shale slopes can represent a critical sliding mechanism during earthquakes. The seismic behavior of preexisting slickensided surfaces in overconsolidated clay was examined by performing dynamic centrifuge model tests of two slickensided sliding block models constructed using Rancho Solano lean clay. Dynamic shear displacements were concentrated along the preformed slickensided surfaces. The peak shear resistances mobilized along the slickensided surfaces during dynamic loading were 90–120% higher than the drained residual strength measured prior to shaking. To accurately predict the displacements of the sliding blocks using Newmark’s method, it was necessary to use dynamic strengths that were 37–64% larger than the drained residual strength of the soil. Dynamic loading caused a positive pore pressure response in the soil surrounding the slickensided planes. The postshaking shear strengths were 17–31% higher than those measured prior to shaking.     

Undrained Shear Strength of K0 Consolidated Soft Soils

On the basis of critical state soil mechanics, this study derives theoretical formulas for predicting the undrained shear strength of K0 consolidated soft soils in triaxial compression and extension. Although the modified Cam-clay model is often utilized to predict the undrained shear strength of soft clays, it is applicable mainly to isotropically consolidated soils. Because of the anisotropy under K0 consolidation, an inclined elliptical yield surface is chosen, which is different from those methods based on the original Cam-clay model. The inclined elliptical yield surface is testified to be appropriate to the K0 consolidated soft soil and results in a better prediction of undrained strength, especially for the triaxial extension test. It is concluded that the analytical solutions obtained in this paper are in good agreement with the available test results and back-analysis of slope failures. On the basis of the investigation of soil properties, a simple formula is proposed for calculating the mean undrained shear strength along the failure surface.     

Drained and Undrained Strength Interpretation for Low-Plasticity Silts

The engineering behavior of low-plasticity silts is more difficult to characterize than is the behavior of clay or sand. Due to their tendency to dilate during shear, establishing a consistent and practically useful failure criterion for low-plasticity silts can be very difficult. Consideration of how the undrained shear strength of silt is related to changes in pore pressure provides a more useful and practical framework for understanding the undrained strengths of these materials and for characterizing undrained strengths for practical purposes. Using a value of Skempton’s as a failure criterion has been found to result in very reasonable values of undrained strength and to reduce scatter in the results as compared to using other criteria. Using a failure criterion based on an appropriate value of results in consistent values of Su/p, and tolerably small values of strain at failure. For low-plasticity, dilative silts that pose the greatest problems with respect to definition of “failure,” using = 0 as a failure criterion is an appropriate and simple choice.     

Analysis of Factors Influencing Soil Classification Using Normalized Piezocone Tip Resistance and Pore Pressure Parameters

This paper discusses the development of a framework for classifying soil using normalized piezocone test (CPTU) data from the corrected tip resistance (qt) and penetration pore-water pressure at the shoulder (u2). Parametric studies for normalized cone tip resistance (Q = qcnet/σv0′) and normalized excess pressures (Δu2/σv0′) as a function of overconsolidation ratio (OCR = σvy′/σv0′) during undrained penetration are combined with piezocone data from clay sites, as well as results from relatively uniform thick deposits of sands, silts, and varietal clays from around the globe. The study focuses on separating the influence of yield stress ratio from that of partial consolidation on normalized CPTU parameters, which both tend to increase Q and decrease the pore pressure parameter (Bq = Δu2/qcnet). The resulting recommended classification chart is significantly different from existing charts, and implies that assessment of data in Q–Δu2/σv0′ space is superior to Q–Bq space when evaluating piezocone data for a range of soil types. Still, there are zones of overlap for silty soils and heavily overconsolidated clays, thus requiring that supplementary information to Q and Δu2/σv0′ be obtained in unfamiliar geologies, including variable rate penetration tests, dissipation tests, CPT friction ratio, or soil sampling.     

Plastic Limit, Liquid Limit and Undrained Shear Strength of Soil—Reappraisal

The concept that plasticity index of soils can be defined as a range of water contents producing a 100-fold variation in undrained shear strength has been experimentally verified with the help of a large number of tests on soils of diverse nature. This has led to the redefinition of the plastic limit as the water content at which undrained shear strength is around 170 kN/m2. Undrained shear strength of a soil at the liquid limit can be considered to be around 1.7 kN/m2. Accordingly, both the liquid limit and the plastic limit have been determined in the present work by a single consistent method, i.e., the Swedish fall cone method. The undrained shear strength-water content relationship has been found to be log-linear for a wide range of water contents beginning from lower than the plastic limit to higher than the liquid limit. This resulted in the formulation of an expression for predicting undrained shear strength of a remolded soil at any water content based solely on its liquid limit and plastic limit.     

Comparison in Undrained Shear Strength between Undisturbed and Remolded Ariake Clays

Triaxial consolidation undrained shear tests are performed on both undisturbed and remolded Ariake clays to investigate the undrained shear strength behavior. When the applied confining stress is larger than the triaxial consolidation yield stress, the strength envelopes expressed in the plot of undrained shear strength versus confining stress of both the undisturbed and the remolded Ariake clays are straight lines through the origin. The strength envelope of the remolded Ariake clays lies above that of the undisturbed Ariake clays when the applied confining stress is larger than the consolidation yield stress. This difference is caused by the difference in water content between undisturbed and remolded states. When the data obtained from triaxial consolidation undrained shear tests of both the undisturbed and the remolded Ariake clays are plotted in the plot of undrained shear strength versus water content, it is found that the undrained shear strength decreases uniquely with the increase in water content.     

Shear Strength and Pore-Water Pressure Characteristics during Constant Water Content Triaxial Tests

Shear strength parameters used in geotechnical design are obtained mainly from the consolidated drained (CD) or consolidated undrained (CU) triaxial tests. However in many field situations, soils are compacted for construction purposes and may not follow the stress paths in CD or CU triaxial tests. In these cases, the excess pore-air pressure during compaction will dissipate instantaneously, but the excess pore-water pressure will dissipate with time. Under this condition, it can be considered that the air phase is drained and the water phase is undrained. This condition can be simulated in a constant water content (CW) triaxial test. The purpose of this paper is to present the characteristics of the shear strength, volume change, and pore-water pressure of a compacted silt during shearing under the constant water content condition. A series of CW triaxial tests was carried out on statically compacted silt specimens. The experimental results showed that initial matric suction and net confining stress play an important role in affecting the characteristics of the shear strength, pore-water pressure, and volume change of a compacted soil during shearing under the constant water content condition. The failure envelope of the compacted silt exhibited nonlinearity with respect to matric suction.     

Strength Measurement for Near-Seabed Surface Soft Soil Using Manually Operated Miniature Full-Flow Penetrometer

A manually operated penetrometer (DMS) fitted with cylindrical (T-bar) and ball penetrometer tips was developed for measuring the profiles of undisturbed and remolded undrained shear strength within box-core samples. This paper summarizes the findings of a series of miniature penetrometer tests and vane shear tests that were carried out on reconstituted clay from a local site in Western Australia. The aim of the tests was to evaluate the potential of the DMS in characterizing the shear strength of seabed surficial sediments. It was found that the DMS gave essentially identical T-bar and ball penetration resistances but these were up to 17% lower than the net cone resistance. From the comparison between the T-bar and ball penetration resistance and the shear strengths measured from vane shear tests, average N factors of 11 and 14 were obtained for intact and fully remolded conditions, respectively. The test results suggest that the DMS is a reliable and efficient means of obtaining intact and remolded shear strength profiles.     

Evaluation of Cyclic Shear Strength of Two Cemented Calcareous Soils

The mechanism controlling the cyclic shear strength of cemented calcareous soils was investigated based on the results obtained from monotonic and cyclic triaxial tests on two different types of calcareous soil. Undrained cyclic triaxial tests performed on artificially cemented calcareous soils with different loading combinations showed that the effective stress path moved towards or away from the origin, due to the generation or dissipation of pore pressure with progressive cycles. Previous investigations have shown that the Peak Strength Envelope or the State Boundary Surface or the Critical State Line forms a boundary beyond which effective stress paths during cyclic loading cannot exist. However, in this study it was observed that the maximum stress ratio (ηmax) obtained from monotonic tests defined the boundary for the cyclic tests. Based on the information obtained from this study, an approach for evaluating the cyclic shear strength is proposed. It was observed that the modified normalized cyclic shear strength had a strong linear relationship with the logarithm of the number of cyclic to failure irrespective of confining pressure, type of consolidation and stress reversal.     

Undrained Shear Strength of Granular Soils with Different Particle Gradations

A series of undrained tests were performed on granular soils consisting of sand and gravel with different particle gradations and different relative densities reconstituted in laboratory. Despite large differences in grading, only a small difference was observed in undrained cyclic shear strength or liquefaction strength defined as the cyclic stress causing 5% double amplitude axial strain for specimens having the same relative density. In a good contrast, undrained monotonic shear strength defined at larger strains after undrained cyclic loading was at least eight times larger for well-graded soils than poorly graded sand despite the same relative density. This indicates that devastating failures with large postliquefaction soil strain are less likely to develop in well-graded granular soils compared to poorly graded sands with the same relative density, although they are almost equally liquefiable. However, if gravelly particles of well-graded materials are crushable such as decomposed granite soils, undrained monotonic strengths are considerably small and almost identical to or lower than that of poorly graded sands.     

Objective Oriented Multistage Ring Shear Test for Shear Strength of Landslide Soil

Ring shear tests were conducted on five samples of different nature with a modified Imperial College type ring shear machine. The three different testing methods used, (1) individual sample testing for each normal stress, (2) increasing load multistage ring shear test, and (3) reducing load multistage ring shear test, all showed similar effective residual internal friction angle for the samples, irrespective of testing method. However, effective residual shear intercept was different according to the testing methodology. The internal friction angle did not vary, particularly after the first minimum point in the stress displacement curve, although the residual shear intercept decreased with increase in the displacement. The thickness of the shearing zone increased along with the displacement. The remolded peak shear strength for saturated conditions at field dry density varied with the consolidation history. Measurement of remolded peak shear strength was possible in a single sample using the increasing load multistage ring shear test at normal consolidation. The equilibrium water content of the sample after the ring shear test was nearly equal to the plastic limit.     

Modified Direct Shear Test for Anisotropic Strength of Sand

This paper presents a simple method to estimate the directional dependency of granular soil strength using a modified shear box and a special specimen preparation procedure. This method is used to investigate the strength anisotropy of granular materials with particle shapes varying from spherical to angular. The experimental results show that the friction angle of granular materials varies with the orientation of shear plane relative to the bedding plane, and the degree of anisotropy is affected by particle shape. Comparison of the data from direct shear tests in this study with those of plane strain and torsional simple shear tests in the literature shows that direct shear using the modified direct shear box can reasonably capture the directional dependency of the friction angle for cohesionless materials.     

Threshold Shear Strain for Cyclic Pore-Water Pressure in Cohesive Soils

Threshold shear strain for cyclic pore-water pressure, γt, is a fundamental property of fully saturated soils subjected to undrained cyclic loading. At cyclic shear strain amplitude, γc, larger than γt residual cyclic pore-water pressure changes rapidly with the number of cycles, N, while at γc<γt such changes are negligible even at large N. To augment limited experimental data base of γt in cohesive soils, five values of γt for two elastic silts and a clay were determined in five special cyclic Norwegian Geotechnical Institute (NGI)-type direct simple shear (NGI-DSS), constant volume equivalent undrained tests. Threshold γt was also tested on one sand, with the results comparing favorably to published data. The test results confirm that γt in cohesive soils is larger than in cohesionless soils and that it generally increases with the soil’s plasticity index (PI). For the silts and clay having PI=14–30, γt = 0.024–0.06% was obtained. Limited data suggest that γt in plastic silts and clays practically does not depend on the confining stress. The concept of evaluating pore water pressures from the NGI-DSS constant volume test and related state of stresses are discussed.