Reinforced Soil Using Cohesive Fill and Electrokinetic Geosynthetics

An electrokinetic geosynthetic (EKG), is a polymeric geosynthetic material, enhanced to conduct electricity, which can be used to transport water in fine-grained soils by electrokinetic means. This paper describes the design, construction details, and analysis of a reinforced soil wall using EKG and wet cohesive fill. In order to establish an initial design layout, a long-term stability analysis of the wall was carried out using the soil’s critical state shear strength parameters. The long-term design was then checked for short-term stability based upon a minimum required undrained shear strength for the clay utilizing four different short-term analytical methods: critical height, Coulomb, discrete theory, and composite theory. The electroosmosis design was then undertaken, based upon the water content—undrained shear strength curve for the fill material ascertained from laboratory testing. Using this curve the difference between the as-placed water content and the water content corresponding to an undrained shear strength of 20?kPa was calculated, giving the volume of water that needed to be removed from each lift of clay fill. Using this volume of water the electroosmosis calculations were undertaken. A simplistic analysis was undertaken using a linear voltage gradient and fixed soil parameters, followed by a more complex analysis using finite difference techniques to establish the voltage gradient. The variation in the value of electro-osmotic permeability ke were estimated using both an empirical model and a graphical interpretation of the actual variation of ke measured in the laboratory. The results of these analyses yielded estimated treatment times and undrained shear strength of the clay.     

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.     

Anisotropic Strength Evaluation of Clay Reinforced with Grout Piles

Grout piles are often used to reinforce the base soil against base heave when carrying out deep excavations in soft clay. However, there is still a lack of an adequate criterion to describe the shear strength of clay reinforced with grout piles. In general, the anisotropic strength characteristic of clay reinforced with grout piles is more significant than that of clay. The objective of this work is to develop an anisotropic strength criterion for the reinforced soil mass. Only four parameters are needed in this anisotropic strength criterion: two are the strength properties of the in situ clay, namely, the axial compressive and axial extensive undrained shear strengths; another is the undrained shear strength of treated soil; and the final is the improvement ratio which is related to the spacing and layout pattern of the grout piles. To be used in two-dimensional undrained stability analysis, the suitability of this anisotropic strength criterion under plane strain conditions is verified by comparing the results with true triaxial test. The maximum difference between the calculated and laboratory measured shear strengths is less than 8%. The results of this study indicate that the anisotropic undrained shear strength of clay reinforced with grout piles under plane strain condition decreases with an increase in the angle between the vertical direction and the major principal stress and decreases with a decrease in the strength anisotropy ratio of clay reinforced with grout piles. However, there will be a greater improvement in the effect if the grout piles are installed in the active zone rather than in the passive zone. This is because the shear strength of a grout pile mobilized in the active zone is close to its maximum level.     

Yield Stress of Super Soft Clays

Super soft clays can be defined as insensitive cohesive soils that have a water content higher than the liquid limit. It is difficult to define and measure the strength of these soils using conventional soil mechanics apparatus. It is proposed that the shear strength be determined using a rotary viscometer and be defined as the shear stress at zero strain (shear strain) rate in this test of viscosity. In this paper a number of potential methods to determine the shear strength or yield stress of super soft clays is considered. The yield stress of four super soft soils, each with varying water contents, have been measured using a rotary viscometer. The results of these tests together with published data are used to develop a relationship between the yield stress and the index properties of super soft clays.     

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.     

Evaluation of Remolded Shear Strength and Sensitivity of Soft Clay Using Full-Flow Penetrometers

The undrained remolded shear strength of soft clays is of importance in geosystem design, particularly for offshore structures. Common methods to estimate remolded shear strength, such as correlations with cone penetration data, direct measurement with an in situ field vane shear device, and laboratory measurements, produce varied results and can be particularly costly and time consuming. Full-flow penetrometers (T-bar and Ball) provide an alternative rapid method to estimate remolded shear strength and soil sensitivity through remolding soil by repeated cycling of the penetrometer up and down over a given depth interval. The cyclic penetration resistance degradation curve inherently contains information regarding remolded strength and sensitivity. The objective of this paper is to assess the ability of full-flow penetrometers to predict remolded strength and soil sensitivity, and to develop a suite of predictive correlations in which these properties can be estimated in the absence of complementary laboratory or in situ test data. To accomplish this, full-flow penetration profiles and cyclic tests were performed at five well characterized soft clay sites, which together represent the broad range of soils in which the penetrometers will be often used. A previously developed model for the reduction in penetration resistance with cycling is modified to predict the entire degradation curve, including the remolded penetration resistance using only measurements obtained during initial penetrometer penetration and extraction. Using field vane shear strength as the reference measurement, correlations are developed to predict soil sensitivity and remolded shear strength based solely on full-flow penetrometer data, which is particularly useful in site investigation programs where site specific data are not yet available or are sparse. Finally, the usefulness of these relationships is demonstrated by implementing them for two additional soft clay sites.     

Effective Stress Behavior of Quasi-Saturated Compacted Cohesive Soils

Important geotechnical structures constructed on compacted cohesive soils often involve compaction either around or on the wet side of optimum water content. In general, at these water content values, water voids are continuous and air voids are occluded, and the soil may be assumed to be in a state termed as “quasi-saturated.” This paper evaluates the effective stress behavior of such quasi-saturated compacted specimens of Gangetic silt and Canyon dam clay in the broad framework of the conventional modified Cam-clay model. The initial state of quasi-saturated compacted specimens is shown to lie on the recompression line in w versus ln(p′) space. The actual recompression line on which the specimen state would lie, and the corresponding equivalent past maximum pressure, are found to depend only on the amount of compaction energy and the soil structure, and are independent of the molding water content or initial dry density. It is observed that, at low effective confining stresses, quasi-saturated compacted soils behave like overconsolidated soils and the effective stress paths during undrained shear lie on the Hvorslev surface. However, at confining stresses greater than the past maximum pressure, these soils behave like normally consolidated soils and the effective stress paths move practically along the Roscoe surface toward the critical state line.     

Behavior of a Loosely Compacted Unsaturated Volcanic Soil

In this paper, the stress-strain relationship and volumetric behavior of a loosely compacted unsaturated decomposed volcanic soil (fill) were studied by conducting three series of triaxial stress path tests: (1) consolidated undrained on the saturated fill; (2) constant water content; and (3) a reducing suction under constant deviator stress on the unsaturated fill. The last two series of tests were designed to simulate the effects of undrained response and rainfall infiltration in initially unsaturated slopes, respectively. It was found that the saturated loose volcanic soil behaves like clay under isotropic compression but it resembles sand behavior when it was subjected to undrained shear. For isotropically consolidated unsaturated specimens sheared under a constant water content, a hardening stress-strain and a nonlinear shear strength-suction relationship are observed. At relatively high suctions, both angle of friction and apparent cohesion appear to be independent of suction. Volumetric contraction during shear is observed in this series of tests. On the other hand, anisotropically consolidated loose unsaturated specimens subjected to a reducing suction change from contractive to dilative behavior as the net mean stress increases. This observed volumetric behavior, unlike the shear strength, is stress path-dependent and cannot be explained by using the existing elastoplastic critical state theoretical framework extended for unsaturated soils.     

Analysis of Soil Strength Degradation during Episodes of Cyclic Loading, Illustrated by the T-Bar Penetration Test

Pipelines and risers form an essential part of the infrastructure associated with offshore oil and gas facilities. During installation and operation, these structures are subjected to repetitive motions which can cause the surrounding seabed soil to be remolded and soften. This disturbance leads to significant changes in the operative shear strength, which must be assessed in design. This paper presents an analytical framework that aims to quantify the degradation in undrained shear strength as a result of gross disturbance—in this case through repeated vertical movement of a cylindrical object embedded in undrained soil. The parameters of the framework were calibrated using data obtained in a geotechnical centrifuge test. In this test a T-bar penetrometer, which is a cylindrical tool used to characterize the strength of soft soil, was cycled vertically in soil with strength characteristics typical of a deep water seabed. Using simple assumptions regarding the spatial distribution of “damage” resulting from movement of the cylinder, and by linking this damage to the changing undrained shear strength via a simple degradation model, the framework is shown to simulate well the behavior observed in a cyclic T-bar test. This framework can potentially be extended to the similar near-surface behavior associated with seabed pipelines and risers.     

Design of Foundations on Sensitive Champlain Clay Subjected to Cyclic Loading

Sensitive clay subjected to cyclic loading may experience gradual loss of its shear strength, which may lead to liquefaction. Foundations built on this clay would suffer extensive settlement and significant loss of bearing capacity or perhaps catastrophic failure. This paper presents an experimental investigation on sensitive (Champlain) clay obtained from the city of Rigaud, Quebec (Canada). Consolidation tests, static and cyclic undrained and drained triaxial tests were performed on representative samples of this clay. The objective of this investigation was to examine the influence of the physical and mechanical parameters, which govern the shear strength of sensitive clay subjected to cyclic loading. Based on the results of the present investigation and those available in the literature, it can be reported herein that the undrained response is the most critical for these foundations; furthermore, the preconsolidation pressure is considered as an important parameter in establishing the shear strength of sensitive clay. A design procedure is developed to determine the safe zone for the undrained and drained responses, within which a combination of the cyclic deviator stress and the number of cycles for a given soil/loading/site conditions can achieve a quasielastic resilient state without reaching failure. The proposed design procedure is applicable to all regions around the world, where sensitive clays can be found. Furthermore, this procedure can be adopted to examine the conditions of existing foundations built on sensitive clay at any time during its lifespan.     

Yield, Strength, and Critical State Behavior of a Tropical Saturated Soil

The paper reports laboratory investigations carried out on a tropical soil profile to study its compressibility, strength, critical state and limit state conditions, and their variation with depth. The soil profile comprises a reddish lateritic layer (horizon B) underlain by a saprolitic soil (horizon C) from which a number of block samples were taken. A series of isotropic and anisotropic compression tests, and drained and undrained triaxial tests, were conducted on specimens sampled at depths between 1.0 and 7.0 m, and also in the exposed saprolitic soil. Special triaxial tests, with the pore pressure increased to induce failure, were performed to investigate the failure at low stress levels. On this basis a tensile cutoff on the failure envelope was defined. In order to assess the influence of the natural soil structure, drained and undrained triaxial tests were carried out on compacted samples obtained from depths of 1.0 and 5.0 m. Higher strength parameters were measured for the horizon C soil, which is consistent with its lower clay content. A nonlinearity in the critical state line in q:p′ stress space was identified, but linear regression was used to obtain critical state parameters. The limit state curves for soils from horizon B are centered on the hydrostatic axis, but limit state curves for horizon C suggested anisotropic behavior.     

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.     

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.     

Determination of Strength and Index Properties of Fine-Grained Soils Using a Soil Minipenetrometer

This paper describes the use of a soil minipenetrometer (SMP) to determine the strength and index properties of fine-grained soils. The SMP has been developed to allow both fall cone and quasi-static penetration tests to be performed. Displacement controlled quasi-static penetration tests can be used for the direct measurement of undrained shear strength, both for remolded and undisturbed samples. In addition the quasi-static penetration test can be used to define an additional lower plastic limit parameter, the PL100, which represents the moisture content of a fine-grained soil with an undrained strength 100 times that at the liquid limit. This approach offers the advantage that removal of the coarse fraction is not required to estimate the PL100.     

Effects of Disturbance on Undrained Strengths Interpreted from Pressuremeter Tests

Although the cylindrical cavity expansion theory should provide a sound basis for obtaining the undrained shear strength of clays from pressuremeter tests, the interpreted strengths are often inconsistent with data measured in high-quality laboratory tests. This paper investigates how the pressuremeter results are affected by disturbances that inevitably occur during device installation. The installation of self-boring and displacement-type pressuremeters is simulated using strain path analyses, with realistic effective stress-strain-strength properties described by the MIT-E3 model. Derived strengths obtained from the simulated expansion of displacement-type pressuremeters tend to underestimate the in situ∕cavity expansion strength by amounts that depend on the relative volume of soil displaced, the time delay prior to testing, and the initial overconsolidation ratio of the clay. Interpretation procedures using the simulated contraction curves give much more reliable estimates of the true undrained shear strength. The simulated disturbance effects of self boring lead to derived peak shear stresses that are significantly higher than the reference undrained shear strengths. This overestimate depends on the volume of soil removed during installation and is enhanced when the finite membrane length is included in the analyses. Self-boring pressuremeter data from a well-documented test site in Boston confirm the general character of the predicted pressuremeter stress-strain behavior. The theoretical analyses underestimate the peak strengths derived from self-boring pressuremeter (SBPM) expansion tests, but match closely the measured postpeak resistance in the strain range of 3–6% (saddle point condition). Saddle point strengths are similar in magnitude to the shear strengths measured in laboratory undrained triaxial compression tests at this site. The current predictions are not able to explain the very high shear strengths derived from the SBPM contraction curves.     

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.     

Engineering Properties of Fibrous Peats

This state-of-the-art paper presents an interpretation of the permeability, compressibility, and shear strength of fibrous peats using data from laboratory tests on undisturbed block samples of two fibrous peats, as well as extensive laboratory and field data from the literature on fibrous peat deposits. Engineering properties of fibrous peats are significantly different from those of most inorganic soils. However, the same fundamental mechanisms and factors determine behavior of both inorganic soils and fibrous peats. Fibrous peat deposits possess very high initial permeability, typically 1,000 times the initial permeability of soft clay and silt deposits. Upon compression, the permeability of fibrous peats decreases dramatically, with a ratio of permeability change index to in situ void ratio equal to 0.25, as compared to 0.50 for soft clay and silt deposits. Fibrous peats display extreme compressibility to the increase in effective vertical stress, with compression index values right after preconsolidation pressure 5 to 20 times the corresponding compressibility of typical soft clay and silt deposits. Among geotechnical materials, fibrous peats display the highest ratios of secondary compression index to compression index, in the range of 0.05 to 0.07. The values of coefficient of earth pressure at rest for normally consolidated young fibrous peat deposits are in the range of 0.30 to 0.35, as compared to 0.45 to 0.65 for inorganic soils. The values of friction angle from triaxial compression tests for fibrous peats are in the range of 40 to 60°, as compared to less than 35° for soft clay and silt compositions. For fibrous peats, the ratios of undrained shear strength in compression to preconsolidation pressure are usually in the range of 0.50 to 0.75, as compared to 0.32 for soft clay and silt deposits. For surficial fibrous peat deposits the ratio of vane shear strength to preconsolidation pressure is near 1.0, as compared to 0.12 to 0.35 for inorganic soft clay and silt deposits. For fibrous peats, the ratio of undrained Young’s modulus to undrained shear strength is in the range of 20 to 80.     

Erosion Study of New Orleans Levee Materials Subjected to Plunging Water

During Hurricane Katrina, overtopping water caused erosion and subsequent failure of several sections of I-type flood walls in New Orleans. Erosion stemmed from the kinetic energy of water falling from the top of the flood wall, unlike the typical surface erosion caused by shear flow. This study evaluated the effects of important parameters of levee soils—fines content, degree of compaction (DOC), clay mineralogy, and water content in relation to the erosion behavior of New Orleans levees subjected to the plunging water. In general, test results showed that a higher fines content contributed to greater erosion resistance. The trend became unclear when fines content exceeded 20–25%. A higher degree of compaction did not necessarily contribute to greater erosion resistance. Underwater soaked soils showed much less erosion resistance than nonsoaked soils. Soils containing expansive clay minerals showed less erosion resistance than soils containing nonexpansive clay minerals.     

Effect of Sand Columns on the Undrained Load Response of Soft Clays

When sand columns are used as vertical drains in soil improvement schemes, the possible reinforcing role that these columns can play in regards to improving the bearing capacity is usually neglected in design. The objective of this paper is to evaluate the degree of improvement in the mechanical properties of soft clays in practical applications involving the use of sand drains or sand columns in clayey soils. For this purpose, 32 isotropically consolidated undrained triaxial tests were performed on normally consolidated kaolin specimens. The parameters that were varied were the diameter of the sand columns, the height of the columns, the type of columns (geotextile encased versus nonencased), and the effective confining pressure. Test results indicated that sand columns improved the undrained strength significantly even for area replacement ratios that were less than 18%. The increase in undrained strength was accompanied by a decrease in pore pressure generation during shear and an increase in Young’s modulus. The drained shear strength parameters were found to be relatively unaffected by the sand column reinforcement, except for fully penetrating columns with high area replacement ratios.