Characteristics of Soils with Low Plasticity: Intermediate Soil from Ishinomaki,Japan and Lean Clay From Drammen,Norway

Two soils with low plasticity are investigated; intermediate soil from Ishinomaki, Japan and lean clay from Dram- men, Norway. Since both the soils were retrieved using the Japanese sampling method, the test results from these samples are comparable. Though they have the same order of plasticity index (I_p), there is a significant difference in the grain size distribution characteristics between these soils. Ishinomaki intermediate soil contains a lot of sand or silt sized particles, its I_p value being nearly proportional to its clay content. On the other hand, Drammen clay consists of a large proportion of rock flour, which contains little clay mineral. The study shows that the unconfined compression test significantly underestimates the undrained shear strength for both soils, and their residual effective stress (p'_r) is also very low. It has been found that to compensate for loss of p'_r, recompression tests are useful methods to evaluate the strength of such soils.     

Peak and residual strength characteristics of cement-treated soil cured under different consolidation conditions

The shear strength of cement-treated soil can be changed by both cementation and consolidation during the early stages of hardening because of cement hydration. Based on the results of triaxial and unconfined compression tests, this paper describes the effects of isotropic and one-dimensional consolidation stress, applied during the curing period, on the undrained peak and residual shear strengths of cement-treated soil. The sample used was a mixture of fine-grained sand and ordinary Portland cement. A consolidated undrained triaxial compression test (ICU) was conducted on the specimens immediately after the cement treatment. Each test was conducted under different consolidation pressures, curing times and delayed loading times. The following conclusions were developed from the results and discussions: (1) the undrained peak shear strength of cement-treated soil, cured under different consolidation conditions, increases with an increase in either the consolidation pressure or the curing time, whereas it gradually decreases with an increase in the delayed loading time. (2) The rate of undrained strength increase resulting from consolidation differs significantly between isotropic and one-dimensional consolidations. (3) For a curing time of between one and seven days, the rate of strength increase by isotropic consolidation exceeds that by one-dimensional consolidation. The simultaneous volumetric change of cement-treated soil during consolidation depends on the stress conditions of the specimen, that is, the difference between isotropic and one-dimensional consolidations. (4) When the test is not conducted under nearly in-situ conditions, the undrained shear strength may be underestimated, depending on the time interval between the cement treatment and the start of consolidation. (5) The shear strength in the residual state is influenced by the consolidation pressure during curing. (6) As the consolidation pressure during curing increases, the specimens exhibit a higher residual strength.     

Effect of principal stress direction during consolidation on the shear strength properties of natural granite residual soil

In numerous real-life civil engineering practices, including multi-stage embankment construction and foundation pit excavation, the direction of the major principal stress σ₁ becomes rotated. In these cases, the granite residual soil may be subjected to inclined consolidation (IC) with σ₁ being inclined, because of the relatively high permeability as a result of the fissures formed during weathering. While the effect of the σ₁ direction during the shear on the strength of granite residual soil (inherent strength anisotropy) has been primarily established, little is known about how the soil strength is affected by the direction of σ₁ during consolidation. This paper presents the effects of IC on the shear strength properties of natural granite residual soil through undrained hollow cylinder torsional shear tests. The effect of the soil structure is also considered by testing remolded soil specimens. The results reveal that while IC changes neither the shape of stress–strain curve nor the specimen features at failure, it leads to an increased ultimate shear strength in terms of both the undrained strength and stress ratio, with the remolded soil being more affected. The presented data provide new insights into the understanding of residual soil strength behaviors.     

Effect of initial water content on undrained shear strength of K0 consolidated clay

This study investigates the effect of initial water content on the pore pressure response and undrained shear behavior of K₀ consolidated reconstituted clay. A series of K₀ consolidated undrained triaxial compression tests were conducted on reconstituted Lianyungang clay. Results were compared to those obtained by isotropic consolidated undrained triaxial tests. The testing results showed that the K₀ consolidated undrained strength envelope of reconstituted soil content is a straight line passing through the origin regardless of the initial water content. The initial water content would affect the undrained strength of K₀ consolidated clay as decreased normalized undrained shear strength was observed with clay at higher initial water content. The slope C of normalized pore pressure and stress ratio is affected by the consolidation method, where C is found to be a soil constant for K₀ consolidated clay and the value would be higher with clay under K₀ consolidation. The pore pressure increases with increasing initial water content at a certain axial strain under given consolidation pressure, and the difference in excess pore pressure increases with the increasing consolidation pressure. Pore pressure coefficient at failure (A_f) increases as the initial water content increases, where a trendline can be well fitted between the pore coefficient at failure and the ratio of initial water content to the liquid limit of clay. The undrained strength indexes, i.e., effective cohesion and effective internal friction angle have decreasing tendency with increasing initial water content; however, changes in the total strength indexes of soil in this study are insignificant with varying initial water content.     

Increase in Undrained Shear Strength of Clay with Respect to Rate of Consolidation

Estimating increase in undrained shear strength s_u of clay is an important purpose of consolidation analysis as well as settlement prediction when multi stage loading for construction of earth structures on soft clay is concerned. The present paper investigates the increase in s_u with regard to rate of consolidation. Since values of s_u are usually evaluated as a function of effective vertical stress σˊ_v, rate effect on shear strength increment ratio in normally consolidated state s_un/σˊ_v is first discussed considering the fundamental concept expressed by the equation of s_un/σˊ_v=s_uf/σˊ_y, in which s_uf is in-situ undrained shear strength and σˊ_y is consolidation yielding stress. The paper also describes two case histories where actual increases in s_u were observed in soft clay deposits. The first site is located offshore Osaka-bay where a large-scale seawall was constructed, and soft clay in the site was improved by sand drains. The second is a reclaimed land in Yanai City where a sand fill and a subsequent test embankment were conducted on a soft clay deposit without improvement by vertical drains. It is found from the study that: (1) rate effect on s_un/σ′_v cancels rate effect on σˊ_y, and the equation mentioned above is valid regardless of the rate of consolidation, and (2) the in-situ values of s_un/σˊ_v observed in the two sites vary from 0.27 to 0.37 as consolidation progresses, and they are well related to rate of consolidation. According to the experience at the two construction sites, values of s_un/σˊ_v with regard to rate of consolidation are proposed for design use at field/construction sites.     

An approach to shorten the construction period of high embankment on soft soil improved with PVD

Prefabricated Vertical Drains (PVDs) are being used to accelerate the consolidation of subsoil for construction of high embankment on soft ground. The construction is carried out in stages and height of the first stage construction depends on in-situ undrained shear strength. Each subsequent stage construction is carried out after completion of either 90% primary consolidation or percent consolidation at inflection point. The height of subsequent stages depends upon the gain in undrained strength of subsoil. In this paper, the authors have advocated an approach to shorten the construction period for high embankments. In this approach, the first stage construction would be carried out based on the in-situ undrained shear strength of subsoil. Instead of waiting for 90% primary consolidation or percent consolidation at inflection point, the embankment is raised in layers of 0.2 m thickness. Based on the time required to gain strength with the construction of the 0.2 m layer, the waiting period is determined for each subsequent layers. The waiting period depends on soil parameters such as subsoil thickness, C_r/C_v ratio and different PVD factors viz. smear, drain spacing and well resistance, pattern of laying of PVD, etc. Using this approach, there would be increase in the consolidation rate and overall reduction in the construction period. A typical practical example has been solved to demonstrate the usefulness of this approach over the two conventional methods. For a 4.5 m high embankment, it is observed that waiting period is reduced by 77% and 43% as compared to the 90% primary consolidation method and inflection point method respectively.     

Shear Strength Properties of a Residual Soil in Singapore

The shear strength and stress-strain behaviour of residual soil are known to be affected significantly not only by the initial porosity and stress history, but also by the bonds between particles. Although residual soil is commonly encountered during constructions in the tropical region, studies on its engineering properties are far from adequate. There is a lack of in-depth study to characterize the strength and deformation behaviour of intact residual soils in Singapore, especially under more representative testing conditions such as plane-strain conditions. In relation to a tunnelling construction project in Singapore, the engineering properties of an intact residual soil were characterised using laboratory tests. Large block undisturbed soil samples taken from a construction site were used. K₀ consolidated undrained triaxial compression (CK₀UC), extension (CK₀UE), direct simple shear (CK₀UDSS), and K₀ consolidated undrained plane-strain (CK₀UPS) tests were conducted. The undrained shear strength ratio c_u/σ′₁₀ and overconsolidation ratio (OCR) relationships were established. The test results indicate that the undrained shear strength (c_u) of the intact residual soil is highly anisotropic in term of c_u/σ′₁₀ ratio. It is also noted from the experimental results that the secant friction angles were highly dependent on consolidation stresses, as well as the testing methods. These anisotropic properties will affect considerably the design methods and the selection of parameters for analyses.     

Yielding and Flow Liquefaction of Loose Sand

The relationship between the shapes of the yield surface and the undrained effective stress path (UESP) of loose sand is investigated for triaxial loading conditions. It is shown that the UESP can be used in the construction of capped yield surfaces for sands. The stress ratio M_p, measured at a point where the UESP of loose sand reaches a peak, has been incorporated as a material parameter in the analytical relationship by which the yield surface is defined. The variations of M_p with void ratio, state parameter, and consolidation stresses are examined and compared with previous studies, in cases where such studies exist. It is shown that M_p is strongly influenced by soil dilatancy and anisotropy and its variation is remarkably consistent with the variation of soil strength and yielding stresses. Quantitative relationships for the variations of M_p are then introduced; these have been used elsewhere in constructing yield surfaces and modeling the constitutive behavior of sands. In addition to their use in modeling sand behavior, the yield surfaces and quantitative variations of M_p obtained here can be used in quantitative assessments of the susceptibility of loose sandy soils to flow liquefaction.     

Consolidation behavior of dredged ultra-soft soil improved with prefabricated vertical drain at the Mae Moh mine,Thailand

The effectiveness of the prefabricated vertical drains (PVDs) in the consolidation of ultra-soft dredged soil with various soil water contents (W) in Mae Moh mine, Lampang, Thailand was researched via a series of large-scale model tests and numerical analysis. Large settlements with the delay of excess pore pressures is a distinct behavior of ultra-soft soil. The PVD dimensions were found to have a significant effect on the rate of consolidation and the delay of excess pore pressure at low total vertical stress (σ_v). The smaller PVD dimension resulted in the smaller rate of consolidation and longer delay of excess pore pressure. The undrained shear strength (S_u) of ultra-soft clay at various degrees of consolidation could be approximated by the vertical effective stress (σ′_v) based on the SHANSEP where the σ′_v was determined from the Asaoka's observational method. The finite element analysis with axisymmetric and plane strain models showed that the axisymmetric model produced an excellent settlement prediction. However, the excess pore pressures were not well predicted by the axisymmetric model, due to the delay of excess pore pressures at the early stages of consolidation. In practice, the plane strain models proposed by Chai et al. and Indraratna and Redana's methods are suggested to predict the consolidation settlement of the Mae Moh dredged soil improved with PVD. The outcome of this research will facilitate the geotechnical design of reclamation of ultra-soft dredged soil in Mae Moh mine and other similar soils.     

Properties of very soft clays: A study of thixotropic hardening and behavior under low consolidation pressure

In a study on the properties of very soft clays, bender element testing was used to evaluate thixotropic hardening behavior; that is, to measure the stiffness with resting time under constant volume and water content. A laboratory vane test, which measures the undrained shear strength of the materials, was also carried out for comparison purposes. To investigate the mechanism of the thixotropic phenomenon, a consolidation test with very low pressure was also performed in a cell equipped with bender elements. The most important findings from this study are as follows: (1) regardless of soil types, the effect of thixotropy was significant around the liquid limit state and less remarkable at the lower and higher ranges; (2) the shear modulus at the liquid limit after 24 h resting is around 200 kPa; (3) the correlation between the shear modulus and the undrained shear strength of very soft clays is similar to that of cement-treated soil proposed by Seng and Tanaka (2011); (4) the increment of the shear modulus developed in the thixotropy process appears to be noticeably higher than that in the secondary consolidation process. It is believed that these findings are very useful to establish a new theory for the consolidation of ground filled by very soft clays or dredged soils with extremely high water content as well as to understand the effects of ageing on the consolidation properties of natural soils.     

Effects of Sampling Tube Geometry on Soft Clayey Sample Quality Evaluated by Nondestructive Methods

Sample disturbance caused by difference in sampling tube geometry was evaluated by two nondestructive methods: the measurement of the residual effective stress (p_r') by ceramic disc; and the use of the bender element to ascertain the shear wave velocity (V_s), and thus the maximum shear modulus (G_BE). Samples were measured under atmosphere, i.e., not under confined stress conditions. The soil samples were obtained from two sources: reconstituted Kasaoka clay prepared in the laboratory, and at the test site at Takuhoku, Hokkaido, Japan. Samplers with different geometrical designs, referring to the Japanese standard stationary piston sampler, were used for the model ground and field sampling. The geometrical effects of the sampling tube, for example, the thickness of the tube wall, the edge angle, and the existence of a piston were carefully examined. The quality of the samples taken with different samplers was evaluated by p_r' and G_BE, values which were normalized by the in situ vertical effective stress (σ'_vo) and G_f measured by the seismic cone test in the field. It was found from these studies that p_r'/σ'_vo and G_BE/G_f vary considerably due to the geometry of the sampler, with the edge angle of sampling tubes being the most important feature in obtaining high quality samples. The wall thickness, and thus, the area ratio of the sampler is not critical to the sample quality if the edge angle is sharp enough. The existence of the piston does not significantly influence the sample quality in field samples. Furthermore, the correlation between G_BE and p_r' was also investigated, and it was found that the two parameters are strongly dependent.     

Stress-Deformation Behavior Under Anisotropic Drained Triaxial Consolidation of Cement-Treated Soft Bangkok Clay

In this study the compression behavior of high water content cement-treated soft Bangkok clay is further investigated by conducting a constant stress ratio (CSR) test at various stress ratios (η). The test utilized cement-treated clay specimens with cement contents (A_w) of 10% and 15%, each of which was in combination with 100% and 130% total clay water contents. The test results confirmed that the ratio of after-curing void ratio to cement content (e_ot/A_w) can effectively characterize the compression behavior of cement-treated clay. The specimens with higher values of e_ot/A_w yielded higher volumetric and shear strains at the same stress ratio. While those with lower values of e_ot/A_w resulted in lower shear strains, with consequent higher values of strain increment ratios (dɛ_v/dɛ_s) both before and after transitional yield points. Significantly, the e_ot/A_w ratio has described the relationship of the compression yield loci of cement-treated clay at various stress ratios and mixing conditions.     

Mechanical Behavior of Bentonite-Sand Mixtures as Buffer Materials

For the purpose of establishing the method for estimating in-situ mechanical behavior of artificial buffer materials, stress-deformation behavior of bentonite-sand mixtures were investigated through oedometer test, consolidated undrained triaxial compression test and expansive stress-strain measuring test by changing the clay content as 30, 50, 70 and 100%, and by changing the range of initial dry density of mixture from 1.4 to 1.8 g/cm³. Oedometer test results suggest that the magnitude of consolidation yield stress almost coincides with the maximum expansive stress (p′_s)_max irrespective of bentonite-sand mix proportion, initial density of mixture and the magnitude of molding stress at the specimen making. Strong correlation between consolidation stress and initial tangent modulus during undrained triaxial compression test is observed, and it is found that the reduction rate of rigidity is hardly dependent on the specimen making method, molding stress and the consolidation stress. From the two series of expansive stress-strain measuring tests, it is recommended to perform the measurement of expansive stress by feed back system with the load cell installed at the base of the specimen. A unique relationship is found between the maximum expansive stress (p′_s)_max versus bentonite specific volume v_b, which is defined as the specific volume calculated by excluding the volume of sand particles. The line showing the unique log v_b versus log (p′_s)_max relationship can be recognized as the state boundary line prescribing one-dimensional expansive stress-strain behavior of the bentonite-sand mixtures.     

Behavior of zeolite-cement grouted sand under triaxial compression test

Permeation grouting with cement agent is one of the most widely used methods in various geotechnical projects,such as increasing bearing capacity,controlling deformation,and reducing permeability of soils.Due to air pollution induced during cement production as well as its high energy consumption,the use of supplementary materials to replace in part cement can be attractive.Natural zeolite(NZ),as an environmentally friendly material,is an alternative to reduce cement consumption.In the present study,a series of consolidated undrained(CU) triaxial tests on loose sandy soil(with relative density D_r=30%)grouted with cementitious materials(zeolite and cement) having cement replacement with zeolite content(Z) of 0%,10%,30%,50%,70% and 90%,and water to cementitious material ratios(W/CM) of 3,5 and 7 has been conducted.The results indicated that the peak deviatoric stress(q_(max)) of the grouted specimens increased with Z up to 50%(Z_(50)) and then decreased.The strength of the grouted specimens reduced with increasing W/CM of the grouts from 3 to 7.In addition,by increasing the stress applied on the grouted specimens from yield stress(q_y) to the maximum stress(q_(max)),due to the bond breakage,the effect of cohesion(c') on the shear strength reduced gradually,while the effect of friction angle(φ')increased.Furthermore,in some grouted specimens,high confining pressure caused breakage of the cemented bonds and reduced their expected strength.     

Basic parameters governing the behaviour of cement-treated clays

Although extensive research has been conducted on the mechanical behaviour of Portland cement-treated soft clays, there has been less emphasis on the correlation of the observed behaviour with clay mineralogy. In this study, experimental results from the authors have been combined with the data found in the literature to investigate the effect of parameters such as curing time, cement content, moisture content, liquidity index, and mineralogy on the mechanical properties of cement-treated clays. The findings show that undrained shear strength and sensitivity of cemented clays still continue to increase after relatively long curing times; expressions are proposed to predict the strength and sensitivity with time. This parametric study also indicates the relative importance of the activity of the soil, as well as the water–cement ratio, to the mechanical properties of cementitious admixtures. Two new empirical parameters are introduced herein. Based on the results of unconfined compression, undrained triaxial, and oedometer tests on cement-enhanced clays, expressions that use these parameters to predict undrained shear strength, yield stress, and the slope of the compression line are proposed. The observed variations in the mechanical behaviour with respect to mineralogy and the important effect of curing time are explained in terms of the pozzolanic reactions. The possible limitations of applying Abrams׳ law to cement–admixed clays are also discussed.     

砂土剪胀剪缩特性及其对抗剪强度的影响

为了研究饱和砂土的剪胀剪缩特性及其对抗剪强度的影响,选取滹沱河细砂,利用空心圆柱扭剪仪较系统地开展了一系列不同初始密度、不同固结压力条件下的排水与不排水纯扭剪试验研究,在总应力保持不变的情况下研究了砂土的剪胀剪缩特性,着重探讨了在排水与不排水试验中,不同密度和不同有效围压的砂土在单调剪切荷载作用下的应力-应变关系、硬化与软化、土体的剪胀剪缩以及强度等特性。结果表明:砂土密度和固结压力对砂土剪胀剪缩特性具有显著的影响;砂土的剪胀剪缩特性对砂土的排水、不排水强度以及应力-应变关系产生显著的影响;由于剪胀剪缩特性的影响,砂土的不排水抗剪强度甚至可能高于排水抗剪强度;研究成果可为今后砂土的本构模型和数值模拟提供试验资料。     

Effect of spatial variability of block-type cement-treated ground on the bearing capacity of foundation under inclined load

Deep mixing methods are widely used for stabilizing soft clayey soils and improving their bearing capacity. However, spatial variability in the shear strength of the cement-treated ground introduces uncertainties in estimating the bearing capacity for design. This paper evaluates the reliability of, block-type, cement-treated foundation under inclined load conditions using random field numerical limit analyses. The undrained shear strength is modelled as a random field which is characterized by a log-normal distribution and a spatial correlation length. Monte Carlo simulations are then used to interpret the stochastic bearing capacity factor and failure mechanisms for inclined concentric loading conditions at selected ratios of the shear strength ratio of cement-treated ground to original clay, the coefficient of variation in undrained shear strength and correlation length of the cement-treated zone. Variability of the undrained shear strength can reduce the expected bearing capacity of the cement-treated ground by 50–70% compared to homogeneously mixed clay.     

Main Factors Governing Residual Effective Stress for Cohesive Soils Sampled by Tube Sampling

The residual effective stress (p'_r) was measured for various clayey soils collected from various parts of the world, including Japan. All samples studied in this paper were retrieved by the same sampling method, i.e., using the Japanese standard sampler. However, measured p'_r/σ'_vo, where σ'_vo is the in situ effective overburden pressure, considerably varied for different sites as well as with depth. This paper examines main factors governing the p'_r value, focusing on location of the sample in the sampling tube; transportation of the soil samples; time duration between retrieval of the sample and extrusion of the sample from the sampling tube; overconsolidation ratio (OCR); clay content and plasticity index (I_p). In addition, the p'_r values are correlated to the volume change generated when the in situ σ'_vo is applied in the oedometer test, which is extensively used for assessment of the sample quality. The largest measured value of p'_r was found at one third of the sample length from the cutting edge of the sampling tube. The effects of the transportation and the time duration from the sampling to the extrusion of the sample are not prominent for the p'_r value. Any clear relations between p'_r and Δe/e_o are not found, where Δe and e_o are the void ratio change caused by applying σ'_vo and the initial void ratio, respectively. Among factors examined in this paper, OCR is the most effective factor: i.e., as OCR increases, p'_r/σ'_vo ratio increases for every studied site. However, when compared at different sites, the p'_r/σ'_vo ratio at the same OCR is considerably different. In spite of some exceptions, there exists a tendency that p'_r/σ'_vo ratio increases with the increase in the clay content as well as I_p.     

A Simple and Unified Three-Dimensional Model to Describe Various Characteristics of Soils

A simple and unified model to describe some features of soil behavior in one dimensional condition is presented in another related paper (Nakai et al., 2011). In the present paper, this one-dimensional model is extended to describe not only the soil features explained in the related paper three-dimensionally (3D), but also to explain other soil features found in multi-dimensional conditions, such as shear behavior considering the influence of intermediate principal stress on the deformation and strength of soils, and the positive and negative soil dilatancy. Firstly, the first step in extending any kind of one-dimensional model to a three-dimensional one is explained in detail: the significance of t_ij concept and its stress invariants (t_N and t_S) is explained and compared with the idea of ordinary stress invariants (p and q) used in the Cam clay model. Then, the advanced elastoplastic relations (stages I to III) in the one-dimensional condition presented in the related paper are re-formulated as three-dimensional models—e.g., a model for over consolidated soil, a model for structured soil and a model which considers time-dependent behavior. The three-dimensional models for over consolidated soil (stage I) and structured soil (stage II) are formulated so as to coincide with the subloading t_ij model developed by Nakai and Hinokio (2004) and by Nakai (2007), respectively. The validity of the models in stage I and stage II is checked by simulations of various shear tests for sands with different void ratios and for over consolidated and natural clays under drained and undrained conditions. The model in stage III is verified by simulations of shear tests with different strain rates, and by simulating creep tests and others, not only for normally consolidated clay but also for non-structured and structured over consolidated clays under drained and undrained conditions.     

Numerical investigation of the ultimate lateral resistance of piles in soft clay

The paper presents a numerical study on the undrained lateral response of a single, free-head, reinforced concrete pile in soft clays. Soil conditions simulating normally consolidated clays are examined—undrained shear strength increasing with depth—and the pile-soil interaction under static lateral loading is analyzed. The nonlinear p-y curves proposed in literature for soft clays are imported into a beam-on-nonlinear-Winkler-foundation simulation in order to predict the pile head lateral load—displacement curve and the distribution of the horizontal displacement and bending moment along the pile. The striking differences among these methods require further investigation via 3D finite element analyses. The determination of the ultimate soil resistance p _ult from the results of the finite element analyses aims at providing the estimation of a range of values for the ultimate soil resistance coefficient N _p with depth and the comparison of the derived values to the corresponding ones proposed by existing methodologies.