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.     

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.     

Undrained Shear Strength of Cement-Treated Soils

In order to evaluate the effects of cementation on the mechanical properties of cement-treated soil, a series of isotropic consolidation and undrained triaxial compression shear tests were performed for cement-treated specimens of Ariake clay, Akita sand, Rokko Masado and Toyoura sand. This paper evaluates factors affecting the shear strength of these cement-treated soils. The following conclusions are obtained: 1) Cement-treated soil has a normally consolidated line in e-ln p' space which depends on the mixing cement content. The consolidation yield stress, p'_y, of cement-treated soil increases with increasing cement content and initial specimen density. 2) Changes in cohesive strength due to cement-treatment can be represented by a tensile effective stress, p'_r. Strength properties can then be normalized by the augmented consolidation stress, (p'_c+p'_r). 3) The shear strength properties of quasi-overconsolidated clay can be represented by the yield stress ratio, R=(p'_y+p'_r)/(p'_c+p'_r). 4) The undrained shear strength of cement-treated soils can be represented as a power law relation of the yield stress ratio, R, and the augmented consolidation stress.     

Effect of shear displacement on the hydraulic conductivity of a fracture

The effect of shear displacement inclined relative to macroscopic water flow on the hydraulic conductivity of a rock fracture was estimated, using synthetic fractures that reproduce a tensile fracture in granite. The results showed that the hydraulic aperture normalized by the mean aperture increased with the angle between the directions of shear displacement and macroscopic water flow, according to a sinusoidal function of twice the angle. Formulae were established to estimate the hydraulic aperture of the fracture as a function of the mean aperture, the standard deviation of the initial aperture, the shear displacement, and the angle between the shear displacement and macroscopic water flow, based on results obtained in both this work and previous work, but neglecting scale effects. By assuming the mechanical properties of the fracture based on experimental results for granite, but neglecting scale effects, the hydraulic conductivity of the fracture with an arbitrary direction under a given state of stress (σ₁=29 MPa, σ₂=25 MPa and σ₃=13.5 MPa) was estimated for macroscopic water flow in the directions of both σ₁ and σ₂. When the contour map of the transmissivity of the fracture is plotted on a stereonet of the normal direction of the fracture in the principal axes of stress, there is a ridge (line of the local maximum) of transmissivity in the circumferential direction, and the inclination angle of the ridge from the σ₃-axis decreases with shear displacement, since shear dilation increases with both a decrease in normal stress and an increase in shear displacement. Furthermore, for the condition of stress given in this study, the transmissivity for macroscopic water flow in the direction of σ₁ is maximum for a fracture with a normal direction within the σ₂–σ₃ plane, while that in the direction of σ₂ is maximum for a fracture with a normal direction within the σ₁–σ₃ plane.     

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.     

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.     

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.     

Applicability of T-Bar and Ball Penetration Tests to Soft Clayey Grounds

T-bar and Ball penetration tests (TPT and BPT, respectively) were carried out at three sites consisting of soft clays. These penetration tests have several advantages over the conventional cone penetration test (CPT): much larger project area and no effects of overburden pressure for obtaining soil parameters such as undrained shear strength. Therefore, TPT and BPT are considered to be suitable in-situ tests for very soft ground. In this paper validity of the assumed mechanism of these cones are examined, using ball cones with different diameters of the ball. In addition, the cone factors for CPT, TPT and BPT are compared based on undrained shear strengths (s_u) measured from field vane and direct shear tests. Cyclic BPT was carried out to measure the sensitivity of soil layers. It is revealed that reduction in the tip resistance with the increase in the number of cyclic penetration can be correlated with the sensitivity measured by the field vane tests. Finally, the possibility of estimating the in-situ effective overburden pressure (σ′_vo) is discussed for an artificial island filled by dredged clayey soil, where consolidation due to self weight of the filling is still continuing.     

Mechanical Properties of Lightweight Treated Soil Cured in Water Pressure

An artificial lightweight soil has been developed as a backfill to reduce the earth pressure behind port and harbor structures. To reduce the unit weight lightening ingredient such as air foam or EPS beads is mixed within slurry of dredged soft clay, while cement is used as stabilizer to warrant compressive strength. This experimental study aims to characterize the strength and deformation properties of lightweight treated soil cured in water pressure. Samples of two types of lightweight treated soil mixed with air foam or EPS were cured under various pressures, and subjected to undrained shearing tests on triaxial apparatus modified to detect volumetric change. Though high pressures inevitably compress lightener and consequently incur increment in unit weight, pressured curing did not reduce the compressive strength, q_max = (σ_a-σ_c)_max. It was also found that the deformation modulus E₅₀ greatly decreases with relative confining pressure σ_c/q_max. The lightweight soils maintained relatively large residual strengths, showing no significant sign of brittle failure as often confronted in unconfined compression test. It was observed that the critical state line exists when subjected to ultimate strains, and that the peak deviator stress envelop was identified in effective stress path plane for air foam mixed cases alone. K₀-consolidation tests were conducted on modified triaxial apparatus, showing that K₀ values from the quasi one dimensional tests decline to as small as 0.1 to 0.15 around axial strain of 0.5~1% at near yielding points. Poisson's ratios based on both undrained shearing and K₀-consolidation are compared in consistent tendency with minimal values of 0.1 to 0.2 near the identical yielding points. Yet it is revealed from the obtained compression curves that the compressibility increases drastically by some 100-fold when comparing before and after yielding for lightweight treated soil. This fact strikes the importance of not overloading lightweight treated soil by its compressive strength.     

国际土力学及基础工程协会简介

国际土力学及基础工程协会(ISSMFE)于1936年成立,同年在美国哈佛大学召开了第一届会议,在这之后,除特殊情况外,都是每四年召开一次国际会议。我国于1957年参加国际土力学及基础工程协会,并派茅以升、陈宗基为代表出席在英国召开的第四届国际会议。此后,二十余年中,主要由于台湾问题,一直没有再出席国际土力学及基础工程协会的会议。历届会议的概要见表1。     

Shear strength response of reinforced pond ash

Reinforced pond ash is a composite material, which can be used as an alternative construction material in the field of geotechnical engineering. To study the shear strength response of reinforced pond ash, a series of unconsolidated undrained (UU) triaxial test has been conducted on both unreinforced and reinforced pond ash. In the present investigation the effects of confining pressure (σ₃), number of geotextile layers (N), and types of geotextiles on shear strength response of pond ash are studied. The results demonstrate that normal stress at failure (σ_1f) increases with increase in confining pressure. The rate of increase of normal stress at failure (σ_1f) is maximum for three layers of reinforcement, while the corresponding percentage increase in σ_1f is around (103%), when the number of geotextile layers increases from two layers to three layers of reinforcement. With increase in confining pressure the increment in normal stress at failure, Δσ increases and attains a peak value at a certain confining pressure (threshold value) after that Δσ becomes more or less constant. The threshold value of confining pressure depends on N, dry unit weight (γ_d) of pond ash, type of geotextile, and also type of pond ash.     

饱和黏土不排水抗剪强度各向异性的热力学本构模型研究

基于饱和土体TTM（Tsinghua thermodynamic soil model）热力学本构模型分析研究了饱和黏土的不排水抗剪强度各向异性问题。模型及试验研究表明：非等向或固结历史是引起土体强度各向异性的重要原因,固结应力比越小,不排水强度各向异性越大。不排水加载过程中主应力轴的方向对土体不排水抗剪强度和变形有着重要影响。一般而言,当主应力轴方向从0°旋转到90°时,土体的不排水抗剪强度逐渐下降,峰值应变却逐渐增大。此外,非等向固结会导致土体主应力与主应变的非共轴性。利用TTM理论模型,对Kaolin Clay 和Boston Blue Clay的不同试验结果进行了模拟验证和预测。结果表明,TTM理论模型具有反映和预测应力引起的饱和黏土强度各向异性和应变软化等特性的能力,同时也具备精准描述主应力轴旋转等复杂路径下饱和黏土的强度和变形特征的能力。     

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.     

不同固结路径温州黏土固有不排水强度性状试验研究

为研究初始条件和应力状态对重塑黏土固有不排水强度性状的影响,采用应力路径三轴仪对室内制备的温州黏土进行一系列不同固结路径下的三轴固结不排水剪切试验。通过室内试验研究探讨温州黏土不同固结路径下不同平均有效应力固结后三轴不排水剪切应力应变关系和不排水强度特性;引入不同平均有效应力p’下的孔隙指数I_v与不排水强度比R^*_su=S_u/ p’,分析不同固结路径下不排水强度S_u的变化规律,并与Chandler提出的固有强度线IS_uL进行比较分析。结果表明：相同平均有效应力p’下,不同固结路径下不排水剪切强度S_u和不排水强度比R^*_su=S_u/ p’随固结路径的变化而变化;当不排水强度比S_u/ p’=0.33时,孔隙指数I_v与固结不排水强度S_u之间的关系与Chandler的固有强度线IS_uL一致。     

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。     

低应力条件下海洋粉土的不排水强度特性及其在海底浅层滑坡分析中的应用

 针对浅层滑坡等海底地质灾害,设计并实施一系列的十字板试验和三轴试验,对低应力条件下典型海洋粉土的不排水强度特性进行研究探讨。试验结果显示：固结度对粉土的十字板强度有明显影响,十字板强度随着超孔隙水压力的升高而降低。剪切应变速率对粉土的固结不排水三轴强度影响很小,而超固结比和剪切围压则影响较大;三轴试验中各个试样都能够达到临界状态,临界状态强度随着超固结比和剪切围压的增大而增大。分析波浪作用下海床土剪切滑动的渐进特性,指出临界状态是滑坡发生时滑带土所能达到的最终状态。采用平均有效应力,对十字板强度和三轴临界状态强度进行归一化处理,两种强度显示较好的一致性,得到低应力条件下海洋粉土的临界强度线。最后,应用所得的临界强度线,对风暴潮作用下黄河水下三角洲粉土海床浅层滑坡及“复活”进行很好地分析和解释。     

Shear banding in torsion shear tests on cross-anisotropic deposits of fine Nevada sand

A series of torsion shear experiments was performed on large hollow cylinder specimens of Fine Nevada sand with major principal stress directions relative to vertical, α, varying from 0° to 90° and with the intermediate principal stress, σ₂, varying from σ₃ to σ₁ as indicated by b=(σ₂–σ₃)/(σ₁–σ₃). The Fine Nevada sand was deposited by dry pluviation, thus producing a sand fabric with horizontal bedding planes and cross-anisotropic characteristics. The various stress conditions were achieved by varying the pressures inside and outside the hollow cylinder specimen relative to the shear stress and the vertical deviator stress according to a pre-calculated pattern. All stresses and all strains were determined from careful measurements so that analysis of the soil behavior could be made reliably. The soil behavior was determined for a pattern of combinations of α varying with increments of 22.5° from 0° to 90° and b varying with increments of 0.25 from 0.0 to 1.0. Thus, 25 test locations were established, but many tests were repeated to study the consistency of the results. The friction angles varied considerably with α and b, thus indicating the importance of the intermediate principal stress and the principal stress directions relative to the horizontal bedding planes. The observed shear bands essentially followed the expected directions, but due to the cross-anisotropy shear bands were also observed in the direction of the major principal stress in regions with high b-values. The strength variation was also influenced by the flexibility of the boundaries in these regions.     

饱和软黏土固结过程中的不排水抗剪强度特性

集成大直径固结仪和微型十字板剪切仪的功能,开发和研制了饱和软黏土固结过程中可以随时开展剪切试验的系统装置,考虑超孔压随时间和空间变化的不均匀性,在微型十字板剪切仪板头处的空心轴杆底端配置微型孔压计,并在大直径固结仪中配置微型土压力计,使其具备自动实时监测在十字板剪切试验测点处有效应力变化的功能。利用该系统装置,开展了饱和软黏土在不同固结压力作用下,固结过程中不同时点的十字板剪切试验,实时监测了固结过程中的变形和孔压变化过程,得到了十字板剪切试验测点处的有效应力和不排水抗剪强度,分析了固结过程中不排水抗剪强度和有效应力之间的相关关系。结果表明,在不同固结压力作用下,固结完成后的不排水抗剪强度与有效应力呈现出传统的线性关系,但是,在某一固结压力作用下,固结过程中的不排水抗剪强度却随有效应力的增长呈非线性增长,而且,在不同固结压力作用下,固结压力越大,固结过程中达到相同的有效应力时所对应的不排水抗剪强度越大。固结过程中的不排水抗剪强度并不仅仅取决于剪前固结有效应力,还与剪前孔隙比相关,孔压消散速率小于变形速率是导致固结初期、剪前固结有效应力较小时,不排水抗剪强度较快增长的主要原因。