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.     

超软土不排水强度研究综述

超软土在我国沿海地区广泛分布,了解超软土的不排水强度特性是诸多沿海工程中需要解决的关键问题。通过回顾现有的研究,从测试方法、影响因素、归一化方法等几个方面总结了前人关于超软土不排水强度的研究成果。同时指出,今后需要进一步探讨自重固结对超软土不排水强度的影响规律。     

中等压缩性粉质黏土不排水剪切强度试验研究

为获得原状中等压缩性粉质黏土的剪切刚度和抗剪强度,开展了直剪固结快剪和固结不排水三轴剪切(CU)对比试验,分析了偏应力随整体、局部轴向应变的发展,研究了整体、局部变形测量获得抗剪强度的差异,探讨了刚度衰减特征,提出了固结快剪和CU试验抗剪强度之间的关系。结果表明:变形测量方式对粘聚力c′影响显著,与整体相比,局部粘聚力c′提高2.9~11.1 kPa,局部内摩擦角φ′低0.4°~1.7°。剪切刚度E_sec随轴向应变ε₁增加呈反“S”形衰减。应变ε₁＜0.005%时,局部测量刚度E_sec高于整体,其衰减幅度显著,但随轴向应变增加,二者刚度差值缩小。直剪试验与CU试验抗剪强度差值随着竖向应力的增加而增大,CU试验抗剪强度明显高于直剪,建立了抗剪强度参数的修正关系。     

扁铲侧胀试验求解土的不排水抗剪强度

不排水抗剪强度是反映软土特性的一个重要指标 ,扁铲侧胀试验 (DMT)的过程能够充分模拟软土的不排水条件 ,是获取土的不排水抗剪强度参数的一种有效方法。本文在分析国内外研究现状的基础上 ,通过与实际工程中相邻孔十字板剪切试验所得Su 值进行对比、修正 ,提出了适用于上海地区的扁铲侧胀试验计算不排水抗剪强度的实用公式 ,并在实际工程中得到验证。     

利用标准贯人试验确定粘性土的不排水抗剪强度

粘性土的不排水抗剪强度是粘性土的重要力学指标,但目前确定该指标的方法存在着取样扰动或测试深度有限的缺点.通过对马来西亚槟州第二跨海大桥的试验结果研究,综合考虑了SPT的修正方式,不排水抗剪强度的确定方法,粘性土的塑性指数等因素的影响,建立了该地区土层的Cu和SPT之间的经验关系,可为该地区或其它地区的类似工程提供参考.     

Evaluation of Undrained Shear Strength of Soils from Field Penetration Tests

The evaluation of undrained shear strength of soils is necessary in determining the possibility of occurrence of flow deformation during earthquakes. The present study is aimed at examining the evaluation of undrained shear strength of silty sands from field with Swedish weight sounding tests and cone penetration tests. Based on the outcome of the previous studies on laboratory triaxial tests, the undrained shear strength ratio is defined as the undrained shear strength divided by the initial effective major principal stress. The undrained shear strength ratio is then formulated with respect to the relative density. The penetration resistances of Swedish weight sounding and cone penetration tests are then formulated with respect to the effective overburden stress and relative density, based on laboratory calibration chamber tests. By combining these formulations, the correlations of the undrained shear strength with Swedish penetration resistance and cone tip resistance are established. The range of values of penetration resistances indicative of soil layers susceptible to flow deformation is discussed. The correlations of the undrained shear strength with field penetration resistances thus derived are then examined from case history studies. Two case history studies are carried out with Swedish weight sounding tests at the sites of flow failures induced during the recent earthquakes. A series of case history studies are reexamined, which were carried out with Dutch cone penetration tests in the past studies.     

An Improved Method for Estimating In-Situ Undrained Shear Strength of Natural Deposits

An equation for estimating in-situ undrained shear strength (q_u(I)) of natural deposits is derived as q_u(I)/2c_u(I)=1.0-0.285 ln p_m/S₀ through the unconfined compression test (UCT) and K₀ consolidated-undrained triaxial compression test (CK₀UC). The q_u(I) of natural clay deposits can be estimated from the q_u value multiplied by the reciprocal number of q_u(I)/2c_u(I) of the equation using the suction (S₀) and q_u obtained from UCT for a specimen, where c_u(I) is in-situ shear strength measured from CK₀UC and p_m is two times the effective overburden pressure divided by three. The q_u(I)/2c_u(I) values were unrelated to I_p, q_u and p_m/S₀ and the mean value of these ratios was 0.98 in the range of I_p=26~110 and q_u=12~178 kPa. The mean values of the ratios for q_u, q*_u(I) and q_u(I) to 2c_u(I) were 0.629, 0.998 and 0.977 and the standard deviation of those ratios were 0.14, 0.10 and 0.16, respectively. Therefore, it can be seen that the improved method is appropriate as well as Shogaki's basic method (q*_u(I)). The mean values of q_u(I)/2c_u(I) were 0.94, 0.99 and 0.91 for Iwai organic and soft clay plus Kahokugata clay, respectively. The coefficient of variations of the q_u and q_u(I) values were (13~14)% and unrelated to soils, q_u or q_u(I) values. Therefore, the applicability of the improved method newly developed in this study can be confirmed for Kahokugata and Iwai clays as well as Iwai organic soils. The proposed method is a simple and easy one for practical engineering usage.     

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.     

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。     

不同试验条件下吹填软土不排水抗剪强度对比分析

饱和软粘土不排水强度在实际工程中应用很多,不同试验方法测得的不排水强度不同。本文基于无侧限抗压强度试验和微型十字板剪切试验,对天津滨海吹填软土从取样深度变化、初始各向异性、不同加荷速率三方面进行了对比试验。试验结果表明:两种不同试验强度随取样深度的增加而增大且呈不同程度线性关联性,吹填软土受荷后应变模式、剪破面的方向、破坏标准的不同是导致试验强度差异的主要原因;初始各向异性对两种不同试验条件下的强度值有不同的规律影响,无侧限抗压强度试验强度在竖直制样方向最大,水平方向次之,斜方向最小,微型十字板剪切试验强度在水平制样方向最大,斜方向次之,竖直方向最小;加荷速率对两种不同试验条件下的强度值有相同的规律影响,两种试验强度都随着加荷速率的增大而增加。利用统计回归原理,拟合得到了原状吹填软土在两种室内试验条件下强度之间的关联表达式,便于实际工程参考。     

基坑中基于CPTU软土不排水强度确定及应用

不排水抗剪强度是土的重要力学特性参数。在岩土工程施工期稳定性分析中,需用不排水抗剪强度值。目前,不排水抗剪强度的确定主要有室内试验和十字板剪切试验。但前者受土样扰动影响大,后者十字板强度是土体各滑动面上抗剪强度的较小值,在基坑工程验算坑底隆起时,结果不够准确。本文首先总结了国内外基于CPTU的Su确定方法,在典型软土基坑工程中进行了孔压静力触探试验,根据不排水试验值与锥尖阻力的关系,反演出圆锥系数的值。基于反演的圆锥系数,把CPTU预测的不排水抗剪强度应用到基坑的隆起分析中,与基于室内试验数据和基于十字板强度的计算结果进行了比较,结果表明:基于CPTU的软粘土不排水抗剪强度指标计算结果合理,符合工程实际,值得在工程设计中推广。     

分级加荷条件下正常固结软土的不排水强度确定

本文提出了分级加荷条件下正常固结软土不排水强度确定的试验法和理论法，指出了它们各自存在的问题和解决办法。     

不同应力下剪切速率对尾矿砂强度特性影响研究

尾矿库的运行稳定性很大程度上取决于尾矿砂的强度特性。为了分析应力及剪切速率对尾矿砂抗剪强度的影响,利用室内三轴仪对云南某铁矿尾矿库内原尾矿砂进行了不同围压及剪切速率条件下的固结不排水(CU)试验,并对试验后的颗粒破碎进行分析。试验结果表明:尾矿砂的抗剪强度在低应力下(小于500 k Pa)基本符合线性摩尔库伦准则,但在高应力下(大于500 k Pa)符合幂函数强度准则。尾矿砂的抗剪强度及内摩擦角在各围压下均随着剪切速率的增加而减小,其减小速率在剪切速率0.08~2 mm/min范围内先增加后减小。试验尾矿砂在各围压下剪切后颗粒均有破碎,且在高应力下固结过程也会造成颗粒破碎,但相对整过剪切过程中的破碎量比值很小。颗粒破碎主要集中在构成骨架结构的粗颗粒段(粒径大于0.25 mm),粒径小于0.074 mm的细颗粒基本不破碎,且颗粒破碎与剪切速率成负相关,但颗粒破碎在围压低于400 k Pa时很少受剪切速率的影响。     

含水量变化对原状土剪切强度的影响

工程实践中,道路的边坡、路基、土石坝。建筑物的地基等丧失稳定性的事故很多一般发生的破坏都是剪切破坏。含水量是土体的一项重要指标,不同含水量的土具有不同的抗剪强度,以原状土为实验对象,在不破坏原状土的情况下改变其含水量,在不同含水量下做直剪实验,研究原状土含水量对剪切强度的影响。     

碎石含量对堆积岩土体抗剪强度的影响

通过对现场取得的堆积岩土及人工制作的不同碎石含量的岩土体分别进行大型直剪试验,结合试验数据及土体抗剪强度计算公式,得出了不同碎石含量下岩土体的抗剪强度变化规律。研究表明:碎石土的粘聚力随着碎石含量的增加呈现先递增后递减的趋势,内摩擦角随着碎石土含量的增加而不断地增大,但呈现增速减缓的趋势。     

孔隙水压力对饱和砂的剪切模量和阻尼比的影响

本文通过动三轴进行饱和砂的液化试验,考察三种砂的剪切模量和阻尼比随孔隙水压力变化的规律。由试验可知:①饱和砂的剪切模量与阻尼比随孔隙水压力的增加而降低;②在一般情况下,剪切模量、阻尼比与孔隙水压力及剪应变幅之间的关系可分别表示为可分离变量的函数:G（u,γ）=f（u）·Gm（γ）及λ（u,γ）=g（u）·λ_m（γ）,对试验所用的三种砂已给出表达式;③在接近液化时,橡皮膜对试验结果有较大的影响,应进行修正。     

含水率和干密度对残积土抗剪强度参数的影响

在分析闽东南地区花岗岩残积土物理力学特性的基础上,对3类非饱和残积土的含水率(ω)和干密度(ρ_d)与抗剪强度指标(c、φ)进行相关分析,建立了针对闽东南地区花岗岩残积土含水率和干密度与抗剪强度指标之间的经验公式。研究结果表明:粘聚力的对数与含水率呈负相关,随着含水率的增大,残积土黏聚力总体呈减小趋势;内摩擦角在含水率增大过程中具有非线性衰减的趋势,且集中在一个范围内变化;3类残积土在高密实度下的ω~φ具有峰值点,土体密实度对粘聚力的影响可用线性正相关来反应;一般情况下,干密度值越大,残积土粘聚力值和内摩擦角也越大。利用建立的经验公式,可为花岗岩残积土边坡的勘察、设计和施工过程中土性参数的合理选取及可靠性分析提供参考。     

初始干密度及掺砂比对膨胀土抗剪强度指标影响

研究了风化砂改良膨胀土对抗剪强度及其指标的影响。通过改变风化砂的掺量,研究了风化砂对膨胀土物理性质的影响。试验表明,随着掺砂比例的增加,最佳含水率逐渐下降,渗透系数逐渐增大,说明风化砂能有效降低膨胀土的塑性指数,增大膨胀土的透水性能,使之更适合用作公路路基填料。通过研究风化砂掺入比例及初始干密度对膨胀土抗剪强度指标c、φ值及抗剪强度的影响,可以得出:在掺砂比例一定时,改良膨胀土的粘聚力随着初始干密度的增大而增大,内摩擦角随着初始干密度的增大先增大后减小再增大,抗剪强度值总体逐渐增大;在初始干密度一定时,改良膨胀土的粘聚力随着掺砂比例的增大而减小,内摩擦角随着掺砂比例的增大先增大后减小,抗剪强度总体变化趋势减小。当初始干密度为2.0 g/cm³,掺砂比例为30%时,抗剪强度达到最大值。     

孔结构对浮石混凝土抗压强度影响的试验研究

试验测得玄武岩纤维和橡胶浮石混凝土的含气量、比表面积、气孔平均弦长与孔径分布等参数,运用灰色理论建立各参数与混凝土的28 d抗压强度之间的联系,探究孔结构参数对浮石混凝土抗压强度的影响,试验表明:纤维掺量增加时,含气量对混凝土抗压强度影响最大,气孔平均弦长影响最小;孔径为200~300μm的孔对混凝土凝土的强度影响最大,而>400μm的孔影响最小。橡胶掺量增加时,气孔间距系数对混凝土抗压强度影响最大,比表面积影响最小;孔径为100~200μm的孔对混凝土抗压强度影响最大,而<100μm的孔影响最小。