Sample Disturbance Effects on Undrained Shear Strengths—Study from Takuhoku Site,Sapporo—

The effects of sample disturbance on undrained shear strength were investigated from samples with various qualities, retrieved by different types of samplers at the Takuhoku site, Sapporo. Sample quality was evaluated by three types of shear tests: unconfined compression, fall cone and triaxial recompression tests. Similar to a previous study conducted by Horng et al. (2010), in which the sample quality was evaluated by two nondestructive tests, the residual effective stress (suction) by ceramic disc and the shear modulus by bender element under unconfined conditions, the present study shows that the small edge angle of a tube sampler is important to obtain high quality sample. In addition, the existence of a piston does not have a significant effect on the strength properties. The recompression technique in the triaxial test, where the specimen is consolidated back to the in situ stresses, was able to duplicate undisturbed soil behavior except when the structure of a soil sample was significantly destroyed. The unconfined compressive strength is apparently governed by the residual effective stress. From the two types of tests simulating sampling processes, however, it was found that a reduction in the unconfined compressive strengths of low quality sample was brought by the loss of the residual effective stress as well as destruction of soil structures.     

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.     

Characterization of a Cohesive Soil Bed using a Cone Pressuremeter

A new cone pressuremeter has been developed at the University of Engineering and Technology, Lahore-Pakistan. The new device is called the Akbar Pressuremeter (APMT). This paper is based on the APMT testing of a cohesive soil deposit comprised of low plastic lean clay (CL) to sandy silty clay (CL-ML). The APMT testing, using a full-displacement technique, standard penetration tests and undisturbed samples, was carried out at two locations. The soil strength and type were determined using the undisturbed samples. The applied pressure-cavity strain curves of the APMT tests performed at different levels were analysed to determine soil strength and stiffness. The undrained shear strength of the undisturbed samples was determined in the laboratory by unconfined compression tests. This paper provides a comparison of the parameters interpreted from the pressuremeter and those determined from other field and laboratory methods.     

灵敏性粉土的压缩及触变特性研究

山西汾河一级阶地的饱和粉土大多具有灵敏性,使得其强度和变形性质在扰动前后差别很大。为了研究灵敏性粉土的压缩及触变特性,对探井取样的原状土和重塑土样进行了一维固结压缩试验和无侧限抗压强度试验。试验结果表明,原状土加载到结构屈服应力处,沉降量突增,固结系数突增,主固结比突降,次固结对沉降的贡献开始上升;原状土与重塑土的差异是原状土的结构强度引起的,在高压固结压力下两者压缩曲线约在0.58e₀处重合;灵敏土无侧限抗压强度试验的应力应变曲线表现出类似超固结土的应变软化特性,其触变后恢复具有时效性,弹性模量受扰动影响较为敏感,可考虑作为扰动指标的量度。     

秦皇岛滨海饱和软土的试验研究

为保持饱和软土的天然结构特征，野外采用开挖探井，环刀取土的方法获得了大量Ⅰ级土试样，并在室内进行有关饱和软土的物理力学性质试验。结果表明：秦皇岛滨海饱和软土的天然含水量介于37％～44％，土性属淤泥质粉质粘土。基于浅层平板载荷试验、静力触探试验以及十字板剪切试验，对饱和软土的天然抗剪强度、灵敏度、承载力进行了试验研究。建立了十字板抗剪强度和试验深度、静力触探比贯入阻力的经验公式，为工程应用提供了便利条件。秦皇岛滨海饱和软土的天然不排水抗剪强度平均值为27kPa，灵敏度为2．4～12．5，具有中高灵敏性。其地基强度的使用应综合考虑建筑物的加载速率、地基容许变形等多种因素。     

Static liquefaction-triggering analysis considering soil dilatancy

The failure of a sloping ground due to static liquefaction occurs when the shear stress applied by a monotonic triggering load exceeds the undrained yield (peak) shear strength of the saturated liquefiable cohesionless soil. Current practices for determining the in-situ undrained yield strength for grounds subjected to static shear stress rely on either a suite of costly laboratory tests on undisturbed field samples or empirical correlations based on in-situ penetration tests, which fail to account for the effect of soil dilatancy in decreasing the degree of strain-softening and the brittleness of cohesionless soils with an increasing penetration resistance. In this study, the effect of soil dilatancy on the static liquefaction failure of cohesionless soils is characterized by an empirical relationship between the soil brittleness index and the undrained yield strength from a database of 813 laboratory shear tests collected from the past literature. The application of this relationship for estimating the static liquefaction-triggering strength of cohesionless soils under sloping ground conditions is validated by comparing several cases of liquefaction flow failures. Finally, a procedure is briefly demonstrated for evaluating the triggering of static liquefaction in a dyke to the north of Wachusett Dam and Duncan Dam which incorporates the dilatancy behavior of cohesionless soils in a semi-empirical procedure based on in-situ penetration tests.     

原状土的无侧限抗压强度试验研究

针对连云港地区滨海相软土的工程特性，通过大量的试验，分析了原状土的无侧限抗压强度的主要影响因素，以确保提高无侧限抗压强度试验的精度。     

灌溉诱发黄土滑坡机理研究

基于斜坡土体的原位应力状态及现场应力路径,利用GDS三轴测试系统,对原状黄土开展了等压/偏压固结不排水剪(ICU/ACU)和常剪应力排水剪(CQD)试验,研究了饱和黄土的应力应变特性,得到了其稳态线和状态边界面,在此基础上探讨了灌溉诱发黄土滑坡的形成机理。     

A Study on the Engineering Properties of Sand Improved by the Sand Compaction Pile Method

In order to directly evaluate the effects of soil improvement by the Sand Compaction Pile (SCP) method on the density, deformation, and static and liquefaction strength characteristics of sandy soils, a series of field and laboratory tests were performed. Laboratory tests were performed on high-quality undisturbed samples obtained from sandy soils both before and after soil improvement by the SCP method. The high-quality undisturbed samples were recovered by the in-situ freezing sampling method. The drained shear strength (internal friction angle, φ_d), liquefaction strength (R₁₅: cyclic stress ratio needed to cause 5% double amplitude axial strain in 15 cycles), and cyclic deformation characteristics (G~γ and h~γ relations) were determined by performing a series of laboratory tests on the undisturbed samples. Both the in-situ density and the relative density were measured on the undisturbed samples used in the laboratory tests. A standard penetration test (SPT) and a suspension-type P-S wave logging test were performed to investigate the soil profile of the test site before and after the sand compaction. Both the static and the liquefaction strengths of the sandy soils obtained in the laboratory tests were also compared with those estimated by empirical correlations used in practice based on the SPT N-value and soil gradations.     

天然沉积结构性黏土的不排水强度性状

通过对福州天然沉积黏土原状样及不同初始含水率重塑样进行三轴固结不排水剪切试验,探讨土结构性对天然沉积黏土强度性状的影响规律及作用机理。研究结果表明：结构性对天然沉积土固结不排水抗剪强度的作用程度受固结压力大小影响可以分为3个阶段：固结压力小于屈服压力阶段,不排水抗剪强度主要由土结构性控制,与应力水平基本无关;土结构逐渐屈服阶段,固结压力大于屈服压力,不排水抗剪强度随固结有效应力的增大逐渐趋于重塑样的强度线;土结构性影响消失阶段,原状样的不排水抗剪强度随固结有效应力的增长规律与重塑样相同。     

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.     

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。     

弱胶结结构性软黏土力学特性的试验研究

对取自不同地区的两种弱胶结结构性软黏土原状(undisturbed)样、重塑(remolded)样和泥浆(reconstituted)样进行了单向压缩和三轴剪切试验,分别得到土样的压缩曲线和应力–应变曲线。试验结果表明:原状样的压缩曲线为陡降型曲线,而不同制样土样的压缩曲线存在明显的差异;由于孔隙比和孔径分布对土体抗剪强度的综合影响,不仅导致相同围压下三轴剪切时孔隙比不同的重塑样和原状样强度差异较大,且孔隙比相近的不同土样的强度也存在不同程度的差异;若同一孔隙比下,两种软黏土的不同制样土样的强度关系均为原状样的强度最高,重塑样的强度最低,并可通过相近孔隙比下孔径大于0.2μm的孔隙体积量和孔径分布均匀性可合理地解释3种制样土样强度高低的关系。由于不同制样土样的孔径分布的差异不会随固结压力的增大而消失,用参考孔隙比e_(10)~*,简单表示土的孔隙比和孔径分布(即组构)参数,对压缩和剪切试验结果进行归一化整理后,发现不同土样的试验结果可归一化为相关度高的e/e_(10)~*-σv曲线和e_f/e_(10)~*-qf曲线,证明结构屈服应力后,不同土样变形和强度差异主要是由孔隙比及孔径分布(即组构)的不同引起的,用参考孔隙比e_(10)~*简单表示土的组构参数是有效的。     

天津滨海软土不同应力路径下抗剪强度对比试验研究

现有基坑支护设计参数试验条件与实际不符。选取天津滨海新区海相软土为研究对象,进行三轴固结不排水试验、三轴不固结不排水试验和减p路径试验并对比分析了几种试验条件下土的强度参数。得到:滨海软土在不同应力路径下的强度与初期固结方式有关。减p路径对总应力状态下抗剪强度有影响,但对有效应力状态下抗剪强度基本无影响。在滨海软土基坑支护设计中,选用CU试验参数进行计算,设计偏于安全。     

结构性软土力学特性的试验研究

对扰动程度不同的原状样和重塑样分别进行单向固结试验和固结排水及不排水三轴剪切试验,研究结构性对上海软土的变形、强度特性的影响。压缩试验结果表明:结构性强的原状样具有明显的结构屈服应力,结构性一般的原状样,结构屈服应力不明显;原状样比重塑样具有更大的压缩指数Cc和膨胀指数Cs。剪切试验结果表明:相同固结压力下,结构性强的土样强度不仅低于结构性弱的土样强度,而且低于重塑样的强度。这是由剪切时土样的孔隙比差异造成的。若消除孔隙比的影响,则结构性将使原状样具有更高的强度,且结构性越强,土体强度越高。     

Effect of Specimen Size on Unconfined Compressive Strength Properties of Natural Deposits

In order to use the advantages of unconfined compression tests, a new testing procedure using S (or Small size) specimens (15 mm in diameter and 35 mm in height) is proposed and a new portable unconfined compression test apparatus with suction measurement is outlined. The effect of specimen size on unconfined compressive strength properties of natural deposits is discussed from laboratory tests. The standard deviations of the ratios of q_u and E₅₀ values of the S specimens to O (or Ordinary size) specimens (35 mm d and 80 mm h) were in the range of 0.09 to 0.16. The 10% variation from the mean value reflects the homogeneity of soils since the coefficient of variations of the undrained shear strength for the undisturbed and reconstituted soils were 8% to 17% (Matsuo and Shogaki, 1988). In an engineering sense, there was no difference in shear strength and deformation characteristics between the S and O specimens for soils having plasticity indexes ranging from 10 to 370 and unconfined compressive strengths of 18 kPa to 1000 kPa, that were taken from 26 different sites in the United Kingdom, Korea and Japan. These soils consisted of Holocene and Pleistocene clays plus diatomaceous mudstone and highly organic soils.     

含水量变化对红粘土变形和强度特性的影响研究

对击实后的红粘土土样在饱和前、饱和后进行了三轴剪切试验和无侧限抗压强度试验,研究其抗剪强度特征的变化规律。针对原状土样和重塑土样在脱湿或加湿过程中无侧限抗压强度的变化进行了研究。研究表明,在脱湿过程中,土体的抗剪强度逐渐增大,而且原状土的应力应变关系曲线一般呈应变软化的特点,而重塑土的应力应变关系曲线则一般呈现硬化的特点;在加湿过程中,原状土无侧限抗压强度峰值并不明显。分析表明,脱湿过程或加湿过程中土样强度随含水率的变化是显著的。     

软土循环扭剪应力应变响应的弹塑性分析

为了分析笔者提出的增量弹塑性模型对描述一般应力状态土单元循环应力应变响应的适用性,进行了软土不排水循环三轴压缩试验与无初始轴向偏应力的循环扭剪试验,得到了两种应力状态土单元的循环应力应变响应。进而依据循环三轴压缩试验结果确定模型参数,再利用模型预测软黏土循环扭剪试验土单元的应力应变响应,并与试验结果进行对比分析。结果表明,预测出的无轴向静偏应力作用的循环扭剪土单元环向剪应力应变滞回曲线与试验结果相比偏小,但是总变化趋势与试验结果较为一致;预测出的循环扭剪土单元环向累积剪应变随循环次数的变化与试验结果基本一致。研究工作表明,该增量弹塑性模型能够较好描述不同应力状态土单元的不排水循环应力应变响应,特别是循环累积应变响应。     

STRENGTH AND CONSOLIDATION PROPERTIES OF BUSAN NEW PORT CLAYS

The sedimentary environment and the effects of sample disturbance on strength and consolidation properties of Busan New Port clays are examined through microfossils and radiocarbon age analyses and unconfined compression, K₀-consolidated undrained triaxial compression and extension tests and consolidation tests. In this study, only one or two samples, 74 mm in diameter and 100 mm in height, obtained from different depths, are used for the whole series of tests to provide small-sized specimens. The sedimentation rates of Busan New Port clays were (3.7-7.8) mm/year and higher than those for the coastal areas of the USA, Thailand and Japan. The in-situ undrained shear strength and consolidation parameters were estimated using Shogaki's method and compared with those of other test results and evaluated. Busan New Port clays are lightly overconsolidated clays. It can be seen that the consolidation settlements, which were greater than those estimated, were observed in Holocene Busan clay, are caused by the underestimation of the compression index and coefficient of consolidation values caused by sample disturbance.