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.     

初始静孔隙水压力对砂土静动力剪切特性影响的试验研究

选取福建标准砂和滹沱河细砂,利用空心圆柱扭剪仪开展了一系列不同初始静孔隙水压力条件下的不排水循环扭剪试验和单调扭剪试验,着重探讨初始静孔隙水压力对超静孔隙水压力发展及其不排水抗剪强度的影响。试验结果表明:初始静孔隙水压力对超静孔隙水压力的发展产生显著的影响,从而影响砂土的静动力剪切特性。具体地,在不排水循环剪切过程中,初始静孔隙水压力越大,其超静孔隙水压力发展和变形发展越快;在不排水单调剪切过程中,初始静孔隙水压力越大,在砂土剪胀阶段产生负超静孔隙水压力越大,从而使砂土的强度显著提高。基于试验结果,初步探讨了初始静孔隙水压力对超静孔隙水压力及静动力剪切特性的影响机理。研究表明,研究地下水位以下土体(准饱和土)静动力剪切特性尤其是研究液化问题时,应充分考虑初始静孔隙水压力对砂土抗液化强度的影响,室内试验应根据砂土所处的地下水位深度来决定初始静孔隙水压力(反压)的大小。     

In-Situ Evaluation of Strength and Dilatancy of Sands Based on CPT Results

In-situ tests have been increasingly used to estimate the shear strength of soils. In this paper, we propose methods to evaluate in-situ strength and dilatancy of sandy soils based on cone penetration test (CPT) results. It takes into account the silt content, relative density and stress state of the sand. A series of laboratory test results from fundamental property tests and triaxial tests are analyzed to develop methods for in-situ evaluation of strength and dilatancy for sands. Based on test results, modified and simplified dilatancy equations, in terms of the cone penetration resistance q_c and intrinsic soil variables, are proposed. Results from proposed and original dilatancy indexes show close agreements for various soil conditions. Values of intrinsic variables for the proposed dilatancy relationships were proposed as a function of silt content. Based on TX test results, a direct CPT-based correlation, applicable to both clean and silty sands, is proposed as well. In order to verify the proposed methods, calibration chamber CPT results obtained in this study and collected from the literature are adopted. It is observed that the results from the proposed methods show good agreement with the measured results.     

Applicability of CPT for Construction Control of Seawall on Soft Clay Improved by Sand Drain Method

A case study is presented of an application of the cone penetration test for construction control of a seawall construction on soft ground improved by sand drain. Effective construction control of such a soil structure requires a systematic way to collect accurate information about the progress of consolidation and gain in soil strength. An empirical correlation is proposed concerning cone resistance measured by a cone penetration test and the undrained shear strength of the ground. The relationship between cone resistance and the undrained shear strength is not affected by the state of consolidation of the ground. Based on the empirical correlation found in this study, gain in undrained shear strength during construction can be accurately evaluated by the cone penetration test. This paper also demonstrates the usefulness of cone penetration test to monitor the degree of consolidation of the improved ground. Also estimated by the cone penetration test are aging effect, classification of clay layer and strength increment ratio in normally consolidated state. Aspects geotechnical behavior of the ground are discussed such as the process of strength increment, aging effect, sequence of various shear tests to examine change in strength and considerations on shear strength of improved ground after deformation. The results of this study clearly show that the cone penetration test is a very useful investigation method and has high applicability for construction control.     

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.     

Static liquefaction and effective stress path response of Kutch soils

The current research is focused on the static liquefaction and effective stress path characteristics of soils of the high seismicity Kutch region, India. In previous studies, the effects of the fines content and the plasticity of the fines on the undrained behavior were explored in a controlled and systematic manner with either non-plastic or plastic fines added to standard or river sands. The undrained characteristics of natural soil deposits having the simultaneous presence of both silt and clay fractions have not been explored. In the present study, the effects of a varying fines content and the nature of the fines on the static liquefaction and effective stress path characteristics of various soil deposits in the Kutch region at their in-situ densities have been studied. Various liquefaction indices were determined to quantify the static liquefaction, namely, the undrained brittleness index, the collapse potential, the liquefaction potential, and the resistance to further deformation. The variation in these parameters was analyzed in the context of the fines content, silt content, clay content, and plasticity. The effective stress paths of the Kutch soils exhibited a strain softening, limited strain softening or strain hardening response and their behavior was controlled by the fines content and its nature. Due to the presence of non-plastic fines, and irrespective of the content, the silty sands exhibited intense strain softening which was captured by the liquefaction indices.     

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。     

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.     

细粒土不均匀分布对粗粒土力学特性的影响

为了研究冻融循环条件下细粒土的不均匀分布特性对粗粒土力学特性的影响,对不同冻融循环次数、冻结温度、围压条件下的含有不同细粒土分布的粗粒土进行常规的静三轴剪切试验,研究冻融循环后具有不均匀细粒土分布的试样的最大剪胀位置、应力–应变关系曲线、起始屈服强度、起始屈服应变、静强度、破坏应变变化规律。研究结果表明,在相同的细颗粒与粗颗粒不均匀分布的条件下,试验后试样的最大剪胀位置随着试样上两层中细粒土含量的增加而沿着试样高度上升,随着冻融循环次数的增加,最大剪胀位置的上升速率是不断增加的。细颗粒与粗颗粒不均匀分布的程度越高,尤其是粗颗粒的富集程度越高,试样的脆性越高,试样的应变软化程度越明显。冻融循环次数的增加,降低了粗、细颗粒较均匀分布的3种试样的应力–应变软化程度的差异性,即冻融循环次数的增加可以使粗颗粒和细颗粒的组合结构趋于稳定。在相同的围压条件下,冻融循环次数的增加削弱了因细粒土不均匀分布而导致的起始屈服强度的差异性和静强度的差异性。试样中的粗颗粒和细颗粒分布越不均匀,试样的起始屈服强度和静强度越大。当围压为100 k Pa时,静强度与起始屈服强度的比值随冻融循环次数波动较大,当围压增加到300 k Pa时,比值随冻融循环次数的波动较为平缓。细粒土不均匀分布是起始屈服应变和破坏应变的最显著性影响因素,但是围压对破坏应变的影响程度明显大于围压对起始屈服应变的影响程度。细粒土不均匀分布对破坏应变影响的显著性明显小于它对起始屈服应变影响的显著性。     

Aging Effects on Small Strain Shear Moduli and Liquefaction Properties of in-situ Frozen and Reconstituted Sandy Soils

In order to investigate the effects of different geological ages on liquefaction properties of sandy deposits, a series of undrained cyclic triaxial tests was performed on three kinds of in-situ frozen and their reconstituted samples which were retrieved from Holocene (Tone-river sand) and Pleistocene (Edo-river B and C sands) deposits. The specimens were subjected to isotropic consolidation at a specified confining stress which is equivalent to the in-situ overburden stress at the depth of sampling, and small strain shear moduli were measured before and during the undrained cyclic loading tests. The liquefaction properties and the small strain shear moduli were affected by not only the natural aging effect of the specimen but also the inter-locking effect that was enhanced by applying drained cyclic loading before the undrained cyclic loading tests. During liquefaction, different tendencies of degradation in the small strain shear moduli which would reflect the aging effects of the specimen were observed between Tone-river Holocene sand and Edo-river B and C Pleistocene sands. The applicability of reconstituted samples as substitutes for in-situ frozen samples was confirmed with Tone-river Holocene sand that has no cementation effect between soil particles, whereas it seems difficult to simulate fully the liquefaction behaviour of Edo-river B and C Pleistocene sands which have higher cementation effect.     

基于固结试验的膨胀土地基变形预测方法

由广西膨胀土的固结试验,确定了广西膨胀土的体积变形指标。对依托工程膨胀土进行了现场静力触探试验,根据比贯入阻力随深度的变化曲线特征,确定了膨胀土活动区深度。给出了膨胀土裂隙开展深度的理论解,膨胀土裂隙开展深度的计算值与静力触探试验确定的膨胀土活动区深度和现场观测的裂隙开展深度基本一致。最后介绍了基于固结试验基础上的膨胀土地基变形计算方法,并在试验基础上给出了膨胀土地基变形的计算结果。以膨胀土地基变形量作为膨胀土地基的分类指标,对膨胀土地基进行了分类。分类结果对膨胀土基础选型具有重要意义。     

Effects of Sample Disturbance on Small Strain Characteristics and Liquefaction Properties of Holocene and Pleistocene Sandy Soils

To investigate liquefaction properties of sandy soils, undrained cyclic loading tests are usually performed. However, it would be difficult to simulate fully the actual soil behaviour through the laboratory tests because the tested sample can be disturbed even though it is taken by in-situ freezing technique. In this study, by using three kinds of in-situ frozen sandy soils which were taken from Holocene and Pleistocene deposits and their reconstituted samples, their volume change properties were measured during freeze and thaw processes at different confining pressures of 30 kPa and 98 kPa. In order to investigate the effects of the possible sample disturbance on the liquefaction resistance, small strain characteristics were measured as well, which would reflect the soil structure. Decreases in the small strain characteristics and the liquefaction resistance were observed in case of the Holocene specimens that were thawed at the confining pressure that was lower than the in-situ overburden stress. On the other hand, in case of the Pleistocene specimens, the effects of the confining pressure during the thaw process on the small strain characteristics and the liquefaction resistance were small. Such contrastive feature between the Holocene and the Pleistocene samples could be linked with the difference in the types of their natural aging effects.     

考虑胶结作用的木质素固化粉土边界面塑性模型

为研究木质素固化粉土的应力–应变特性,通过无侧限抗压强度试验和微观结构分析,探讨木质素固化土的胶结特性。基于边界面塑性理论,引入硬化参数、应力剪胀参数和胶结破坏速率等参数,提出考虑胶结作用的木质素固化土边界面塑性模型,采用非相关联流动法则和改进映射法则描述土体的不同破坏模式,并阐述模型中各参数的意义及计算方法。根据室内固结试验和三轴压缩试验,对木质素固化粉土的应力–应变、应力剪胀和超孔隙水压力变化特征进行分析,并验证了本文所提模型的有效性。研究表明:木质素产生的胶结作用是土体工程性质改善的主要原因之一;12%掺量木质素固化土屈服应力和不排水抗剪强度较素土分别提高约90%和40%,高、低围压下土体应力剪胀特性不同,围压对超孔隙水压力的变化影响较大;通过试验验证了模型计算的准确性,该模型可描述土体在不同受力状态下的应变特征,具有原理简单,参数明确的特点,可为固化土应力–应变的数值计算提供相应的理论基础。     

Analytical investigation of disturbance of seabed-sampled soil specimens and its influence on unconfined strength

In view of the need to utilize ocean space and to develop seabed resources, the assessment of the stability of deep seabed soil has emerged as an important challenge in the field of geomechanics. To study seabed stability, the strength and stiffness of the natural ground must be ascertained. Accordingly, it is necessary either to conduct laboratory testing on soil specimens sampled from the seabed or to estimate the strength and stiffness by in-situ tests. While in the future it may be reasonable to conduct in-situ tests to estimate the stiffness and strength of seabed soil, it will still be necessary to compare the physical properties measured by in-situ testing with those measured by laboratory testing in advance of these determinations. In short, soil specimens must be sampled from the actual deep seabed, and laboratory mechanical tests must be conducted on the sampled soil specimens. However, soil sampled from the ocean bottom is subject to effects that differ from those exerted on soil sampled from the earth. More specifically, the non-negligible effects of disturbance are expected with soil sampled from the ocean bottom. The effects of disturbance occur during the sampling process due to the vaporization of dissolved gases, as these soil specimens are under relatively higher pressure and contain pore water with a high amount of dissolved gases. Therefore, numerical simulations were conducted in the present study to investigate the effects of vaporized dissolved gases on the mechanical behavior of soil specimens during sampling and on the undrained shear strength as determined by laboratory tests. The analyses revealed that the combination of the decreasing effective stress caused by the sampling and factors such as overconsolidation and unsaturation is attributable to the difference between the soil strength ascertained by laboratory testing and the in-situ soil strength.     

Modeling of the Steady-State Response of Granular Soils

The paper presents a study of the static liquefaction of granular soils, particularly the existence and uniqueness of the so-called steady-state and the conditions leading to the steady-state. The study is carried out using a simple constitutive model for granular soils established using a few well-known relationships based on results of drained triaxial tests on sands and silts. The paper clarifies the necessary theoretical and experimental conditions in determining whether a soil has achieved the steady-state. The results of the analytical study strongly emphasize the importance of dilatancy and deformability on the steady-state response of granular soils. It is shown that, in the absence of instability, liquefaction should be treated simply as another aspect of the stress-strain response of granular soils, and liquefaction should be treated not just as a strength problem but also as a deformation problem.     

Earthquake-Induced Flow Slides of Fills and Infinite Slopes

Earthquake induced flow slides of saturated or partially saturated sloping ground have taken place in man-made earth structures during past earthquakes. Two series of dynamic centrifuge model tests (1:50 scale) were conducted to simulate the behavior of embankment fills and infinite slopes resting on a stiff sloping base. Conditions that yield earthquake-induced flow slide were investigated. The test parameters included soil density, slope angle, level of phreatic water and shaking intensity. The test results showed that downward deformation increased with decreasing soil density, increasing phreatic water elevation and shaking intensity, and that there existed a threshold soil density below which flow slides occurred. Undrained monotonic loading torsional simple shear tests were also conducted. Conditions inducing very large deformation or flow slide that were obtained by the undrained shear tests were consistent with those observed in the model tests.     

砂土液化势剪切波速简化判别法的改进

结合压电陶瓷弯曲元波速测试技术开展了饱和标准砂的不排水循环三轴试验,并根据试验结果改进了以往提出的利用剪切波速进行砂土液化势判别的简化方法。这种方法本质上基于砂土抗液化剪切强度与弹性剪切模量之间良好的相关性。用改进后的方法全面评价了26次地震、70多个液化场地的液化势,并与国内外其他液化势简化判别法的判别结果作了比较。分析结果表明,改进后的简化方法的评价结果与现场震害调查数据更趋一致。最后,通过一个实例分析]示了利用该改进方法进行土层液化势判别的一般步骤。该改进方法仍有待于深入研究,尤其是对密实砂土场地在强震下的液化评价,需要进一步的试验和现场数据加以佐证。     

广州地铁砂土层液化判别

在广州地铁工程砂土地震液化判别过程中,考虑了地铁结构与液化土层的相互作用。通过大量的现场实验、室内动三轴实验,总结了水平场地、区间、车站土层液化分布情况和液化特点;为了提高液化判别精度,进一步详细地对比和检验了现场和室内的判别结果,分析了液化土层与结构的空间相对位置以及结构对液化势的影响,所采用的多参数和多手段的液化判别技术为合理的抗液化设计提供帮助。     

三维胶结结构性土UH模型

为反映胶结对结构性土剪切最终应力比以及剪胀规律的影响,将p–q坐标中静态的临界状态线(CSL)扩展为与CSL平行并随结构性衰减而从左侧移向CSL的动态临界状态线(MCSL),构造与MCSL匹配的屈服面和剪胀方程,从而将以考虑加载体积垮塌为主的结构性土统一硬化(UH)模型扩展为能考虑胶结影响的胶结结构性土UH模型。在此基础上,应用变换应力三维化方法,将所提模型应用到三维应力空间。相对于结构性土UH模型,三维胶结结构性土UH模型增加了1个模型参数描述初始胶结程度。该参数可由无侧限压缩试验近似确定。通过4种结构性土的试验结果与模型预测对照表明,三维胶结结构性土UH模型能够较合理地反映受胶结影响的结构性土等向压缩、常规三轴剪切与真三轴剪切等特性。     

Aging effects on liquefaction resistance of sand estimated from laboratory investigation

Liquefaction resistance is known to increase concomitantly with the increase in time after construction or sedimentation. Nevertheless, the mechanisms of its aging effect on liquefaction have not been completely elucidated. To clarify the mechanisms of aging in sandy soils, the liquefaction resistance (CRR), initial and secant shear moduli (G₀ and G_sec), and laboratory penetration resistance of long-term consolidated sand specimens were examined using cyclic undrained triaxial tests, local small strain (LSS) tests equipped with bender elements (BEs), and penetration index tests, respectively. Based on the existing reports, the CRR was inferred from G₀, G_sec, and the laboratory penetration resistance. In the case of Toyoura sand of D_r = 40%, the CRR increased by about 14% with a 360-day consolidated specimen in the cyclic undrained triaxial tests. However, increases in the CRR evaluated from G₀ and the laboratory penetration resistance were nothing and only 2%, respectively. G_sec started to degrade at greater shear strain in the long-term consolidated specimens. An increase in the CRR, evaluated from the G_sec of 0.01% shear strain, had a much better agreement with that obtained from the cyclic undrained triaxial tests.