Field and Laboratory Measurements of Small Strain Stiffness of Decomposed Granites

Non-linear stress-strain characteristics and stiffness-strain relationships of sedimentary soils and sands at small strains have been reported by many researchers. Research work on the behaviour of weathered or decomposed granites at small strains, however, has rarely been reported. This paper compares some stiffness measurements of decomposed granites from field investigations involving crosshole seismic, self-boring pressuremeter (SBPM), and high pressure dilatometer as well as results from laboratory tests using bender element and internal transducers. The in-situ crosshole measurements show that the elastic stiffness of Moderately Decomposed Granite (MDG, approximately 7000 MPa) is about 25% greater than that (about 5500 MPa) of Highly Decomposed Granite (HDG), which is in turn approximately 18 times higher than that (about 300 MPa) of Completely Decomposed Granite (CDG). This is likely attributable to the materials' different bond strengths and structures. A new method has been adopted to interpret the SBPM data. Measured data from crosshole seismic and self-boring pressure meter tests for CDG are found to be consistent. Bender element laboratory tests on CDG indicate that the measured A-coefficient in the expression of G₀/p_r = A(p/p_r)ⁿ lies between the results from clay and sand as reported in the literature. However, the measured n-value for CDG is generally larger for clays and sands. The measured bender element results are consistent with data from internal transducers. Highly non-linear characteristics of CDG were observed in both the laboratory and field tests. Generally the elastic stiffness of CDG as determined by laboratory tests is about 50-80% of that from field tests. Some possible reasons are discussed.     

Fem Simulation of the Viscous Effects on the Stress-Strain Behaviour of Sand in Plane Strain Compression

A stress-strain model called TESRA (Temporary Effects of Strain Rate and Acceleration), described in a non-linear three-component framework, has been proposed to simulate the effects of viscous property on the stress-strain behaviour observed in drained plane strain compression (PSC) tests on clean sands. According to the TESRA model, the current viscous stress component is obtained by integrating for a given history of irreversible strain increments of viscous stress component that developed by respective instantaneous irrecoverable strain increment and its rate and have decayed with an increase in the irreversible strain until the present. The TESRA model was implemented into a generalized elasto-plastic isotropic strain-hardening non-linear FE code. The integration scheme to obtain the viscous and inviscid stress components according to the TESRA model in FEM analysis needs some specific considerations including the relevant choice of the suitable rate parameter. The shear stress—shear (or axial) strain—time relations from five drained PSC tests on saturated Toyoura sand and air-dried Hostun sand were successfully simulated by the FE code embedded with the TESRA model. It is shown that the FE code can simulate the time-dependent stress-strain behaviour of sand accurately without spending any significant extra computational time or storage. The results of simulation using one element and multi-element are essentially the same.     

土体小应变特性的研究进展

大量实测资料表明 ,在工程实际中大部分土体的变形在 0 0 1%～ 0 3%之间。但由于仪器精度的限制及人们意识的不足 ,直到近年来人们才逐渐意识到研究小应变土体特性的重要性 ,本文较为系统地介绍了这方面的研究情况     

降雨作用下碎石土滑坡变形演化过程及孕灾模式研究

在总结碎石土滑坡地质模型的基础上,采用有限元数值方法研究不同降雨强度下碎石土滑坡的变形演化过程,并对降雨过程中及降雨后期1～7 d的应力应变情况进行分析,得出典型碎石土滑坡的变形破坏过程及其孕灾模式.研究表明,降雨过程中,滑坡中前部以水平剪切变形为主,中后部以沉降拉剪变形为主;碎石土滑坡在降雨作用下的孕灾模式可划分为三个阶段:后缘拉裂孕灾阶段、中部破裂扩展孕灾阶段、滑面贯穿孕灾阶段.     

Rate Effects on the Stress-Strain Behaviour of Eps Geofoam

The rate-dependency of the stress-strain behavior of EPS (Expanded Polystyrene) geofoam with densities of 19.3 and 28.0 kg/m³ was investigated by performing unconventional unconfined compression tests. A set of monotonic loading (ML) tests were performed at different constant values of vertical (axial) strain rate, ε_v. The ε_v value was stepwise changed many times and several sustained loading (SL) tests were performed during otherwise ML at a constant ε_v in other tests. A number of SL tests were performed during global unload and reload cycles to infer the stress-strain relation when ε_v=0. The elastic properties were evaluated by applying minute unload/reload cycles during otherwise ML. The rate-dependent stress-strain behaviour observed in these tests was described by an elasto-viscoplastic model (i.e., a non-linear three-component model), for which the vertical (axial) stress, σ_v, consists of inviscid and viscous components, σ_v^f and σ_v^v, while ε_v consists of elastic and irreversible components, ε_v^e and ε_v^ir. It is shown that the viscous property of EPS geofoam is of Isotach type in that, under the loading conditions where ε_v^ir is always positive, the current σ_v^v value is a unique function of instantaneous ε_v^ir and ε_v^ir, therefore the strength increases with ε_v. This viscous property was quantified based on the test results and incorporated into the model. The rate-dependent stress-strain behaviour, including the creep behaviour, observed in the experiment is simulated very well by the proposed model. In particular, the fact that the creep strain becomes significant when the sustained σ_v value becomes larger than the inviscid yield vertical stress is well simulated.     

堆石在平面应变条件下的强度和应力-应变关系

应用大型平面应变仪,进行了灰岩及细砂岩两种堆石的风干料及饱和固结排水平面应变试验。试验结果表明:1.堆石在平面应变条件下的强度比三轴试验强度高;2.饱和试样强度低于风干料试样强度;3.平面应变条件下的应力-应变关系与三轴试验结果相比较,差别较大,前者呈明显的应变软化特性;4.平面应变条件下堆石泊松比基本上是一常数,而且小主应力σ₃对它影响较小。     

某加筋碎石土边坡变形破坏模式分析

通过某加筋碎石土边坡的工程实例,现场踏勘初步分析其变形破坏的主要原因,判断其可能的几种破坏模式;采用野外钻探、原位试验及室内土工试验等手段获取边坡各岩土层的物理力学指标,利用Geo-Studio软件建立二维地质模型,并进行边坡渗流和边坡稳定性计算。从正常工况和最不利工况两个角度来验证该边坡产生变形破坏的原因及模式,最后得出结论,并预测未来边坡变性破坏的发展趋势,以期为同类型加筋碎石土边坡的设计和施工提供一些参考。     

Modelling and Simulation of Rate-Dependent Stress-Strain Behaviour of Granular Materials in Shear

A constitutive modelling of the elasto-viscoplastic stress-strain behaviour of geomaterials in shear that has been developed within a non-linear three-component model framework is validated by simulating a comprehensive series of drained triaxial compression (TC) and direct shear (DS) tests on a wide variety of granular materials. Illustrative simulations of rate-dependent stress-strain behaviour of geomaterial under typical laboratory test conditions were performed to analyse the structure of the model. The versatility of the proposed model and its applicability to a wide variety of shear loading histories is examined and demonstrated by these simulations. The following results are shown. Commonly with different basic viscosity types, Isotach, TESRA and P&N, the viscous stress component has a positive component that increases with an increase in the irreversible strain rate, which makes feasible stable and realistic simulations of rate-dependent stress-strain behaviour, including creep deformation, based on the proposed model. With different unbound granular material types having similar relative densities, the creep strain in TC tests and creep shear displacement in DS tests that develop by sustained loading at a given shear stress level for a given period tends to decrease with an increase in the particle roundness. This trend of behaviours is explained by a decrease in the viscosity type parameter, θ, associated with an increase in the particle roundness based on the simulations of these tests.     

Monotonic and Cyclic Tests of Interface between Structure and Gravelly Soil

Monotonic and cyclic behavior of the interface between a structure and gravelly soil was investigated using systematical tests. A series of monotonic and cyclic tests of the interface between a steel plate and gravel were conducted using a large-scale test apparatus through varying gravel types, surface roughnesses of steel plates, normal boundary conditions, magnitudes of normal stress and displacement amplitudes. Microscopic movements and crushing process of soil particles were measured coupled with macroscopic stress-displacement relationship response. Based on the results, it is concluded that monotonic and cyclic behavior of the interface between a structure and gravelly soil is significantly different from that of gravelly soil itself. Main behaviors of the interface includes: (1) shear strength is proportional to normal stress; (2) the interface between a steel plate and gravel exhibits insignificant strain softening; (3) normal displacement accumulates and the accumulation rate decreases with increasing shear cycles, but varies in well-regulated manner within a single shear cycle; (4) shear deformation is composed of indispensable slippage component on the contact surface and soil deformation component constrained by the structure nearby while the latter mainly contributes to volumetric change due to dilatancy; (5) mechanical response is dependent on shear direction due to cyclic shear application after an initial shear application; (6) evolution of stress-displacement relationship response are governed by the evolution of physical state including particle crushing and soil compression due to shear application; (7) main factors influencing on behavior include surface roughness of the structure, characteristics of the soil and magnitude of normal stress.     

非线性有限元模拟中的应力、应变关系

提出一种方法,可以确定直至结构破坏时,与单元长度相关的应力、应变关系,并适用于有限元分析模型。采用已测出的表面位移计算结构直至破坏时的应变。从颈缩处已测出的力和横截面面积可以推导出应力的数值。此外,由于光学测量的数字特性,将应变的参考长度清晰地定义为像素的函数。在数值模拟中有限单元的长度与这个参考长度相等。此法可以对拉伸试验进行精确的数值模拟,除非在结构破坏时曲线无法拟合,或者缺少根据所取的分隔尺寸来调整材料关系的迭代方法。精确的材料关系有助于提高非线性数值模拟的准确性。     

Modeling of Gravelly Soil with Multiple Lithologic Components and its Application

The behavior of a gravelly soil with multiple lithologic components was investigated using large-scale compression and triaxial tests based on the research scheme “separating before mixing”. A series of test results showed that the behavior of a gravelly soil with multiple lithologic components was significantly dependent on the identity and proportions of the lithologic components. A new constitutive model was established to capture the main behavior of a gravelly soil with arbitrary lithologic proportions using few parameters based on the presentation of an elasto-plasticity model and the deformation mechanism of a complex soil. The parameters of this model can be determined using triaxial tests of a simple soil with a single lithologic component. The numerical efficiency of the model was confirmed by successful application to several high rockfill dams. Three-dimensional finite element (FEM) stress-strain analysis results for the Jishixia concrete-faced rockfill dam were obtained as important references for the selection of rockfill and optimizing the design of the dam.     

碳纤维增强卷边槽形钢柱的非线性性能和承载力

对碳纤维增强卷边槽形钢柱的试验结果、非线性性能的数值分析以及承载力进行研究。基于GBT的基础研究,涉及柱弹性抗弯性能的研究集中在下列方面:i)布的位置(腹板、翼缘、卷边);ii)碳纤维的方向(纵向、横向或倾斜)以便对加固效果(性能价格比)进行优化。对19个端固定的短、长卷边槽形钢柱的试验过程进行描述。采用碳纤维布对钢柱的不同部位(腹板、翼缘、卷边)进行加固,纤维的方向包括纵向和横向:因为研究的目标是评估碳纤维加固对柱的结构响应的影响,对未加固钢试件也进行了试验。基于壳有限单元模型并采用Abaqus规定,由包含非线性平衡路径(荷载与轴向压缩的比值)和极限强度(大部分与局部/屈曲失稳破坏机理相关)组成的试验结果被用于校准和验证几何及物理非线性数值分析。最后,根据试验和数值结果,对于受到局部/屈曲失稳影响的卷边槽形钢柱,得出碳纤维布加固的有效位置和纤维方向的相关结论。     

Large-Scale Monotonic and Cyclic Tests of Interface Between Geotextile and Gravelly Soil

A series of monotonic and cyclic shear tests, as well as pullout tests, were conducted on gravel-geotextile interfaces using a large-scale apparatus, with development of a new special pullout test element. The macroscopic response of stress and displacement, as well as the movement and crushing process of soil particles, were observed and measured. The interface exhibited evident strain-softening and aeolotropic normal displacement, which were significantly influenced by normal stress. Shear strength decreased and normal displacement increased with increasing number of shear cycles. Shear deformation was composed of slippage at the contact surface and deformation of the soil constrained by the geotextile; and the thickness was estimated at 5-6 times the average soil grain size. There was significant evolution of physical state due to shear application, including soil particle crushing and soil compression, as well as damage to the geotextile. The pullout test underestimated shear stiffness of the interface due to significant deformation of the geotextile itself. Shear strength increased with increasing normal stress, described by a logarithmic equation, according to the pullout tests, rather than the linear relationship obtained using direct shear tests. Therefore, an appropriate test method should be selected with careful consideration of the site conditions.     

基于边界面理论的小应变土体本构模型

处于小应变范围（0.01％-0.3％）的土体在市政工程中广泛存在，近年来人们对土体小应变特性的研究表明，这一范围内土体的特性难以用传统的本构模型加以描述。通过大量实验，在揭示小应变范围内土体特性的基础上提出基于边界面理论的小应变模型，并通过实验结果对模型加以验证，结果表明模型是适用和可行的。     

由压缩试验分析砾质花岗岩残积土的结构特性

通过引入应力综合结构势来分析砾质花岗岩残积土的结构性.以深圳南山区太子花园施工工地的砾质花岗岩残积土为例,对土样在不同含水量及干密度条件下进行原状、饱和、重塑状态的压缩试验,由压缩曲线得到应力综合结构势,并由此做出结构性参数曲线,据此考察含水量和干密度的变化对砾质花岗岩残积土结构性的影响.得出含水量与干密度的变化对砾质花岗岩残积土的结构性有显著影响的结论.     

三峡库区碎石土地基浸水试验研究

碎石土广泛存在于三峡库区.三峡水库蓄水以后,库区常年水位将会比蓄水前抬高50～100m,大量原来在天然状态下的碎石土地基将会受到水的长期浸泡,从而导致其承载力下降,这将给库区蓄水后碎石土地基的稳定性评估及承载力验算带来一个新的难题.文章采取两种浸水试验方案来模拟库区碎石土地基在库水长期浸泡下的状况.试验结果表明,两种浸水试验方案是有效的,碎石土地基在水长期浸泡下其承载力大大降低,可达30%左右.     

Estimation of the Small-Strain Stiffness of Clean and Silty Sands using Stress-Strain Curves and CPT Cone Resistance

The initial, linear elastic range of a soil stress-strain curve is often defined by the small-strain elastic modulus E₀ or shear modulus G₀. In the present study, simpler and effective methods are proposed for the estimation of the small-strain stiffness of clean and silty sands; these are based on triaxial compression test results and the CPT cone resistance q_c. In the method based on stress-strain curves obtained from triaxial compression tests, an extrapolation technique is adopted within the small-strain range of a transformed stress-strain curve to obtain estimates of the small-strain elastic modulus. Calculated small-strain elastic modulus values were compared with the values measured using bender element tests performed on clean sands and sands containing nonplastic fines. The results showed that the method proposed produces satisfactory estimates of the small-strain elastic modulus for practical purposes. In the CPT-based method, two G₀-q_c correlations available in the literature were evaluated. For isotropic conditions, both correlations produced reasonably good estimates of G₀ for clean sands but overestimated it for silty sands. A G₀-q_c correlation which is proposed takes into account the effect of silt content of the sand and stress anisotropy.