Characterization of a profile of residual soil from granite combining geological, geophysical and mechanical testing techniques

This paper summarizes the results of an experimental site investigation and characterization survey, on a residual (saprolitic) soil from granite, in the framework of a research project led by the Faculty of Engineering of the University of Porto (FEUP). This project aims at characterizing these unusual soils in the context of the development of an International Prediction Event (Class A) on the behaviour of different types of piles. A very extensive site characterization campaign, including a large variety of in-situ tests and field methods, has been held. These investigations comprised the application of several geophysical borehole and surface methods, namely P- and S-wave seismic refraction, reflection, cross-hole (CH), down-hole (DH), electrical resistivity imaging and ground probing radar (GPR), as well as mechanical tests, namely SPT, CPT and DMT, among others. The site is geologically formed by an upper layer of heterogeneous residual granitic soil, overlaying rather weathered granite contacting a gneissic migmatite. Direct and indirect results from some of the referred surveys were compared between them and with some of the available geological and geotechnical information, namely those obtained from seismic, electrical and GPR profiles, conducted adjacent to three boreholes in which undisturbed soil samples were collected previously to geophysical data acquisition. In addition, a comprehensive laboratory testing program was carried out using the collected undisturbed samples. A discussion of the obtained results is hereby presented, giving emphasis to the correlations encountered between the different tests, specific of saprolitic soils with weak relic structures.     

天然沉积粉质黏土的应力路径试验研究

天然沉积土在沉积过程中产生了结构性和各向异性,使其受力变形特性与重塑土存在明显的差异。实际工程中的天然土体往往在受荷过程中会经历不同的应力路径,因此,需要开展考虑结构性和各向异性影响的应力路径试验。通过研究不同应力路径下土体的力学特性,为建立复杂应力路径下的合理的本构模型提供试验依据。采用大直径PVC管取样器获取张家港地区地下2.5 m深的粉质黏土不扰动土样,通过GDS三轴仪对土样进行了K0固结不同排水应力路径试验。结果表明,应力路径对不扰动土样的体积变形和剪切变形均有显著影响,且球应力和偏应力对土的体应变和剪应变存在交叉影响。无论以体积变形为主还是剪切变形为主的应力路径下,应力-应变曲线都有明显的屈服性状。通过描绘试验所得各应力路径下的屈服点,获得张家港不扰动土样的屈服轨迹大致呈倾斜的椭圆形状,采用Wheeler模型的屈服面与试验屈服点的吻合程度要优于Nakano模型。     

非饱和上海软土水力和力学特性耦合弹塑性模拟

目前大多数非饱和土的弹塑性本构模型用非饱和击实土的试验结果进行验证,但现场其他类型的土,如沉积土经常有在非饱和状态下外部环境变化的情况。现有的非饱和土弹塑性模型是否适用于沉积土一类的现场土是需要研究的课题。进行非饱和上海第③层土的吸力控制排水排气三轴剪切试验,使用文中提出的能统一考虑非饱和土水力性状和力学性状的弹塑性本构模型,预测上述三轴试验结果,并与试验数据进行比较。比较结果显示,建立的本构模型能够很好地预测非饱和上海软土的水力和力学性质,说明该模型不仅可以适用击实土的预测,还能够很好地适用于其他类型非饱和土的水力和力学性质的模拟。     

Unidirectional cyclic resistance of Ticino and Toyoura sands from centrifuge cone penetration tests

The evaluation of the undrained cyclic resistance of sandy deposits is required to forecast the soil behaviour during an earthquake (liquefaction, cyclic mobility); due to the difficulties in obtaining undisturbed samples of most liquefiable soils, it is usually deduced from field test results such as cone penetration tests. This paper proposes a methodology to evaluate the undrained cyclic resistance from normalised cone resistance of two well-studied silica sands (Ticino and Toyoura), with different mineralogy, one mainly composed of feldspar, the other of quartz. The determination of the cyclic resistance of Ticino and Toyoura sands was achieved through undrained cyclic triaxial tests on reconstituted specimens. The tip resistance was deduced from CPTs performed in centrifuge with a miniaturised piezocone on homogeneous reconstituted models. Both the undrained cyclic and tip resistances were correlated with the state parameter ψ. Results of centrifuge and triaxial tests were combined through ψ to deduce the cyclic resistance ratio CRR directly from the normalised cone resistance. The shape of the curve relating CRR to the normalised cone resistance resulted unusual respect to all the recognised curves widespread in the geotechnical literature. The aim of the proposed correlations is to provide a useful instrument to improve the actual knowledge on liquefaction and to give a contribution based on the critical state soil mechanics framework to the development of refined correlations between the cyclic resistance of a sand and the results of cone penetration tests.     

Creep behaviour of frozen soils in uniaxial compression tests

The stress—strain behaviour of frozen soils is often described by means of creep curves from uniaxial or triaxial creep tests. Ten or twelve tests of similar samples are required to obtain a good relationship between stress σ, strain ε, temperature T and time t or strain rate ε. To reduce the required number of samples it is possible to apply the compression load in creep tests stepwise. Therefore creep curves for different stress levels can be obtained from one sample.

In this paper the bearing capacity of cylindrical samples of frozen medium sand under constant uniaxial compression stress and under stepwise increased stresses is compared. It is shown how to use these different creep curves to describe the stress—strain behaviour of frozen soils.     

残积土抗剪强度的环剪试验研究

残积土风化剧烈,研究其大变形下的工程特性很有必要。利用环剪仪的试验特点,可以研究土体在较大剪切位移下抗剪强度的变化规律。通过对残积土残余和峰值强度的环剪试验测定和对试验数据的整理与对比分析,得到了含水率与残余强度指标之间的关系,证明大变形下残积土具有浸水软化的特性;从应变阈值角度分析了残积土应变软化的性质,不同剪切位移下的残积土具有不同的抗剪强度,所研究的残积土达到峰值强度对应的应变介于0.02～0.06之间,而到达基本稳定的残余强度所需要的应变介于0.06～0.20区域内。研究还发现,矿物成分不同直接影响到残积土的残余强度,是残积土不均匀性的内部因素     

土−水特征曲线测量过程中孔隙分布的演化规律探讨

测量了在宽广吸力范围内原状样和压实样的脱湿持水曲线,对比分析了单双峰结构持水性能的差异;并利用压汞试验测试两种土样在脱湿过程的孔隙分布,分析了两者的差异并探讨了脱湿过程孔隙的演化规律;在考虑收缩变形的基础上,基于孔隙分布曲线确定了土?水特征曲线的基本参数。试验结果表明：原状样在宽广吸力范围内基本上呈单峰孔隙结构;饱和压实样具有单峰孔隙结构,随着吸力的增加,双峰结构越来越明显,当吸力达到很大时,演化成完全双峰孔隙结构。原状样的持水曲线为经典的S形,而压实样的持水曲线在过渡段出现了水平台阶状;低吸力段,压实样的持水曲线低于原状样,而高吸力段,两者的持水曲线基本重合。基于孔隙分布曲线确定了控制持水曲线进气值和残余值的孔径,并计算出对应的吸力值,其值更符合实际物理意义。     

Influence of Cyclic Wetting Drying on Collapse Behaviour of Compacted Residual Soil

The climatic zones where residual soils occur are often characterized by alternate wet and dry seasons. Laboratory studies of earlier workers have established that the alternate wetting and drying process affects the swell-shrink potentials, water content, void ratio and particle cementation of expansive soils. The influence of cyclic wetting and drying on the collapse behaviour of residual soils has not been examined. This paper examines the influence of alternate wetting and drying on the collapse behaviour of compacted residual soil specimens from Bangalore District. Results of such a study are useful in anticipating changes in collapse behaviour of compacted residual soil fills. Experimental results indicated that the cyclic wetting and drying process increased the degree of expansiveness of the residual soils and reduced their collapse tendency. Changes in the swell/collapse behaviour of compacted residual soil specimens from wetting drying effects are attributed to reduction in water content, void ratio and possible growth of cementation bonds.     

Characterisation of a volcanic residual soil and its implications for large landslide phenomena: application to Tenerife,Canary Islands

Large landslides are common processes during the evolution of volcanoes and individual events can exceed several cubic kilometres in volume. Volcanic slope failures are a significant risk for the neighbouring population due to their huge volumes and great runout distances. Around the Canary archipelago, a total of seventeen deposits of large landslides have been found, and on Tenerife, seven large landslides have affected the subaerial and submarine morphology during the last ∼6 Ma. However, the causes of such mass movements are still poorly understood. This work analyses the events around the Canary Islands and focuses on the ones that occurred on Tenerife in order to obtain new insights into the mechanisms of large volcanic landslides. The study is divided into a first part that includes site investigations examining the general features favouring large-scale failures at volcanoes. The second part describes the laboratory tests used to analyse a residual soil that may be the potential slip surface of the slides on Tenerife. The site investigation revealed that regional tectonics and the climate have a significant influence on the spatial distribution of the landslides. Moreover, morphological and geological features such as deep fluvial canyons, a high coastal cliff and persistent dike intrusion may favour the initiation of slope failure. A typical residual soil sample from the lateral scarp of the La Orotava amphitheatre on Tenerife was studied by carrying out standard laboratory tests. The microstructure was analysed using environmental scanning electron microscopy and a particular bonding was found. This bonding was also detected by the geotechnical tests. Consolidation tests and direct shear tests revealed that the mechanical behaviour of the residual soil changes greatly if the bonding of the soil is broken. The bonded structure generally fails when the effective normal stress surpasses the yield strength of the bonding. In the case of large volcanic landslides with thicknesses up to several hundred meters, the high overburden easily exceeds this yield strength and generates a broken bonding. Therefore, volcanic residual soils, such as the one analysed in this study, are perfect candidates for the potential failure surfaces of large volcanic landslides. Referring to the La Orotava events, we assume that residual soil layers and morphological, geological and climatic features reduced the slope stability to critical conditions, whereas a strong earthquake associated with a caldera collapse episode may have finally triggered the landslide. The results obtained indicate that the residual soils play an important role in affecting the stability of volcano slopes and their destabilising influence significantly favours large-scale sliding. We suggest that the results obtained from this study can be applied to other locations since volcanic residual soils are common in volcanic areas.     

Aspects of sand behaviour by modified constant shear drained tests

Constant shear drained tests (CSD) are probably the most suitable to simulate the strength and deformation behaviour of soils in slopes under water infiltration conditions or lateral stress relief. This is significant because soil behaviour following a CSD stress path could differ from that of traditional compression triaxial tests. In this paper, CSD tests on sand following an alternative procedure are presented and discussed. The modified CSD tests were conducted by increasing the pore water pressure at a constant rate from one end of the specimen with water free to drain from the opposite end. Among the results from specimens consolidated at variable initial void ratios and principal stress ratios it was revealed that specimens showed a tendency to dilate even for loose sands; failure was reached at low axial strains; and a pre-failure type of instability could be identified. The modified procedure has the potential to provide new insights into the failure mechanisms of slopes under a water infiltration condition.     

三峡泄滩滑坡体滑动带土的蠕变试验研究

三峡古滑坡的变形和长期稳定的控制性因素,在于滑动带土力学特性的强度损伤积累和时间效应。为了滑坡建模和稳定数值分析的需要,进行了泄滩滑坡体滑动带土的三轴排水蠕变试验研究。并在此基础上,给出了排水条件下粘性土的Mesri蠕变模型,即剪应力-应变关系采用双曲线模型,应变-时间关系采用幂函数来描述土体的弹粘塑性。     

Stress deformation properties of macroporous loess soils

Milovic, D., 1988. Stress deformation properties of macroporous loess soils. Eng. Geol., 25: 283–302.

Herein are presented the results of laboratory and field tests which were carried out on loess in Yugoslavia.

Experience gained during recent decades show that the loess soil in some cases undergoes structural collapse and subsidence due to inundation. In order to find the explanation of such behaviour, numerous laboratory and field load tests on loess soil have been performed. Using the obtained results, several correlations have been established.

On the basis of the unconfined test results, a correlation between the initial dry density, initial water content and unconfined compression strength has been established.

A relatively large number of the consolidation subsidence tests (about 550), carried out on undisturbed loess samples, made possible the determination of the degree of subsidence and the corresponding values of the dimensionless coefficients of subsidence. These coefficients have been determined for several values of the initial dry density, for various degrees of saturation and for several stress levels upon wetting. Using these coefficients, the values of the additional settlement caused by flooding of loess under the foundation can easily be calculated. It is of particular interest to note that, in general, water penetrates under one part of the building, producing differential settlements, which are in most cases very dangerous.

By the comparative laboratory investigation of the undisturbed loess samples obtained by thin walled sampler and samples obtained from pits, it has been shown that the mechanical disturbance is an extremely important factor which governs the shear and deformation parameters of loess soils.

The laboratory test results obtained on undisturbed loess samples, cut from blocks in the vertical and horizontal direction, have shown that this soil exhibits anisotropic properties. For this reason a stress deformation problem in an anisotropic medium has been treated by the finite element method.

Static penetration tests and field load tests in loess soils with natural water content and also after saturation have been performed and are described.

The results of the observed settlements for two statically identical multi-storey buildings are also presented. Using the coefficients of subsidence for the undisturbed samples cut from blocks, very good agreement between the calculated and the observed settlements has been obtained.     

Strength and structure of laterites and lateritic soils

Ten soils from Puerto Rico representing various states of weathering were subjected to engineering index tests, strength and density measurement, mineralogic analyses, and scanning electron microscopy.Engineering classification systems used for temperate soils tend to underestimate the engineering behavior of these undisturbed lateritic soils and in the undisturbed state the soils exhibit low densities and high strengths.The engineering properties of the undisturbed soils show systematic trends in relation to the degree of weathering and the parent rock. For a given rock, the void ratio is decreased and cohesion increased as weathering proceeds. This is the result of increasing crystallization and cementation by the sesquioxides.The specific gravity of the solid particles is a promising index property to characterize these lateritic soils insofar as it reflects iron oxide content. The decrease in void ratio and increase in cohesion with increasing specific gravity suggests a model for the engineering classification of tropical soils.     

Effect of cyclic loading on monotonic shear strength of fine-grained soils

In this study the effect of cyclic loads on monotonic shear strength has been studied on torsional apparatus. Tests have been conducted on both reconstituted and undisturbed fine-grained hollow soil specimens. The plasticity index of soils is in the range from 2 to 33 for reconstituted specimens, prepared by dry pluviation, and from 5 to 13 for undisturbed soil specimens. The existence of a critical shear strain level, called yield shear strain, where softening starts, is determined from cyclic tests. The level of cyclic yield strain is ± 0.75% for the reconstituted soil specimens and ± 0.5% for the undisturbed soils. If soil undergoes a cyclic shear strain level below the cyclic yield strain, reduction of monotonic strength of reconstituted and undisturbed specimens is limited, but when cyclic shear strain level is larger than yield strain monotonic strength decreases down to 40% of its initial strength.     

Adaptation of an existing constitutive model for describing mechanical behaviour of cemented granular soils

In this paper, an existing elastoplastic constitutive model, originally developed for granular soils, is adapted to describe the stress–strain behaviour of cemented granular soils. The existing model (CJS), due to its modular formulation, can be easily developed to take into account different supplementary behavioural aspects in soil mechanics. In the present study, the failure mechanism of the CJS model is modified by introducing the essential aspects in the behaviour of cemented granular soils in its formulation. All of the model parameters have clear physical meaning and can be identified using classical laboratory tests. A set of direct relations between model parameters and famous mechanical parameters of soils such as internal friction angle and cohesion at peak and residual states is presented. In order to validate the model, the results of triaxial and uniaxial tests in the compression and extension performed on cemented granular materials are used. The validation results indicate the good capability of the proposed model.     

A theoretical and experimental study on the behaviour of lignosulfonate-treated sandy silt

The effectiveness of an environmentally friendly stabilising agent for soil, namely, lignosulfonate was examined through a series of laboratory tests. A simple bounding surface plasticity model was developed to capture the bonding effects induced by lignosulfonate. One of the appealing aspects of the model is that it can incorporate the mechanical behaviour of the bonded soil during shearing, including the brittle and ductile failure modes. Validity of the model was verified by experimental results of lignosulfonate-treated soils under different stress path conditions. The mechanical behaviour of chemically treated soil was adequately captured by the model.     

基于能量平衡法的原状冻土Ⅰ型非线性断裂韧度测试研究

基于能量平衡的原理,采用冻土弯曲断裂试验模型,根据不同深度的冻胀量、冻深与时间变化规律,严格控制试样制作及试验时的温度,对原状冻土进行了断裂韧度测试试验。得出Ⅰ型直裂纹试样的 和 值,从而为原状冻土的非线性断裂破坏问题的研究进行了有意义的尝试,拓宽了思路。     

Modelling of shearing behaviour of a residual soil with Recurrent Neural Network

Modelling of shear behaviour of residual soils is difficult in that there is a significant variability in constituents and structures of the soil. A Recurrent Neural Network (RNN) is developed for modelling shear behaviour of the residual soil. The RNN model appears very effective in modelling complex soil shear behaviour, due to its feedback connections from an hidden layer to an input layer. Two architectures of the RNN model are designed for training different sets of experimental data which include strain-controlled undrained tests and stress-controlled drained tests performed on a residual Hawaiian volcanic soil. A dynamic gradient descent learning algorithm is used to train the network. By training only part of the experimental data the network establishes neural connections between stress and strain relations. Although the soil exhibited significant variations in terms of shearing behaviour, the RNN model displays a strong capability in capturing these variabilities. Both softening and hardening characteristics of the soil are well represented by the RNN model. Isotropic and anisotropic consolidation conditions are precisely reflected by the RNN model. In undrained tests, pore water pressure responses at various loading stages are simultaneously simulated. With a RNN model designed for a special drained test, the network is able to capture abrupt changes in axial and volumetric strains during shearing courses. These good agreements between the measured data and the modelling results demonstrate the desired capability of the RNN model in representing a soil behaviour. © 1998 John Wiley & Sons, Ltd.     

饱和黄土的静态液化特性试验研究

利用GDS三轴测试系统,对原状黄土进行了等压/偏压固结不排水剪（ICU/ACU）、常剪应力排水剪（CQD）试验。ICU和ACU试验表明：土体的应力-应变模式为强烈应变软化型,得到了其稳态线和潜在不稳定区,提出液化发生的必要条件就是土体状态位于或被带到潜在不稳定区域内。CQD试验表明,土体破坏的启动摩擦角远小于稳态摩擦角。当其应力路径试图穿越不稳定区时发生不完全排水剪缩破坏,具有突发性,检验了提出的不稳定区域的有效性。基于潜在不稳定区域的位置和斜坡土体的应力状态,提出了饱和黄土静态液化分析方法。