A multi-stage triaxial testing procedure for low permeable geomaterials applied to Opalinus Clay

In many engineering applications,it is important to determine both effective rock properties and the rock behavior which are representative for the problem’s in situ conditions.For this purpose,rock samples are usually extracted from the ground and brought to the laboratory to perform laboratory experiments such as consolidated undrained(CU)triaxial tests.For low permeable geomaterials such as clay shales,core extraction,handling,storage,and specimen preparation can lead to a reduction in the degree of saturation and the effective stress state in the specimen prior to testing remains uncertain.Related changes in structure and the effect of capillary pressure can alter the properties of the specimen and affect the reliability of the test results.A careful testing procedure including back-saturation,consolidation and adequate shearing of the specimen,however,can overcome these issues.Although substantial effort has been devoted during the past decades to the establishment of a testing procedure for low permeable geomaterials,no consistent protocol can be found.With a special focus on CU tests on Opalinus Clay,this study gives a review of the theoretical concepts necessary for planning and validating the results during the individual testing stages(saturation,consolidation,and shearing).The discussed tests protocol is further applied to a series of specimens of Opalinus Clay to illustrate its applicability and highlight the key aspects.     

Experiments on thermo-hydro-mechanical behaviour of Opalinus Clay at Mont Terri rock laboratory,Switzerland

Repositories for deep geological disposal of radioactive waste rely on multi-barrier systems to isolate waste from the biosphere.A multi-barrier system typically comprises the natural geological barrier provided by the repository host rock e in our case the Opalinus Clay e and an engineered barrier system(EBS).The Swiss repository concept for spent fuel and vitrified high-level waste(HLW)consists of waste canisters,which are emplaced horizontally in the middle of an emplacement gallery and are separated from the gallery wall by granular backfill material(GBM).We describe here a selection of five in-situ experiments where characteristic hydro-mechanical(HM)and thermo-hydro-mechanical(THM)processes have been observed.The first example is a coupled HM and mine-by test where the evolution of the excavation damaged zone(EDZ)was monitored around a gallery in the Opalinus Clay(ED-B experiment).Measurements of pore-water pressures and convergences due to stress redistribution during excavation highlighted the HM behaviour.The same measurements were subsequently carried out in a heater test(HE-D)where we were able to characterise the Opalinus Clay in terms of its THM behaviour.These yielded detailed data to better understand the THM behaviours of the granular backfill and the natural host rock.For a presentation of the Swiss concept for HLW storage,we designed three demonstration experiments that were subsequently implemented in the Mont Terri rock laboratory:(1)the engineered barrier(EB)experiment,(2)the in-situ heater test on key-THM processes and parameters(HE-E)experiment,and(3)the full-scale emplacement(FE)experiment.The first demonstration experiment has been dismantled,but the last two ones are on-going.     

Study of the micro-mechanical behaviour of the Opalinus Clay: an example of co-operation across the ground engineering disciplines

The high degree of scientific cross-fertilisation possible between the three geo-engineering disciplines soil mechanics, rock mechanics and engineering geology, is demonstrated by means of a micro-mechanical model of the Opalinus Clay. After a brief review of Terzaghi’s effective stress principle and the importance of micro-mechanical models in general, a conceptual study of a micro-mechanical model of a claystone is presented in some detail. The model is based on the Particle Flow Code (PFC) developed by Itasca Corp. It introduces into the model the pertinent composition and structure of the Opalinus Claystone established in the local engineering geology of Switzerland and SW Germany. This includes elongated clay platelets, various layers of densified water around the platelets, free water in the pores and a specific texture of the platelets after consolidation. The model is numerically subjected to a series of loading stages. It is shown that the micro-mechanical model reproduces a number of features which have been known for a long time in soil and rock mechanics but which are often intractable in conventional generic models. The features include non-linear stress–strain curves with pre-failure damage and post-failure strain softening, a non-linear increase of the particle contacts with loading, distinct clustering of deformations, clustering of micro cracks leading to the development of shear bands and hysteresis in cyclic loading. It is concluded that micro-mechanical models are promising tools for further development of our understanding of the mechanical behaviour of geological materials. They offer an excellent opportunity for scientific co-operation between engineering geologists and soil and rock mechanics engineers.     

Shear behaviour of a geogrid-reinforced coarse-grained soil based on large-scale triaxial tests

In China, weathered mudstone geogrid-reinforced coarse-grained soil is used extensively for road embankments. However, the microstructure and disintegration process of weathered mudstone remain unclear. Furthermore, few studies have investigated the shear behaviour of this kind of geogrid-reinforced fill through large-scale triaxial tests against grain size effects. To bridge this gap, this study reports results from large scale consolidated undrained (CU) and consolidated drained (CD) triaxial tests as well as scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and disintegration tests on weathered mudstone geogrid-reinforced coarse-grained soil. EDX spectrograms and SEM images show that coarse grains disintegrate rapidly mainly owing to the high clay mineral content and loose microstructure. Therefore, a suitable disintegration time (∼15 days) is recommended for embankment sits. The shear behaviour of this geogrid-reinforced fill is investigated in detail through large-scale triaxial tests. The shear deformation tends toward strain hardening behaviour with an increase in the number of geogrid layers and the confining pressure. Geogrids significantly improve the apparent cohesive strength of coarse-grained soil. The pore water pressure is found to develop rapidly in the 0%–4% axial strain phase but dissipate slowly in the 4%–12% axial strain phase. During shear, the pore pressure coefficient A values of 0.2–0.4 are indicative of the partial saturation of specimens. Consequently, pore water pressure development is mainly attributed to the movement and rearrangement of coarse particles in coarse-grained soil. Experimental data show that the geogrid-reinforcement coefficients increase with the number of geogrid layers, and a 20-cm separation between geogrid layers is recommended for embankment construction sites. The number of geogrid layers influences the geogrid–soil interface’s mobilization and the slip surface type. Test results revealed three types of slip surfaces related to the failure shapes of specimens. Then, based on CU experimental data, the parameters of the Duncan–Chang constitutive model are discussed.     

超深三轴水泥土搅拌桩技术及在深基坑工程中的应用

随着沿海软土地区深基坑工程的发展,深度超过50m的超深三轴水泥土搅拌桩技术成为深层承压水控制的有效措施。通过工程实例介绍了超深三轴水泥土搅拌桩技术及其在深基坑工程中的应用情况。阐述了超深三轴水泥土搅拌桩的施工工艺、施工要点和质量控制措施,结合上海和天津地区的超深三轴搅拌桩的工程实例进行了说明,验证了超深三轴水泥土搅拌桩技术的可行性和可靠性。     

Numerical simulation of the swelling behaviour around tunnels based on special triaxial tests

This paper is to contribute to the understanding of the behaviour of tunnels in swelling ground. An Italian case study of a tunnel, collapsed due to swelling of a stiff clay, is taken as an example. The stress paths during excavation of elements of ground around the opening are computed in order to evidence the significant difference to that reproduced by usual swelling tests in the laboratory. An innovative triaxial testing procedure is developed and the stiff-clay tested. A numerical simulation of the swelling phenomenon induced by the excavation of the tunnel, based on the experimental results obtained, is then compared to site observations.     

浅淡影响三轴压缩试验结果的因素

针对三轴试验中遇到的问题，通过对影响三轴试验结果的一些因素分析，指出了试验过程中应注意的事项。     

Coulomb stress changes: From earthquakes to underground excavation failures

Evidence from shallow tunnels and mines indicates that, in certain cases, deformation and failure are not confined to the vicinity of the excavation front, as is widely believed. This situation has recently been described as lateral strain propagation from a “source” to remote parts of the excavation through low-strength rocks. A similar situation is observed in deep excavations, mainly expressed by rock bursts and micro-earthquakes. We suggest that (slow) propagation or (seismic) radiation of deformation away from the excavation front can be explained on the basis of the theory of Coulomb failure stress changes, in analogy to faulting in large earthquakes: local increase of stresses due to the excavation or failure in a certain point leads to a slight increase of stresses around this point. If adjacent points are already at a critical level for failure, this slight increase triggers a laterally, domino-style propagating new failure. Yet, in contrast to earthquakes, triggered deformation in mining can, in principle, be predicted and counteracted.     

A methodology for modelling energy-related human behaviour: Application to window opening behaviour in residential buildings

An energy simulation of a building is a mathematical representation of its physical behaviour considering all the thermal, lighting, acoustics aspects. However, a simulation cannot precisely replicate a real construction because all the simulations are based on a number of key assumptions that affect the results accuracy. Above all, the real energy performance can be affected by the actual behaviour of the building occupants. Thus, there are great benefits to be derived from improving models that simulate the behaviour of human beings within the context of engineered complex systems. The occupant behaviour related to the building control potentialities is a very complex process that has been studied only in the last years with some focuses related to natural ventilation (window opening behaviour), space heating energy demand (in particular the adjustments in the temperature set-point) and natural light (focusing on window blinds adjustments). In this paper, a methodology is presented to model the user behaviour in the context of real energy use and applied to a case study. The methodology, based on a medium/long-term monitoring, is aimed at shifting towards a probabilistic approach for modelling the human behaviour related to the control of indoor environment. The procedure is applied at models of occupants’ interactions with windows (opening and closing behaviour). Models of occupants’ window opening behaviour were inferred based on measurements and implemented in a simulation program. Simulation results were given as probability distributions of energy consumption and indoor environmental quality depending on user behaviour.     

Validity of continuous-failure-state unloading triaxial tests as a means to estimate the residual strength of rocks

The residual strength of rocks and rock masses is an important parameter to be constrained for analysis and design purposes in many rock engineering applications.A residual strength envelope in principal stress space is typically developed using residual strength data obtained from compression tests on many different specimens of the same rock type.In this study,we examined the potential for use of the continuous-failure-state testing concept as a means to constrain the residual strength envelope using a limited number of specimens.Specifically,cylindrical specimens of three rock types(granodiorite,diabase,and Stanstead granite) were unloaded at the residual state such that a full residual strength envelope for each individual specimen was obtained.Using a residual strength model that introduces a single new strength parameter(the residual strength index,or RSI),the results of the continuous-failurestate unloading tests were compared to conventionally obtained residual strength envelopes.Overall,the continuous-failure-state residual strength data were found to be consistent with the conventional residual strength data.However,it was identified that the primary factor limiting an accurate characterization of the residual strength for a given rock type is not the amount of data for a given specimen,but the variety of specimens available to characterize the inherent variability of the rock unit of interest.Accordingly,the use of continuous-failure-state testing for estimation of the residual strength of a rock unit is only recommended when the number of specimens available for testing is very limited(i.e.5).     

紧邻大直径管线的深基坑工程设计实践

上海漕溪路站地块公共开放空间项目位于密集城市中心,周边邻近诸多城市干道、市政管线、高架道路、轻轨线路和车站,环境保护要求较高,其中基坑北侧紧邻的大直径污水管为本工程的重点保护对象。文章介绍了该基坑工程的工程概况及总体设计方案,重点说明针对邻近污水管保护所采取的搅拌桩中套打灌注桩、架空栈桥等关键技术,以及基坑实施前制定的诸如预埋跟踪注浆管等必要的应急预案,并对主要的监测结果作了分析。该基坑工程已顺利完成,设计方案的实施为主体地下结构的施工和实现预定的工期目标创造了有利的条件,有效地保护了基坑周边的环境。     

复杂环境下深基坑工程设计实践

结合济南普利门超高层商业综合体项目的工程概况和周边环境条件,提出了两墙合一地下连续墙结合预应力锚索基坑支护方案,并分析了实施效果,指出该设计方案有效保护了基坑周边的环境,创造了良好的社会经济效益。     

Clay soil behaviour due to long-term contamination by liquid petroleum fuels: microstructure and geotechnical properties

Bulletin of Engineering Geology and the Environment - The article presents the results of qualitative and quantitative microstructural analysis, and the basic properties of clay soils subjected to...     

Thermal behaviour of masonry walls: Mathematical modelling and field monitoring

This Canadian study shows that the potential use of their procedure is very broad since the data computed can be used to investigate deterioration problems, to check risks associated with any proposed thermal upgrading and to perform stress analysis. The variation between the computed data and recorded data is small, except at peaks where the maximum differences occur.     

激光断面仪在隧道开挖与初期支护中的应用

简述了隧道激光断面仪检测的基本原理和使用方法,并介绍了激光断面仪在开挖与初期支护断面轮廓检测中的目的和工程实例,针对标准断面选取的原则不同,定性地描述了检测出的两种断面轮廓的数据可能出现的偏差.     

Cross-section crushing behaviour of hat-sections (Part II: Analytical modelling)

Hat-sections are often used to experimentally investigate building sheeting subject to a concentrated load and bending. In car doors, hat-sections are used for side-impact protection. For building sheeting, two types of crushing (by moving yield lines) exist. This article shows that cross-section behaviour can partly be used to explain total section behaviour. This is shown by finite element models of a very small strip dx of the hat-section. The quality of the finite element models for describing moving yield lines is verified.     

PLAXIS在基坑开挖对周边环境影响评估中的应用

介绍了某基坑支护的设计方案,采用PLAXIS有限元软件,模拟分析了基坑开挖对周边构筑物的影响,预测了周边构筑物的变形规律,对基坑支护方案进行了评估,并给出了一些基坑施工建议,为保证工程的顺利进行奠定了基础。     

Housing market segmentation: An application of convergence tests to Los Angeles region housing

Various techniques have been offered to measure the extent of a market. This paper uses a technique that has gained momentum in the economic growth literature, namely time-series convergence, to identify housing markets in the LA region. Using single-family housing price data over the 1992:1 to 2001:3 period, we find that convergence of housing prices is specific to various county clusters. This suggests that there are unique housing markets throughout the LA region.Received June 2002 / Accepted: January 2003     

Optimality criteria for materials with nonlinear behaviour: application to beams in bending

Structural optimization problems involving materials with nonlinear behaviour are dealt with and constraints on displacements are taken into account. The optimality criterion for such problems is shown to involve an adjoint problem and a dual material, whose constitutive law is linear, depending on the stress state of the actual structure. Beams in bending are studied and analytical and numerical methods of solution are discussed.