1,720,997 research outputs found
Stiffness of geomaterials at very small strains
No full textExperiments have been conducted in the triaxial apparatus to observe the behaviour at very small strains of three widely different natural materials: Bothkennar clay, London clay, and a high-porosity Chalk, Local strain measurements made using LVDTs were calibrated by a Fabry-Perot laser interferometer. a displacement accuracy as high as +/-0.027 mum, was obtained, leading to an axial strain accuracy of the order of +/- 10(-4)%. Special care was taken to ensure that the rates of creep reduced to acceptably small Levels before loading commenced. At very small strain levels the observed stress-strain behaviour appeared to be linear for all three materials. The extent of this linear behaviour was similar (0.002-0.003%) for all three materials, despite the maximum stiffness (E-max) ranging over two orders of magnitude. The initial influence of stress change and strain level on secant modulus was relatively small, with stiffnesses at 0.01% strain being of the order of 80-95% of E-max. Effects of loading path were observed, giving different rates of stiffness degradation as a function of strain, but recent stress history effects did not occur even though stress excursions went well beyond the range normally thought to induce plastic strains. Stiffnesses measured in the triaxial apparatus at very small strain levels were found to be similar to those obtained using held geophysical techniques, despite assumptions of isotropy; and the effects of sampling. The implications of these observations for practice are discussed
Soil stiffness in stress paths relevant to diaphragm walls in clay. Powrie, W., Pantelidou, H. & Stallebrass, E. (1998). Geotechnique 48, No. 4, 483 ± 494
No full textThe authors report an extensive triaxial testing programme on reconstituted kaolin to investigate the influence of stress paths on the stiffness of soil elements in the vicinity of diaphragm walls. They state that 'the effect of the recent stress history of the soil on its stress-strain response is now well established'.In support of this statement, they present data that appear to show that the stiffness of their kaolin, when tested under triaxial conditions, is strongly influenced by a change in stress path direction. Fig. 8 of their paper shows that a specimen that experienced a reversal in total stress path direction (1808) has anormalised stiffness 4-8 times higher than that of specimens that undergo no reversal in total stress path direction. This finding is in broad agreement with results from reconstituted London Clay reported by Atkinson et al. (1990), who showed an increase in stiffness of an order of magnitude when comparing the stiffness after a reversal in effective stress path direction with no change in direction
Block sampling of soils: Some practical consideration
No full textBlock sampling of soils can produce samples of the highest quality. However during block sampling attention has to be given to a number of practical aspects to minimize disturbance to the material. Mechanisms by which disturbances can be introduced include strains imposed to samples during the act of sampling, swelling, stress relief and moisture content changes during storage.This paper describes a number of techniques to obtain block samples. Techniques referred to include sampling the soil from test pits, auger holes and tunnels as well as down hole block sampling under high water table conditions. The mechanisms by which disturbance to samples can occur are discussed, and practical guidelines to minimize the level of disturbance are suggested
Laser interferometry to evaluate the performance of local displacement transducers
No full textThe authors have developed a Fabry-Pérot laser interferometer system for displacement measurement, which is compact and can be used as a stand-alone local strain measuring system. Interferometry operates on the principle of interference of a coherent light source and therefore provides an absolute standard of measurement. The purpose of this development is to investigate the extent of the linear-elastic range of geomaterials in triaxial stress space, and to obtain values of stiffness which can subsequently be compared with those obtained in other ways, both in the laboratory (using resonant column or bender elements) and in the field (using field geophysical techniques). The interferometer is also suitable as a high-accuracy instrument for calibration of other linear displacement transducers. This paper describes the instrument configuration and evaluates the performance of a commercial LVDT calibrated against the interferometer
Triaxial ultra-small strain measurements using laser interferometry
No full textThis paper describes a prototype device for ultra-small strain triaxial stiffness measurement, using laser-optic interferometry, and based upon the Fabry-Perot principle. The results of a test on an intact specimen of weak rock are presented, and comparisons made between measurements using this device and a submersible LVDT system. The displacement resolution of the interferometer was 0.0006 μm and the strain resolution 8 × 10^-7 %. It is shown that the measurement uncertainty of the instrument was low compared with the magnitude of the displacements measured and in addition it was sufficiently accurate to detect the linear stress-strain behavior of a weak rock.<br/
The value of stiffness measured in field seismic surveys
No full textStiffness parameters are required for many geotechnical calculations, including numerical modelling using the finite element or finite difference methods. The measurement of stiffness is not trivial, yet the actual values of stiffness adopted during analyses can have a very significant impact on predictions, and hence the perceived viability of a project. Measurement of stiffness in the laboratory is now well established, using techniques such as advanced triaxial testing with local strain measurement, cyclic triaxial testing, and the resonant column apparatus. Field geophysical techniques, although not as well established, can add significantly to the determination of input parameters for deformation analyses and prediction. This paper considers the strengths and weaknesses of these methods, using examples from a range of projects and ground conditions, and discusses how seismic geophysics can most effectively contribute
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Performance of push-in pressure cells in overconsolidated clay
No full textLow profile push-in pressure cells (spade cells) are commonly used to measure total horizontal stresses in overconsolidated clays. It is known that spade cells over-read the in situ stress in such deposits as a result of the complex localised stresses created during installation. Previous investigations into the magnitude of over-read are limited in number and have produced scattered results. During construction of the Channel Tunnel Rail Link at Ashford, Kent a spade cell was installed horizontally in an overconsolidated clay, aligned to measure initial vertical stress, and the changes in vertical stress were measured as the material above the cell was removed. The results of this experiment are presented and interpreted with reference to a finite element analysis and a comparison of spade cell and self boring pressuremeter test data of horizontal stresses at the same site, to assess the over-read of this type of spade cell in the Atherfield Clay
- …
