Several laboratory studies have shown that the hydraulic conductivity of compacted clay may increase up to three orders of magnitude when subjected to freeze-thaw. In this paper, methods to freeze and thaw specimens of compacted clay are reviewed and compared. Methods to measure the hydraulic conductivity of the specimens are also reviewed. Only naturally formed clay soils are considered; soil-bentonite mixtures and other amended soils are not included.
A review of testing conditions present during freeze-thaw and their effect on hydraulic conductivity is also included. Testing conditions that are addressed include availability of an external supply of water (closed vs. open system), dimensionality of freezing (one-dimensional vs. three-dimensional), rate of freezing, ultimate temperature, number of freeze-thaw cycles, and state of stress. The rate of freezing, number of freeze-thaw cycles, and state of stress appear to have the largest effect on hydraulic conductivity.
The effect of sampling disturbance on the hydraulic conductivity of compacted clay subjected to freeze-thaw is also presented. Specimens removed in Shelby tubes may be disturbed during sampling and extrusion. As a result, the effects of freeze-thaw can be masked. Collecting block specimens of thawed clay or taking core specimens of frozen clay are suggested as alternative procedures. A method to collect block specimens is presented.
a b s t r a c tAn explosion on the ground surface may cause significant damage to an underground structure, such as a tunnel or a pipeline. The extent of damage would depend on the intensity of blast, the material and configuration of the structure, as well as the nature and geometry of the intervening material.An underground structure may be protected by means of a protective barrier, installed directly above the structure. The effectiveness of using a compressible barrier, made of polyurethane geofoam, to mitigate the effects of surface explosion was investigated.The effects of a surface explosion were studied through a combination of physical model tests and numerical modeling. Reduced-scale (1:70 scale) physical model tests were conducted using a geotechnical centrifuge, where the scaling law for explosions was utilized to model the effects of a large explosion using a relatively small mass of explosives under a high gravitational field (70 g, in this case). The results of the physical model tests were used to calibrate a three-dimensional numerical model in which a fully-coupled EulereLagrange solver was utilized to model the explosions.Tunnel configurations with and without protective barriers were studied to assess the mitigation provided by protective barriers. Material properties for polyurethane geofoam barriers were evaluated from laboratory tests. The influence of barrier thickness in reducing the strains, stresses, and pressures on the tunnel induced by an explosion was studied. The beneficial effects of a protective geofoam barrier were found to increase with increasing barrier thickness only up to a certain thickness, beyond which, further increase in thickness did not result in additional reductions. The results will help in design optimization, while planning protection systems for new tunnels, as well as for retrofitting existing tunnels.
The dynamic frictional properties of different geosynthetic interfaces play an important role in the stability analyses of various geotechnical structures that incorporate geosynthetics. The paper presents and discusses laboratory test results on eight different interfaces, formed through various combinations of three geosynthetics (a geotextile, a smooth geomembrane, and a geonet). The dynamic frictional properties were estimated using cyclic direct shear tests, shaking table tests conducted at a normal g-level of 1g as well as at high g-levels, and on a 100 g-ton geotechnical centrifuge. The centrifuge simulated high normal stress levels, commonly encountered by geosynthetics comprising base liners of landfills or base isolators for large structures. The tests revealed various important characteristics regarding the dynamic frictional properties of the geosynthetic interfaces, including a dependence of some of the interfaces on the level of normal stress and the excitation frequency. It is felt that these differences should be considered when selecting proper test methods in relation to the application for which the geosynthetic is considered. It was concluded that proper simulation of field conditions in laboratory experiments is important to obtain suitable friction angle values to be used in design.
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