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Authors: K. J., Tharesh
Issue Date: 2002
Abstract: Earthquakes are one of the natural disasters. India has experienced many destructive earthquakes in the past. Its entire area is earthquake prone. Liquefaction of soils due to seismic vibrations is of great concern for earthquake engineers. Widespread damage due to liquefaction was observed during the recent Bhuj Earthquake of Jan. 26, 2002. Liquefaction occurred even at sites where the ground water table was very deep below ground level. Many structures as well as natural ground settled down and some light underground tanks floated to ground level due to liquefaction. As such, study of soil liquefaction is important. It has attracted attention of many research workers all over the world. Complete Liquefaction of soil occurs when excess dynamic pore pressure in soil exceeds or equals the initial effective overburden pressure. If excess dynamic pore pressure is smaller, there will be partial liquefaction. Both situations may lead to severe damage. Present day analytical methods do not-predict soil liquefaction satisfactorily due to many complex parameters affecting liquefaction. Inertia forces induced by earthquake ground vibrations result into oscillatory shear stresses and induce corresponding dynamic simple shear strains also. This is also associated with rotation of principal planes. It is desirable to simulate these field conditions in laboratory testing also. These are not simulated in dynamic triaxial testing. Oscillatory simple shear test apparatus simulates these conditions. However, the sample size is too small and leads to testing errors. Rectangular tanks with rigid walls placed on shake tables house larger test samples. However, rigid sides and friction between longitudinal sides of tank and the soil sample flout simulation of free field conditions. Not many large shake table facilities reported in the present day state of the art allow free field simple shear ground deformation conditions. This dissertation deals with design and fabrication of a large shake table test facility for experimental study of soil liquefaction and demonstrate its performance. The test tank houses 2.5m -iii- long, 1.25m wide and 1.25m tall cohesionless soil sample. Each transverse side of the tank consists of a 50 mm tall mild steel (MS) strip rigidly connected to base of the tank and 8 more strips of 150 min height each side capable of rotating about horizontal transverse axis of the strip. These strips are connected to the corresponding horizontal longitudinal movable strips through hinged connections at junctions. This allows free field simple shear deformations of the soil sample in the longitudinal vertical planes. It also allows development of shear strains of different magnitude at different elevations with in the sample, which is desirable. Each longitudinal side of tank consists of 2.7m long MS strips. The bottom most strip is 50 mm tall and rigidly connected to base of the tank. The remaining eight strips are 150mm tall and10 mm wide and movable in horizontal directions, Balls moving in V-grooves attached to each of the adjacent plates resting one over the other facilitate horizontal movements. Each longitudinal plates are supported laterally by two columns. Ball bearings are placed at points of contact between longitudinal strips and side-supports to reduce friction. The tank is made watertight by providing a rubber sheet all along the longitudinal and transverse sides of the sample. Top surface of the soil sample is open. The table with wheels moves on rails and is actuated by a 7.5 H.P. AC motor-mechanical oscillator assembly generating simple harmonic dynamic force. The speed of the motor is 1000 rpm. The frequency of the shake table vibrations is controlled by using a suitable combination of gears. This facility successfully created simple shear conditions in the soil within the test tank. The rubber lining provided rendered the tank watertight and stopped leakage of water. It is one of the largest facilities of its kind reported in present day state of the art. The facility is useful for experimental studies on soil liquefaction and for study of dynamic behaviour of relatively large models of pile foundations and isolated
Other Identifiers: M.Tech
Appears in Collections:MASTERS' DISSERTATIONS (Earthquake Engg)

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