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Shaking Table Studies on the Conditions of Sand Liquefaction

served.sand bed alone. In this two layered model, densities of both the layers were kept same. The overburden pressure was controlled by varying the thickness of the dry sand layer with respect to the total thickness of the sand bed. The ratio between the dry sand layer (h) and the total thickness of the saturated sand bed (H) was termed as overburden height ratio. The over burden height ratio was varied from 0.05 to 0.15 in this series. Third series of the tests were conducted to find out the effect on building surcharge on the liquefaction response of the saturated sand bed. To simulate the surcharge load imposed by the building, concrete slab of size 300 mm × 250 mm × 50 mm (length × width × height) was placed on the saturated sand bed, keeping the centerlines of the sand bed and the concrete slab matching exactly without any

eccentricity. The tests were carried out at two different contact surcharge pressures,

1.1 kPa and 2.2 kPa.

RESULTS AND DISCUSSIONS

Series 1: Free field liquefaction studies at different input wave parameters

This series of tests were conducted to find out the minimum base acceleration

required to initiate liquefaction at a particular frequency. The base accelerations were

varied from 0.1g to 0.15g with constant frequency as 1 Hz. The variations in pore

water pressures with respect to variations in accelerations were measured. Initiation

of liquefaction is defined in terms of excess pore water pressure developed during

dynamic loading and initial vertical effective stress. The ratio of excess pore water

pressure to initial vertical effective stress is termed as pore water pressure ratio ru.

When pore water pressure ratio becomes one, the sand bed behaves like a liquid. The

shear strength of soil at liquefaction will be zero due to excess pore water pressure

development. The number of cycles required to change the pore water pressure ratio

from zero to one govern the liquefaction potential of the sand.

In this series of shaking table tests, liquefaction of sand bed was observed at an

acceleration of 0.15g. Liquefaction was physically visualized in this test, where the

sand bed started flowing like a liquid. Figure 2 shows the photographs of sand bed

taken before and after this test. In all other tests below this acceleration, at 1 Hz

frequency, complete liquefaction did not occur, though there was increase in the pore

water pressure with increase in the acceleration, which was measured with the

number of cycles.

FIG. 2. Sand bed before and after liquefaction

Shaking Table Studies on the Conditions of Sand Liquefaction

Shaking Table Studies on the Conditions of Sand Liquefaction

Shaking Table Studies on the Conditions of Sand Liquefaction

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