LIQUEFACTION RESISTANCE OF GEOGRID REINFORCED SOIL

Abstract

Liquefaction of saturated soil is one of the complex geotechnical phenomena which have caused devastations in past earthquakes in all over the world. The catastrophic damages due to liquefaction during some of the past earthquakes such as Long Beach (1933), Niigata (1964), Alaska (1964), Kobe (1995), Bhuj (2001) and Fukushima (2011) have attracted the attention from researchers and engineers and considerable research has been done to evaluate the liquefaction susceptibility of soil. Liquefaction is likely to cause differential settlement and tilting of buildings, damages of pavements, airports, embankments, and disruption of buried pipe lines and failure of pile foundations. Liquefaction is triggered by external excitations which result in generation of pore water pressure in saturated soil. As a result, the effective stress of the soil layer is reduced and vanishes in complete liquefaction. The effects of liquefaction are reduction in stiffness and shear modulus of soil, development of sand boils and settlement. Fine grained cohesion-less soils in saturated conditions when subjected to cyclic loading undergo volumetric compression, as the duration of earthquake excitation is of the order a few seconds and dissipation of excess pore water pressure does not take place immediately, the process reflects undrained condition. The dissipation of excess pore water pressure follows with consolidation settlement change. The use of geosynthetics reinforcement is a modern practice in mitigating liquefaction and settlement problems. In a reinforced soil, the soil mass is reinforced by incorporating reinforcement that is strong in tensile resistance. Through soil-reinforcement interfaces bonding, the reinforcement restrains lateral deformation of the surrounding soil, increases its confinement, reduces its tendency for dilation, and consequently increases the stiffness and strength of the soil mass. In the present work, Solani sand (a fine sand) is used for liquefaction studies. Two types of geogrids of different mesh sizes were used to study the liquefaction behaviour. The experiments were carried out on indigenously developed Horizontal Shake table (1.05m x 0.6m x 0.6m). The shake table vibrates in uni-axial direction (horizontal) which simulates the undrained condition of cyclic loading. iii The effectiveness of geogrid sheets were also investigated on the compressibility of dry sand at various frequency and acceleration conditions. It was observed that settlement of dry sand is higher for cyclic load under 5 Hz frequency than cyclic loads under 3.5 Hz and 2 Hz keeping the number of cycles and acceleration constant. Samples reinforced with 3 layer geogrid were tested at various frequency and acceleration and it was observed that, settlement decreased considerably with the inclusion of , geogrid reinforcement. The liquefaction potential of saturated sand samples at relative densities 25% and 40% were studied. The effects of geogrid reinforcement were studied at the same relative density of 25% and 40% at acceleration of 0.25g. The tests were carried out keeping the frequency of dynamic load (2 Hz) in most of the tests. The consolidated saturated samples were subjected to series of load cycles (in steps) after the initial shaking to observe volume changes and change in pore water pressure. The effects of geogrid on saturated samples under repetitive cyclic loads were evaluated and analysed. The geogrid sheets having dimension 1000mm x 550 mm were used in three different • combinations of 2 layers, 3 layers and 4 layers at different depths within the sample. It. was observed that due to reinforcement of geogrid sheets, the liquefaction resistance of sand increased at both the relative densities. In the present work, the effect of reinforcing materials on the settlement and liquefaction resistance of sand was studied experimentally and validation of the settlement in dry state was done.