Effect of Geogrid in Saturated Sand Against Liquefaction Chauhan Rajiv, Research Scholar Department of Civil Engineering, Indian Institute of Technology, Roorkee Online published on 29 May, 2012. Abstract Liquefaction is an earthquake associated natural phenomenon, which causes widespread damage to man made structures. One of major reasons for collapse of structures due to liquefaction is loss of strength due to generation of excess pore pressure in sandy soils. The pore water pressure response of sandy soils controls the liquefaction behavior. Various kinds of earthquake- resistant methods are available now a days. However, all of these methods are expensive and normally require advance construction techniques, which may not be affordable sometimes. The liquefaction studies with geogrid are scanty. The present study addresses the effect on liquefaction behavior of sand duly reinforced with HDPE geogrid. The uniaxial geogrid made from high strength polyester yarns with black PVC coating having carbon black as 2% and ultimate tensile strength of 88.7 kN/m was used as reinforcing material in the study. It was a stiff grid with rectangular openings of size 220 mm × 17 mm and unit weight of 6.5 N/mm2. Tests had been conducted in a rigid steel tank of size 1060 mm length, 600 mm width and 600 mm height mounted on a shake table. The amplitude of motion could be changed through two eccentric shafts. By changing the relative position of two shafts, the amplitude could be fixed as desired. The maximum amplitude of horizontal acceleration which can be generated in the shake table was up to 0.3g.The hand brake assembly is used for stopping the shake table instantaneously. The pore pressure measurement was performed with the help of 3 nos. glass tube piezometers each of 5 mm diameter, attached to the tank through rubber tubes at heights of 80 mm, 180 mm and 260 mm from bottom of tank and were denoted as B, M and T respectively. To check the entry of soil particles into the piezometer tubes, porous stones duly wrapped with filter paper were tied at mouth of piezometer tubes. For each acceleration, the relative densities of the sample were also varied from 35% to 50%.The density in the sample was maintained as per guidelines of ASTM D5311–92(reapproved 2004) and Ishihara (1996).The geogrid layers of size 800 mm x 400 mm were placed in shake table at a vertical spacing of 100 mm c/c in 500 mm depth of sample. This size of geogrid layer was adopted to eliminate the boundary effects on it. When soil was reinforced with 4 nos. geogrid layers placed equidistant vertically and shaking was imparted, the average excess pore pressure decreases from 3.31 kN/m2 (for virgin soil) to 2.41 kN/m2(for reinforced soil) respectively for 35% relative density at 0.1g. This trend was further observed at higher acceleration values i.e. 3.64 kN/m2 (for virgin soil) to 2.87 kN/m2 (for reinforced soil) at 0.3g for same density. Thus average decrease in excess pore pressure was 27% at 0.1g and 21% at 0.3g at 35% relative density. A new parameter i.e. Liquefaction Resistance Factor (LRF) has been introduced which is ratio of excess pore pressure to effective overburden pressure. If this value is equal to or greater than 1.0, liquefaction is likely to occur, and if it is below 1.0 then there will not be any liquefaction. The liquefaction resistance of sand increases due to addition of geogrid layers. For 35% relative density, the average increase in liquefaction resistance was about 23% with 4 nos. geogrid layers at 0.1g.Further by increasing acceleration to 0.3g, the liquefaction resistance was 20% for same density. For 50% relative density, the average liquefaction resistance factor comes down from 1.18 (for virgin soil) to 0.893 (for reinforced sand) at 0.3g. Effect of surcharge was also studied on reinforced sand to simulate field conditions. The surcharge applied was through precast concrete blocks. Each concrete block was weighing approximately 1800 N. These were placed on a 10 mm thick steel plate placed on the soil sample for applying the load on to the soil sample uniformly. These concrete blocks were loaded and removed after test with the help of a chain pulley block (Mittal, 1988). The blocks were rigidly connected to each other through 4 Nos. steel anchor bolts and two steel channels; so that their position was not disturbed under the action of dynamic loads. Different surcharges were applied on sand sample. With the application of 5.94 kN/m2 surcharge, the average liquefaction resistance factor increased to 28% at 0.3g at 50% relative density. This shows that it works in good arrangement under overburden conditions also. Settlement behavior of sand with and without reinforcement was also studied. In case of virgin soil, 18.8 mm of settlement reduces to 15.2 mm with inclusion of 4 nos. geogrid layers at 35% relative density for 0.1 g.This trend was further observed at higher acceleration values also. Excess pore pressure build up time i.e. time to reach maximum pore pressure was also studied. For reinforced sand, the pore pressure build up time increased from 17.6 seconds to 48.3 seconds for 35% relative density at 0.3g. Similarly excess pore pressure dissipation time also increased from 80 seconds (for virgin soil) to 119 seconds (for reinforced sand) at 35% relative density for 0.1g. From this study, it is concluded that liquefaction potential of fine sand can be significantly reduced by use of geogrid reinforcements. Therefore, such study suggests that prior to construction of new embankments (railways or highways) and underground structures e.g., reservoir etc., if ground is reinforced with geogrids, the severity of damages could be minimized to a great extent. Top |