LIQUEFACTION STUDIES OF COMPOSITE MATERIALS

Abstract

The liquefaction of saturated loose sand during earthquakes has been the cause of severe damages to various buildings, embankments and retaining structures. The catastrophic damages due to liquefaction during some of the past earthquakes such as Alaska (1964), Niigata (1964), Northridge (1994), Kobe (1995), Kacaeli (1999), Chi-Chi (1999) and Bhuj (2001) have attracted the attention of researchers in this area and considerable research work has been done to evaluate the liquefaction susceptibility of soil. Therefore, improving site conditions to eliminate liquefaction is anecessity. It has been reported in the literature that the addition of randomly distributed fibers, sheet elements and geogrid sheets to the soils increased its strength and stiffness significantly. However, the effect of reinforcement on liquefaction susceptibility of sand and pond ash is rarely reported. Keeping this in view, the major aim of the present research work was to evaluate experimentally the effect of different reinforcing materials on the liquefaction resistance of sand and to devise a suitable technique for strengthening and improving the liquefaction resistance ofpond ash. In the present study, the locally available Solani river fine sand and pond ash from a thermal power plant were considered for laboratory investigation. The reinforcing materials used for sand are geogrid sheet, synthetic fiber and coir fiber. Stone-sand columns were considered for strengthening of pond ash. The tests were conducted on a simple but indigenously fabricated one dimensional vibration table. The test bin was awatertight tank 1050 mm long, 600 mm wide and 600 mm high, in which sample of sand, sand reinforced with three different materials, fly ash and fly ash with stone-sand columns were prepared. The pore pressure measurements were performed with the help of glass tube piezometer and stop watch. The pore water pressure was measured at four different depths. In the present study, firstly anumber of tests were performed on sand imparting horizontal harmonic excitation of varying amplitude (from 0.1 gto 0.4 g) to samples prepared at various relative densities (25 %, 40 %and 60 %). The effects of different reinforcements on sand were studied at 25 %relative density. The saturated samples of pond ash were prepared at relative density of 20 %and the effect of stone-sand columns on the liquefaction resistance was studied in In each test, the pore water pressure was recorded at three different depths at a regular interval of time, keeping the frequency of dynamic load constant (5 Hz) in most of the tests. A number of parameters for liquefaction study such as excess pore water pressure (U), maximum excess pore water pressure (Umax), time taken to build up maximum excess pore water pressure (ti), maximum excess pore water pressure stay time (t2) and maximum excess pore water dissipation time (t3) were observed experimentally. The liquefaction resistance was evaluated in terms of maximum excess pore water pressure ratio (rumax) which is the ratio of maximum excess pore water pressure (Umax) to the effective overburden pressure (crv0) i.e. rumax = Umax/crv0. It was observed that the Solani sand is susceptible to liquefaction only at relative density of 25 %and at other two relative densities there was only reduction in shear strength but did not liquefy (for complete liquefaction rumax =Umax/ vv0 >1). It was also observed that at a particular relative density, the maximum excess pore water pressure ratio (rumax) increases with amplitude of excitation. Also for the same level of excitation, the rumax decreases with relative density. These trends of the results were as expected and validate the experimental setup and procedure. The geogrid sheets having the dimension equal to plan dimensions of the tank i.e. 1050 mm x 600 mm were used in three different combinations of 3 layers, 4 layers and 5 layers at different depths within the sample and tests were conducted at two relative densities i.e. 25 %and 40 %. It was found that due to reinforcement with geogrid sheets, the liquefaction resistance of sand increased at both the relative densities and this increase is significant at small amplitude of excitation. The average increase in liquefaction resistance of sand for relative densities of 25 %and 40 %were observed to be 31 %and 40 %respectively corresponding to 5layers ofgeogrid sheets at 0.1 gacceleration. The percentages of fiber (synthetic or coir) by weight of dry sand were taken as 0.25 %, 0.5 % and 0.75 %. Corresponding to each percentage, the samples were prepared at relative density of 25 %. It was found that at 0.75 %fiber content, the percentage increase in liquefaction resistance of sand for synthetic fiber and coir fiber was about 88 %and 91 %respectively at 0.1 g acceleration. In case of pond ash, the time taken to build up maximum pore water pressure (ti), the duration of time for which maximum pore pressure stay (t2) and maximum pore water dissipation time (t3) were observed to be very high in comparison to sand. IV For pond ash improved with stone-sand columns, the liquefaction resistance was increased significantly and the percentage increase was about 92 %in case of columns spaced at three times the diameter of the column. The reinforcement techniques considered in the present investigation for Solani sand and pond ash were found effective in increasing their liquefaction resistance.