LIQUEFACTION MITIGATION MEASURES THROUGH GROUTING

Abstract

Earthquake is one of the major natural vulnerabilities that threatens life of human by damaging high cost structures. One of the drastic causes of destruction to engineering structures has been the development of liquefaction in saturated sand deposits during earthquakes resulting in loss of life and property. The catastrophic damages especially to various buildings, embankments and retaining structures due to liquefaction were caused during some of the past earthquakes such as Niigata (1964), Alaska (1964), Loma Prieta (1989), Northridge (1994), Kobe (1995), Kocaeli (1999), Chi-Chi (1999) and Bhuj (2001). India has witness such disaster in Ahmadabad Gujarat during 2001 Bhuj earthquake, where most of reinforced cement concrete buildings suffered great damage. Earthquake induced liquefaction was the main cause of catastrophic damage, which has made liquefaction a primary topic of concern in the arena of geotechnical engineering. Therefore, great occurrence of damage in the course of earthquake due to liquefaction with its potential to damage has attracted the attention of investigators in this area and extensive research has been done to assess the liquefaction susceptibility of soil. Hence improving the site situations to reduce liquefaction is a necessity. It has been reported in numerous literature review that stabilization of soil by grouting techniques has been suggested by several investigators to improve the soil against liquefaction. However, the literature on liquefaction susceptibility of saturated sand due to neat cement grout is scanty. Keeping this in view, the aim of the present research work was to study experimentally the effect of neat cement grout on the liquefaction resistance of saturated sand. In the present study, locally available fine sand collected from bed of Solani river was used for laboratory investigation. Neat cement grouts were used as the stabilizing material under dynamic conditions. The tests were conducted on one dimensional vibration table. The test tank was water tight bin of length 1060 mm, width 600 mm with height of 600 mm, in which saturated samples of sand and sand improved with neat cement grouts, were prepared. The pore pressures were measured with the help of glass tube piezometers and a stop watch at four different depths. A number of tests was performed with varying amplitude of 0.1g, 0.2g and 0.3g at constant dynamic frequency (4 Hz and 8 Hz) respectively. Saturated samples of sand and grouted sand were prepared at different relative densities varying from 35% to 80% to study the stratification effects. The effect of neat cement grout on liquefaction resistance was studied. The saturated sand and grouted sand iii samples were tested for their effect on excess pore pressure rise, settlement variation and liquefaction potential subsequently in saturated conditions. In each test, pore water pressure was recorded at four different depths at a regular interval of time at 4 Hz and 8 Hz frequency of dynamic load in the entire test. A number of parameters like excess pore water pressure (Umax) and settlement were observed experimentally. The excess pore pressure increases with increase in acceleration but decreases with increase in relative density due to increase in strength of the soil. The liquefaction resistance was evaluated in terms of a parameter called excess pore pressure ratio (Ru) which is well-defined as ratio of excess pore water pressure to effective over burden pressure (Ru = Umax/σv’). It was observed that Solani river sand was susceptible to liquefaction at relative density of 35%, 50% and 65% at all levels of accelerations. For all relative densities, Excess pore pressure ratio (Ru) was found to be greater than 1.0 (for complete liquefaction, excess pore pressure ratio is larger than or equal to one). Whereas, no liquefaction was observed at 80% relative density for ungrouted Solani river sand at all acceleration levels. The 1:3 (cement: water) ratio of 43 Grade Ordinary Portland cement (OPC 43) and a clean water were used as grout. Tests for grouted sand were conducted for all four relative densities and it was observed that due to inclusion of grouts the liquefaction resistance of sand increases at all the relative densities and the increase was significant. The average increases in liquefaction resistance of sand for relative densities of 35% to 65% were observed to be 28% and 44% respectively at 0.3g acceleration. It was noticed that, utmost increase in liquefaction resistance was observed at 50% relative density of sand at all level of excitations. Similarly, the settlement of sand shows significant decreases with inclusion of neat cement grout. A parametric study has been done to study the small scale test on sand samples prepared with and without grouting. Numbers of tests were conducted on cyclic triaxial test apparatus. The study was conducted to study the effect on pore pressure generation, deviator stress and subsequent liquefaction behavior due to inclusion of grouts. It can be concluded that, the severity of liquefaction of saturated sand can be controlled appreciably by use of neat cement grout. Hence, the ground improvement techniques considered in the present investigation for Solani river sand were found effective in increasing their liquefaction resistance.