LIQUEFACTION OF SANDS DURING EARTHQUAKES

Abstract

Liquefaction of saturated sands has often been the main cause of Catastrophic damage to structures resulting in loss of life and property. In case a foundation soil is liable to liquefy during an earthquake extra precaution taken in design of the superstructure is not of any help in the event of an e arthquake. Areview of literature shows that the liquefaction behaviour of saturated sands is influenced by the method of test and test equipment used in the study. Avery compre hensive tests have been carried out on small samples under triaxial and simple shear conditions under cyclic loads. Few investigations were also done on large size samples on vibration tables. But no uniform agreement to date (1977) has been achieved from different tests performed by different investigators. Small sample data has been used to predict the field behaviour regarding its liquefaction characteristics and a method of analysis is available to estimate the extent of depths to which it will occur during an earthquake. Since uniform agreement has not been achieved from tests carried out on different equipments a comprehensive test programme was called for on large size sample. However the available method also does not consider the progressive nature of liquefaction during the earthquake. ii in this thesis the investigations were carried out on horizontal vibration table to study the behaviour of sands regarding its liquefaction characteristics. The tests were carried out on four different sands varying in size from fine to very coarse. The effect of the following variables have been studiedi 1. Grain size 2. Initial relative density 3. initial surcharge i) Air pressure surcharge condition ii) Dead weight surcharge condition 4. Acceleration of vibrations 5. Number of cycles 6. Drainage characteristics Amethod has been developed for carrying out the liquefaction analysis of a site during an earthquake. This analysis uses the test data obtained on large size samples on vibration table. The method presented herein takes into account the progressive nature of liquefaction during the earthquake by estimating loss in effective overburden pressure which may lead to liquefaction of deeper layers also. This process requires the laboratory testing on sand sample obtained from the site under expected conditions of accelerations and frequencies of earthquake at corresponding <?o< Ill relative densities and effective overburden pressures. The case study of Niigata site has also been made using the proposed method. An artificial earthquake was generated for the site by using the Koyna earthquake of 1967 the only well recorded Indian earthquake. For obtaining the artificial earthquake a time wise response analysis was made by considering the upward propagation of shear waves from the base rock motion underlying the alluvium deposits. An excellent agreement has been found between the analysis so performed and the behaviour observed in heavy damage zone during the Niigata earthquake of 1964 at both places where liquefaction was observed and also where it was not observed. Acomparison of liquefaction analysis from this study is also made with the analysis proposed by Seed and Idriss (1971) which uses data from small sample tests. In this study it has been brought out that: Model studies requiring simulation of model laws may not be suitable for liquefaction studies. Laboratory tests with air pressure surcharge conditions are not suitable for liquefaction studies. The sands may not liquefy if they are at relative densities greater than 65%. ? IV Dead weight surcharge device which has no holes at bottom plate can be considered to represent field conditions in vibration table studies. The data obtained from shake table tests can be successfully used to predict the possibility of liquefaction in field during an earthquake. With the use of such a data the analysis can be performed in a more rational manner. A rational simple method of laboratory investigation for liquefaction analysis has been developed which can be standardized for professional use of field engineer.