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Liquefaction Potential by Locally Available Sand and Silt


Published on Feb 27, 2019

Abstract

Liquefaction is derived from a Latin word „Liquefacere‟, meaning to melt, to dissolve, or to weaken [1].Soil liquefaction is a major concern for structures constructed with or on saturated sandy soils. Soil liquefaction has first become worldwide by its damaging nature in 1960‟s, after the occurrence of two destructive earthquakes, the Niigata and Alaska earthquakes in 1964.Soil liquefaction describes the phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid.Silts and silt-clay mixtures are found in several parts of the world. Their liquefaction behaviour during earthquakes has been critically examined.

The liquefaction behaviour of clean sand has been studied extensively. If fines are added to sands, their resistance to liquefaction increases if the soils are tested at the same voids ratio, however occurrence of tremendous damages due to liquefaction was observed in the silty soils during past earthquake (1988 Saguenay Earthquake, 1999 Chi-Chi Earthquake and in 2001 Bhuj Earthquake)[2]. These earthquakes draw attention towards liquefaction potential of silty sands.The present work mainly focused on the liquefaction resistance of silty sand and to know the effect of silt content variation on its resistance.The natural sand commonly consists of fines of particle size less than 0.075mm with different proportion.

The presence of fines significantly influences and place important role in the liquefaction potential as well as engineering properties of sand.Silt grains and sand grains both have a common origin that is rock forming minerals. The shape silt grains and sand grains belong to same form and also the attraction forces, such as hydrogen bond and Vander waals bonds are negligible between silt grains similar to that of sand grains[3]. The issue of liquefaction resistance of silty sand has become controversial in the recent days. All the available research work shows the conflicting opinion and conclusion seems to be associated with confusion about liquefaction potential of silty sand.This work is mainly focused on variation of relative density in liquefaction process. Relative density explains about voids ratios of a cohesion less soil in its loosest state. An increase in relative density results in increase in cyclic stress ratio there by making the soil more resistance against liquefaction.

Keywords : Liquefaction, earthquake, strength loss, horizontal shake table, sand silt mixture, clay content.

Objectives:

The main objective of this work is to assess liquefaction potential of locally available sand with varying silt content as(0%, 10%, 30% and 50%), tests have been conducted on different relative densities like loose, medium, dense as 12%, 44%, 54% respectively with varying frequency.

Methodology

The materials required for the experiment study are locally available sand and silt.Some basic tests are conducted on them to determine the index properties of sand and silt The sand is selected as soil medium, with various percentage of silt content by varying relative density to determine the soil liquefaction resistance of sand.In present study the sand sample were collected from our college and silt samples fromTeggi near Bilagi. Silt is granular material of size between sand and clay, whose mineral origin is quartz and feldspar. It exist as soil deposited at the bottom of a water body, like mudflows from landslides.Shake table is an equipment to stimulate the field conditions during earthquake with known frequency and amplitude of vibrations. For the present work horizontal shake table[10] instrument of electronic cam type.The instrument mainly consists of Drive unit, Eccentric cam unit, shake table and control unit.Simple pore water pressure measuring device is used. Pipette with graduations marked upto 10cc with the least count of 1cc is used to measure the water head. Bottom end of the pipette is connected to the pore water pressure measuring point through a flexible tube.

Sand bed preparation is done by conventional rainfall technique and height of the bed is about 15cm.Sand bed is saturated by supplying with pre-calculated amount of water through inlet of liquefaction container without any leakages. Finally saturated bed is subjected to the uniaxial , horizontal vibration of known amplitude and frequency. Variation of porewater pressure is noted.The liquefaction resistance of a particular sand-silt mixture is studied for different frequency values, 2Hz, 3.5Hz and 5Hz.

Conclusion:

From the graphical results,Maximum pore water pressure in loose condition is higher than in the dense condition, hence maximum pore water pressure is inversely proportional to relative density. So in case of loose condition of relative density, the number of voids are more and as vibration continues due to settlement of sand dissipation of water becomes difficult hence their should be increase in pore water pressure but in case of dense condition due to easy dissipation of water further increases in the pore water pressure is not allowed. The liquefaction resistance of sand-silt mixture decreases with the increase in frequency. The maximum pore water pressure during the liquefaction of sand-silt mixtures increases with increase in frequency of vibration. Maximum pore water pressure develops at 30% silt content.Liquefaction resistance of sand-silt mixtures decreases upto silt content 30%, silt content beyond which the resistance increases, hence 30% silt is considered as limiting value.

Scope For Future Work:

The main aim of present work was to study the effect of silt content and base vibration on clean sand with varying relative density as loose, medium and dense conditions.

This work has some limitations due to time and equipment constraints.

1. Study on liquefaction resistance of sand-silt mixtures by varying amplitude and frequency.

2. Determination of liquefaction resistance of sand-silt mixtures by adding some percentage of clay to the mixture.

3. This study can be carried out by using the Triaxial vibratory equipment as they better represent earthquake conditions.

4. The study can be carried out by measuring the pore water pressure top, middle and bottom of container.

5. The study on liquefaction resistance can be carried out at by keeping different loads on sand bedwith different grades of sand.

6. The study can also be carried out by layered soil.(loose-medium, medium-dense and vice-versa)

Reference

[1] Choy Soon Tan, Amination Marto, Tiong Kung Leong and Lie Siang Teng, “The Role of Fines in Liquefaction Susceptibility of Sand Matrix Soil”, EJGE, Vol.18, 2013.

[2] K. Rangaswamy, A. Boominathan and K. Rajagopal, “Comparative Study on Liquefaction Resistance of Sand and Silty soi”, GEOTIDE, Indian Geotechnical Conference, Guntur, 2009.

[3] Desmond C A Andrews and Geoffrey R Martin, “Criteria for Liquefaction of Silty soils”, Journal of 12WCEE, 2000.

[4“Horizontal Shake table manual”- Eccentric cam type by Milenium Technologies PVT LTD. Bangalore

Project Done By Aishwaraya Chatnihal, Aishwaraya , Akshata Patil, Kavya Bennur






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