Engineering Geology

Engineering Geology

Comparison of Standard Penetration Test (SPT) and Shear Wave Velocity (Vs) Methods in Determining Liquefaction Hazard along Tabriz Metro Line 2

Authors
Delara Oshnaviyeh
Rouzbeh Dabiri2
10.18869/acadpub.jeg.12.2.183
Abstract
Introduction

When loose sand is subjected to seismic shaking, it tends to volume reduction and settlement. The density of the under layers is revealed in the settlement of the ground surface that causes the destruction of the structures located on ground surface. In dry sand layer, settlement in severe shakings occurs under a constant and effective stress condition and very rapid stress. In this regard, the sand deposit settlement is completed before the end of an earthquake, but if the sandy soil layer is saturated and drainage is limited the condition is prepared of fixed volume situation and the major effect of the seismic shocks is generation of exceed pore water pressure. Therefore, the deposit settlement of saturated sand requires a longer time, varying from a few minutes to a few days, depending on the permeability and compressibility of the soil and the length of the drainage path. The main purpose of the present study is to evaluate liquefaction hazards along Tabriz Metro Line 2 with using Standard Penetration test (SPT) and shear wave velocity (Vs) methods. Also, the probable rate of settlement in the soil layers in study area and correlation with liquefaction potential index (LPI) according to both procedure have been determined and discussed in the following paragraphs.

Material and Methods

In order to evaluate the liquefaction potential of soils using two field methods, geotechnical information of 54 boreholes in Tabriz Metro Line 2 were collected. The types of soil and geotechnical properties can affect the liquefaction potential. In this study, the gravely sand, silty sand, silty and sandy soils were studied. Ground water level is one of the main parameters in in soil liquefaction potential evaluation of soils. Variation of water level in boreholes is 2 to 30 meters. The peak ground acceleration (PGA) is necessary for the analysis of boreholes to evaluate liquefaction potential of soils. PGA values were selected in each boreholes position according to the Iranian Code of Practice for Seismic Resistant Design of Buildings (Code-2800-ver.4) equal to 0.35g (for return period 475 years and design life 50 years). Liquefaction potential of soil layers based on SPT results with appliying Idriss and Boulanger (2010) method has been assessed. Andrus, Stokoe and Jung (2004) procedure was used in shear wave velocity (Vs) method (with assuming cementation and un cementation condition in soils). Liquefaction potential index (LPI) of soil layers was calculated for both field tests results. Then, probable rate of settlement due to liquefaction in saturate soil layers was determined. Tokimatsu and Seed (1978) method applied for SPT results, Yoshimine (1992), Yoshimine et al. (2006) and Yi (2009) procedures have been used in Vs test. Finally, correlation between rate of settlement and LPI results were determined.

Results and discussions

Outcomes of this study can be explained in below:

1. Results obtained from comparison of both methods in liquefaction potential evaluation have been showed, agreement between two methods have been happened rarely. Specially, with assuming cementation condition in soils, LPI obtained from Vs method is more than SPT. Although, different factors can be affected at uncertainties in SPT results such as type of drilling machine, energy efficiency and accuracy of test performing. Also, in shear wave velocity method, maximum velocity for occurring liquefaction in soil layers () related to fines content percentage. It is possible that boundary values in procedure not compatible with geotechnical properties in study area.

2. Evaluation of probable rate of settlements in soil layer in study area have been showed that settlement values obtained from Vs is more than SPT. This condition is compatible with LPI amounts.

Conclusions

In sum up, settlement due to liquefaction in saturate soil layers is one of the important phenomena in geotechnical earthquake engineering. Maximum rate of settlement in soil layers in study area is equal 0.45m based on SPT method and 0.9m according to Vs procedure which should be considered. Accordingly, serious damages can be inflicted to buildings, underground structures and life lines in study area. Therefore, it is suggested in future researches with using empirical and numerical (or soft computing) methods based on field and experimental tests results a detailed assessment conducted and influence of various parameters on settlement of soil layers be determined and the items listed below should be considered:

- Cementation parameter (C) values of soils in shear wave velocity method maybe not compatible with geotechnical properties in study area. It should be evaluated exactly.

- In this research, peak ground acceleration (PGA) value was selected based on code 2800-ver.5. As regard to Tabriz Metro Line 2 is beside to Tabriz North Fault, PGA value according to historical earthquake catalogue and seismic risk analysis should be evaluated and seismic hazard have to determine with accuracy.
Keywords
Liquefaction
Settlement
Tabriz Metro Line 2
Standard Penetration Test (SPT)
Shear Wave Velocity (Vs)
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