evaluation of liquefaction potential of soil using cpt and...

ISBN 978-93-84422-56-1 Proceedings of International Conference on Architecture, Structure and Civil Engineering

(ICASCE'15) Antalya (Turkey) Sept. 7-8, 2015 pp. 1-7

Evaluation of Liquefaction Potential of Soil using CPT and SPT

Alketa Ndoj1, Neritan Shkodrani

1, Veronika Hajdari

1

1Polytechnic University of Tirana, Faculty of Civil Engineering, Tirana, Albania

Abstract: The process of saturated granular soil strength and resistance reduction due to increase of pore water

pressure during the earthquake, is named liquefaction. Liquefaction of saturated sandy deposits is one of the natural causes of earthquake disasters. Nigata earthquake, in 1964, in Japan, and the earthquake of Alaska, in 1964 caused dramatic damages, but they are considered as a big help on identification of liquefaction phenomena, as an important problem of Earthquake Engineering. The aim of this study is to evaluate the liquefaction, based on the data collected by Piezocone Test (CPTU) and Standard Penetration Test (SPT). These data are used to evaluate the liquefaction for a case study, in a coastal area of Albania, located in from Lalezi Bay to Hamallaj area, near Durresi city, where several residential buildings and resorts useful during the summer season are foreseen to be built. The site investigation was carried out in August - December 2014 and included 8 CPTU tests until a maximal depth of 20 m. Corrected values of SPT are taken by the derived geotechnical parameters by CPTU results. According to this study, in this area sands, gravelly sands, silty sands and clays are present. The water level is located very close to ground surface, at around 0.5 - 2.0 m under the ground surface. The highest moment magnitude registered in this area is 6.2 and it belongs to Durresi city, for the earthquake happened in December 1926. Liquefaction potential estimation as a function of the factor of safety against triggering of liquefaction, based on the results of site investigations, is conducted for a given moment magnitude of earthquake (M=6.2) based on Cyclic Stress Method. Cyclic stress ratio is determined based on Simplified Procedure by Seed and Idriss 1971. Cyclic resistance ratio is defined based on half empirical methods, by using the results in situ tests (CPT and SPT method). In Lalezi bay - Hamallaj area, liquefaction estimation from CPT based method show that all the soils are classified as “like sands”, below the water level where the liquefaction may occur. Also the SPT based method show that the liquefaction may occur in these soils, but the thickness of liquefiable layers of soils is smaller.

Keywords: Liquefaction, Standard Penetration Test, Cone Penetration Test, Factor of safety.

1. Introduction

Earthquakes are the most catastrophic natural phenomena. They cause damages in engineering structures and loose of human lives. The process of saturated granular soil strength and resistance reduction due to increase of pore water pressure during the earthquake, is named liquefaction. Liquefaction of saturated sandy deposits is one of the natural causes of earthquake disasters. Numerous methods have been proposed for evaluating the liquefaction potential of clean and silty sands soils, such as Cyclic Stress Method (Seed and Idriss 1971), Cyclic Strain Method (Dobry and Ladd 1980; Dobry et al., 1982), Effective Stress Method (Martin et al., 1975, Ishihara and Towhata 1980, Finn and Bahatia 1981) etc. The most used method by many authors is the Cyclic Stress Method. This method was initially developed by Seed and Idriss 1967 and then it was improved by other authors as Seed and Idriss 1971, Shibata 1981, Tokimatsu and Yoshimi 1983, Seed et al., 1985, Youd et al., 2001, Cetin et al., 2004, Idriss and Boulanger 2004[1]. This method compares the cyclic stress ratio induced by the earthquake (CSR) with cyclic resistance ratio (CRR) in the ground. Liquefaction is expected to occur in those depths where the cyclic stress ratio exceeds the cyclic resistance ratio. Cyclic stress ratio is determined based on Simplified Procedure by Seed and Idriss 1971[2]. Cyclic resistance ratio can be defined based on half empirical methods, by using the results of laboratory or in situ tests. In situ tests that are widely used to estimate the soil potential liquefaction are Standard Penetration Tests (SPT), Piezocone Tests (CPTU), Shear wave velocity

http://dx.doi.org/10.17758/UR.U0915314 45

measurements (Vs), Seismic Dilatometer Marchetti Tests (SDMT), etc. Based on this tests are developed several methods to estimate the liquefaction, such as SPT, CPTU, and Vs based methods. Standard Penetration Tests (SPT) and Piezocone Tests (CPTU) are two of the most used in situ testing techniques to estimate the liquefaction. Nowadays CPTU tests are very used for liquefaction evaluation, because the results of these tests are very reliable and continuous. The CPT and SPT methods are used in this paper for evaluating the potential of soil liquefaction in Lalezi bay - Hamallaj area, in Albania.

2. Site Description

The site is located in Lalezi Bay and Hamallaj, near Durresi city, in Albania. A map of this area is shown in Figure1. This area is selected to estimate the liquefaction potential, because it has a great construction potential, which are very frequented during the summer season. The studied area is laid along the Adriatic coast, in the area of sandy beach, with a total length of approximately 10km.

The site investigation was carried out in August - December 2014 and included 8 CPTU tests until a maximal depth of 20 m. The positions of these tests are also shown in Figure1. According to this study sands, gravelly sands, silty sands and clays are present in this area. The water level is located very close to ground surface, at around 0.5 - 2.0 m under the ground surface [3].

This area is situated near Adriatic lowlands, always under the action of repressive movements post Plyocen (PL zone), which includes the hilly and field terrains of molasic basin near Adriatic. It is laid from Lezha district in the north to Vlora district in the south and it is characterized from a high seismic activity.

According to seismic active map of Albania, the expected moment magnitudes of the earthquakes varies from 5.5 to 6 [4]. The highest moment magnitude registered in this area is 6.2 and it belongs to Durresi city, for the earthquake happened in December 1926. The qualitative analysis for the factors that control the liquefaction phenomena, based on soil type, hydro geological conditions, seismic activity show that these soils are susceptible to liquefaction.

3. Liquefaction Assessment

Liquefaction assessment requires the evaluation of the parameters as below:

Earthquake induced Cyclic Stress Ratio

(CSR) Cyclic Resistance Ratio (CRR)

Factor of safety against triggering liquefaction (FS)

The Peak ground accelerations calculated using a probabilistic method (PSHA) for risk levels such as 50% in 50 year, 10% in 50 year and 2% in 50 year are 0.22g, 0.356g and 0.475g, respectively. These calculations are

performed based on maximal moment magnitude, Ms ≥ 4 for 44 years, from year 1970 until year 2013, using the information taken by earthquake catalogue for this area. According to Design Albanian Code KTP-N.2-89, the soils in Lalezi Bay and Hamallaj area are classified at Category III, based on geotechnical report. For this soil category, the Peak Ground Acceleration for maximal expected intensity in this area (for I = 8ballë, MSK-64 according to seismic regionalization of Albania) is 0.26g, which can be considered as the first level of risk (serviceability earthquake). 3.1. Earthquake induced Cyclic Stress Ratio

Earthquake induced Cyclic Stress Ratio is calculated based on Seed-Idriss method, using the recommendations of Idriss and Boulanger. In this area, CSR is estimated for the moment magnitude of the earthquake equal to M = 6.2, which is the maximum magnitude, and the Peak Ground Acceleration equal to

= 0.26g. 3.2. Cyclic Resistance Ratio

CRR is calculated according to CPT and SPT methods, based on in situ tests data. Both methods used in this paper for soil liquefaction evaluation are presented by Idriss and Bulanger [1][5]. The equations proposed by authors, as a function of equivalent values of penetration resistances of SPT and CPTU ( , ), are


used to evaluate the liquefaction in this area. These resistances take into consideration the fine content effects therefore equations are used to estimate the liquefaction in case of clean sands and silty sands. The used data in these calculations are: cone tip resistance, sleeve friction, corrected N value of SPT, ( ), taken by the derived geotechnical parameters by CPTU results, moment magnitude of earthquake, Peak Ground Acceleration, water level from ground surface and bulk density of soil. 3.3. Factor of safety against triggering liquefaction (FS)

Factor of safety against triggering liquefaction is determined by dividing the Cyclic Resistance Ratio with the Earthquake induced Cyclic Stress Ratio stress ratio. The soils are susceptible to liquefaction if the values of this factor result lower than 1.

Fig. 1: Lalzi Bay map and in situ site investigation plan.

4. Results

The results of calculation are shown in graphs in figures 2, 3, and 4 for 8 CPTU performed in study area.

The CPTU tests data are Con End Resistance qc and Sleeve Friction fs. Based on Robertson et al. 1986 is estimated the type of soils encountered in this area. The equivalent values of penetration resistances of SPT and CPTU ( , ) are also presented in these graphs. At the end factor of safety against the triggering of liquefaction is presented according to CPT and SPT methods.


Fig. 2: Liquefaction evaluation in CPT 01, CPT 01A and CPT 05.


Fig. 3: Liquefaction evaluation in CPT 03, CPT 02 and CPT 06.


Fig. 4: Liquefaction evaluation in CPT 07 and CPT 04.

5. Discussions and Conclusions

In this paper the liquefaction potential for Lalezi Bay and Hamallaj area, in Albania is evaluated based on

CPT and SPT methods. Quantitative analysis based on the factor of safety against the triggering of liquefaction

calculated by both methods show that the liquefaction may occur in this area. Liquefaction estimation in this

area is carried out also for Peak Ground Accelerations = 0.20g, = 0.18g, = 0.16g. The results

show that the liquefaction in this area can begin for a Peak Ground Acceleration equal to = 0.16g-018g. CPT method indicates that the liquefaction may occur in all the soils met in testing profile, below the water level that are classified as "loose sands" and “medium dense sands”, except the dense sands. The relative density in these soils is =10-70%, the fine contents FC=25-40% and the factor of safety FS=0.5-0.99. The thickness of the soil layers where the liquefaction can occur is 5.5m in CPT-05 up to 15m in CPT-04.


Also SPT based method indicates that the liquefaction may occur, but the thickness of the soil layers is smaller. The factors of safety using this method are FS=0.73-0.99. It is observed that Cyclic Resistance Ratio based on CPT method is higher than one calculated based on SPT method. Both the methods show that the liquefaction can occur to depths18-20m. So it is necessary to improve the soils in this area by different site techniques before construction of engineering structures.

6. References [1] Idriss, I. M and Boulanger, R. W, SPT-based liquefaction triggering procedures, Report No. UCD/CGM-10-02,

Center for Geotechnical Modeling, Department of Civil and Environmental Engineering, University of California Davis, California, December 2010, 259p.

[2] Seed, H.B. and Idriss, I.M. (1971) Simplified Procedure for Evaluating Soil Liquefaction Potential. Journal of the Soil

Mechanics and Foundation Division. ASCE 97(9). pp. 1249-1273. [3] In Situ Balkans Sh.P.K., Factual Report of Cone Penetration Tests (CPTU) in Lalezi Bay – Hamallaj, Albania, Report

No. B1140106R002LDH, 2014, 117p. [4] Aliaj Sh., Koçiu S., Muço B and Sultarova E, Seismicity, Seismotectonis and Seismic Hazard Evaluation in Albania,

Publication of Academy Of Sciences, 2010, Tirana-Albania, 310p. [5] Idriss, I. M and Boulanger, R. W, Soil liquefaction during Earthquake, Earthquake Engineering Research Institute,

EERI Publication, 2008, Monograph MNO-12, Oakland, CA, 261 p.


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