Keywords: saturated sand, effective stress analysis, nonlinear soil modelling, response spectrum, structural dynamics, reinforced concrete frames, water table depth.

In case of saturated sands, the sand-pore water couple constitute a two phase medium. Under cyclic conditions, the pore water pressure increases due to the densification of the sand particles. Consequently, both the effective stress and the shear modulus decrease. This behaviour of the saturated sands may lead to considerable differences in the surface amplification depending on the depth of water table [1,2,3,4].

Currently seismic codes classify the subsoil conditions based on the shear wave velocities. The spectra used in codes are based on the earlier research done by Seed et al. [5] and Newmark and Hall [6]. In both studies, actual field data recorded in various earthquakes at different locations namely with different site conditions and different water table depths.

This study investigates the potential effects of ground water level on the seismic response of buildings through a three stage approach. The first stage of the study is the determination of seismic motion at the base rock level. In the second stage, using the records obtained, surface accelerations and response spectra are calculated for varying levels of ground water table using an effective stress analysis software, LASS-IV [7]. In the final stage, the surface motions obtained in the second stage are applied to reinforced concrete frames of different heights to demonstrate the effects of ground water table depth using SAP2000.

The results of the study showed that the existence of ground water in the sand type subsoil has a damping effect on the surface response of the amplified base motion and consequently affects the seismic response of the buildings.

1

K. Ishihara, I. Towhata, "One-dimensional Soil Response Analysis During Earthquake Based on Effective Stress Method", Journal of the Faculty of Engineering, University of Tokyo, XXXV(4), 656-700, 1980.

2

M.D. Trifunac, M.I. Todorovska, "Nonlinear Soil Response as a Natural Passive Isolation Mechanism - The 1994 Northridge California Earthquake", Soil Dynamics and Earthquake Engineering, 17(1), 41-51, 1998. doi:10.1016/S0267-7261(97)00028-6

3

G.T. Zorapapel, M. Vucetic, "The Effects of Seismic Pore Water Pressure on Ground Surface Motion", Earthquake Spectra, EERI, 10(2), 403-437, 1994. doi:10.1016/S0267-7261(97)00028-6

4

T.L. Youd, B.L. Carter, "Influence on Soil Softening and Liquefaction on Spectral Acceleration", Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 131(7), 811-825, 2005. doi:10.1061/(ASCE)1090-0241(2005)131:7(811)

5

H.B. Seed, C. Ugas, J. Lysmer, "Site Dependent Spectra for Earthquake Resistant Design", Bulletin of the Seismological Society of America, 66(1), 221-243, 1976.

6

N.M. Newmark, W.J. Hall, "Earthquake Spectra and Design", EERI Monograph, Berkeley, California, USA, 103, 1983.

7

S.U. Dikmen, J. Ghaboussi, "Effective Stress Analysis of Seismic Response and Liquefaction: Theory", Journal of Geotechnical Engineering, ASCE 110(5), 628-644, 1984. doi:10.1061/(ASCE)0733-9410(1984)110:5(628)

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