International Journal of Civil and Structural Engineering Research ISSN 2348-7607 (Online) Vol. 7, Issue 2, pp: (116-124), Month: October 2019 - March 2020, Available at: www.researchpublish.com
Seismic Response of Structures Rested on Improved Soils Ayman Abd-Elhameda,b a
Faculty of Engineering at Mataria, Helwan University, Cairo, Egypt.
b
Faculty of Engineering & Technology, Egyptian Chinese University Email: aymanm79@hotmail.com
Abstract: The objective of the current study is to study the seismic response of structures on improved soil condition using stone columns as a liquefaction countermeasure. For this purpose, numerical simulations of a building modelled as single degree-of- freedom (SDOF) is performed using time history analysis considering two different cases of the beneath the model. The first case deal with the original unimproved soil, while the second one considers an improved base soil supporting the SDOF building model. An efficient discrete springs and dashpots to represent the sway and rocking movements of the soil mass. Moreover, the equivalent-linear approach is used to numerically simulate the seismic soil response. Three different records of different intensities are used to excite the SDOF model to perform the analysis. The examined cases indicate that the use of improvement technique may simultaneously have a beneficial or harmful effect on the global seismic response of the system during earthquake loads. Keywords: Soil improvement; Vibro stone columns; Dynamic soil properties; SDOF; Liquefaction.
I. INTRODUCTION During interdisciplinary studies of past and recent earthquakes, soil-structure interaction (SSI) has been found to significantly affects the seismic response of superstructures and foundations [1, 2]. When a structure is subjected to strong ground shakings, foundation oscillates depending on the supporting soil type, the foundation, and inertia of the superstructure. Recent studies proved that seismic response of structures founded on soft soil may significantly differ from those that founded on improved soil [3]. This fact is principally due to the influence of soil properties modification on the seismic response of the structure. In the geotechnical engineering field, liquefaction has been a matter of great interest for more than four decades. Liquefaction of supporting soil is a phenomenon occurs in saturated cohesion less soils subjected to ground shakings of long duration. Most structures whose foundations stand directly on liquefiable soil will experience significant damage. In addition to this, foundations settled due to dominant liquefaction-induced building displacement [4, 5]. Discrete vibro-stone columns are an acceptable method of subsoil improvement. It is often used for liquefaction mitigation of loose sand potentially subjected to severe earthquakes. The stone column technique is an acceptable method that is used since late 1950s to improve the strength parameters of soil and mitigate seismicallyinduced liquefaction hazards to structures rested on liquefiable soils. The construction of stone columns involves adding crushed or cobbled stones vertically that can be arranged to form a group of columns of either triangle or square forms with specified gap distances, diameters and distances. These formed groups are put below the level of base foundation of depth no less than 4 m [6, 7]. Unlike other techniques of soil improvement, the use of vibro technique induces slight vibrations that guarantees an appropriate choice for mitigating hazards due to the induced liquefaction. Moreover, these traditional techniques for improvement of ground requires a relatively short period installation time. The present paper aims to investigate the performance of the coupled soil -structure system supported on unimproved soils under seismic actions. For simplicity, a SDOF structure and improved-soil-foundation structure system shown in figure 2 is also considered in the analysis to examine the influence of installation of stone columns on the global responses of foundations and structures. Different seismic records from different regions are used to excite the building model. The equation of motion is derived, and numerical attempts to evaluate the seismic responses of coupled soil -structure system are utilized using a developed discrete time state space equation in time domain. Moreover, this paper focuses on the effects of reinforced soil by stone columns on the induced responses and peak responses for both base and the superstructure of a SDOF model founded on original and improved soil and seismically excited by three different ground motions.
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