a low-cost base-isolation system on artificial soil ...

Published by Elsevier Science Ltd. All rights reserved 12th European Conference on Earthquake Engineering Paper Reference 661 (quote when citing this paper)

A LOW-COST BASE-ISOLATION SYSTEM ON ARTIFICIAL SOIL LAYERS WITH LOW SHEARING RESISTANCE I.N. Doudoumis1, P. Papadopoulos1 & T. Papaliangas2 1

Aristotle University of Thessaloniki, Department of Civil Engineering, 54006 Thessaloniki, GREECE. 2 Techn. Educ. Instit. of Thessaloniki, Department of Civil Engineering, 14561 Thessaloniki, GREECE.

ABSTRACT In this paper the concept of interposing an artificial soil layer between the superstructure and the natural foundation soil of the buildings is studied. This soil layer has a low shearing resistance, which allows the slipping of the building under the action of strong seismic motions. The reduction of the inertia forces at the superstructure, as well as the size of the expected values of the basement slipping, were investigated through an analytical study in a series of building models with varying number of storeys. The buildings were subjected to the excitation of certain characteristic earthquakes taken place in Greece during the last decades and the results obtained showed the effectiveness of this base-isolation concept. Keywords: Base isolation; slipping foundation; sliding structures

INTRODUCTION Recent strong earthquakes all over the world (El Salvador, India, etc.), which have caused extensive collapse to the totality of buildings over certain regions, put in mind the necessity that the buildings must be able to resist in strong seismic motions (probably substantially higher than those specified by the codes) without using specialised and expensive construction techniques. The application of traditional base-isolation devices is an efficient alternative, which however demands an increased financial cost for the analysis and construction phase of the load-bearing structural system and consequently it can not be easily applied to a wide range of ordinary buildings. In the present paper, the concept of interposing a slender artificial soil layer between the superstructure and the natural foundation soil of new buildings is studied analytically. This artificial soil layer is characterised by a low shearing resistance, which allows (under the action of moderate and strong seismic motions) the relative slipping between the structure and its foundation soil, thus behaving as a low cost base-isolation and energy-dissipation system. The system can be applied when the slipping of the building can take place without any limitations caused by the lateral passive reaction of the surrounding soil or other adjacent buildings. In such cases, proper construction details and additional devices can control the maximum acceptable value of the relative slipping and keep the mechanical properties of the interposed soil layer to be unchanged in time.

The idea of interposing a soil layer between the superstructure and the natural foundation soil, was first introduced on 1909 by Calantarients [1], who suggested that in seismic regions the buildings can be built on a layer of fine sand, mica, or talc allowing sliding during an earthquake. In his proposed system, special care was given to the construction details of the utility lines (gas and sewage) to withstand large relative displacements. Since, this idea though an appealing one, it has not been widely accepted and it has been applied on rather a limited number of building structures. Considering the analytical investigations done on the topic of sliding structures, it is noted that a lot of effort has been spended rather for the case of slipping foundations without uplifting posibility [2], than for the case of slipping foundations with uplifting posibility. In this paper a quantitative analytical evaluation of the effectiveness of the interposed soil layer is attempted, considering both the uplifting and slipping possibility of the thereby seated buildings, under the action of characteristic seismic motions taken place in Greece during last decades.

MECHANICAL PROPERTIES OF THE INTERPOSED SOIL LAYER Low shearing resistance of the interposed artificial soil layer can be provided by suitable natural materials, for example granular products of rocks containing low friction minerals (talc, chlorite, serpentine etc.), or high plasticity clays (e.g. montmorillonitic clays) or a combination of them in a appropriate arrangement. Granular rock products of this type will provide on one hand a basic frictional behaviour with relatively low shearing resistance and on the other hand the required strength in compression. The shearing resistance can be reduced if a substance with “lubrication” action is added. Wet bentonite is a natural material, which presents such lubrication properties and negligible cohesion, so that the overall shear behaviour remains essentially frictional. Further reduction in shearing resistance can be achieved if an arrangement allowing for sliding along a predetermined flat surface, for example a concrete slab, is used. In this study the following soil types and arrangements were examined, in order to determine the sliding friction coefficients: 1) Three types of gravel of different origin, mineralogy and surface roughness • Thessaloniki talc schist gravel. • Chalkidiki serpentinite schist gravel and • Axios river gravel. 2) Three mixes of gravel and bentonite • Thessaloniki talc schist gravel with bentonite, • Chalkidiki serpentinite schist gravel with bentonite, and • Axios river gravel with bentonite 3) Interfaces between soils and rock or concrete slab . • Thessaloniki talc schist gravel with bentonite on a talc schist slab • Chalkidiki serpentinite schist gravel with bentonite, on a cement mortar slab • Axios river gravel with bentonite on a cement mortar slab The two rock types chosen, i.e. the talc schist from Thessaloniki area and the serpentinite schist from Chalkidiki area, were selected because they consist of platy minerals with low shearing resistance and adequate strength in compression. The average grain size distribution

of the two types of schist gravel is: 98%