Toward developing an engineering procedure for evaluating building performance on softened ground

by Dashti, Shideh, Ph.D., UNIVERSITY OF CALIFORNIA, BERKELEY, 2009, 907 pages; 3383604

Abstract:

Seismically-induced settlement of buildings with shallow foundations on liquefiable soils has resulted in significant damage in past earthquakes. Previous studies have identified factors such as shaking intensity, building weight, foundation width, and liquefiable soil layer properties as being important in the response of structures founded on liquefied soils. However, the state of practice still largely involves estimating building settlement using procedures developed to calculate post-liquefaction consolidation settlement in the free-field. Estimating building settlement based on free-field post-liquefaction reconsolidation volumetric strains neglects the importance of other mechanisms that could damage buildings. Current procedures ignore these important mechanisms, and thus they do not capture the true consequences of liquefaction, nor do they offer a sound approach for evaluating proposed mitigation schemes.

Without a sufficient number of well-documented field case histories, key parameters that affect soil and structural response need to be identified and studied through carefully performed physical model tests. Well-calibrated analytical tools that capture correctly the effects of key parameters can then be developed to evaluate soil response and the influence of each settlement mechanism. Liquefaction remediation techniques can only be designed effectively if they are shown to control the dominant settlement mechanisms.

A series of four centrifuge experiments involving buildings situated atop a layered soil deposit were performed to identify the dominant mechanisms involved in liquefaction-induced building settlements and to investigate the role of key testing parameters. The results indicated that building settlements were not proportional to the thickness of the liquefiable layer and that considerable building settlement occurred during earthquake strong shaking. The development of high excess pore water pressures, localized drainage in response to the high transient hydraulic gradients, and earthquake-induced ratcheting of the buildings into the softened soil were observed to be important effects that should be captured in procedures developed to estimate liquefaction-induced building settlement.

The effective application of liquefaction mitigation techniques requires an improved understanding of the development and consequences of liquefaction. The last centrifuge experiment in the series of four was designed to study further the relative importance of the dominant liquefaction-induced building settlement mechanisms while simultaneously providing insights on the effectiveness of potential mitigation strategies. In this experiment, installation of a buried stiff structural wall minimized shear-type soil deformations under the building, which reduced total building settlements by up to 55%. Use of a flexible impermeable barrier that inhibited horizontal water flow without preventing shear movements did not reduce permanent building settlements significantly, but did increase building tilt. The relative importance of each mechanism was shown to depend on the characteristics of the earthquake motion, liquefiable soil, and building. The initiation, rate, and amount of liquefaction-induced building settlement depended greatly on the rate of ground shaking. Engineering design procedures should incorporate this important feature of earthquake shaking, which can be represented by the time-rate of Arias Intensity (i.e., the Shaking Intensity Rate, SIR). An improved understanding of the effects of testing parameters on various settlement mechanisms is intended to advance the predictive capabilities of numerical simulations and design procedures. This is also a necessary step toward mitigating effectively the consequences of liquefaction on buildings.

AdviserJonathan D. Bray
SchoolUNIVERSITY OF CALIFORNIA, BERKELEY
Source TypeDissertation
SubjectsGeotechnology; Civil engineering
Publication Number3383604

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