CHAPTER 11 LIQUEFACTION PERFORMANCE OF SHALLOW FOUNDATIONS IN PRESENCE OF A SOIL CRUST George Bouckovalas1 and Panos Dakoulas2 Geotechnical Engineering, National Technical University of Athens, Greece gbouck@central.ntua.gr (www.georgebouckovalas.com) 2 Department of Civil Engineering, University of Thessaly, Volos, Greece dakoulas@uth.gr 1 Abstract. Liquefiable soils are currently categorized by all seismic codes as extreme ground conditions where, following a positive identification of this hazard, the construction of surface foundations is essentially allowed only after proper treatment soil. This article examines to what extent this situation may change in presence of a non-liquefiable soil crust, between the foundation and the liquefiable soil. Means are provided for analytical evaluation of the degraded bearing capacity and the associated seismic settlements for the specific case of strip foundations on a cohesive (clay) crust. Furthermore, the conditions are explored which ensure a viable performancebased design, and the issue of a critical soil crust thickness, beyond which liquefaction effects are minimal, is addressed. 1. Introduction Building a well engineered surface foundation directly upon the surface of a liquefiable soil layer, without prior improvement or reinforcement, is clearly out of the question, since settlements will be excessive and uneven, leading to structural, as well as operational failure. The foundation failures shown in Figure 11.1 are merely some typical examples from the ensuing hazard. However, it is possible that such a solution becomes feasible in the presence of a sufficiently thick and shear resistant non-liquefiable soil crust (e.g. clay, dense or dry sand and gravel, improved soil) between the foundation and the liquefiable subsoil. The reason is simple: as the thickness of the non-liquefiable soil crust increases gradually, beyond the maximum depth of a Prandtl type failure mechanism, failure is likely to develop exclusively into that layer and consequently any liquefaction of the subsoil will have a minor effect on the post-shaking failure load and the associated settlements. Thus, the question is not whether there is a beneficial effect of the non-liquefiable soil crust, but what this effect is and whether it is of engineering interest. In providing a satisfactory overall answer to these questions, one must first resolve the following design issues: (a) What is the bearing capacity of surface foundations on a liquefied subsoil, in the presence of a non-liquefied soil crust? (b) What are the liquefaction-induced settlements of the footing in the above case? 245 K.D. Pitilakis (ed.), Earthquake Geotechnical Engineering, 245–276. c 2007 Springer.  246 George Bouckovalas and Panos Dakoulas Fig. 11.1. Liquefaction induced failures of shallow foundations. (a) Caracas Venezuela, 1967, M = 6.5, (b) Dagupan, Philippines, 1990, M = 7.8, (c) Chi–Chi Taiwan, 1999, M = 7.3, and (d) Adapazari, Turkey, 1999, M = 7.4 (c) Is it meaningful to seek an acceptable (in terms of bearing capacity and seismic settlements) allowable bearing pressure, or the range of the corresponding soil and foundation conditions is too narrow for any practical application, so that soil treatment is always inevitable? This article deals with the above issues, based on published evidence, as well as on nonpublished research which is currently underway at the Geotechnical Division of N.T.U.A. To remain focused, and also to respect the length limits of the presentation, the article will address only the case of strip foundations resting upon the free surface of a liquefiable soil profile with a cohesive (clay) crust. However, many of the concepts, methods and data presented herein have more general applicability, and can be independently used to extend the findings of the article to other foundation and soil profile types. 2. Existing background 2.1. PERFORMANCE-BASED DESIGN REQUIREMENTS Figure 11.2 presents results from a typical numerical analysis, similar to the ones which will be presented later in this article, which simulates the static and the seismic loading of a rigid foundation on liquefiable soil. It may be observed that: (a) During shaking (part bc of the load–settlement curve), the footing settles without any change of the static load Q. Seismic settlements may become considerably