Assessment of Liquefaction-Induced Hazards for Immersed Structures
Authors: Travasarou Thaleia , Chacko Jacob , Chen Weiyu , Fernandez Alfredo 
Published: April 2012

The behaviour of structures founded in liquefiable soils is an area that isoften encountered in the offshore industry for facilities such as pipelines orshallow foundation elements acting either in compression or tension. Lightimmersed structures surrounded by liquefiable soils are vulnerable toliquefaction-induced hazards such as uplift and horizontal displacementdemands. Large uplifts of immersed structures have been reported followingstrong earthquakes, and observed in the laboratory. The dynamic response of animmersed structure surrounded by liquefiable soil is complex and involves thesimultaneous occurrence of phenomena such as pore pressure generation anddissipation, soil dilation and contraction, water flow, and soil structureinteraction. Due to these complexities, liquefaction induced demands onimmersed structures are generally poorly understood despite their potentialcatastrophic consequences. This limited understanding may adversely affect theability to optimize design of new structures and assess the truevulnerabilities of existing structures. Indeed, current practice often usessimplistic approaches, with binary decision making related to whetherliquefaction maybe triggered or not. However, tools and techniques are nowavailable to better evaluate the consequences of liquefaction on buriedstructures. This paper presents a case history that illustrates the applicationof these techniques for a nearshore project.

Extensive analytical and experimental work was performed to study thevulnerability to liquefaction-induced uplift and lateral displacements of anexisting relatively light (i.e., relative to the surrounding soil) immersedtube tunnel in California. The studies were performed to guide decisionsregarding the need for retrofit and subsequently to select optimal mitigationmeasures. The numerical evaluations were conducted with advanced soilconstitutive models appropriately calibrated against laboratory cyclic simpleshear and centrifuge model tests, and actual case histories. The experimentalstudy involved the design and execution of two centrifuge tests modelingrepresentative tube-soil cross sections. The favorable comparison betweennumerical results and experimental observations allowed the use of thenumerical tools in vulnerability evaluations along the whole tunnel length. Incontrast to the general belief that liquefaction-induced uplift is aforce-controlled phenomenon governed by buoyancy forces on immersed structures, both analytical and experimental studies revealed that the prevalent upliftmechanism is kinematic in nature and is associated with the ability of theliquefied soil to move around the structure. This important finding resulted inretrofit decisions with significant associated cost savings for this project. Additional uplift contributions may occur from heaving of soft soils or byvolumetric expansion due to water flow in response to a pressure gradient, which may be reversible. The results and findings of this study, are notrestricted to the specific project and can be used in analyses and design ofother structures founded within liquefiable soils.

Introduction

Liquefaction-induced soil displacements can impose significant demands onstructures surrounded by or in the vicinity of liquefiable soils, withpotential catastrophic consequences. For light or hollow structures, orstructures subjected to tension forces liquefaction-induced uplift, oftenassociated with large displacements and emergence of the initially buriedstructure at the ground surface, has been observed following strong earthquakesand during laboratory shaking table and centrifuge experiments. Conversely, heavy structures or heavily loaded structures will settle or displacedownwards. Moreover, buried structures with large extent such as immersedtunnels and pipelines may be vulnerable to horizontal demands associated withliquefaction induced lateral spreading of granular saturated soils, especiallyat the boundaries where the structures cross the spreading zone.