EEFIT mission to Haiti following the 12th January 2010 earthquake

The Mw= 7.2 Haiti earthquake of 12th January 2010 caused extensive damage to buildings and other infrastructure in the epicentral region in and around Port-au-Prince. The Earthquake Engineering Field Investigation Team (EEFIT), which is based in the United Kingdom, organised a field mission to Haiti with the authors as the team members. The paper presents the geotechnical findings of the team including those relating to soil liquefaction and lateral spreading and discusses the performance of buildings, including historic buildings, and bridges. Unprecedented use was made of damage assessments made from remote images (i. e. images taken from satellites and aircraft) when planning the post-earthquake relief effort in Haiti and a principal objective of the team was to evaluate the accuracy of such assessments. Accordingly, 142 buildings in Port-au-Prince were inspected in the field by the EEFIT team; damage assessments had previously been made using remote images for all these buildings. On the basis of this survey, the tendency of remote assessments to underestimate damage was confirmed; it was found that the underestimate applied to assessments based on oblique images using the relatively new technique of Pictometry, as well as those based on vertical images, although to a lesser degree. The paper also discusses the distribution of damage in Port-au-Prince, which was found to be strongly clustered in ways that appear not to have been completely explained. © 2012 Springer Science+Business Media B.V.

How Well Do We Understand Earthquake Induced Liquefaction?

Soil liquefaction following large earthquakes is a major contributor to damage to infrastructure and economic loss, as borne out by the earthquakes in Japan and New Zealand in 2011. While extensive research has been conducted on soil liquefaction and our understanding of liquefaction has been advancing, several uncertainties remain. In this paper the basic premise that liquefaction is an 'undrained' event will be challenged. Evidence will be offered based on dynamic centrifuge tests to show that rapid settlements occur both in level ground and for shallow foundations. It will also be shown that the definition of liquefaction based on excess pore pressure generation and the subsequent classification of sites as liquefiable and non-liquefiable is not satisfactory, as centrifuge test data shows that both loose and dense sand sites produce significant excess pore pressure. Experimental evidence will be presented that shows that the permeability of sands increases rapidly at very low effective stresses to allow for rapid drainage to take place from liquefied soil. Based on these observations a micro-mechanical view of soil liquefaction that brings together the Critical State view of soil liquefaction and the importance of dynamic loading will be presented. © 2012 Indian Geotechnical Society.

Excess pore pressures around underground structures following earthquake induced liquefaction

Underground structures located in liquefiable soil deposits are susceptible to floatation following an earthquake event due to their lower unit weight relative to the surrounding saturated soil. This inherent buoyancy may cause lightweight structures to float when the soil liquefies. Centrifuge tests have been carried out to study the excess pore pressure generation and dissipation in liquefiable soils. In these tests, near full liquefaction conditions were attained within a few cycles of the earthquake loading. In the case of high hydraulic conductivity sands, significant dissipation could take place even during the earthquake loading which inhibits full liquefaction from occurring. In the case of excess pore pressure generation and dissipation around a floating structure, the cyclic response of the structure may lead to the reduction in excess pore pressure near the face of the structure as compared to the far field. This reduction in excess pore pressure is due to shear-induced dilation and suction pressures arising from extensile stresses at the soil-structure interface. Given the lower excess pore pressure around the structure; the soil around the structure retains a portion of this shear strength which in turn can discourage significant uplift of the underground structure. Copyright © 2012, IGI Global.

Re-mobilization of pile shaft friction after an earthquake

During strong earthquakes, significant excess pore pressures can develop in saturated soils. After shaking ceases, the dissipation of these pressures can cause significant soil settlement, creating downward-acting frictional loads on piled foundations. Additionally, if the piles do not support the full axial load at the end of shaking, then the proportion of the superstructure's vertical loading carried by the piles may change as a result of the soil settlement, further altering the axial load distribution on piles as the soil consolidates. In this paper, the effect of hydraulic conductivity and initial post-shaking pile head loading is investigated in terms of the changing axial load distribution and settlement responses. The investigation is carried out by considering the results from four dynamic centrifuge experiments in which a 2 × 2 pile group was embedded in a two-layer profile and subjected to strong shaking. It is found that large contrasts in hydraulic conductivity between the two layers of the soil model affected both the pile group settlements and axial load distribution. Both these results stem from the differences in excess pore pressure dissipation, part of which took place very rapidly when the underlying soil layer had a large hydraulic conductivity.

Lateral spreading-induced abutment rotation in the 2011 Christchurch earthquake: Observations and analysis

A series of strong earthquakes near Christchurch, New Zealand, occurred between September 2010 and December 2011, causing widespread liquefaction throughout the city's suburbs. Lateral spreading developed along the city's Avon River, damaging many of the bridges east of the city centre. The short-to medium-span bridges exhibited a similar pattern of deformation, involving back-rotation of their abutments and compression of their decks. By explicitly considering the rotational equilibrium of the abutments about their point of contact with the rigid bridge decks, it is shown that relatively small kinematic demands from the laterally spreading backfill soil are needed to initiate pile yielding, and that this mode of deformation should be taken into account in the design of the abutments and abutment piles.

On the use of densification as a liquefaction resistance measure

The use of densification to improve the performance of shallow foundations during the centrifuge modeling of earthquake-induced liquefaction on level sand deposits is discussed. The densification of liquefiable ground provided protection against or significantly reduces liquefaction-related damage. Propagation of accelerations in the deposit exhibited considerable distinct features according to the relative density of the sand in the model. It was found that during the first couple of cycles, the dense soil amplifies the fundamental frequency component of the earthquake and preserves the higher frequency components.