Stiffness of sands through a laboratory test database

Deformations of sandy soils around geotechnical structures generally involve strains in the range small (0·01%) to medium (0·5%). In this strain range the soil exhibits non-linear stress-strain behaviour, which should be incorporated in any deformation analysis. In order to capture the possible variability in the non-linear behaviour of various sands, a database was constructed including the secant shear modulus degradation curves of 454 tests from the literature. By obtaining a unique S-shaped curve of shear modulus degradation, a modified hyperbolic relationship was fitted. The three curve-fitting parameters are: an elastic threshold strain γe, up to which the elastic shear modulus is effectively constant at G0; a reference strain γr, defined as the shear strain at which the secant modulus has reduced to 0·5G0; and a curvature parameter a, which controls the rate of modulus reduction. The two characteristic strains γe and γr were found to vary with sand type (i.e. uniformity coefficient), soil state (i.e. void ratio, relative density) and mean effective stress. The new empirical expression for shear modulus reduction G/G0 is shown to make predictions that are accurate within a factor of 1·13 for one standard deviation of random error, as determined from 3860 data points. The initial elastic shear modulus, G0, should always be measured if possible, but a new empirical relation is shown to provide estimates within a factor of 1·6 for one standard deviation of random error, as determined from 379 tests. The new expressions for non-linear deformation are easy to apply in practice, and should be useful in the analysis of geotechnical structures under static loading.

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.

Cyclic jacking of piles in silt and sand

Jacked piles are becoming a valuable installation method due to the low noise and vibration involved in the installation procedure. Cyclic jacking may be used in an attempt to decrease the required installation force. Small scale models of jacked piles were tested in sand and silt in a 10 m beam centrifuge. Two different piles were tested: smooth and rough. Piles were driven in two ways with monotonic and cyclically jacked installations. The cyclically jacked installation involves displacement reversal at certain depth for a fixed number of cycles. The depth of reversal and amplitude of the cycle vary for different tests. Data show that the base resistance increases during cyclic jacking due to soil compaction at the pile toe. On the other hand, shaft load decreases with the number of cycles applied due to densification of soil next to the pile shaft. Cyclic jacking may be used in unplugged tubular piles to decrease the required installation load. © 2013 Taylor & Francis Group, London.

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.

Reliability analysis of deep excavation based on a semi-empirical approach

The design and construction of deep excavations in urban environment is often governed by serviceability limit state related to the risk of damage to adjacent buildings. In current practice, the assessment of excavation-induced building damage has focused on a deterministic approach. This paper presents a component/system reliability analysis framework to assess the probability that specified threshold design criteria for multiple serviceability limit states are exceeded. A recently developed Bayesian probabilistic framework is used to update the predictions of ground movements in the later stages of excavation based on the recorded deformation measurements. An example is presented to show how the serviceability performance for excavation problems can be assessed based on the component/system reliability analysis. © 2011 ASCE.

Estimation of soil properties and deformations in staged constructions based on mcmc method

This paper presents a Bayesian probabilistic framework to assess soil properties and model uncertainty to better predict excavation-induced deformations using field deformation data. The potential correlations between deformations at different depths are accounted for in the likelihood function needed in the Bayesian approach. The proposed approach also accounts for inclinometer measurement errors. The posterior statistics of the unknown soil properties and the model parameters are computed using the Delayed Rejection (DR) method and the Adaptive Metropolis (AM) method. As an application, the proposed framework is used to assess the unknown soil properties of multiple soil layers using deformation data at different locations and for incremental excavation stages. The developed approach can be used for the design of optimal revisions for supported excavation systems. © 2010 ASCE.

Bayesian updating of a unified soil compression model

Some amount of differential settlement occurs even in the most uniform soil deposit, but it is extremely difficult to estimate because of the natural heterogeneity of the soil. The compression response of the soil and its variability must be characterised in order to estimate the probability of the differential settlement exceeding a certain threshold value. The work presented in this paper introduces a probabilistic framework to address this issue in a rigorous manner, while preserving the format of a typical geotechnical settlement analysis. In order to avoid dealing with different approaches for each category of soil, a simplified unified compression model is used to characterise the nonlinear compression behavior of soils of varying gradation through a single constitutive law. The Bayesian updating rule is used to incorporate information from three different laboratory datasets in the computation of the statistics (estimates of the means and covariance matrix) of the compression model parameters, as well as of the uncertainty inherent in the model.

Wave propagation under vertically excited surface foundation

Recently developed equipment allows measurement of the shear modulus of soil in situ as a function of level of strain. In these field experiments, the excitation is applied on the ground surface using large scale shakers, and the response of the soil deposit is recorded through embedded receivers. The focus of this paper is on the simulation of signals which would be recorded at the receiver locations in idealized conditions to provide guidelines on the interpretation of field measurements. Discrete and finite element methods are employed to model one dimensional and three dimensional geometries, respectively, under various lateral boundary conditions. When the first times of arrival are detected by receivers under the vertical impulse, they coincide with the arrival of the P wave, related to the constrained modulus of the material, regardless of lateral boundary conditions. If one considers, on the other hand, phase differences between the motions at two receivers the picture is far more complicated and one would obtain propagation velocities, function of frequency and depth, which do not correspond to either the constrained modulus or Young's modulus. It is thus necessary to apply some care when interpreting the data from field tests based on vertical steady state vibrations. The use of inverse analysis can be considered as a way of extracting the shear modulus of soil from the field test measurements. © 2008 ASCE.