Y. Xu

Calibration of models for pile settlement analysis using 64 field load tests

Due to the presence of uncertainties, errors inevitably arise with the estimations of pile settlement. To properly consider serviceability requirements in limit state design, it is necessary to characterize the performance of commonly used settlement prediction models. In this work, information from 64 cases of long driven steel H-piles from field static loading tests in Hong Kong is utilized to evaluate the errors of three settlement prediction models for single piles: two elastic methods and a nonlinear load-transfer method. Commonly adopted soil parameters recommended in two Hong Kong design guidelines are used to reflect the uncertainty arising from evaluation of soil properties. The model error is represented by a bias factor. A conventional statistical analysis was first conducted to study the variability of model bias. A regression analysis method was then proposed as a supplemental analysis of model bias when only limited test data were available or when the measured settlement data distribute in a large range. Both methods result in very similar mean biases. The mean bias of each prediction model tends to vary with the load level and the bearing stratum at the pile toe; while the coefficient of variation of model bias only varies in narrow ranges.

Assessment of flood risks in Pearl River Delta due to levee breaching

A levee system may pose enormous risks to the people protected by the system. Risk analysis is at the heart of levee risk mitigation and engineering decision-making. An explicit methodology for the analysis of flood risk due to levee breaching is desirable. In this paper, a case study on the risks of the North Pearl River Levee System (NPRLS) in Guangdong Province, China, is conducted to illustrate an explicit procedure of flood risk analysis. The performance of the levee upon a 100-year flood at milestone 7 +330 near Shijiao is evaluated. The failure probabilities are evaluated for three failure modes: overtopping, piping and slope sliding. The flood scenario resulted from a levee breach in a 100-year flood event at water level 15.5 m above the mean sea level is simulated. The fatality rate is estimated using a model for estimating the vulnerability of human lives in floods based on Bayesian networks, HURAM. The conditional loss of life is finally estimated according to the risk analysis procedure and the fatality rates from HURAM. Possible measures to mitigate the risks of the levee are also discussed.

Localizing and Quantifying Leakage Through CFRDs

AbstractLarge leakage occurred in several concrete-face rockfill dams (CFRDs) around the world recently, resulting in economic losses and safety problems. To rehabilitate these dams, it is necessary to identify leakage locations on the concrete face slabs and understand the severity of leakage at each location. This technical note presents a method for localizing and quantifying leakage through CFRDs. Analysis models are developed to estimate the leakage rate from horizontal and vertical leakage sources. The relationship between water level and leakage is described for these two leakage sources. Based on a monitored water level-leakage relation, the elevations of leakage are determined, and the leakage rate at each elevation is estimated using these models. A case study on the Buxi CFRD is presented to illustrate the method. The calculated water level-leakage relation is in good agreement with the measured one before rehabilitation, and the leakage has decreased over 80% after rehabilitation.

Testing and Analysis of Erodibility of Hongshihe Landslide Dam

The Great Wenchuan Earthquake of Ms 8.0 in Richter scale on 12 May 2008 caused the formation of 34 large and numerous smaller landslide dams. Hongshihe Landslide Dam is one of the large-scale dams. The erodibility of fresh landslide deposits plays an important role in evaluating the breaching process of such landslide dams due to overtopping. A landslide dam typically comprises freshly deposited mass of heterogeneous, unconsolidated or poorly consolidated earth materials and is vulnerable to overtopping failure. The landslide deposits are usually broadly graded with particle sizes ranging from clay to boulders. Moreover, their grain size distributions are highly heterogeneous along depth and along the run-out direction of landslide debris. Due to the variation of soil properties, the soil erodibility also varies significantly along the run-out direction and depth. This paper describes a series of field jet index tests conducted at two landslide dams shortly after the earthquake to investigate the erodibility of freshly deposited landslide soils. The basic soil parameters (i.e., grain-size distribution, bulk density, water content, and Atterberg limits), as well as the coefficient of erodibility and critical erosive shear stress at different locations were also measured to examine the variation of soil erodibility of Hongshihe Landslide Dam along the run-out direction, depth, and the water-flow direction. The results show that the coefficient of erodibility increases significantly along the run-out direction but decreases slightly with depth and along the water-flow direction, whereas the changes in critical erosive shear stress are limited along the run-out direction, depth, and the water-flow direction.