Engineers Australia

Predicting instability of embankments on soft ground from monitoring data

Dominic Trani and Patrick Wong

Abstract

Predicting impending failure of embankments on soft ground remains a challenge to geotechnical engineers conducting routine monitoring as a way of controlling the rate of embankment construction to avoid failures. A literature study has been carried out on available methods of predicting embankment performance. The Authors then propose a method of plotting the inverse of incremental lateral displacement rate at the embankment toe against embankment height. The idea is that the plotted data can be extrapolated to find the maximum embankment height to cause failure. That is, when the inverse rate ratio reaches zero, the embankment is collapsing at infinite rate. In practice, however, embankment distress is likely to have occurred before this ratio reaches zero. Based on several case studies, the Authors propose that an iterative prediction procedure be adopted as the embankment height increases and more data points become available. A projected limit of 0.05 days/mm or 50 days/metre (i.e. incremental lateral displacement rate of 20 mm/day following an embankment lift) be used as a guide to forecast the impending failure height, together with limiting the height of construction to between 80% to 90% of the predicted failure height at any time to control the rate of embankment construction to reduce the risk of embankment failure. This method has been tested only on limited examples, and needs to be further tested on more cases. This method should not be regarded as suitable in all circumstances. In rate sensitive soils for example, failure may occur sometime after embankment construction even though it may appear stable at its final lift. It is important that a number of different methods be used to assess the performance of embankments on soft ground. An essential element in adequately controlling the rate of construction to avoid embankment failure is making sure that there are sufficient levels of instrumentation and monitoring. During embankment construction, it is essential that daily readings be taken and reviewed carefully, and the contractor is prepared to unload the embankment by removing fill if there are tell-tales of impending failure.