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Embankments on Soft Soil

Background

Soft soils are problematic since, under load, they can produce large settlements over a very long time span. These

soils are widespread in the coastal region of Eastern Australia, where the population is increasing fast and much

infrastructure has to be constructed.

One research project carried out by the Centre concerns the 'Ballina Bypass', which is being constructed on an

estuarine soft soil deposit in Northeast New South Wales as part of the multi-billion dollar Pacific Highway upgrade

program. In order to assess the likely foundation behavior, two trial embankments were constructed during 1998.

They were instrumented with settlement plates, vertical settlement profilers, vibrating wire piezometers and horizontal

profiles. The site investigation, sampling, installation of instruments were performed by the RTA and Robert Carr and

 Associates Australia, while advanced laboratory tests were performed in the Centre’s Geotechnical laboratories. The

project aims to improve our understanding of soft soil foundation behaviour and, hence, to establish a predictive

numerical model for estimating long-term settlement. This, in turn, directly affects the construction sequence and

maintenance of the highway.

Site Investigation

Subsoil stratums and soil properties are determined

from borehole logs and laboratory tests. However, the

Centre’s CPT can be employed as a supplementary

approach to achieve extra data at a low cost.

Laboratory tests

Laboratory tests were conducted at the University of

Newcastle. These tests including index tests,consolidation tests and triaxial compression tests and

provided data on soil density, void ratio, moisture

content, permeability, compressibility, and shear

strength of the subsoil layers for stability and settlement

analysis.

Numerical modelling

The numerical predictive model is based on the finite

element method with fully coupled Biot consolidation

theory. The soil behaviour was modelled with an

improved critical state model, which can capture the

key features of the behaviour of normally consolidated

and lightly over-consolidated clays. The numerical

model has been calibrated against the measured data

that have been collected for a period of more than three

Figure 1: Teven Road site trial embankment instrumentation

-cross-section view

Figure 3: Calibration of the numerical model and

predictions: Settlement

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years from the instrumented embankment. The aim of

the model is to be able to predict the long term

behaviour of the embankment.

Figure 2: Soil stratification and the tip resistance from

CPT

Figure 4: Calibration of the numerical model and

predictions: Excess pore pressure

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Figure 5: FE mesh and predicted total displacement