INVESTIGATIONS INTO LATERITIC SOILS AT MILLSTREAM DAM

PAPER ABSTRACT

Millstream Dam is located in the South West Region of Western Australia and is founded on lateritic soils, which are a common surface material throughout the region. These lateritic soils have been formed by weathering of basic or acidic parental rocks, in a tropical or semitropical climate. As a consequence of intensive long term leaching, some of the lateritic horizons at Millstream Dam have been modified and altered. 

As Millstream Dam is to be raised to increase the storage capacity, a thorough understanding of the foundation behaviour is essential to achieve an economical design without compromising the dam safety. It has been found that the soils at Millstream Dam exhibit behaviours that are not consistent with conventional models and correlations. 

These behaviours are attributed to a complex structure of the soil matrix, which comprises clay particles bonded together into larger aggregates. These aggregates are also bonded to each other, forming a matrix of silty or sandy appearance, which is characterised by low dry density and high void ratio. Under shearing, these soils demonstrate high initial strength, which gradually reduces as the inter-aggregate bonds are broken and the relative position of the aggregates changes. 

The materials therefore display both the characteristic of clayey and granular soils. Due to the strain softening, the rate of construction will need to controlled.