Soil stiffness for shallow foundation design in the Perth CBD

Foundation systems for high-rise structures in the Perth CBD include the whole range of footing types: individual spread footings, single rafts, piles, and piled rafts. Of these, raft foundations are the most common. The design of raft foundations (and indeed all foundation types) relies heavily on calculations of the anticipated total and differential settlements. For these calculations, the most crucial material parameters are the stiffnesses of the soils underlying the foundation. In the Perth CBD, the soil types consist of interbedded layers of dense to very dense sand or fine gravel, and stiff to hard clays, overlying bedrock. In the period since the 1970s, when most of the current high rise structures in the CBD were built, a number of methods of determining the soil stiffness have been used. Very little information is available regarding the actual settlement performance of these structures. However, two important publications from the 1970s provide back-analysed stiffness parameters from the measured performance of 4 moderate rise structures (up to 40 storeys high) and these are regarded as benchmark values. The paper discusses the various methods used in Perth for determining stiffness, both ‘traditional’ and ‘modern’, and the results obtained using these methods are compared to the benchmark values. Data from a number of sites, mostly at the west end of the CBD, are discussed in detail, as a number of insitu test methods for determining stiffness have been used at some of these sites, including seismic CPT, Marchetti dilatometer (DMT) and self-boring pressuremeter (SBP). Some comments are also included about stiffnesses of sands in other parts of the Perth area, compared to the CBD area.