Design of shallow foundations on soft lacustrine silt in Rotorua, New Zealand

J.R. Flynn
WSP NZ Limited, Tauranga.

R.P. Orense
University of Auckland, New Zealand.

ABSTRACT

Thick layers of soft lacustrine silts are often encountered below a shallow surface crust around the southern margins of Lake Rotorua and can present challenges for engineering projects in the region due to their soft and compressible nature. In recent years, a common approach to increase the bearing capacity and reduce settlement is to over-excavate the surface crust and replace it with a lightweight backfill, such as compacted pumice sand or layers of EPS geofoam, to create a compensated foundation. This paper aimed to evaluate the performance and applicability of the two geomaterials as a method for addressing settlement and bearing capacity-related issues related to the soft silts found around the margins of Lake Rotorua. In this paper, 2D & 3D numerical model tests were carried out on strip and rectangular foundations resting on both geomaterials to investigate the influence of the thickness and properties of the replaced zone on the load-settlement behaviours. The results indicated that an increase in the thickness of the pumice sand or EPS geofoam resulted in an increase in allowable bearing pressure and a reduction of settlement. However, the EPS had better overall performance based on settlement criteria and a greater load-compensating effect than the pumice sand.