Client name: Ridings Road, Kerepehi

Duration: October 2014 – January 2015

Location: Hamilton, New Zealand 

Coffey provided a foundation design solution to deliver stability in challenging ground conditions, including reduced settlement and adverse environmental impacts, and improvement in overall constructability – on time and within budget.


As part of their consent application our client needed a clearer understanding of the local ground conditions to ensure their effluent tank foundation design would tolerate future level changes caused by settlement.

Tests revealed the presence of a shallow water table (1.0- 1.4m) and natural soils of fluvial clays and marine deposits. Combined, these conditions would not support the tank structure in terms of engineering specifications.

In order to provide adequate foundation support, a detailed engineering design was required.

Settlement is a key issue in terms of the ‘soft ground’ encountered at the site. If the ground settled following the construction and loading of the tank, this could lead to structural weakness and possible failure of the tank over time, resulting in additional construction and remediation costs.

A worst case scenario was adopted for the design analysis.


We considered several possible design options including driven pile foundations, ultra lightweight polystyrene block foundations and densified gravel raft foundations.

To reduce costs and construction time, we ultimately recommended the densified gravel raft option around the perimeter of the tank. This would support the areas of maximum loading and limit the amount of earthworks required.


The central portion of the tank was raised, or ‘hogged up,’ in order to allow for eventual settlement of loads within the centre of the tank.

The analysis of the geotechnical data gathered from Cone Penetrometer Tests (CPT’s) and boreholes were used in combination with the tank design parameters in order to generate an effective design solution.

The foundation design resulted in the creation of stabilised blocks of stiffened ground below and immediately adjacent to the tank foundation structures.

Throughout the course of the foundation construction we provided inspections and advice in order to ensure design specifications were met.