Potential savings in construction costs of retaining walls

Significant construction savings are the potential outcome from a guidelines project now underway thanks to UC Quake Centre funding.

Dr C Y Chin and Dr Claudia Kayser have been undertaking research over the past five months into the seismic design of retaining walls. The results of their work will form the basis for recommendations in a Retaining Walls Design guideline to be published later this year. Both Dr Chin and Dr Kayser are staff at AECOM New Zealand, and were appointed as Industry Fellows on secondment to the Quake Centre.

The project has been a collaboration between the New Zealand Geotechnical Society (NZGS), Ministry of Business, Innovation and Employment (MBIE), EQC and the Quake Centre, and is aimed at improving the seismic design of retaining walls in New Zealand.

NZGS Chairman, Gavin Alexander, says the collaboration is a welcome one, exploring areas of a great deal of value to industry.

“We’re very pleased to have the support of both the Quake Centre and EQC to enable this piece of work. It will form the basis of the new guideline on the design of retaining walls for earthquake loading, which is an area where current New Zealand guidance is fragmented and incomplete.”

The guideline is part of ongoing work, which began prior to the September 2010 earthquake, to create a suite of guidance documents around Earthquake Geotechnical Engineering.

The Project

Current common design methods use simplified pseudo-static methods employing free-field Peak Ground Accelerations (PGA) to derive dynamic earth pressures acting behind walls. Recent research has provided some evidence that only a fraction of PGA needs to be used in calculating pseudo-static methods which would result in lower dynamic earth pressures and significant construction cost savings.

This research, through the use of programs such as OpenSees - a finite element analysis program - will employ acceleration time histories appropriate for New Zealand to determine dynamic earth pressures behind retaining walls. A variety of site soil classes, wall heights and types of walls will be explored. Comparisons of results from this analyses will be correlated against the simplified pseudo-static analysis to provide guidance on reductions (if any) of PGA in such analysis.

Analyses using OpenSees is now progressing, and the current programme is looking to complete the work in mid-June 2015.

Dr Chin and Dr Kayser would like to acknowledge the generosity of the University of Auckland hosting them by allowing them to work at their Newmarket campus.