Residential development on coastal clifflines

The threshold criteria for determining when a detailed stability analysis is required is dependent on a number of factors but primarily the geology of the site. Where previous studies have been undertaken guidelines may be present to determine when detailed studies are required. A case study of a detailed stability analysis of a redevelopment located on a cliffline is given.

Introduction

The principle geotechnical hazard affecting residential development on coastal clifflines is stability of the cliffline. Properties located on coastlines are a finite resource within any locality and usually command premium real estate values. With increased demand new development and redevelopment of existing properties on clifflines becomes more profitable.

Increased demand for coastal properties have resulted in pressure to develop marginal sites and intensively redevelop existing residential properties, by demolition of existing structures and replacement with a number of townhouses. Such redevelopment can result in a demand to locate structures as close to the cliff edge as possible to maximise usage of the site. Over the last 20 years well publicised cliffline failures affecting existing residential development has resulted in an increased awareness of the risks associated with cliffline development. These factors have lead to increased demand for stability assessments of sites located on clifflines to satisfy the requirements of the Local Authority and the New Zealand Building Code.

In the past Local Authorities have accepted stability assessments from registered engineers. Recently some Local Authorities have introduced a policy of having a vetted list of professionals from whome stability assessments will be accepted. The issue of who should be undertaking such assessments is clearly, suitably qualified and experienced professionals. For the purposes of this paper it is assumed that the person undertaking a stability assessment has the requisite qualifications and experience.