What lies beneath – Mitigating the risk from buried services to geotechnical investigations

Explorations of the ground for geotechnical purposes entail a risk of striking buried services. Without mitigation the risk of striking a buried service is greater especially in urban areas where the number of buried services is increasing plus a larger proportion of exploration is undertaken and value of those buried services is higher. Consequently the ground investigation industry needs to minimise those risks. This paper discusses a series of approaches for dealing with buried services and presents a protocol to eliminate, isolate or manage the risk. All good investigation practitioners will obtain the latest plans of utility networks but non-intrusive methods to clear services and use of a service locator specialist are also wise approaches. In the end however, the ground has to be penetrated and good practice has evolved to undertake some gentle initial probing of the upper layers before commencing the main exploratory effort, especially in the congested streetscape of the modern city. The paper discusses the merits of various probing techniques but the authors have found some form of hydroexcavation to be particularly effective. An alternative philosophy to utilities avoidance is positive identification, which can yield wider engineering and economic benefits as illustrated by examples. For major infrastructure investigations in the Auckland Central Business District, the authors present their experience of development of the advance excavation technique into the formation of a Ground Access Portal (GAP) that consists of a vertical void held open by a polythene tube through which follow-on drilling can take place.