Detection of Earth Anchors at building sites

Earth anchors are steel cables used in the construction of earth-retaining walls to transmit load to earth or rock at greater depth away from the structure. In the construction of large multi-storey buildings it is common for percussion drilling to be conducted in close proximity to buried anchors. If an anchor is intercepted, there is a significant risk that the retaining wall and supported site offices would collapse. The detection of earth anchors beneath pavement in an urban environment is a difficult geophysical problem because the target has a dimension of about 0.01 m at a depth of about 10 m. Since one end of the anchor is exposed, the detectability of the target can be enhanced by injecting electrical current into it. Modelling of a buried inclined source has shown that the electric potential at ground surface is diagnostic of the anchor, and that the half-width of the anomaly is proportional to depth. A field trial has been conducted at one of the major building sites in the Sydney Central Business District. Various combinations of point sink and line sink current electrodes were tested in the field, but the most effective combination proved to be a pair of line electrodes set up by electrifying two earth anchors about 2 m apart in a vertical plane. Measurements of total potential and potential difference were made at electrodes driven into the contact between the concrete gutter and asphalt road. Because the anchor plates are in contact with the network of steel girders which form the skeleton of the retaining wall, most of the injected current was short-circuited through the shoring and the terminals of the transmitter. The magnitude of the observed response is consistent with about one per cent of current entering the ground through the anchors. The mean depth of the anchors is estimated at 8.5 m, compared to an expected depth of 7.8 m if the anchors are inclined at 45 degrees.