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ENGINEERING

Foundation investigation

The in-situ determination of elastic moduli such as Young’s Modulus and Poisson’s Ratio form an important part of geotechnical investigations ahead of the design and construction of new buildings or other engineered structures.

Typically measurements are obtained using a combination of conventional downhole, uphole and crosshole P-wave and S-wave seismic techniques, although recent technologies such as multi-spectral surface wave and micro tremor analysis (both of which can be undertaken at surface without the need for a borehole) are becoming popular.

The image below illustrates the result of a borehole tomography survey on the site of a proposed road bridge across a railway line in Yorkshire. The aim of the survey was to remotely image the P-wave velocity of the bedrock beneath the railway line, thereby removing the need close the line to rail traffic. The tomography survey was combined with both P- and S-wave downhole and crosshole surveys to determine Poisson’s Ratio and the Shear Modulus of the bedrock beneath the proposed support columns.

Figure 2: P-wave velocity tomogram between two pairs of boreholes either side of the railway line. Low velocity overburden material appears blue in the image whilst bedrock appears green through red. A zone of higher velocity bedrock is apparent at around 15m on the right-hand side of the image.

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Figure 1: Crosshole seismic survey in progress.

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