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Geological Society, London, Engineering Geology Special Publications; 1997; v. 12; p. 141-151;
DOI: 10.1144/GSL.ENG.1997.012.01.12
© 1997 Geological Society of London

Section 3: Site Investigation

An example of the use of crosshole tomography in dam wall foundation studies

S. J. Cartmell1, P. J. Conn2 & T. D. Pugh3

1 Dept. of Trade and Industry, Oil & Gas Directorate, 1 Victoria Street, London SW1H 0ET, UK (formerly of J. Arthur & Associates Ltd)
2 J. Arthur & Associates Ltd, The Gatehouse, 2 Holly Road, Twickenham, Middlesex TW1 4EE, UK
3 Hydrosearch Associates Ltd, Chandler House, Anchor Hill, Knaphill, Woking, Surrey GU21 2NL, UK (formerly of J. Arthur & Associates)

Understanding the material properties upon which a man-made structure is to stand is essential to ensure its post-construction stability. The method of imaging of the rock mass between boreholes by crosshole seismology is a well established in situ test utilized in many civil engineering site investigations. A case history is presented from a site investigation for a hydro-electric dam project in the Lesotho Highlands of Southern Africa.

Ten boreholes were drilled along the line of a proposed dam wall, to determine rock properties within the wall foundation. Uphole seismic and wireline logging techniques were used to determine seismic velocity and the dynamic elastic properties within the vicinity of the boreholes. Crosshole shooting and first arrival picking of both Compressional (P) and Shear (5) waves was used to determine the average dynamic elastic rock properties within the inter-borehole mass. Tomographic processing derived velocity information within cells with horizontal and vertical dimensions of 5 and 2.5 m respectively.

Dynamic elastic properties were calculated for individual cells using smoothed density values and velocity data from tomographic processing. Contouring of these values indicated trends in dynamic moduli between boreholes, a technique that allowed identification and location of anomalous zones.