Engineering geologists and geotechnical engineers will welcome Graham West's review of the current practices in the site investigations for tunnels, and their effectiveness.

West points out some discrepancies between the computer-predicted model of the geology along the route of the Jubilee Line extension and the results of the site investigation boreholes.

Those discrepancies were a necessary consequence of simplifications needed to design a computer model used by Sir William Halcrow & Partners and Mott, Hay & Anderson (1973). The model was based mainly on logs of water boreholes from the London area which generally contained simple descriptions of lithology without giving stratigraphic provenance of the formation. Therefore, a deliberate decision has been made to treat the Blackheath Beds and Woolwich and Reading Beds as a single lithological unit.

My recent Australasian experience prompts me to comment on two other points made by West. Where, on mountainous sites like Dinorvic, access to borehole positions is difficult, helicopters can be used to advantage; the whole operation of dismantling the rig into air-portable parts, moving it and reassembling it ready to drill on a new site can be carried out in a single day.

In difficult strata, such as very weak friable Pliocene sandstone or volcanic rock with clay-filled joints, full core recovery can often be achieved using large diameter (PQ3) triple tube wireline core barrels. Cores of firm to stiff clays obtained in this way are considered (when sealed on site) to be more suited for triaxial testing than samples from normal

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