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A3 cores clear way for embankment design


DETAILED DESIGN of the A3 Hindhead improvement scheme in Surrey can now be finalised with the completion of a £600,000 ground investigation by Norwest Holst Soil Engineering.

The Phase 1B ground investigation focused on earthworks, deep cuttings, structures and revised tunnel alignment for the 5.7km route (GE December 2003).

Norwest Holst's contract, carried out for Balfour Beatty under the instructions of consultant Mott MacDonald, involved describing, classifying and characterising the geology, along with investigating the lateral extent, thickness and behaviour of superficial deposits on which embankments may be formed and through which cuttings may be excavated.

The rock mass properties, groundwater levels and qualities, permeabilities, frequency of chert bands, discontinuity sets and stability of existing slopes were also analysed.

Cable tool boreholes were put down along the route to depths of up to 30m, a number of which were extended by rotary coring.

Rotary open and cored holes were also formed to between 40m and 75m depth. A total of 400m of 100mm diameter cores were produced by triple tube drilling with a predominantly foam flush. The foam was carefully controlled and suppressed in several cases when expelled at the surface because of landowner concerns.

Several of the exploratory hole positions were within environmentally sensitive Sites of Special Scientific Interest and Special Protection Areas which required Norwest Holst to plan preventative measures and supervise site work to prevent any contamination and disturbance.

One rotary borehole was drilled to 95m at an inclination of 45infinity using the Geobore S wireline system. Tolerance requirement was 1:100 through weak, unstable and fractured sandstone with an average core recovery in excess of 95%. This allowed full geological descriptions and very detailed fracture logging and orientation which was within 1infinity in both inclination and direction.

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