The constraints of the adjacent highway interchange, environmental area and cost of traditional bore logging prompted UEA to recommend a horizontal investigative bore to prove the ground conditions. The bore showed decomposed sandstone in a 20 metre band close to the drill shaft and dense clay for the remainder of the alignment. The resulting ground conditions proved the viability of Pilot Tube Microtunneling, or laser guided boring machine (GBM), for the insertion of the carrier pipe as opposed to the more costly method of TBM Slurry Microtunneling.
To accurately design and cost a crossing it is always suggested that a geotechnical investigation is carried out close to the prescribed bore route, preferably to a depth exceeding that of the initial bore design. Traditionally a series of bore holes, or bore logs, are undertaken along the full length of the bore, not just at either end.
The horizontal investigative bore for this particular project took less than one day to complete at half the cost of a single traditional bore logs, and eliminated the need for another 3 additional bore logs which were originally proposed for this site.
The GBM pilot has since been successfully completed and work is underway inserting the 450mm steel enveloper pipe and 300mm Polycrete carrier.