DIFFICULT GROUND conditions complicated a major directional drilling project under the Dart Estuary in South Devon.
South West Water needed the 750m long River Dart crossing to provide a sewage transfer route between Kingswear on the east side of the river and Dartmouth on the west.
Allen Watson was drilling subcontractor for the project. The consultant was Pell Frischmann and main contractor was Alfred McAlpine Civil Engineering.
The entry point for the pilot bore was in the car park of the Darthaven Marina in Kingswear.
The bore path ran 22m beneath the river bed, under a railway viaduct and surfaced in the main car park in the centre of Dartmouth.
Work was further complicated by the need to avoid sheet piles on the Kingswear side and to rise steeply on the Dartmouth side at the crossing's exit point.
Ground conditions were varied, with gravel and loose filled ground at the marina, followed by soft fine sand and silt alluvial deposits and slate bedrock beneath. On the Dartmouth side alluvium overlaid the slate.
Work began with a 250mm diameter pilot bore, drilled using a custom built maxi horizontal directional drilling rig fitted with a 170mm mud motor assembly.
When the bore reached the slate 90m along the route, a 406mm diameter casing was installed to seal off the gravel, stabilising the bore and preventing drilling fluid being lost through the loose material.
Once drilling entered alluvium on the Dartmouth side, the mud motor unit was replaced with a jetting assembly for drilling in silt and fine sand. Progress and alignment of the bore was monitored using a magnetic guidance system.
A bentonite-based drilling mud was used throughout, continually recycled to minimise the amount of bentonite in circulation. The fluid was monitored and optimised for each ground material to ensure efficient spoil removal and hole stability. Mud formulation was also adjusted to minimise fluid loss through the loose ground.
The hole was reamed to 362mm diameter and 219mm diameter steel casing pulled in. Finally the 160mm diameter polyethylene product pipe was installed inside the casing.