Earthworks & sub-surface drainage

Sadleir supervised this landslip stabilisation scheme at Blaina, in South-East Wales, for consulting engineers Ward Jackson Associates.

Disturbed ground in the steep slopes of the glacial valley had been further weakened by surcharge from earlier ironworks and colliery spoil tips, since largely removed, but leaving a continuing ground movement to threaten the residential properties in the valley floor.

The weakness was exacerbated by an adverse ground-water régime, itself complicated by a combination of natural geological faulting and abandoned mineworkings at higher levels. Local rates of movement had reached several metres per month. Substantial parts of previous drainage schemes had been totally destroyed by ground movements.

The remedial scheme — designed and managed by Blaenau Gwent County Borough Council's Engineer, specialist ground engineering consultants Ward Jackson Associates — entailed the re-profiling of a large area of the valley side, …

…in combination with extensive ground drainage.

Proceeding necessarily with caution on unstable and disturbed ground, and working typically on 1 in 2 gradients, safe and efficient execution of the works owed much to the considerable skill of the machine operators assigned to the project by main Contractor Walters UK Ltd.

Throughout the earthworks operations, the Engineer maintained regular monitoring of surface and sub-surface movements, rainfall, piezometric pressures and drainage flow rates.

Extensive rock-filled buttress and toe drains were complemented by rotary-bored horizontal relief drains, penetrating up to 90m into water-bearing strata.

New surface collector channels were installed with flexible revetments.

The ¼–million tonne earth-moving operation required a temporary ‘Bailey’ bridge, spanning between reinforced-earth abutments, by which the fleet of 25t 6wd dump trucks could carry excavated material across a busy A‑class road to the disposal area.