Prevention of Internal Erosion in Homogeneous Dams—A Case Study

Under the 1975 Reservoirs Act, United Utilities (UU) is the statutory undertaker for 140 impounding reservoirs (IRs) retained by earth-fill embankment dams.

Many of the dams were constructed around the turn of the 19^th century, before the phase associated with the ‘Pennine’ style clay core. Therefore, owing to their early construction, the dams are substantially homogeneous, comprising locally available materials, including a mixture of peat, sand, gravel and clay.

UU has utilised a ‘Portfolio Risk Assessment’ (PRA) method, in combination with the seepage ‘Toolbox’ (Rigby et al (2014)), to provide quantification and ranking of various potential failure mechanisms in order to allow a targeted approach to risk reduction.

To provide the highest risk reduction possible and remove three such dams (Blackstone Edge, Whiteholme and Springs IRs) from the ‘intolerable’ zone, Mott MacDonald Bentley (MMB) was appointed to address the potential risk of internal erosion via a poorly compacted or high permeability zone, either within the embankment or surrounding an existing conduit.

This paper presents case studies of remedial works which have been undertaken at these sites to reduce the risk of failure to an acceptable level. The schemes comprised a variety of methods, including a granular filter combined with partial sheet pile cut-off wall and permeation grouting by both ‘Tube-à-Manchette’ (TaM) and end-of-case techniques. Grouting operations have included the innovative addition of dyed grouts to allow permeation to be traced.