Geophysical Investigation to Inform the Extent of Grouting Works at Marchlyn Bach Dam, UK-Paper

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Marchlyn Bach Dam in North Wales is a 10 m-high concrete-core dam with rockfill shoulders. Persistent leakage and downstream sinkholes had kept the reservoir below its normal operating level. A 2012 Willowstick magneto-metric resistivity survey was used to map likely seepage pathways and guide remedial work.<br /><br />The survey indicated that most seepage escaped around the ends of the concrete core and existing grout curtain, rather than through them. Leakage was more substantial at the left abutment, where fractured rock channels water beneath and around the spillway; a smaller pathway was identified near the right abutment. Later boreholes confirmed highly fractured, laminated slate beneath the site.<br /><br />In 2020, Bachy Soletanche carried out targeted grouting, including polyurethane injection in the valve chamber, void filling, soil permeation grouting, and rock-fissure grouting. End-of-casing grout holes were installed first to contain the subsequent tube-à-manchette injections, avoiding sleeve-grout losses that had affected earlier work. The project was completed from February to September under Covid-19 restrictions.<br /><br />Grout takes were consistently higher in locations corresponding to the geophysically identified pathways, supporting the survey’s interpretation. Verification boreholes and permeability tests showed that all treatment zones met or exceeded the specified target of 3 × 10⁻⁷ m/s. Drainage measurements indicated substantial reductions in leakage, and the client reported an average reduction of up to 98%.<br /><br />The geophysical results enabled the team to focus treatment on localized areas, reducing the planned drilling scope and cost while supporting a more efficient and environmentally sustainable solution. Following the successful works, the reservoir could be refilled toward its normal level.

Keywords

Marchlyn Bach Dam

North Wales

seepage

leakage

grouting

Willowstick survey

rockfill dam

fractured slate

abnormal drainage

reservoir remediation