Built in 1978, Wimbleball Dam is a 50m high concrete buttress dam constructed across the River Haddeo, a headwater of the River Exe in Somerset. The resulting reservoir is a strategic water resource for both South West Water & Wessex Water but since its construction, it has suffered leakages. Following an engineer's inspection, it was considered the leakage was untenable and the recommendation was made that measures should be put in place to reduce it.

Geotechnical specialist Bachy Soletanche (BSL) was contracted to put in place a programme to repair and extend the existing grout curtain below the south flank, dam crest and south abutments of the dam, to reduce the leakage to sustainable levels, and by so doing minimise the loss of fines from beneath the dam. The contract also extended to the installation of a novel and complex water treatment system to capture and treat all discharge. However, the project was particularly challenging as the reservoir could not be emptied during the works and the grouting had to seal flowing features in very high permeability rock.

As principal contractor, BSL was also responsible for the management of the project from site set up to completion of the reinstatement. The designers of the works, CH2MHILL, were responsible for detailing the employer's requirements, including modelling the dam and the grout curtain solution. BSL, CH2MHILL and the client worked closely together throughout the ECI phase to develop a realistic construction programme and to refine the optimum design solution. These deliberations included the rationalisation of the GIN (grout intensity number), procedures selected to ensure that the specification requirements would be achieved.

Working in this collaborative manner allowed the teams to modify the grout mixes and injection parameters as the works progressed, resulting in the success of the project.

BSL employed an innovative approach to model the advance of cracks along the foundation contact. In order to target the boreholes upstream of the dam foundation and existing, ineffective, grout curtain, 3D modelling of the complex dam geometry and borehole alignment was developed.

Prior to commencing the main works, a series of full-scale grout trials were undertaken on site to validate the proposed drilling and grouting techniques. The trial works included the drilling of inspection holes, followed by drilling and grouting of primary, secondary and tertiary holes for grout injection and design validation.

Drilling and grouting operations were executed from three locations. Two between the dam buttresses at a level of 25m below the reservoir head, requiring holes to be drilled through blow out preventers, and a third location on the crest road 5-6m above reservoir water level. Once the drill rig was correctly positioned a hole was drilled through the concrete buttress and dam foundation (average depth 25m), to the base of grout stage 1 in the rock.

Grouting was carried out by traditional descending stage methods, commencing at the dam foundation level and progressing stage-by-stage to full-depth of the borehole, with typical stage lengths of 3-5m, and with six to eight stages per borehole. Within each drilled stage a modified, stable, cement-bentonite grout, of low water cement ratio, but high penetrability was injected. Injection grouting was carried out using a computer controlled electro-hydraulic grouting unit, the grouting instructions for each injection stage of each borehole being downloaded daily.

Post grouting validation holes were drilled and tested for approval. BSL was also required to install a series of pressure relief wells, comprising a 75mm diameter slotted pipe (10 per cent slots), encased in granular stone and terminating at ground level within a secure box.

The project was highly successful, with leakage reduced to the lowest levels since the construction of the dam. Validation testing demonstrated that the mean permeability of the new curtain reduced to approximately 3x10-8m/s, ensuring the longevity of the curtain.

Work undertaken by Bachy Soletanche

Using drilling and grouting Bachy Soletanche:

• Determined the required quantities of grout using investigative techniques and analysis
• Injected the grout with state-of-the-art grouting control procedures

To achieve a durable grout curtain:

• A low target permeability grout was formulated, tested and underwent site trials
• Blast furnace slag was used to resist the aggressive reservoir water
• Grout mixes included micro and ultrafine fine cements and silica fume

The principles of the GIN method were adopted to control the grout injections. The GIN method uses:

• Stable grouts and constant injection rates
• Real-time monitoring of flow, volume and pressures
• Termination based on a curve reflecting energy expended

This project successfully grouted the fissures by employing:

• The GIN grouting method, using state of the art computer-controlled grout injections
• An observational design approach using geophysics, televiewer logging and high-quality hydro testing as well as grouting trials and continual review of the grout injections
• Continual review of injections as work progressed
• Real-time monitoring of the pore-water pressures and dam movements to ensure safety