Back on the mend - refurbishing Lower Carno dam

In the picturesque surroundings of Ebbw Vale, South Wales, Bachy Soletanche Limited is conducting essential remedial works at the Dwr Cymru Welsh Water owned Lower Carno Dam. Built in 1911, the dam has been plagued with a number of leakages and complications eventually leading to the adjacent reservoir being emptied in 2005. Now in order to refill the reservoir, the geotechnical specialist will construct a slurry wall and provide comprehensive rock injection grouting to reinforce the dam in a multi million pound two-phase project.

It was essential to get to the root of the problem, as the reservoir increases the water supply to the local towns and future developments in the community. However it was only after the reservoir was emptied that one of Welsh Water’s partners, Black and Veatch, got the opportunity to determine a permanent solution to the dam’s ongoing problems. The firm initially conducted a study to identify the possible mechanisms for the leakages and after carrying out an intrusive investigation, developed an appropriate design for the remedial works in conjunction with Bachy Soletanche.

Primarily, the project required the construction of a slurry cut off wall from the crest of the dam using a diaphragm wall grab technique to prohibit the migration of groundwater when the reservoir is refilled. A crane-mounted hydraulic grab was considered the most appropriate method to use, as it maintains verticality and accuracy through onboard software and detects the slightest of movement during excavation which is particularly essential when the depth of the slurry cut-off wall is 42m.

The installation progresses in stages or ‘panels’ along the length of the dam and directly through the existing clay core. A cement bentonite slurry mix is pumped into each panel during trench excavation which keeps it stable and creates the finished impermeable barrier. The dam slurry wall is 200m long (with an 800mm width).

The grouting stage of the project firstly requires the installation of a grout curtain wall. This goes into the rock at the dam base to seal the interface between the clay and the rock, and improve the integrity of the rock itself. The grout will extend 10m into the rock, therefore reaching as far as 55m below the crust of the dam. It is here where Bachy Soletanche will install the grout tubes using the overlapping method.

Further grouting is also taking place within a 1.7m diameter brick and concrete lined culvert. It carries water from the reservoir’s drawdown tower into the river on the downstream side of the dam and runs right through the base of the dam’s clay core. It is considered that the culvert could also leak when the reservoir is reinstated. Bachy Soletanche is therefore sealing the interface of the culvert and surrounding clay by grouting around it.

All the remedial work conducted by Bachy Soletanche was completed in November last year, however the project hasn’t gone without its challenges. Daniel Barnard, the project’s Contract Manager explained: ‘It was a very sensitive project in regards to environmental considerations as a nearby river is in close proximity to the site. This required extremely careful coordination on our part in order to avoid contaminating the water with the grout or bentonite fluid used on site. Flexibility with our design was also key to the process due to the poor ‘as built’ construction records from 1911. It led to a 3D CAD model of the dam and surrounding ground being designed in conjunction with Black and Veatch which is proving to be an extremely beneficial tool to the design of the project.’

www.bacsol.co.uk