TENDER VALUE: £60K

CONTRACT: SEATHWAITE TARN IMPOUNDMENT RESERVOIR

CLIENT: UNITED UTILITIES

The site was situated in a remote location on high ground, with The Old Man of Coniston summit situated just 1500m to the east. A meandering steep access track linked the nearest road to the reservoir location. With respect to access, only 4WD vehicles (e.g. Landrover, Ford Ranger etc.) were suitable throughout the works.

The investigations encompassed the advancement of ten rotary drillholes in an extremely remote location within the Lake District Park, Cumbria. Eight rotary drillholes (in two arrays of four holes) were drilled at various inclinations (between 30o and 70o) into the face of the dam impoundment structure (concrete) in order to install vibrating wire piezometer (VWP) instruments. The remaining two drillholes were positioned downstream from the dam and were instrumented with traditional casagrande type piezometers.

Twelve heavy duty VWP were installed (in two arrays of six) within the dam concrete and immediate sub-formation (foundation) zone to facilitate constant monitoring of in-situ pressure conditions. Instrumentation cabling was routed via a ducted slit-trench to an existing Pump House.  A multichannel control box was installed within the Pump House which received the data stream from the VWP and allowed downloading of information on a regular basis. Proof of system function and training was provided to the client at the end of the project.Innovation was applied to the investigation with respect to lowering the overall drilling costs. By advance planning and careful execution of drillhole set-up, inclination, depth  and VWP positioning the technical objectives were achieved with a significant cost saving.

Particular constraints included no mobile or fixed telephone services - a satellite phone was needed for emergency situations, such as the possibility of extreme weather due to the height and location. The main geo-hazards was related to the risk of intersecting excess seepage forces/high pressure water during the drilling process into the dam wall. Operations were undertaken in an area of outstanding natural beauty so the works had to ensure that minimal impact was left on completion.

EXPERTISE FOCUS: Specialist Installations on a Dam Site