Installation of Attenuation Tanks

C R Reynolds were appointed by Costain to construct and install two number attenuation tanks measuring 4.2m in diameter and 14m in length. Works were carried out within a pre piled cofferdam measuring 32m in length, 7m wide and 8.5m in depth from existing ground levels to formation level.

Excavation was carried out in stages using a 45T excavator which was also used to lift in and install the three levels of shoring. Due to the depth of the excavation, an 8t excavator was lifted within the cofferdam to remove and final form the last 2m of spoil, which in turn was loaded into a 2000L boat skip and removed from the excavation. Once all excavation works had been completed, a steel reinforced concrete base was installed at a depth of 1.5m, and the bottom frame removed once concrete had cured. The two attenuation tanks where installed and connected together within the excavation. To do this the hydraulic struts had to be removed in a specific sequence to allow working clearance for the tanks and then reinstated. After installation, the tanks where backfilled with approximately 1000m3 of concrete over 4 concrete pours allowing for the ground supports to be removed as the works progressed. The turrets sections where installed to the access chambers to the tanks and then backfilled. The sheet piled cofferdam was then subsequently removed.

A return visit to site saw the installation of approximately 140m of 280mm rising main pipe, installation of the valve chamber and connecting ductile iron pipe work and the installation of hardstanding areas above the tanks.