Pre-cast units from specialist manufacturer Kijlstra have
Groundworks contractor Mulholland Contracts installed the 31 m long, 8.65 m wide and 0.88 m deep underground tank in just four days, allowing other construction works to get started and minimising delay.
The attenuation tank is a vital element of the sustainable urban drainage system (SUDS) serving a new Taylor Wimpey development called The Scholars, located at Kippielaw, near Dalkeith, in Midlothian.
The tank stores surface-water run-off temporarily during periods of heavy rainfall. The stored water is then gradually released into the drainage system over a period of time, thus avoiding the risk of surcharging the main sewer.
Taylor Wimpey specified the Kijlstra precast system after searching for a system which could be installed quickly and efficiently.
“The tank is located across the temporary site entrance, which had to be closed during the installation” explains Kijlstra sales engineer Wieger Faber. “That meant that all construction traffic had to use the permanent entrance, which is shared by the local school. As you know, schools and construction traffic don’t go well together, so it was important that the temporary access road was re-opened as soon as possible.”
The tank comprised Kijlstra precast wall units which were placed in the prepared excavation and levelled with shims so that the face of the wall lined up with the proposed inside of the tank. The wall elements were secured temporarily while an insitu concrete floor was poured between them.
After the concrete floor had reached sufficient strength, the temporary supports were removed and the vertical joints sealed with a non-shrinkable grout. The tank was then completed with the installation of precast roof elements, bolted to the walls and sealed with grout.
“Mulholland had never used the system before, but they did an outstanding job” says Wieger Faber. “They installed the walls on the Monday, poured the floor on Tuesday and started installing the roof on Wednesday. The whole thing was completed the next day.
“A traditional insitu concrete tank of this size, requiring formwork and reinforcing steel, would probably take two-and-a-half or three weeks from beginning to end” adds Wieger.
With the tank installed and the excavation back-filled, Mulholland re-opened the access road to site traffic. Taylor Wimpey is now forging ahead with the development of four-bed detached homes and apartments.
In the meantime, two more Kijlstra attenuation tanks are scheduled for installation early in 2012. These are both smaller tanks, each measuring 8.0 m x 6.0 m with depths of 1.16 m and 0.76 m respectively.
“All the tanks on this site are very shallow due to the relatively low level of the site in relation to the main drainage system” explains Wieger. “There needs to be sufficient fall in the pipeline from the tanks to the sewer and if the tanks were any deeper there would not be sufficient flow along the pipes