Project summary
A tricky installation with groundwater issues
Having already established a professional working relationship with the Shaftesbury Estates, MJ Abbott were employed to design and install a suitable sewage treatment plant to serve an existing 5 bed dwelling and a further 2 no. 3 bed proposed conversions at All Hallows Farm.
The ground conditions were largely chalk and a mixture of river gravel, however at this site the ground below a depth of 2.0m was a very unstable saturated chalk which would not support the weight of the concrete sewage tanks.
Knowing in advance the groundwater issues and the prolific nature of the water running through the chalk, standard pumps would not have been sufficient to keep the excavation dry. Wellpoint specialists Dewatering Services Ltd were employed to assist and drilled 6 no. 6m deep boreholes around perimeter of the proposed excavation. Each of the boreholes were linked with a 6” umbilical pipe system and connected to 3 no. 4” pumps which pulled the groundwater down around the proposed works sufficiently to start excavating.
The excavation was formed to 10.0m long x 4.0m wide x 5.0m deep to expose the non-supporting chalk sub strata. A large hydraulic trench support system consisting of 3 no. hydraulic beams and interlocking sheets was installed to stabilise the excavation, with anti-fall equipment to allow safe access to the excavation.
Working with structural design consultants Such Salinger Peters, a piling mat was designed and installed to allow a drilling rig to be lifted into the excavation to drill 15 no. deep steel reinforced concrete piles through the base of the excavation down to the level of the supporting strata.
Each of the reinforced steel piles was cut off and the steel was integrated into a steel raft and encased in a concrete plinth at a depth of 4.5m to create a supporting platform for the tanks.
The balance tank which received all the raw effluent consisted of a 2.4m diameter x 4.0m deep concrete tank supplied by Carlow Concrete, carefully lifted into place by a 40 tonne crane. The reactor tank which was installed adjacent consisted of a 2.8m diameter x 4.0m deep concrete tank also supplied by Carlow Concrete and installed by crane.
As the site was close to the River Allen and the ground was known to be seasonally saturated, anti-floatation measures were required to stop the concrete tanks from floating out of the ground during the winter months, so steel reinforcement and concrete were used to ballast the tanks.
The tanks were fully backfilled, concrete cover slabs were fitted, and a new stainless steel control cabinet was installed adjacent.
Waste water treatment providers Advanced Aeration Ltd were employed to install their “Bio Bubble” advanced aeration technology into the tanks to form the high quality sewage treatment plant.
To complete the installation, MJ Abbott were contracted to install 110mm uPVC foul drainage pipes around the site to connect each of the properties to the sewage treatment plant, and French drains to remove the groundwater.
