In the design of the pond I felt that the strong circulation provided by the TPRs and the action of the diffuser drains negated the need for benching the base. Instead of the heavy benching I have seen on many ponds, a slight dish effect was created around each bottom drain to provide a slight fall. The base is between 6" and 9" thick with A142 steel reinforcing grid laid at half depth.
Note that the concrete finished around 1/2" below the lip of the drain. The final render coat will bring this level.
After the base had hardened the wall construction began. The area is very stable in terms of soil movement therefore a 4" thick wall with concrete backfill of around 4" is the design. High density blocks were used for the whole construction and the shape of the pond meant that most blocks were split in half before laying to ensure a smooth contour.
This picture show the overall shape and design of the pond. The contours of the base can bee seen. The concrete used was a 'Readymix' type mixed to the required specification. I used a 'semi-dry' mix. At ths time the footings for the outer walls were also filled with concrete.
