Below is an example construction specification for a traditional soakaway taken from the Soakaway Ultimate Design Guide.
- These notes shall be read in conjunction with the construction drawings with all dimensions in millimetres.
- Each location considered for the installation of a soakaway should be investigated by means of constant or falling head soakage tests.
- All precast units shall be in accordance with BS 5911. The size and depth of the installation shall be dependent on the design of the system.
- All brickwork and chamber covers shall comply with Clause 507 of the Department of Transport's "Specification for Highways Works".
- All inlets to the soakaways shall pass through a silt trap.
- Pipeline connections to the chamber shall be in accordance with Clause 507 (3 and 15) of the Department of Transport's "Specification for Highways Works".
- The soakaway rings shall be supported on a concrete base not less than 200mm thick with a diameter of not less than the external diameter of the rings The concrete used shall be Class ST4.
- The perforated rings shall not commence less than 1 metre below the interface between the overlying unsuitable strata and the suitable strata with 8No 75mm diameter holes per 300mm height of soakaway ring.
- Except where the Engineer permits, the granular backfill to the chamber shall be 50mm nominal single sized hard, clean, crushed rock or gravel complying to BS 882 and compacted in accordance with the 600 series clauses of the Department of Transport's "Specification for Highway Works".
- The granular backfill shall be sealed at the level of the highest perforated ring by a layer of 500 gauge polythene. This shall be abutted and extended 450mm up the sides of the chamber and
- The polythene shall be capped with Class ST4 concrete to a nominal thickness of 0.15m, or as directed by the Engineer, Where the concrete is positioned such as to conflict with Note 6, the final details are to be at the direction of the Engineer.
- The remaining excavation shall be backfilled according to the 600 series clauses of the Department of Transport's "Specification for Highway Works".
- All joints, soakaway holes and lifting eyes in the precast concrete rings that are located in unsuitable strata and extending 1 metre into the suitable strata below the interface with the unsuitable strata should be sealed with mortar or other sealing materials approved by the Engineer.
- A safety sign shall be anchored below the manhole cover comprising a 650mm long by 10mm thick safety chain with a 250mm by 125mm by 2mm thick plate attached to the chain with 2No 6mm thick links. The writing stamped onto the plate in 25mm letters shall be “Soakaway x metres deep”.
- Chambers (manholes, shafts & soakaways etc) greater than 1500mm internal diameter and located within the highway boundary will require Technical Approval by KCC Network Management Bridge Maintenance through their agents Babtie Group (Highway Structures The design and detailing shall be in accordance with the Approval In Principle (AIP) submitted to the Technical Approval Authority (TAA) for each chamber. The use of precast products to BS5911 will not necessarily be accepted. Type approval is available for the design of some pre-cast supplier's products with further details of these from the TAA.
The following additional requirements are required for TAA approval:-
(a) The chamber cover slab shall be set a minimum 450mm below the finished surface level.
(b) The buried surfaces of the cover slab shall receive two coats of bituminous emulsion.
(c) Non perforated shaft rings shall receive a minimum 150mm ST2 concrete surround.
(d) The base slab thickness shall be determined by the design criteria contained in the AIP.