During bulk earthworks and construction of the Amberfield Switching Station in Centurion, Gauteng, Kaytech‘s reinforcement and drainage products were specified for the construction of a CRB (Concrete Retaining Block) wall.

A switching station is a substation without transformers and operates at a single voltage level. It may be used as a collector or distribution station and may also be used for switching current to back-up lines or for parallelizing circuits in case of failure.

Retaining wall specialists, Powergate Construction, were contracted to build a CRB wall against a bulk fill platform previously constructed using G7/G8 backfill material compacted to 93% mod AASHTO or better. Since the platform was required to support heavy steel structures, a consultant from Remacon, manufacturers of concrete retaining wall blocks, specified X-Grid geogrid to provide reinforcement for the G7 fill material.

Manufactured from polymer coated, high tenacity, polyester yarns X-Grid geogrids act to structurally increase the load bearing capacity and tensile resistance of the soil. Benefits include high tensile strength, low creep characteristics and, compared to stiff, extruded geogrids, X-Grid provides high fibre interlock strength. Suitable for installation in a wide variety of soil types, X-Grid is ideal for long term (>100 years) reinforcement applications.

Although the on-site geology was highly variable, including areas of very poor fine soils, the overall geology is highly weathered granite. Fortunately the CRB wall was founded on this competent granite material so did not pose a stability problem.

Earthmoving contractors, Advance Projects originally had a cut and fill challenge where material was removed from one side of the platform to build up a section on the opposite side of the site, so they trimmed off the sloped overburden of the platform to expose the competently compacted bulk fill, which allowed a minimum gap of 2 to 2.5 metres between the cut face and the block wall facing.

The design was therefore based on a ‘gravity’ bulk fill wall to a maximum height of six metres with X-Grid reinforcing the fill. To allow for a 10kPa surcharge loading above the wall, the geogrid would be extended up to, and benched into, the cut face to a minimum horizontal depth of two metres. Commencing in March 2018, Powergate Construction installed 4 000 m2 of X-Grid 40/20.

Because an increased number of CRB walls have failed in the recent past, usually due to water ingress, a subsoil drainage system was incorporated into the design.  This system consisted of Kaytech’s Flo-Pipe 110 (120 m) and bidim A2 (396 m2), a continuous filament, nonwoven, needlepunched geotextile made from 100% recycled polyester. Manufactured from high quality HDPE in a twin wall sandwich design, Flo-Pipe is optimally slotted for maximum infiltration with minimal blockage. The smooth inner wall ensures high flow velocity while the corrugated outer wall withstands massive confining pressure. The benefits provided by the needlepunching process of bidim include appreciable thickness, high porosity even under heavy loads, high resistance to puncture, tearing and burst, as well as a high drainage capacity in both the transverse and normal to the plane.

Quick and easy to install, the three products supplied by Kaytech for this project provided an economical solution with guaranteed long term performance. The incorporation of X-Grid into the CRB wall at Amberfield Switching Station not only offers significant stability but also erosion protection against the elements.

For more information on Kaytech products and systems, visit www.kaytech.co.za