MICRO PILES have been used to tie back a secant pile retaining wall through difficult ground at Southern Water's new pumping station and screen house at The Stade, near Folkestone in Kent.
Because there were problems with installing conventional ground anchors in the underlying Sandgate beds, main contractor Tilbury Douglas looked for an alternative solution from design consultant Applied Geotechnical Engineering. Along with anchor supplier PSC Freyssinet, the consultant sug -gested using Titan micro piles.
Manufactured by Ische- beck Titan, the micro piles were installed at the lower two of five anchor levels tying the wall back into the cliff face on the sea front. The anchors consist of only one rod which acts as both the drill rod and the reinforcement tendon. This, claims Ischebeck, removes some of the installation steps including the removal of the drill rod and casing and the insertion of an anchor rod, increasing productivity by as much as 200%.
The continuously threaded, hollow tendon piles are made from high yield steel offering good corrosion resistance. Ischebeck says they are easily welded, are unaffected by hydrogen embrittlement and as they are hollow, bottom-up pressure grouting can be carried out, ensuring total filling of the annulus as well as all fissures and cracks.
At Folkestone, corrosion resistance was increased by use of circumferential rod spacers attached to the rod couplers which ensured a minimum grout cover of 35mm was achieved. And as the micro piles are uncased, drilling contractor Saxton Deep Drillers was able to insert them up to 16.8m into the difficult ground. They were supplied with sacrificial drill bits (either screwed or welded on depending on ground conditions) and have safe working loads of 750kN and ultimate loads of 1,950kN.
The pumping station and screen house form part of Southern Water's £120M Dover & Folkestone wastewater treatment scheme, which is expected to provide cleaner seawater and beaches and improve flood protection in the area. The new facilities will stop untreated wastewater being discharged into the sea by pumping it to a new underground treatment works at Broomfield Bank near Dover.