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Technical paper: Load tests on post-grouted micro piles in London Clay

By D A Jones and M J Turner, Ground Anchors

This paper was first publsihed in GE’s September 1980 issue

Introduction

Gewi piles are small diameter micropiles, typically between 100 and 150mm diameter, with a main load-carrying member consisting of a 50mm diameter high yield steel bar surrounded by cement grout.

The Gewi pile was developed by Dyckerhoff and Widman in 1972, from the idea that ground anchor techniques could be used to form micro-piles with working loads of up to 50 tonnes. The system was developed around their 50mm diameter Gewi steel reinforcing bar, which has a patented thread-like deformation rolled onto the bar during manufacture. The use of this “thread-bar” enables short lengths to be coupled together to form piles of any required length and to allow working in restricted headroom, It also allows the pile to be designed to withstand both tensile and compressive loads.

The steel core assembly can also incorporate a post-grouting system of valves and pipes which, in weak soils, allows additional quantities of cement grout to be injected several hours after installation. Such post-grouting operations, as instanced by Ostermayer (1975), allow higher load-carrying capacities to be attained.

One feature of such post-grouted piles, however, is that at the present state of the art the design is mainly empirical. It is known that the use of post-grouting will increase the load-carrying capacity of the pile, but it is more difficult to predict the magnitude of such an increase. In addition, the work on clay soils cited by Ostermayer has tended to be based upon harder and less plastic clays then those such as the London Clay of the south-east of England.

When Ground Anchors became the UK licensees of the Gewi pile system, therefore, it was decided to carry out a short test pile programme in typical London Clay, in order to verify predictions of the load-carrying capacity of the piles in such a material. With the permission of the British Railways Board, a site was made available in West London which was underlain from the surface by London Clay, 

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