Comparison of Drilling Methods used for TaM installations in London Clay

Summary

This paper seeks to compare and contrast observations made of different techniques for drilling horizontal holes within London Clay, and to make recommendations as to how subsequent drilling operations from grout shafts should be undertaken in the future.
The drilling operations discussed in this paper were part of the compensation grouting works for the Crossrail project. The two drilling methods were trialled adjacently, in the first two grout shafts in the area of Bond Street Station.
The first method described is the partially cased, water-flush rotary method. This method of drilling has been previously used on similar projects in central London. The use of this method is also implied by clauses within the works specification, as it has been the standard method of drilling for similar projects in the United Kingdom. The water flush rotary method is known to produce heave while drilling to install the tube manchettes pipes
The second drilling method discussed is the partially cased, auger rotary method. This method is commonly used in Germany but due to the long horizontal drilling lengths, this has not been used previously in the United Kingdom. This drilling method, in contrast with the water flush method, generates some minor settlement in the drilling area.
This paper also reviews other aspects of the works including; drilling accuracy, asset management, safety, spoil disposal and the control of ground movements. The control of ground movements is based on the monitoring system which has been installed for the project works and has demonstrated reliability and providing a short response time between detecting settlement and user interaction. Monitoring is primarily comprised of Hydrostatic Levelling Cells installed in the basements of the overlying structures.
These findings illustrated by site measurements following the initial trial in these two grout shafts, have been conclusive for the choice of the dry auger method as the preferred method of drilling for the remaining grout shafts.