CEMENTATION FOUNDATIONS Skanska has developed an innovative drilling bucket for large diameter bored piles.
The bucket has side-cutting blades to ensure the pile shaft is cut to the correct diameter, minimising contamination of the drilling fluid (either bentonite or polymer) by sands and silts.
In most cases it also eliminates the need to clean the fluid before concreting. It has been used to install up to 1.8m diameter piles on two contracts in London.
Cementation claims that the barrels of standard drilling buckets tend to be 100mm smaller than the pile diameter, r elying on the outer pair of teeth being set at the correct radius to produce the desired pile diameter.
These teeth have horizontal cutting edges that often produce a 'scroll' on the internal bore of the pile, rather than a shaft of consistent diameter. To over come this, reamer blades are fitted to the head of the buckets to achieve the correct diameter.
The reamed soil can fall into the annular gap around the bucket, contaminating the drilling fluid which then has to be cleaned before concreting takes place. These reamer blades are also vulnerable to damage.
Cementation says the new bucket deals with these issues by using side cutters placed at the base of the bucket with vertical edges to cut the full pile shaft to the required diameter. Spoil generated by the cutters is drawn into the bucket along with the spoil from those on the base.
When the bucket is full, it is rotated anticlockwise against the base of the hole. This causes a swivel base plate to rotate relative to a fixed base plate, covering both side and base openings and trapping the spoil for removal (the vertical side cutters then act as covers for the side openings).
As the cutting process hardly contaminates the drilling fluid, valuable time is saved, as the fluid does not have to be cleaned before concreting begins. When the drilling fluid is displaced during concreting it is passed over desanders in the conventional way. With contamination rates low, only small quantities of fluid need be replaced during piling.
The firm now plans to license the tool for use in other countries.