Challenging ground conditions on the site of a housing development in south Wales have led foundation specialist Roger Bullivant to specify a new type of pile formed from both concrete and steel.
This new design is known as the Combi-Pile and is created from joining together a leading section of durable hollow steel tube with following lengths of precast concrete. The steel section is designed to be driven into poor ground to clear hidden debris. Following lengths of concrete complete the foundation and, on this site, provide a secure connection with precast concrete ground beams.
Bullivant operatives installed 173 Combi-Piles on a new residential site at Llantwit Fardre in south Wales over seven days earlier this month.
Each pile was created by first driving a 3m length of tubular steel into the ground before 3m to 4m lengths of precast concrete, 250mm by 250mm in plan were added to create piles between 9m and 14m long.
The Combi-Piles were driven using a 5t hammer and were designed to accommodate loads of up to 425kN.
The advantage of using the steel sections at the pile base was that they could push through boulder and rock more effectively than concrete piles.
'Driving purely concrete piles into ground that is of a variable nature can cause difficulties as the pile may divert off course if it hits an obstruction, ' says Bullivant contracts manager Derek Murphy. 'On the other hand, we could specify piles formed entirely of steel, but the cost of installing foundations would rise considerably.
'The use of our new pile, combining both concrete and steel, provides clients with a cost effective and highly durable alternative.' Steel pile sections were joined to concrete lengths using a special spigot and socket connection and the two concrete sections were connected together using a mechanism known as InstaLock, where a reinforcing bar from one section of pile is locked into a socket in the other.
The noise and vibration that is usually associated with driven foundations were kept to a minimum by the use of Bullivant's own Quiet Hammer piling rig.
The foundations company is currently engaged in installing precast concrete ground beams and floors for a total of nine houses and 11 apartments at the site on behalf of client Redrow Homes.