Project Description

Abbey Pynford were instructed by the NHBC to complete a design and build underpinning scheme which consisted of a Piled Raft Underpinning project in Bushey. Existing clay ground conditions caused significant heave to the existing building once the existing ground strata became saturated causing the clay to swell and heave the building up.

Abbey Pynford’s design and build directive was to divorce the property from its existing foundations whilst ensuring the structural stability of the building during the construction of an entirely new foundation.

Prior to Abbey Pynford arriving on site, the house was completely stripped leaving only a shell in order for the programme to commence. The use of a specialist mini piling rig facilitated access throughout the building with the ability to fit through standard door openings. The first operation was to install 200mm R.C auger bored piles. This involved creating a bore approximately 12m deep and then installing steel reinforcement prior to placing concrete.

The following work was completed in two halves to ensure safety of the structure.

Specially designed stainless steel props were installed to support the load bearing walls and subsequently encased with polystyrene to ensure efficient removal at a later date Clay board was laid to create a voided area to allow future heave and swelling of the existing soils without affecting the new concrete raft installed above. This was constructed in accordance with Abbey Pynford in house design using layers of reinforcement prior to the concrete being poured. A stop end was used to ensure a professional finish and to ensure work on the second area was unaffected by concrete spillage.

Having left the concrete to gain strength for approximately 7 days the props encased with polystyrene were removed allowing the building to sit entirely on its new foundation. Any empty pockets had clay board inserted and were filled with concrete to complete the works

Testimonial

A site with tight access shared by the residents of the private road made this underpinning job all the harder. Despite this the piled raft underpinning works were carried out to programme and the highest standards with the works being undertaken through some tough winter weather conditions including freezing weather and driving rain.

Construction Phase Highlights

200mm auger piles prior to installation. These created the auger bore which was subsequently reinforced with steel and filled with concrete to design specifications.

To the left the concrete piles with steel reinforcement can be seen. Further left the toe detail prevents an unsightly finish external to the building, concealing the concrete externally. To the back the clay board where the anti-heave protection is in situ.

The polystyrene encases the props to allow easy removal post concrete pour. The layers of reinforcement are put in place as per the detailed design around the props and above the clay board.

Concrete is poured in the first half of the works and kept neat with use of a stop end. To the right the second half is commenced with clay board and layers of reinforcement above prior to the second concrete pour.