ASUC specialist-contractor Abbey Pynford recently worked on a project in Westminster, which underwent a complete refurbishment and fit out of an existing 1890s 7-storey mid-terrace property. The site is only 7 metres wide but relatively long at around 33 metres, with buildings each side and to the rear. It had most recently been used as offices.
As part of a major refurbishment to return the property to residential use, Abbey Pynford were commissioned to construct a basement below the whole of the site. The maximum depth of excavation was generally 4.7m, extending to 7.0m for the swimming pool and plant room.
Ups and downs
The client desired a unique feature for the project and settled on the inclusion of a hydraulic elevator to service every floor and extend out onto the roof! To facilitate the installation of the multi stage lift ram, a 750mm diameter bore was required to a depth of 11m below the basement floor level.
When the initial enquiry was received by Abbey Pynford, the basement construction techniques had already been ascertained and locked in through the planning process. A preliminary pile layout using sectional flight augers was developed to avoid the existing structures, such as chimneys and other architectural features.
The project also presented geotechnical challenges, such as providing the optimum design to meet the client’s requirements and sourcing an appropriate piece of plant to achieve the desired outcomes within the constraints of the site.
Unfortunately, the proposed layout had not fully considered the impact of piling rig guarding on the plan position nor the effect of the surcharge loads from adjoining properties on the design of the piles, particularly their diameter. The RC panels, spanning between piles due to a bottom up construction sequence, also had the potential to impact on the pile design.
An Italian job
It was also necessary to source bespoke piece of equipment to work within the site restrictions, which required a great deal of research and took the Abbey Pynford team overseas! An appropriate rig was identified in Italy that was capable of boring up to 450mm diameter piles to 11.5m deep using rotary piling techniques within the headroom restrictions. Training was undertaken in Italy and the rig imported in time for the desired start date. The rig was adapted to operate safely within the confined basement space.
In spite of these challenges, Abbey Pynford’s in-house Structural Engineers, in close collaboration with the client’s team, were able to provide a 150m2 basement, 15% larger than originally envisaged for the clients anticipated budget.
This project highlighted Abbey Pynford’s commitment to working collaboratively with clients and their professional teams to achieve a value engineered solution, while managing and hopefully improving upon their expectations.