Basement Construction

To measurably increase the footprint of this four-bedroom Victorian property, ACT was engaged to construct an expansive 2,100 sq ft single-level basement under the full extent of the existing lower ground floor and beneath the front and rear gardens.

The basement works formed part of a larger scheme, which entailed completely restructuring the house on every floor and adding a new pitched roof. To make way for a feature staircase and new extension terrace, we opened up the rear façade at the lower ground floor level and the side façade at the upper ground floor level. The fit-out included all new external glazing, new mechanical, electrical and audio-visual services, crittal style doors, a concrete staircase cast in-situ, terrazzo and stone finishes and bespoke joinery.

This project required close engagement with immediate neighbours to the property, as minimising disruption experienced by local residents in this prime and exceptionally tranquil location was a key focus of ACT’s team.

The basement construction process was subject to a series of significant excavation challenges. Firstly, the building structure to the upper levels needed to be temporarily propped up to enable the removal of the existing lower ground floor structures and allow the basement piling and underpinning to begin.

Given the age of the house and its close proximity to surrounding historic homes, great care was taken when supporting the property by using a combination of temporary sacrificial piles and 15 tons of temporary steel work to support and brace the existing and surrounding structures. A 50 point +/- 2mm movement monitoring system was inspected bi-weekly to detect even the slightest of movements.

Shortly after excavation commenced, it became apparent that the local water table level was considerably higher than had been concluded from a trial bore hole. As the water table was circa 1.5m above the basement formation level, we engaged with specialist de-watering contractor and a geotechnic ground engineering consultancy to form a plan to draw down the water table to enable the excavations to take place safely and without causing undue risk to the neighbouring properties. Six wells were excavated to pump out water, drawing out the equivalent of an Olympic swimming pool of groundwater every 25 days.

This volume of water was pumped out over eight consecutive months, with checks performed at regular intervals for silts and movement monitoring. Groundwater discharge licensing was arranged with Thames Water to enable discharge into the local mains drainage. Four further well pumps were required towards the end of the excavation phase to enable deeper excavations for the basement sump chambers. The waterproofing works scope to the new basement was revised to take into consideration the raised water table.

Thanks to an exceptional team, this project concluded on schedule whilst maintaining ACT’s uncompromising commitment to craftsmanship and attention to detail.

The newly constructed basement enables work and family life to take place in a luxurious and thoughtfully designed setting, benefitting from natural daylight through large, well-placed lightwells, adding a modern touch to this period home. Its generous footprint houses a cinema and playroom area, an office and study, one guest bedroom and ample storage. The basement also includes all the mechanical and electrical plant needed to run the property.

“This was our first home renovation and one marked by considerable planning and architectural complexity. As a result, the work itself required frequent interaction not just with our Project Manager but with the team from ACT itself. They were patient with our questions, gave excellent client-centric advice, made themselves available for weekend tours of the construction site and guided us through difficult decisions relating to our basement dig and unexpected problems on site. The patience and professionalism of the entire team and especially the managers on site was exemplary and, ultimately, decisive in enabling us to finish our dream home in the face of considerable challenges.”