Multi-phased engineering for Luton 40 unit residential block of flats

Project Overview

Client: Oak Developments
Architect: Benchmark Architects
Project type: Commercial flat block comprising 50+ units
Project size:
Phase 1 comprised 40 residential units. While construction was underway, planning approval was secured for Phase 2, which added 11 more units above car parking and a double-height underpass.
Location: Crawley Green Road, Luton

Our team was engaged to provide structural and foul water drainage engineering, along with external retaining wall design, for a commercial flat block development.

The project was delivered in two phases: Phase 1 included 40 residential units, while Phase 2 introduced an additional 11 units over car parking and a double-height underpass. We were recommended by the architect and appointed directly by the client following planning approval.

Project requirements

Phase 1

Phase 1 consisted of a 4-storey traditional load-bearing masonry construction, divided into five blocks (A1, A2, A3, B1, B2) with 1.3m steps between the blocks creating level differences of up to 5.5m throughout the building.

Foundations were trench fill, with the underlying strata found to be solid chalk at shallow depths. Beam and block was used for the ground floor, meanwhile hollowcore planks were used for upper floors (first / second / third), and engineered roof trusses were supported by steel purlin beams spanning up to 10m. External galvanised gully balconies were designed as standalone structures to eliminate cold bridges.

Phase 2

Phase 2 introduced an additional 11 units. Three-storeys were constructed using traditional load-bearing masonry construction over a steel-framed ground floor to minimise structural impact on car parking spaces which featured car park columns to manage car impact loading.

Pile and ground beam foundations were used due to localised loading constraints, avoidance of existing drainage, and proximity to a gabion retaining wall’s piling exclusion zone. The upper floors featured 10mm deep hollowcore planks, meanwhile the roof was engineered using 10.5m span trusses supported by steel purlin beams.

Cantilever ground beams were designed and incorporated within the structure from phase 1 which extended the footprint of the building into the exclusion zone.

The double-height underpass (built off the pile and ground beams) required significant structural consideration, including resistance to torsional and shear loading from adjacent retaining walls. A steel frame was utilised for the underpass to ensure a suitable lateral stability and accommodate the narrow space resulting from the double height structure.

External retaining walls

The site featured several retaining walls, with the highest reaching 3.6m. These were constructed using Anderton ‘Stepoc’ blocks with a wide base and reinforced concrete toe to provide passive resistance against sliding.

Additional retaining walls varied in construction method, including hollow block systems and flat laid blocks. Graded subfloor voids minimised retention near the main building. The freestanding boundary wall also required significant foundation stepping to accommodate the sloped site.

Drainage

The foul drainage system was designed to navigate the perimeter of the building, utilising backdrops to manage level changes. Discharge was directed over the boundary through a cemetery, which was topographically lower, allowing for gravity-fed drainage.

Outcome

The implementation of horizontal movement joints and a well-integrated masonry support system successfully addressed the challenges associated with the height of the gables. This solution ensured long-term durability, minimised the risk of cracking, and maintained the structural integrity of the building.

By taking a strategic approach to movement control, foundation design, and load-bearing structures, we delivered a solution that balanced structural stability with the practical needs of the site, ensuring the success of both project phases.

Crawley Green, Luton