Feature

Make's Maze

The Big Data Institute (BDI) in Oxford is a cutting-edge research centre dedicated to the analysis of large, complex data sets that could hold the key to the treatment of a variety of diseases. But its users may go about their pioneering work completely unaware of one of the most innovative aspects of the building. With such huge quantities of data to be processed – up to 600 trillion computations per second – the 7,500m2 facility needed a vast server room, which in turn required an innovative cooling strategy in order to meet the project’s target of a BREEAM Excellent rating. Service engineer Long and Partners, together with sustainability consultant RES, proposed a first for a UK research building: a subterranean labyrinth. This eventually helped to surpass expectations, securing BREEAM Outstanding – the first ever such rating for both Make and the University of Oxford.

The building

The design of the BDI, Make’s fourth completed project at Oxford’s biomedical research campus, was shaped by the need to encourage collaboration between researchers; to be adaptable to future uses; and to keep energy use to a minimum. Four levels of perimeter offices and open-plan spaces are arranged around a timber-lined atrium, with social and breakout areas interspersed throughout. Not only does this layout foster a sense of openness, it is also key to the passive energy design, as it maximises cross-ventilation by drawing air through the building using controlled atrium ventilators.

  The other Make buildings on the campus have an in-situ concrete frame with flat slabs in order to minimise vibration, maximise clearance for high-level services, and give future flexibility for service penetrations. This also has the advantages of inherent fire protection and thermal mass. At the BDI, post-tensioning was used to minimise slab depths and speed up construction, and an asymmetrical structural grid of 7.5m x 6m, working with a planning grid of 1.5m, was deemed the best solution in terms of cost and flexibility of space. This allowed the mid-span columns to be aligned with the walls of the perimeter offices. The concrete soffits have been left exposed to optimise thermal mass and guide the building’s aesthetic.

Externally the facade echoes the neighbouring Nuffield Department of Medicine Building, also by Make, with which it forms the Li Ka Shing Centre for Health Information and Discovery. It is veiled in horizontal grey polyester powder-coated aluminium bands and glazing, with the arrangement more open along the north elevation and more solid on the south and west, thereby limiting solar gain.

The labyrinth

Fundamental to the passive design is a subterranean concrete labyrinth located beneath the lower ground floor, which uses the thermal mass of the concrete to provide mechanically assisted summer cooling and winter heating. Labyrinths are still very unusual as a cooling strategy in the UK, and this is the first time that one has been used beneath a research building with such a high cooling requirement. The design team worked closely together from the outset and used fluid dynamic modelling to determine the best overall solution for the building.

The labyrinth is designed and used as the primary cooling medium. The building is naturally ventilated with assisted mechanical ventilation, supplied via the thermal labyrinth. No additional mechanical cooling is provided in occupied areas.

The labyrinth works by pulling outside air in through three separate air corridors that are 1km long in total. In summer, it uses the stable temperature of the concrete tunnels to pre-cool the air before it enters the air-handling units. This air is then distributed via floor plenums and extracted via the atrium using the stack effect. The conditioned air from the labyrinth is supplied via an underfloor displacement ventilation system, with the air supply rate calculated to offset the thermal heat gains of the occupied spaces.

In winter, the air is warmed by the tunnels, before it is heated by waste heat from the data centre, and by warm air returning through the process via a thermal wheel. Once this process is completed, any additional heat is provided by a conventional heating coil using high-efficiency condensing boilers, thereby significantly reducing the energy typically required to condition air and achieve thermal comfort. The payback time of the labyrinth in terms of energy cost savings is estimated to be approximately 26 years; the building’s design life is 60.

The initial design intent for the labyrinth was to have an in-situ concrete outer structure with the “maze” of walls built out of blockwork. The nominal design height to achieve the necessary volume of air movement was only 1,200mm. However, to increase air volumes and performance, and to satisfy CDM regulations, this was increased to a safe working height of 2,100mm.

The building sits in an area with a very high water table, and waterproof concrete was used to create the basement cofferdam, which included the raft slab and perimeter walls of the labyrinth, directly below the basement. This was designed so that no groundwater could penetrate the labyrinth. For the internal walls, vertical precast panels were lowered onto locating lugs cast into the in-situ raft slab. These vertical units were cast with a heavily textured surface finish on both sides in order to increase the surface area of the concrete, thus maximising its exposure to the air and further improving the efficiency of the thermal mass.

The concrete lid to the labyrinth was also made of horizontal precast panels that took support from the precast walls below. The precast option reduced the installation time to just four weeks. This was then covered with insulation and a layer of in-situ concrete to create a platform for the basement. One of the challenges faced during construction was rainwater ingress, because any long-standing water could create bacterial growth that might contaminate the airflow in the finished building. The labyrinth, being at the base of the building, would act as a natural sump for rainwater until such time as the building above was weather-tight.

To reduce this risk, a temporary waterproof membrane was applied to the basement slab above the labyrinth. This, combined with regular checks, ensured that water ingress was kept to a minimum. However, in hindsight, it would have been better to put a waterproof concrete topping over the B1 slab and reduce the subsequent thickness of void former and insulation that sat on it.

The key lesson for anyone considering using a labyrinth is to incorporate it from the concept stage so that it can be captured within the project cost plan and factored in to all planning details.

Photos: Make Architects

PROJECT TEAM
Architect: Make Architects
Structural engineer: Peter Brett Associates
Services engineer: Long and Partners
Main contractor: Mace
BREEAM consultant: RES Design