Graven Hill Primary School is a new build development in Bicester. The two-form entry school provides 12 classrooms, specialist facilities for art, design, and technology, and more than 13,000 sq m of pitches, play areas, and multi-use games areas. As the first primary school in the eco-development in Oxfordshire, sustainability was key. The brief required the building to be designed to reduce energy and water consumption, enhance ecology, protect biodiversity, and maximise natural ventilation. The building has been designed with enhanced thermal values and high-performance materials according to the principles of Net Zero Carbon and has achieved a BREEAM* Very Good rating.
The photovoltaic panels provide power to the local grid when the school is not in use. In addition to reducing the amount of heat and energy the school will use, the design also maximises natural ventilation to deliver a healthy place to learn.
The structural solution was detailed from the start to be appropriate for Passivhaus requirements, with thermal detailing to avoid all thermal breaks and cold bridging. The school’s location is a former MOD site with significant slopes to the north and east of the site, with previous military buildings having been demolished. The ground investigation revealed high levels of methane (characteristic situation 5) due to previously buried organic matter and topsoil up to 2m below current ground level. The principal founding strata fell away below ground to the north following the original undeveloped ground levels.
Extensive ground modelling was undertaken to optimise the school levels for access at multiple points across the site, while balancing the cut and fill to create a level platform for the school and playing fields. Piling or deep large mass concrete footings were recommended in the ground investigation report provided to us. However, we developed an alternative approach that would also resolve the issues with the methane levels to characteristic situation 2. This approach involved removing the buried organic matter and incorporating recycled engineering rubble from the wider estate development. The construction rubble was separated, graded, tested, and then installed in engineered layers to create a designed platform for a raft slab.
The raft slab solution significantly reduced the amount of concrete and embodied carbon compared to the piled or mass concrete solutions originally recommended to us. In discussions with the contractor, this solution was also more cost-effective and quicker to construct, saving on programme. The raft solution also has benefits for the Passivhaus detailing. The platform allowed the floor insulation to be installed in a single continuous layer under the raft, eliminating all thermal breaks between the superstructure, raft slab, and ground. With the insulation and membranes located under the slab, this meant a screed was not needed, making the raft slab suitable for direct finishes build-up, removing wet trade, saving time, money and carbon.
Initial structural modelling in conjunction with the M&E engineers compared the pros and cons of different structural materials. A precast concrete floor with a steel frame solution was the preferred option for the thermal mass benefits and positive contribution to the overall internal environment and energy performance.
The brief included Sport England Type 4 drainage to the sports pitches adding additional flow to the drainage systems. The site ground conditions are not suitable for on site soakaways, so we designed a water management features into the landscape, including conveyance swales, water features and an attenuation basin to manage the surface water flows and enhance biodiversity.
Image above: Steel frame install to Phase 1 with raft reinforcement placement to Phase 3.
Image above: First floor steel frame and precast arrangement for open plan class arrangement.
