Design and construction challenges of a new inner city school
The successful completion of a brand new £4.5m primary school, built on a tight inner city former factory site, is testament to the skill and expertise of a wide range of engineers involved in the project.
The 420 place independent Dolphin Primary School, part of Colston’s Girls School in Bristol, is built over three floors and includes 14 classrooms; a large hall; a studio hall; on-site kitchen; library; playground with undercover areas; and a spectacular roof garden for outdoor learning.
Clarkebond provided a wide range of engineering services for the scheme including site investigation, flood risk assessment, transport and planning advice, below ground drainage and structural advice from concept to completion.
‘Dolphin School was unusually challenging,’ commented Martin Hathaway, Clarkebond Director, ‘We’re used to managing all sorts of issues but they don’t usually come all on the same site! Here we had a combination of a very constrained site, a sensitive location, planning restrictions, problematic historic retaining walls, a diverted stream and an un-mapped deep public sewer along the line of the old river bed!
The school is located behind a row of houses in Bristol City centre with only a very narrow vehicle access through a gap in the terrace. The small road to the front of the terrace was very narrow. Vehicle movements onto the site were therefore restricted. Clarkebond used tracking software to explore what vehicle movements might be possible with various road closures and parking restrictions and this was followed by full scale trials.
The site itself was very small and the footprint of the new school covered the entire site. Construction therefore had to be sequenced to build out to the only vehicle access.
The site is on the low side of extremely high historic retaining walls that extend along two boundaries. The existing industrial buildings to be demolished buttressed these walls. Clarkebond surveyed the buildings and the walls and developed a staged demolition process to retain essential buttressing until alternative support to the walls could be put in place.
Maintaining stability of the retaining walls at all times was essential. Avoiding loss of passive resistance to the base of the retaining walls resulted in Clarkebond controlling ground levels at all stages of demolition and construction, balancing the need to avoid excavations near wall bases and raising building levels in contravention of planning conditions. Excavating for foundations was particularly problematic. Clarkebond worked with the contractor BAM to develop a solution with contiguous piled retaining walls to protect the foundations to the historic walls.
A stream formerly ran across one boundary of the site. This had been diverted into a culvert upstream of the site and replaced with a deep public sewer. Following negotiations with Wessex Water, the sewer was diverted under the playground and off site. Clarkebond designed piled foundations to this area so the local authorities could replace it if needed. A below ground storm and foul drainage system including below ground storm water holding tanks was designed such that excavations for pipe runs did not undermine the old retaining walls.
Planning issues resulted in the need for minimum storey heights but at the same time the layout required classrooms over large open school halls. Clarkebond’s structural engineers developed a steel framed solution with shallow pre cast concrete planks clear spanning over the large rooms below and pre-cast planks for the roofs. This created flat roofs which form the base of the play areas with rooftop gardens and the roof at the highest level is used for solar panels.
The new school will result in considerably more people using the site than the previous industrial use. Clarkebond highways worked with all parties to assess vehicle movements and design highways improvements to minimise the impact of the change of use. Off-site highway improvements were designed for which Clarkebond gained S278 technical approval.
Cost Consultant: Mace