Buro Happold delivers ground engineering solution for new secondary school, due to open this summer
Pragmatic ground engineering expertise from international consulting engineers Buro Happold has resulted in the delivery of a simple but effective foundation solution for a site in Southwell, Nottinghamshire. The project is especially complex, as it is essential to keep the existing Minster School buildings open while the new college is being built, and to keep noise and vibration from groundworks to a minimum.
Ground investigations allowed Buro Happold’s Ground Engineering (BHGE) Team to identify the presence of artesian water in the bedrock strata underlying the site, leading to a decision not to opt for a traditional piling solution, which could well have led to significant buildability and cost issues during construction.
In cases like this, the cost of a comprehensive ground investigation is more than recompensed by the later cost savings delivered during the construction works; to this end Buro Happold delivered a comprehensive desk study, ground investigation and interpretation of findings to allow a cost effective foundation design to be developed at an early stage.
Piling would have caused cost and buildability issues due to artesian water underlying the site
The proposed £24m development, designed by architects Penoyre and Prasad and currently under construction by Galliford Try, is being built on the site of an existing secondary school, and is due to open later this summer. The proposed development pulls together the current split campus school into a single site and, most importantly, into a single 2/3 storey structure which complements the attractive scenery of the local area; the site is in the picturesque village of Southwell, known for its twin-spire minster, 20km northeast of Nottingham.
The completed structure will take the form of an in-situ reinforced concrete frame with large spans, to open up the interior spaces of the building, and in direct contrast from the narrow and dense column layout of the existing school buildings. The wide spans, fewer columns and concrete construction, however, also produce relatively heavy loadings on the foundations.
Wide spans and fewer columns will mean heavier loads on the foundations
Under the direction of the BHGE Team, Geotechnical Engineering Limited undertook combined dynamic sampling and rotary drilling of five boreholes to a depth of 24–26m below ground level. The combined drilling system was specifically utilised to maximise recovery and recover high quality samples of the Mercia Mudstone strata which is known to be notoriously variable in terms of its weathering profile and strength.
The Mercia Mudstone was found to underlie alluvium including peat and completely weathered mudstone clay, which has a transitional boundary into the solid bedrock strata. However, during the drilling works, confined artesian water was encountered 7–8m or more below ground level in the northern boreholes on site.
Artesian water and a fault were found, making piled foundations a difficult and costly proposition
Secondly, during analysis of the ground investigation data, a difference in strata sequence in the central area of the site was also seen. Combined with increased rock fracturing in this area, and the increase in weathering of the upper mudstone layers, a fault towards the southern boundary of the site seemed probable. Further investigation confirmed this, along with the absence of artesian water pressures in the upper 6m of strata.
This knowledge enabled the team to make final groundwork decisions. The artesian pressures, combined with the variable weathering profile and strength of the Mercia Mudstone strata, were not considered ideal for a Continuous Flight Auger (CFA) pile solution, while a conventional rotary bored pile solution would be unlikely to be cost effective. The noise and vibrations associated with driven piles were also considered undesirable, given that the existing school will remain fully operational throughout the construction works.
Shallow pad foundations were the obvious solution, leading to cost savings and increased buildability
Therefore, pad foundations were ultimately chosen to be installed within the weathered Mercia Mudstone – below the poor near surface material but above the artesian bedrock – offering the most attractive alternative in terms of cost and buildability.
The shallow foundations were designed to incorporate the maximum allowable bearing capacity, while maintaining total settlement at an acceptable level.
The building sections were categorized into areas of similar geological profile, taking into account the thickness of the alluvium, strength of the mudstone, and implications of fault action. In conjunction with the structural team, representative loading for each group was then assigned to enable an appropriate and representative loading scenario, and total settlement analysis was undertaken for each block.
The overall excavation for the foundations was also minimised by reprofiling the site prior to foundation construction. In potential trouble spots, the pads were resized or increased in depth to chase down to a stiffer stratum, and so the new Minster School is indeed being built on solid foundations, and not on sand – or indeed on a fault line or weak mudstone!
Project team:
Client: Galliford Try
Architect: Penoyre and Prasad
Buro Happold services: Building structures, services, ground, water, infrastructure, acoustics, SAT (sustainability), LiT (lighting), CoSA (specialist consulting) and communications engineering.
Main contractor: Galliford Try
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Buro Happold is a multi-disciplinary international practice of consulting engineers established in 1976. We now employ over 1,700 staff in 21 offices worldwide, and our aim is to produce high quality engineering design in concept, in detail and in execution, on time, to programme and delivering excellent value for money. Our distinctive culture and ethos is still based on the same principles of care, value and elegance that were established when the practice was founded.
We offer structural, building services, civil, infrastructure and façade engineering, as well as a broad range of specialist consultancy services including sustainability, ground and environmental engineering, fire and security design, health and safety management, inclusive and urban design, project management, and specialist CAD and computer simulation provision.
