Innovative Post-Tensioning Solutions for Complex Construction Challenges

Park Street Cambridge is a mixed-use redevelopment of a former Cambridge City Council multi-storey carpark by Marick Real Estate, in collaboration with Cambridge City Council and Staycity.

The Park Street development is a prime example of how post-tensioning technology, innovative engineering, and a collaborative mindset can transform even the most complex construction challenges into cost-effective, high-quality solutions.

The development, which includes a triple basement for public parking, retail units, and a StayCity aparthotel, was initially designed with traditional reinforced concrete (RC) slabs. However, as structural design entered RIBA Stage 4, rising material costs and project complexities led the main contractor, Gilbert-Ash, to approach us for an alternative post-tensioned (PT) slab solution. The goal? To streamline the construction process while bringing the project back within budget.

Interspan provided the design, supply and installation of the Post-Tensioning works for the project, including the PT raft slab.

Geotechnical analysis and modelling were subcontracted by Interspan to geotechnical engineering consultant Richter Associates.

Interspan partnered with Rascor to provide a robust solution to the basement waterproofing and ensure the post-tensioning raft slab details were fully coordinated with the waterproofing solution including at complicated sections at steps, construction joints and interfaces with vertical structure.

Innovative Post-Tensioning Solutions: Transforming the Park Street Development

Interspan’s work at the Park Street development in Cambridge is a testament to how value engineering and innovative structural design can bring significant improvements to construction efficiency, cost savings, and long-term performance.

Reducing Slab Depths and Optimizing Design

Interspan’s first step was to review the structural floors from the upper levels down. As is typically the case with RC vs. PT comparisons, we were able to significantly reduce slab thicknesses. For instance, the upper floors’ slab depth was reduced from 225mm thick RC to 200mm thick PT. In some areas where slabs were required to support transfer columns, reductions in thickness weren’t feasible, but the overall design benefited from significant material savings.

In the basement levels B1 and B2, we were also able to reduce slab thicknesses from the original 450mm/300mm RC design to a slimmer 375mm/225mm PT design.

The real game-changer, however, came at basement level B3, where we proposed a flat soffit PT raft slab, replacing the initially complex piled RC raft. This simplified design featured a 700mm thick PT raft slab, which was easier to install and more cost-efficient.

Innovative Waterproofing Solutions

Basement structures, especially those below groundwater levels, require robust waterproofing systems. In collaboration with our waterproofing partner, Rascor, we ensured that the waterproofing for the PT raft slab met all the project’s rigorous specifications. By utilizing a white tank waterproofing system, Rascor engineered the slab to crack in controlled, predetermined locations, making it easier to inject waterproofing materials at the right spots.

The use of post-tensioned concrete provided another advantage: PT slabs have inherent crack width control due to the pre-compression forces applied by stressed tendons. This means the slab cracks are naturally controlled, reducing the need for additional waterproofing interventions. However, Rascor used injection channels to ensure a Grade 2 waterproofing standard for construction joints where tendons terminate.

Optimizing Foundations: A Weight-Driven Solution

The reduced slab thicknesses and weight savings from the upper levels allowed us to revisit the foundation design. Initially, the B3 basement slab was supported by 70 large-diameter bearing piles. However, following detailed geotechnical analysis, we proposed removing many of these piles. For lighter-loaded sections, such as beneath the courtyard zone, we introduced tension piles to resist uplift pressures caused by groundwater and heaving clay.

Finite element analysis played a crucial role in determining the uplift pressures and load effects at each construction stage. This advanced modelling allowed us to fine-tune the design and confidently remove piles where appropriate, resulting in a simplified, efficient foundation solution that maintained structural integrity while reducing material costs.

Successful Project Completion

Interspan’s collaborative approach to post-tensioning design, foundation optimization, and waterproofing coordination ensured the successful completion of the Park Street project.

Our value engineering solutions helped bring the project back within budget, while the use of post-tensioned slabs provided long-term benefits in terms of structural efficiency, reduced material usage, and improved waterproofing performance.

The Park Street development is a prime example of how post-tensioning technology, innovative engineering, and a collaborative mindset can transform even the most complex construction challenges into cost-effective, high-quality solutions.