WSP: When Soil Meets Concrete

“We recently worked on a project where a stormwater basin had to be built in the middle of a secant pile excavation. The pit was located close to existing buildings, and we faced challenges with a high groundwater table and permeable soil layers – particularly a sand layer further down that caused upward water pressure and risk of base heave,” Jesper explains.

An overlying clay layer helped slow down the water flow, but the upward pressure still had to be addressed. The solution was to cast a reinforced concrete slab at the bottom of the excavation to counteract buoyancy while also acting as a seal.

A bigger challenge, however, was the design of the secant piles, which WSP had integrated into the final design of the basin.

“There was a bit of technical finesse in how we modelled the secant pile wall. Normally, you can assume a certain degree of flexibility, which reduces bending moments – but since we were dealing with a proper concrete wall, we initially modelled it as rigid. And that gave us some issues, because the results didn’t reflect the actual strength and stability,” Jesper says.

When you model something as fully rigid, the software doesn’t allow any deformation – not even the micro-movements that concrete still experiences. This can lead to unrealistically high stress concentrations, especially when the concrete is in contact with more yielding materials like soil.

“In models with a high stiffness contrast like this, it’s often necessary to adjust contact conditions or the shear strength of the soil to maintain numerical stability. So, we reached out to support, who helped us modify the model using plate elements,” says Jesper.