When five residential properties in Banbury began to exhibit significant structural movement, the client needed an engineering solution that would permanently address the underlying cause, unstable clay soils.
Overview: Piled Raft Foundation Design & Installation
When five residential properties in Banbury began to exhibit significant structural movement, the client needed an engineering solution that would permanently address the underlying cause, unstable clay soils. As a trusted engineering consultancy and specialist contractor, we were commissioned to deliver a robust and value-engineered foundation system capable of resisting volumetric soil changes and providing long-term stability.
The Challenge: Ground Movement Due to Volumetric Clay Instability
Comprehensive geotechnical assessments revealed that the properties were situated on highly plastic clay soils prone to seasonal expansion and contraction. Structural monitoring identified that moisture fluctuations, driven by weather cycles and water extraction from nearby deep-rooted vegetation, were causing substantial subsidence and heave.
Root systems extended up to 3.6 metres below ground level, intensifying soil desiccation during dry periods. Laboratory testing, including Atterberg limit analyses and moisture content testing, confirmed that volumetric soil movement was the primary factor contributing to the property damage.
The Solution: Engineered Piled Raft System with Anti-Heave and Radon Protection
To counter the effects of soil movement, the client specified a piled raft foundation. We designed and delivered a bespoke solution engineered for vertical bearing capacity, heave protection, and radon control in line with BRE BR211 guidance for protective measures in new buildings.
Each property was reinforced using:
- 22 Ischebeck Titan grouted piles, extending into stable Whitby Mudstone and Marlstone Rock formations beyond the active clay layer.
- Sacrificial Carbide Y-Cross heads to ensure a secure bore through the upper clay stratum.
- 270mm Cordek Cellvent system, providing up to 150mm of heave protection for clay soils with plasticity indices between 40–60.
These combined measures ensured the new foundation could accommodate soil movement without structural distress while preventing radon ingress.
Complex Floor Level Adjustments
Several of the adjoining properties featured a 450mm variance in floor levels, demanding precise coordination during floor removal and reinstallation. Our design team implemented specialised support stooling to facilitate the new piled raft installation, ensuring level accuracy and long-term performance.
Adaptive Engineering: Real-Time Design Refinements
As excavation progressed, voids and fissures were discovered within the rock formation. Leveraging our in-house design capability and long-standing collaboration with Ischebeck, Shire implemented rapid design adjustments to prevent grout loss and maintain performance integrity.
This agile approach minimised downtime and avoided costly project delays, demonstrating the benefit of choosing an engineering consultancy and contractor with integrated design and delivery expertise.
Project Outcome
The Banbury Piled Raft Solution successfully:
- Stabilised five residential properties affected by volumetric clay movement
- Prevented future structural damage from seasonal shrink-swell cycles
- Enhanced radon resistance and ground stability
- Delivered a value-engineered solution within programme and budget
Through careful planning, expert engineering, and efficient collaboration, our teamresolved a complex geotechnical challenge with a durable, future-proof design.
Results at a Glance
- Long-term structural stability achieved
- Minimal project downtime due to adaptive design
- Radon-safe and heave-protected foundation system
- Reliable performance on high-plasticity clay soils
Key Project Insights
The final solution incorporated Ischebeck Titan grouted piles with radon and heave protection measures tailored to the local ground conditions. Each pile was designed to penetrate beyond the active clay layer into solid rock formations, ensuring the load was transferred to stable strata.
The inclusion of a 270mm thick Cordek Cellvent layer provided up to 150mm of heave allowance, critical for clay soils with a plasticity index of 40–60. The design also integrated measures outlined in BRE BR211, ensuring compliance with best practice standards for radon protection.
This combination of engineering precision and material selection produced a robust foundation system designed to endure the full range of seasonal soil movements expected within the Banbury region.
