Stone Meadow

This project at Stone Meadow, Oxford was designed by Bourne9 Architects, with a focus on delivering a high-quality residential extension in a challenging ground condition environment.

The scheme required not only architectural enhancement but also a robust and carefully engineered substructure solution due to ground movement and gas considerations.

The construction package included:

• Full architectural layouts and construction details
• Structural design for a piled reinforced concrete raft foundation
• Specialist ground gas protection and ventilation strategy
• Integration of modern construction systems to mitigate ground heave and gas ingress

At the heart of the design was a piled raft foundation system, combining structural stability with long-term protection against soil movement and environmental risks.

As main contractor, this project required a high level of technical coordination, particularly at foundation stage where multiple specialist systems were integrated.

Piling & Structural Foundation

To address ground conditions, the build utilised:

• Screw pile foundations, designed and installed to specialist engineer specifications
• Approximately 9 piles installed to an estimated depth of 6m, depending on ground resistance
• A reinforced concrete raft slab (250mm thick) tying the structure together and distributing loads across the piled system

This approach ensured structural stability while minimising excavation and risk associated with variable ground conditions.

Ground Heave Protection – Cellcore / Cellvent System

A key element of the project was the use of a Cellvent HX (Cellcore) system beneath the slab.

This system:

• Provides ground heave protection, allowing for clay expansion without affecting the structure
• Acts as a void former, preventing upward pressure on the slab
• Incorporates gas venting capability, allowing hazardous gases to dissipate safely

The panels are lightweight, easy to install, and designed specifically for use beneath reinforced concrete slabs, making them ideal for this type of foundation.

Gas Protection Strategy

Given site conditions, a full gas protection system was installed, including:

• Cordek Tri-Gas Membrane
  • Protects against methane, carbon dioxide, and radon
  • Also acts as a damp-proof barrier
• Gas Resistant DPC
  • Installed within wall build-ups to maintain continuity of protection
• Sealed laps and detailing systems
  • Using specialist tapes and detailing strips to ensure airtight continuity across all joints and penetrations
• Ventilation system
  • Integrated cavity vents and connectors allowing gases to safely vent to atmosphere
  • Linked to the subfloor void created by the Cellvent system

This layered approach ensured full compliance with current gas protection standards and long-term durability.

Coordination & Delivery

This phase of the project required close coordination between:

• Structural engineers
• Piling contractors
• Gas protection specialists
• Building control

We managed:

• Sequencing of piling, raft preparation, and membrane installation
• Strict quality control around membrane integrity and detailing
• On-site coordination of multiple specialist trades

The completed foundation package provides a highly robust and future-proof base for the new extension.

By combining:

• A piled raft system
• Ground heave protection
• Integrated gas mitigation

…the project delivers both structural reliability and environmental safety.

From above ground, the finished extension integrates seamlessly with the existing property, but beneath the surface lies a carefully engineered solution that ensures:

• Long-term stability in challenging soil conditions
• Protection from ground gases
• Compliance with modern building standards