Skip to content

Raft Foundations

Facebook
LinkedIn
Twitter
WhatsApp
Why Raft Foundations Are a Cost-Effective Solution for Challenging Sites

Raft foundations can be an economical and effective option for ensuring stable building footings, depending on site-specific soil and loading conditions. A recent example is the construction of the Cirtex warehouse facility, which demonstrates the suitability of geogrid-reinforced rafts in managing variable loading from machinery, plant, and office areas.

Ground Conditions – Why Choose a Geogrid Reinforced Raft?

For any load-bearing structure, the geotechnical and structural design process begins with a critical question: Can the most economical footing option—a simple, shallow spread footing—be used? When the answer is no, more costly alternatives like ground improvement or piling often come into play.

The Cirtex site, located in a low-lying alluvial area, presented specific challenges. Loose to medium-dense sands and silts extended down to at least 9 metres below ground level (BGL), accompanied by a high groundwater table. Predicted reconsolidation settlements of up to 300mm and liquefaction potential during seismic events necessitated a foundation system that could mitigate two key risks:

  1. Liquefied bearing capacity failure, where footings might “punch” through a non-liquefied crust into weaker liquefied layers below, leading to serious structural consequences.
  2. High differential settlements between adjacent footings, which could cause uneven stresses and damage.

A geogrid-reinforced raft was selected as the ideal solution to manage these risks effectively and cost-efficiently.

Geogrid Reinforced Raft – Design and Construction Process

Cross Section-Geogrid Reinforced Raft

Designing the geogrid-reinforced raft required close collaboration between geotechnical and structural designers. Both static and liquefied soil conditions were analyzed. In scenarios without the geogrid raft, the structural performance was deemed unacceptable, making the geogrid raft the optimal solution. This approach eliminated the need for costly ground improvement or piling.

Key Features of the Geogrid Raft Design:

  • A single layer of DuraForce® AS410 geotextile at the base of the excavation.
  • Three layers of DuraGrid® X 40/40 biaxial geogrid at 200mm vertical spacings.
  • GAP65 granular fill, well-graded and compacted between the geogrid layers.
  • A 1-metre “wrap-around” detail extending beyond the building footprint for additional stability.
Raft Construction

Cirtex -Geogrid Raft foundation

During construction, any soft spots or unsuitable material in the raft formation were removed and replaced with GAP65 granular fill. The process involved:

  1. Laying the DuraForce geotextile and the first layer of DuraGrid.
  2. Placing and compacting the GAP65 fill in layers, ensuring optimal density using Nuclear Density Gauges in both Back Scatter and Direct Transmission modes. The parent material, Greywacke, achieved a Maximum Dry Density of 2.2T/m³.

Each layer was tested for compaction compliance before proceeding to the next, ensuring that the raft would meet performance specifications.

Supervision and Certification

Compliance was ensured through stringent observation of Hold Points outlined in the specifications and construction drawings. These Hold Points formed the basis for issuing a Producer Statement No. 4 (PS4), certifying that the raft foundation met the required standards. The certification provided assurance to local authorities, paving the way for warehouse construction to proceed.

A Cost-Effective Solution for Challenging Soil Conditions

This case highlights the effectiveness of geogrid-reinforced raft foundations in addressing bearing capacity and settlement challenges common to liquefiable soils. By using three layers of DuraGrid X 40/40 biaxial geogrid and adopting a well-engineered design, Cirtex was able to construct a cost-efficient and stable foundation. With the raft successfully completed, the warehouse construction commenced on schedule, showcasing the practicality of this innovative solution for similar projects