During the construction of shallow traditional foundations in variable glacial soils, over-deepening occurred due to poor temporary stability in a localised area being misinterpreted as a sign of insufficient bearing capacity. Due to shallow groundwater, the collapsed ground extended some 3m outside of the building footprint and the collapsed, saturated soils had potentially significant geotechnical and structural implications.
Remedial action was required to enable the construction of shallow foundations. A bespoke method statement for the reinstatement was prepared, considering the construction challenges and performance requirements. The specification was designed such the resultant engineered fill would be equivalent or better than the original ground.
Disturbed soils were removed and then replaced with aggregate in a systematic controlled manner to the level of the foundation base. Due to the difficult ground conditions and the potential sensitivity of the soils, particularly to changes in water level, some degree of on-site variation to the method was expected, with all parties required to react to changing conditions as they occurred.
Consideration of collapse compression was required within the re-engineered soils due to groundwater inundation of the fill post completion and therefore:
- A durable hard limestone aggregate was chosen to eliminate potential weakening/softening of particles;
- In accordance with BR424, systematic compression of the upper layers of granular fill in accordance with the Specification for Highway Earthworks reduced voids within the filled mass to “eliminate or greatly reduce the subsequent vulnerability to collapse compression”.
Following the initial pumping of the excavation, removal of the wet disturbed sands commenced. The coarse aggregate was placed in a controlled manner into and above the saturated ground until a dry basal layer was formed. Once a sufficient height above the reduced water level, the aggregate was then systematically compacted in layers using remote plant in accordance with the Specification for Highway Works.
On compeltion the performance of the re-engineered soils were assessed to check that the original foundation recommendations were still valid. This was verified through in-situ plate load testing and desk based bearing and settlement calculations and enabled NHBC approval of the works and the provision of a warranty for the property. Services included:
- Rapid site attendance to provide Client support during a potentially costly project emergency;
- Design of a bespoke method statement for remedial earthworks below groundwater and with poor temporary stability;
- Development of a construction phase plan and toolbox talks pre-commencement to inform the contractor providing support of the potential risks and the mitigation measures that were to be implemented. This included the use of the remote plant within the excavation due to temporary stability risks;
- Supervision of remedial works and reactive design decisions in response to emerging conditions and practical challenges during the implementation of the remedial works;
- Validation of the remedial works, including bearing capacity and settlement calculations to verify in-situ performance test results;
- Technical liaison and justification with designers and regulators to achieve approval for the works, warranties and Client satisfaction.
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