Harrow Council Clay Shrinkage Subsidence Stabilisation

Solution

Geobear proposed an alternative: targeted geopolymer clay shrinkage treatment based on detailed site investigation information designed to:

• Reduce the bulk permeability of the clay, by filling the natural fissures present in the London Clay with hard geopolymer
• Reduce water loss from the soils by the action of tree roots, which preferentially grow along the natural fissures in the clay.
• Reduce seasonal vertical movements.
• Provide controlled lift and re-levelling where required.

Figure 2 and 3 show the natural fissuring in London Clay and the presence of tree roots in these fissures.

Figure 2 London Clay Stress Relief Fissures

Figure 3 Roots in London Clay Stress Relief Fissures

Engineering Design

The site investigation data showed that the degree and depth of desiccation varied around the residential block, due to the variable impact of the trees on the soil moisture content, see Figures 4 and 5.

Figure 4 Soil Moisture Content and Shear Strength Variation with Depth

Figure 5 Casagrande Classification Chart for Liquid and Plastic Limits

Analysis of the data in Figure 3 and 4 show that the desiccation depth varied between 3.0m and 4.0m below ground level, which allowed the treatment depth to be varied across the structure. The Geobear clay treatment solution utilizes closely spaced injections spacings at 0.5m in both vertical and horizontal directions to ensure an intensive treatment. The Geopolymer selected for clay shrinkage projects is a specialist high density long cream time geopolymer which can flow into the cracks and fissures as narrow as 0.1mm.

The treatment plan for the project is shown in Figure 6.

Figure 6 Treatment Plan - Ford Close, Harrow.

Field Delivery

Geobear geopolymer technology is a non disruptive solution, as the geopolymer is installed using light-weight hand held equipment. The process starts by setting out the exact location of the injections in accordance with the treatment plan (Figure 6) and after all locations have been cleared to be free from buried services. Holes are then drilled at 16mm diameter through the concrete foundations and underlying soil to the specified depth. Injection tubes of 12mm diameter are installed to the drilled holes to the specified depth.

The van containing the geopolymer components and pumping equipment was parked within a distance of 100m of the injection points (work area). Injection hoses were run from the van to the work area. The hoses are connected to injection tubes by an injection gun that allows technicians to very accurately control the amount of geopolymer injected into the soil, whilst immediate lift at the point of injection is constantly monitored using laser levels, which have an accuracy of +/- 0.25mm.