A preliminary engineering geological assessment of the potential for soil shrinkage due to field-tile drain installation in the Marlborough Crescent area of Richmond

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Franks, C.A.M. 1988 A preliminary engineering geological assessment of the potential for soil shrinkage due to field-tile drain installation in the Marlborough Crescent area of Richmond. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 416 38 p.

Abstract: This report forms a preliminary engineering geological assessment of the potential for shrinkage of near surface soil materials at Marlborough Crescent, Richmond. A limited site investigation programme has revealed the presence of highly plastic clay soils beneath the site and soil index classification tests on a number of samples indicate their potential for expansion/shrinkage with moisture content variations. X-ray diffraction testing on selected samples was used to determine the mineralogy of the soils and confirm the presence of smectite clays with shrinkage capability. Soil shrinkage is discussed in general terms and the theoretical magnitude of potential shrinkage of the in situ soils is determined. This ranges from 3% to 15% in volume terms and is considered to be low to very high for surcharge loads of 48 kPa and 7 kPa respectively. Drainage of the soil materials, by the installation of field tile drains, is discussed in terms of its capacity to cause shrinkage. In general, the material is considered to be relatively impermeable and it is thought to be unlikely that significant shrinkage will result from drainage of this material. The exception to this is those locations where fissures in the clay may have increased the secondary permeability of the soil mass significantly and the in situ moisture contents are high. It is concluded that drainage of the soil materials in these locations may result in some shrinkage and hence settlement of the ground surface. This preliminary assessment does not allow the limit of the area that may be affected by shrinkage to be defined. Detailed site investigation and soil testing will be required to determine this with any confidence