Chemical stabilisation of Whaka Terrace loess, Christchurch.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The dispersive and erodible nature of some loess soils on the Port Hills results in a variety of mass movement, surface and sub-surface erosion features. These features produce engineering problems for residential development, and chemical stabilisation techniques have been used to reduce and/or prevent further erosion by rendering the soils non-dispersive and non erodible. Field and laboratory investigations were undertaken to characterise the dispersive and erodible nature of Whaka Terrace subdivision loess, as well as to assess changes in chemistry and mineralogy of the chemically stabilised soils. Field investigation included engineering geological mapping, geophysical surveying, hand auger hole drilling and logging, and sampling. The application of 1%, 2%, and 4% hydrated lime, quicklime, Portland cement, gypsum, and a mixture of hydrated lime and gypsum (on a 1:1 basis) was used to stabilise erodible loess-colluvium in the laboratory. Evaluation tests included permeability, pinhole erodibility, jar slaking, uniaxial swelling, crumb dispersion, unconfined compressive strength, undrained shear strength, and the determination of optimum moisture content, Atterberg limits and grainsize distribution. Soil chemistry was analysed using pH, soluble salts, organic content, cation exchange capacity and XRF, while XRD, SEM and EDAX were used to study soil mineralogy and fabric. Application of 1 and 2% quicklime and hydrated lime produces a non-erodible, non-dispersive and durable material which resists slaking and swelling during saturation, and wetting and drying cycles. Application of Portland cement at higher concentration (2 and 4%) produces similar results to that of hydrated lime and quicklime, while gypsum fails to produce a non-erodible and durable material. Furthermore, although the application of a mixed hydrated lime and gypsum stabiliser produces a non-dispersive and non-erodible material, it fails to produce a durable material and mixed stabiliser soils possess a high potential for swelling and slaking. The unconfined compressive strength and shear strength of all stabilised samples except gypsum increases by 2-3 times relative to the untreated samples. This increase in strength is related to the production of pozzolanic cementing agents (pozzolanic reaction) in the chemically stabilised samples. The presence of cementing agents also changes the strain deformation of treated samples by producing a more brittle material relative to the untreated samples.