Assessment of the reactivity of expansive soil in Melbourne metropolitan area

Expansive or reactive soil refers to clay soils that undergo significant volume change in response to changes in soil suction. This volume change occurs as swelling upon wetting, and shrinkage upon drying. In Australia, the design of residential slab constructed on expansive soil is based on the estimated ground surface movement, ys. To calculate ys, it is essential to know the reactivity index. In this study, a series of laboratory tests which include shrink-swell test, soil-water characteristic curve, liquid limit, plastic limit, linear shrinkage, soil suction measurement and X-ray diffraction (XRD) test have been performed in the geotechnical laboratory of the School of Civil, Environmental & Chemical Engineering at RMIT University to assess the distribution and nature of expansive soils in the greater metropolitan area of Melbourne. More than 70 soil samples were collected from 47 field sites across 37 suburbans in Victoria, covering a range of geological/geographic conditions and areas designated for future residential development.

The results of the laboratory tests are used to establish a library of shrink-swell indices. An interaction reactive soil map for Melbourne has been developed, which provide (1) the site location, (2) depths of soil samples collected, (3) site geology information, (4) shrink-swelling indices (Iss) and (5) traditional index properties of soil (liquid limit, plastic limit, linear shrinkage). The interaction map and database can be used by practitioners to assess and calibrate the shrinkage index estimated on the visual-tactile basis. The empirical equation for calculation of soil shrink-swell index and two assumptions (i.e. an empirical correction factor of 2 for axial swelling test and assumed suction change range of 1.8 pF) adopted in AS1289.7.1.1 (2003) were assessed experimentally in this study. The results suggest that the values of correction factor (for one-dimensional swelling) between 2.6 and 2.9 are likely to be appropriate. The analysis of soil suction data at effective saturation and air dry after shrink-swell tests indicates that a value of soil suction change within the range 1.8 to 3 pF may be more appropriate and less. To evaluate the effects of surcharge pressure, sample size and initial water content on shrink and swell tests, further laboratory tests have been performed and the results show (1) an increase in surcharge pressure led to a decrease in swelling strain, (2) The swelling strain increased slightly with the diameter of soil samples and (3) the higher the initial water content, the lower the swelling strain and the higher the shrinkage strain In addition, a number of correlations were attempted in this thesis between the shrink-swell test and other soil tests including linear shrinkage, plastic index and liquid limit.

The results show that there is no obvious correlation between the shrink-swell index and traditional soil induces such liquid limit, plastic limit, plastic index and linear shrinkage. The main contributions of this research are (1) a database/interaction map which would be a very useful tool for design engineers and building industry and (2) evaluation of the empirical equation and assumptions introduced in the Australian Standard which can lead to a better understanding of shrink-swell test and subsequently improving the calculation of the shrink-swell index.