Investigation of Potentially Expansive Soils, 'The Birches' Subdivision, Rangiora, New Zealand (2001)
AuthorsClendon, Nicholasshow all
'The Birches' is a recently developed subdivision in the township of Rangiora. Early in 1997 a Benkelman Beam test on Lowes Place, one of the two major access roads into 'The Birches', produced deflection results as high as 12.76 mm. In addition to this was the raising of a section of footpath 10-20 mm up from the curb. Early 1998 saw longitudinal shrinkage cracks appear in a house access driveway, while later in 1998 two house experienced cracking and movement of the interior wall linings, subsequently requiring redecoration. These types of damage are typical of the damage caused by expansive soils, and an investigation was put in place to evaluate these potentially expansive soils. There are no previous cases of swelling soil problems in Rangiora or on the Canterbury Plains, so a field investigation program using crack monitoring, shallow moisture pits and trenches was implemented. A range of samples were gathered from three trenches, including bulk, long and short tube, and block samples. The laboratory methods for analysing these samples included a scanning electron microscope for the identification of microscopic layering, the plotting of grading curves to establish grain distribution, the establishment of dry density, and laterally confined vertical swell levels. The aim of this was to establish both a cause, and the controlling factors of the observed soil volume expansion. The trenches revealed massive, homogenous, silty clay units, with numerous rootlets throughout. The SEM study showed no layering or bedding to be present, but showed evidence of possible bioturbation or leaching. XRD analysis discerned the clay mineralogy was, on average, 20% kaolinite and 80% muscovite. Both of these are very stable minerals, and showed no swelling properties when glycolated. This indicates the causes of volume expansion in these soils are structural. Remoulded samples were also tested, and proved to be more susceptible to volume expansion when moisture content was increased. This is because the process of remoulding destroys the stablility of the lattice structure of the soil, which has formed through repetition of the shrink/swell process. The presence of leaching and bioturbation, and the presence of kaolinite, indicates acidic leaching. The historical data, combined with the evidence of previously high levels of vegetation in the area, as indicated by the presence of rootlets in the silty clay unit, suggests the depositional environment was that of a swamp margin.