Mineralogy and geotechnical properties of Singapore marine clay at Changi

An engineering geological study was undertaken to determine the geotechnical properties and mineralogy of Singapore marine clay at Changi in the Republic of Singapore. This soft soil is a quartenary deposit that lies within submarine valleys cut in an old alluvium formation. The marine clay comprises a soft upper marine clay layer overlying a stiffer lower marine clay layer. An intermediate stiff clay layer is sandwiched between these two marine clay layers; it is believed to be the dessicated crust of the lower marine clay layer. In the present study, morphological and mineralogical observations of Singapore clay were taken by scanning electron microscopy, X-ray diffraction and photographic identification. The geotechnical investigation included physical, compression, permeability and field vane shear tests. The upper marine clay was found to be soft with undrained shear strength values ranging from 10 to 30 kPa, while the lower marine clay was found to have undrained shear strength values from 30 to 60 kPa. The sensitivity of the marine clay at Changi varied from 3 to 8 and is described as highly sensitive marine clay. The upper clay layer had a coefficient of consolidation of 0.47–0.6 m2/year due to the vertical flow (cv) and a coefficient of consolidation of 2–3 m2/year due to the horizontal flow (ch). The lower marine clay had a cv of 0.8–1.5 m2/year and a ch of 3–5 m2/year, while the intermediate stiff clay had a cv of 1–4.5 m2/year and a ch of 5–10 m2/year. The primary clay mineral was kaolinite, followed by smectite and mica. With this high kaolinite content, the activity of the Singapore marine clay was found to be low at approximately 0.5–1.3 and classified as inactive to active clay. With reference to the intrinsic state line, natural Singapore clay (upper, intermediate and lower layers) has been classified as structured clay and is in a meta-stable state. The compression behaviour can be modelled by the Intrinsic State Line and the destructuring framework. The predicted and the measured compression curves were comparable for various depths. The successful modelling of compression curves will be useful for settlement calculations and for further research on the development of a constitutive model for Singapore marine clay.