SOFT SOIL ENGINEERING

Key Researchers: Jubert Pineda, Richard Kelly, Laxmi Suwal, Lachlan Bates

PROBLEM DESCRIPTION

Natural soft soils are typically weakly cemented and highly compressible. These features increase the likelihood of failure of civil engineering infrastructure built on them. Even if failure is not of concern, large settlements may occur during and after construction due to their high compressibility. These excessive settlements can deform structures and pavements increasing the cost of maintenance.

SCIENTIFIC AND ENGINEERING APPROACHES

The Ballina Bypass Alliance (RMS, Leighton Contractors, AECOM, SMEC and Coffey) generated a legacy initiative where land acquired for the bypass, in Ballina northern NSW, could be used as a National Field Testing Facility (NFTF). The idea was taken up by the ARC Centre of Excellence for Geotechnical Science and Engineering (CGSE), who has utilized the NFTF to study several topics of soft soil engineering which have direct impact on current practice:

  • Advanced in situ and laboratory characterization of soft soils: Figure 1(a) shows the undrained shear strength profile obtained from advanced laboratory and in situ tests.
  • Assessment of sampling disturbance in soft soils: a wide arsenal of sampling techniques, including tube and block samplers, have been used at the NFTF to assess sample quality. Figure 1(b) shows the disturbance caused by different tube samples in Ballina clay.
  • Embankment behaviour: three embankments have been constructed at the NFTF to evaluate construction methods and embankment behaviour. An International Embankment Prediction Symposium (EPS) was organized by the CGSE in 2016 to assess current practice. Figure 1(c) compares predictions versus measurements used in the symposium.
  • Behaviour of shallow footings: a number of shallow footings have been constructed to assess current design methods. In parallel to the EPS, an International Prediction exercise was organized by the CGSE.
  • Development of new in situ testing tools and constitutive models for soft soils: the NFTF has contributed to the development of new tools, technology and interpretation methods including fixed-piston samplers, full flow penetrometers and advanced constitutive models for soft soils.

Figure 1 Figure 4Figure 5

Figure 1. Undrained shear strength profile at the NFTF obtained from laboratory and in situ tests (left). Sample quality assessment using CT scans (middle). Settlement prediction vs performance for the embankment used in the International Embankment Prediction Symposium (right).