Assessment of Methodologies for Reburial of Excavated Acid Sulfate Soft Soils Within Shell Cove Boat Harbour Zone

Kourosh Kianfar, Scott Morrison and Glenn Colquhoun

A study was carried out to assess the practical aspects of placement of excavated disturbed Acid Sulfate Soft Soils (ASSS) within the future Shell Cove Boat Harbour excavation zone at the proposed Shell Cove Boat Harbour development. The objective of the work was to assess and comment on the methodology from a geotechnical perspective for the placement of ASSS material ‘in the moist’ rather than as ‘dredged slurry’, during Stage 2 of construction. Two large field trial cells (approximately 15m by 15m at base, and 4.5m high) were constructed to practically observe, test and assess different placement scenarios. The following three scenarios were assessed:

  • Placement of untreated ASSS material
  • Placement of Agricultural lime treated ASSS material to modify soil shear strength and consistency
  • Placement of Hydrated lime treated ASSS material to modify soil shear strength and consistency

Track rolling via long reach excavators and swampy dozers were utilised to place, spread and remove the large air voids between clumps/clods of the ASSS material within the trial cells. The neutralisation capacity of the various limes for treatment of Acid Sulfate Soil was also assessed in a preliminary manner. It should be noted that the focus of the work from the addition of lime was on geotechnical modification/stabilisation aspects rather than assessment of Acid Sulfate Soil neutralisation capacity.

Shear strength of ASSS material was assessed using a hand-held shear vane. The shear strength of the soils was measured at the borrow pits prior to excavation of material. It was then compared with the measured shear strength of the soil in different layers placed within the trial cells for different scenarios. Comments are provided in relation to the effects of the mixing agent on the shear strength of the soil.

Construction of bunds within harbour excavation zone including bund sizing and stability of bund to be used by earthmoving plant was also assessed and commented on. Change in soft soil pH due to addition of agricultural and Hydrated limes was also assessed. Waiting time to place subsequent layers within the cells and the effect of weather conditions on the placed soft soils within trial cells is also discussed to some extent in this paper.

Utilisation of a new type of long reach excavator not previously used in Australia is also discussed. This innovative construction technique enabled the excavator to travel directly onto soft disturbed soils and to spread the soils within the harbour excavation zone in a manner that provided enormous cost and time savings for the client.