Mentor: Alex Scheuermann
Project Team: Julius Lumsden, Carlos Sailema Cobo, Chelsea Vincent and Serena Selveraj.
Fluidisation of saturated or nearly saturated soft soils is of great importance for a wide range of natural processes and engineering applications, concerning particularly slope stability, land reclamation, and management of sewage sludge and mine tailings. Fluidisation is the cause of many disasters, including tailings dam failures and debris flows. Other engineering disciplines are affected by soft soils as well. For example, in marine engineering, fluidisation of fine-grained, moist ore materials induced by the cyclic movement of the ship can result in the sinking of the ship itself.
Soft soils with the tendency towards fluidisation are difficult to dewater and consolidate. In geotechnical engineering, soft soils need to be preconditioned for improvement of the mechanical strength prior to the construction of overlying structures such as buildings and roads. Existing techniques for treating soft porous materials include surcharge with and without the use of wick drains, evaporative drying or the application of suction and electro osmosis.
High saturation of predominantly fine-grained materials causes soft consistencies with little shear strength, leading to a reduced workability and an increased fluidisation tendency of the material.
The project aims to receive an improved understanding on the behaviour and the characteristics of soft soils. Participating students will have the chance to work with different types of soils (reference materials and natural soils) to characterise their specific hydraulic and mechanic behaviour and to test their reaction under certain loading conditions. There is the chance to work together with Port of Brisbane on materials from their site and to contribute on the development of a field laboratory on the premises of Port of Brisbane.
Top figures: Amphirol for remoulding the surface of soft soils for accelerating evaporation (left) and surface of soft soil treated with the amphirol on a site at the Port of Brisbane (right).
It is important to understand the behaviour and characteristics of soft soils in order to predict the mechanical strength of the soil in different natural and engineering applications. Such applications include slope stability, management of sewage sludge, mine tailings and land reclamation. The soft soil challenge 1 is a project designed to test the effects of flocculants on both the hydraulic and mechanical behaviour of the dredged material in the land reclamation area at the Port of Brisbane. Ultimately the tests will help to determine whether flocculants will reduce the dewatering time in the dredged material paddocks and make the land reclamation process more efficient. It is also important to know the effects of the flocculants on the shear strength of the dredged material throughout the dewatering process.
Above Pic 1 & 2 respectively: sticky mud in intermediate paddock and slurry in new paddock at the Port of Brisbane.
To begin the Soft Soil Challenge 1 a site visit was undergone to the Port of Brisbane in order to familiarise the students with the task at hand and to visualise the types of soil being tested. Soil samples were collected from two different paddocks, one intermediate and one new containing sticky mud and slurry respectively for initial soil characterisation. The initial tests completed for soil characterisation were water content and Atteberg limits; it’s liquid limit (See Fig 1) and plastic limit. Both new, intermediate and almost completely reclaimed areas were visited to discuss the different stages of dewatering.
Above Fig 1: Showing the Liquid Limit determined using the Casagrande cup method
An Information session lead by a flocculants expert from Nalco was held in the Geomechanics Laboratory at the University of Queensland. The instructive session covered the safety equipment required to handle the flocculants, and a demonstration illustrating effective flocculant concentrations, efficient mixing times and speeds and important reliability checks.
Testing was completed using a Time Domain Reflectometry device (TDR) to determine the soils electrical conductivity, apparent dielectric constant and volumetric water content with and without the use of flocculants.
It was expected that the soil samples taken from the Port of Brisbane would have a high electrical conductivity due to the high concentration of dissolved salts and high moisture contents. This was confirmed from the recorded results where both the sticky mud and slurry had high electrical conductivities with the slurry being the higher of the two. It was also expected that the slurry with flocculant mixture would have the highest electrical conductivity out of the three samples due to the high concentration of free water (See Fig 2).
Above Fig 2: electrical results found using TDR device.
Tests to determine the apparent dielectric constant were then completed with the expectation that the slurry and flocculant mixture would have the highest result followed by the slurry and then the sticky mud. This is due to the apparent dielectric constant being highly sensitive to moisture content. It can be seen in Fig 3 that the results reflect the expected outcome except that the slurry did not show a higher apparent dielectric constant than the sticky mud.
Above Fig 3: apparent dielectric constant found using TDR device.
The volumetric water content was then determined using the TDR device. It was expected that the volumetric water content would increase from the sticky mud to the slurry finishing with the slurry and flocculant mixture. It was found that the slurry and flocculant mixture did in fact have the highest volumetric water content, however the sticky mud and slurry samples showed negative results. This could be due to the difference in calibration between the TDR device sensor and the site conditions (See Fig 4).
Above Fig 4: volumetric water content results found using TDR device.
In the near future we hope to further develop our understanding of the effects of flocculants on the dewater of soft soils both on the specific hydraulic and mechanical behaviour and to test the soil reactions under different loading conditions at different moisture contents and flocculant concentrations. These tests will be conducted in the hope of reducing the land reclamation time and increasing the shear strength during the dewatering phase at the Port of Brisbane’s land reclamation site.