Journal article

Cost and embodied carbon reductions in cutter soil mix walls through fibre reinforcement

20 Feb 2017

Cutter soil mix (CSM) walls are created by mixing soils with cement and bentonite slurry to produce a soil–cement mix with modest tensile and compressive strengths. CSM walls may be stabilised using internal steel beams. Presented here are the results of a CSM wall field trial and laboratory testing programme. Polypropylene fibres were added to the soil–cement mix in order to explore whether or not fibres increase wall resistance to bending, reduce the required quantity of steel and thus reduce cost and embodied carbon. The trial involved mixing fibres into a single CSM wall panel. Samples were taken and tested at 28 days and 2 years to assess unconfined compressive strengths, indirect tensile strengths and flexural tensile strengths. The fibre orientation distribution was also assessed. The testing confirmed that the mixing technique resulted in a uniform orientation distribution of fibres and significantly improved tensile strength. Also presented is a hypothetical design of a fibre-reinforced CSM wall supporting an 8 m excavation to show that the steel quantity can be reduced while maintaining stability. A cost analysis, limited to the steel beams, fibres and anchors, shows that a CSM wall with 0.5% fibres by dry weight has a cost of $2930 AUD per linear metre of wall. This is significantly lower than the cost when no fibres were included, which is $4070 AUD per linear metre. The embodied carbon analysis, limited to the steel beams, fibres, cement and anchors, is also presented. The analysis shows that a CSM wall with 0.5% fibres by dry weight produces an emissions saving (having a total of 755 kgCO2e/m3) compared to the CSM wall without fibres (having a total of 929 kgCO2e/m3). Increasing the fibre content from 0.5% does not significantly alter the cost or embodied carbon when the cement content is constant. However, a higher fibre content does have the potential to reduce embodied carbon and cost further if an adequate tensile strength can be achieved with a lower cement content.

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