Innovative design of reinforced soil wall on a steep slope subject to land slip risks

Qijing Yang and Simon Yau

Abstract

This paper presents a case study of an innovative reinforced soil wall (RSW) design on a steep slope using site-won material that is not in full compliance with Roads and Maritime Services (Roads and Maritime) QA specification R57. Firstly, the local geology and the landslips that occurred along the project corridor are briefly described. The key design and material requirements for RSW are reviewed with respect to the use of site-won material. A detailed design strategy is then presented on how to mitigate the potential risks of using the non-compliant site-won fill material. Comprehensive laboratory testing of the site-won material and large-scaled pullout tests of RSW reinforcement were undertaken. With the test results and engineering judgement, the design proceeded with the following key assumptions/factors: 1) An allowable fines (<75 microns) content of up to 25%; clay (<2.4 microns) content of up to 7%; 2) the use of reduced friction angle of 30 degrees; 3) a 10% reduction in the calculated pullout capacity of reinforcement; and 4) 1.5 times sacrificial allowance required by R57. These are to cater for long term corrosion on steel reinforcement and to ensure longevity and integrity of the RSW. Heavy rainfall in February 2010 resulted in two significant landslips within the project corridor, with the larger one being immediately down slope of the proposed reinforced soil wall (RW01). As a result an additional geotechnical investigation was carried out and landslip remedial works implemented to ensure the long term global stability of RW01. A three dimensional effect was considered in the assessment of the slope instability during the construction stage review.