Report on an Australian Geomechanics Society or Footings Group LectureOn the 17th August 2009, I attended a lecture titled 'Revetment Walls in Unsaturated Clays' presented by a team of geotechnical professionals to the SA Chapter of the Australian Geomechanics Society. As a part of the proposed South Road upgrade and future North-South Corridor, Richard Herraman (Geotechnical Engineering Group Manager, DTEI) and his department have received funding to lead a study into more cost-effective ways of stabilising excavated slopes in stiff clays. This study has involved numerous parties including; John Woodburn (Soil Mechanics Instrumentation) who discussed a feasibility study and possible excavation methods; Dr William Kaggwa (Adelaide University) who discussed the geotechnical testing of unsaturated clay soils; and Chris Ward (Parsons Brinckerhoff) who discussed his modelling of the design suction profile.
Often the construction of a retaining wall in stiff clay involves an excavation (often at a steep slope), building the wall and then backfilling. However many stiff clay excavations, such as that shown in figure 1, will stand for decades without any support (unless it gets wet and softens). This leads to the question of whether there is any need to retain and if it is possible to simply protect the slope.
Figure 1: Steep slope excavations in stiff clay often stand up without any support [Source: SA Chapter Australian Geomechanics Society, URL: http://www.australiangeomechanics.org/common/files/sa/20090817-RevetmentWalls.pdf]
A revetment wall utilises the inherent strength of the soil mass and comprises a protective covering on an embankment of earth which is designed to maintain the slope at a steeper angle than the material would naturally assume. According to AS4678, a structure is classed as a revetment wall if the angle of inclination is less than 70 degrees from the horizontal. Richard Herraman highlighted the 60 degree inclination revetment wall of the Millswood underpass on Goodwood Road as an example of a cost-effective method of retaining slopes (figure 2). This wall comprises 100mm of shotcrete with no soil nailing and has stood since 1915 with only relatively minor maintenance work performed (installation of weep holes and individual concrete panel replacement).
Figure 2: The Millswood underpass was used as a case study for the potential future use of revetment walls for underpasses [Source: Google Maps]
John Woodburn then discussed how saturated soil mechanics predicts the maximum height of a vertical cut to be equal to the depth of cracking, but much greater heights are predicted by unsaturated soil mechanics. Luckily I was able to follow this discussion as we had only recently discussed unsaturated soil mechanics and the influence of total suction.
It was then outlined that John's study involved investigating the range of soils along Adelaide's north-south corridor and the depth to groundwater, as well as modelling the design suction profile at the wall and away from the wall of the Millswood underpass case study (modelling performed by Chris Ward).
This modelling was performed using SEEP/W for the suction profile and SLOPE/W for the slope stability investigation and considered 3 cases; a deep watertable, shallow watertable, and a leaking service. My notes from the meeting are a little sketchy on the results of this modelling (my handwriting couldn't keep up with the presentation), but the final message was that revetment stability in unsaturated clays is realisable, given a deep watertable.
Another component of the presentation, which I found interesting, related to the field investigations and detailed testing required for the design of a revetment wall. Dr William Kaggwa discussed the use of SPT/CPT/Dilatometer data, but believes that many companies do not test soil suctions adequately, even though we deal primarily with unsaturated soils.
The possible construction technique of a revetment wall was also discussed and John Woodburn outlined the need to monitor for the presence of perched watertables, blocky clays, pockets of loose dry sands and saturated sands from old stream channels. He suggested that a staged excavation take place with an initial cut of 1:1, which can then be cut further after investigation of the factors outlined previously. It was also made clear that the wall should be flexible with articulation and adequately drained to ensure saturation does not occur. The importance of proper drainage appears to be a recurring theme throughout my study of geotechnical engineering.
John also warned that potential shrinkage behind the wall may lead to a loss of facing support and that although vertical movement (i.e. settlement) will be low, it should be accounted for in the design. He also advised that using pre-cast panels allowed for rapid construction of revetment walls in a cost effective manner. Somewhat unfortunately, the major theme of the presentation was on saving money and questions were raised about whether we (geotechnical engineers) should be sacrificing safety, particularly given the cost of professional indemnity insurance.
Dr. Peter Mitchell also raised the question of slickensides in fissured clay soil, cracks and slippage planes and how these can possibly go undiscovered during the geotechnical testing phase and lead to a potential failure of a revetment wall. John countered the question by arguing that these would be discovered during the staged excavation (i.e. cut 1:1 then dig further).
Overall, I found this presentation useful, interesting and ultimately rewarding as it directly related to what was being taught during Geotechnical Engineering N and my potential future career. I believe the use of revetment walls in stiff clay has merit, but is in need of further investigation before it can be fully adopted as a method of stabilising excavations for road construction.
