Marc Hendrickx and Associates Pty Ltd

We were asked to present at the 2015 NSW RMS Geotechnical Conference held at the Power house Museum in March. Subject of our talk was on the geotechnical aspects of the recently completed slope works at Clyde Mtn on the Kings Highway.

Abstract is outlined below:
Clyde Mountain, Kings Highway: Slip sliding away…Not

Abstract
Kings Highway is the main link between Canberra and the NSW coast at Bateman’s Bay. The section of the highway across the top of Clyde Mountain, east of Braidwood follows a route laid out by Sir Thomas Mitchell in the 1850s and cuts through very steep, densely forested terrane. The road in this section has a long history of instability and is affected by landslides from above and embankment failures below.

The main landslides above the road have their origin in construction works in the late 1950s and early 1960s, where widening the road to a modern standard undercut unfavourably inclined meta-sedimentary rock strata. This provided the potential for large scale planar failures on bedding to occur. The first landslide event involved about 30,000m3 and occurred on December 15, 1959 while widening works were in progress. Another landslide of about 5000m3 occurred on October 28, 1993. At this time workers installing concrete drains at below the slope reported the mountain groaning and observed small rock falls. A subsequent engineering inspection found an open tension crack above the slope. The road was closed and the slide occurred an hour or so later. On April 20, 2012 a smaller slide of about 930m3 occurred following heavy rain and damaged rock fall fences. On Tuesday June 25, 2013 a section of the crest of the 1993 failure slipped following a >200mm rainfall event resulting in about 600m3 coming to rest against the existing rock fall fence which burst in a number of sections but prevented most of the landslide debris reaching the road. In addition to these larger failures smaller rock falls and slips are common but generally go unreported. The area has also been affected by embankment failures below the road with repairs undertaken in 1975.

Prior to the early 2000’s the only preventative measures in place were a series of light weight rock fall fences. These were replaced in the early 2000’s by a series of RMS Type 3 rigid rock fall fences with an estimated capacity of about 350kJ. At the same time a series of draped mesh installations, shotcrete and rock bolts were installed to reduce landslide risk.

Following the 2012 event RMS engaged Marchgeo to undertake an investigation to better understand the geology, history of failures and to develop remedial measures to reduce the likelihood of failures affecting the road over the short to medium term.

The large landslides are planar failures on bedding planes best developed where the road and bedding are parallel. Where there is a slight difference in orientation, potential wedge style failures dominate. Rainfall is a major factor in triggering some of the slide events, but lack of an onsite rain gauge (until recently) has made it difficult to match historical events to precise rainfall figures. The study highlighted the importance of keeping accurate records of past failures, with the dates of previous slides being obtained from third party sources.

Remediation focused on reducing the likelihood of further large slides. This was done in two stages in 2013 and 2014. Initial work involved extensive de-vegetation, hazard mapping, and extensive scaling with blasting, machine and hand tools. The 2013 landslide occurred during this period of works. The second phase involved further development of the scope with the main construction phase of long term measures occurring between April-September, 2014. Works included installation of a series of reinforced shotcrete toe buttresses pinned back into the slope with long rock anchors that cross cut potential failure surfaces, along with additional scaling, spot bolting and shotcrete. Draped mesh (Geobrugg – Deltax) was used to control small rock falls across the slope. In addition to reducing the likelihood of small rock falls reaching the road the mesh is intended to reduce the rock fall risk to workers involved in clearing debris from behind the fences. Prior to the mesh being installed the fences were subject to puncture damage from high velocity impacts from small rock falls that contributed to a loss of capacity over time. The mesh now prevents these high velocity impacts and helps extend the life of the fences that were also extensively repaired.

Repair of a fill embankment downhill from Pooh Bear corner by soil nailing was also undertaken as part of the remedial works.

One of the figures used:

Clyde Mountain: simplified geotechnical model for 2012 failure zone.