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Recent Projects
Bakun Dam Access Road Rock Slopes Construction Phase, 1996-1997 (on-going)
The Bakun Dam will be one of the largest earth filled dams retaining a lake the size of Singapore.
The present forestry roads from the river side town of Tabau require upgrading to provide a site access road for workers and construction material. The road runs through the typical landscapes of sub-tropical forest, rugged terrain and residual soils. The geology consists primarily of sandstone and mudstone. The picture below is one of many impressive views of the route winding through some difficult terrain.
Along the route many tens of embankments and cuttings had been constructed. The largest of which is called, appropriately, the Deep Cut which rises 80 metres from road level. Stability problems had been experienced in several cuttings ranging in scaled and complexity from localised bench failures and soil slope instability to larger multiple bench mechanisms.
On behalf of High Point Rendel for KTA Consultants in Sarawak, soil and rock engineering advice was provided for many rock and soil slopes, including the Deep Cut.
It was soon noted that not only is weathering a problem for these soils and rocks, but also, stress relief may be playing a more significant role in such large cuttings. When possible, some layout changes were also made to minimise any consequences of potential instability as pictured below. The photograph shows one of many stronly curved cutting sections. One can imagine that a good appreciation of discontinuities in three dimensions is required here.
Many rock and mixed soil slopes exist, some up to 30 metres high. Below is a typical example of a circular failure in a five metre high slope of mudstone.
The Deep Cut has between twelve and sixteen benches and runs north-south. The bedded sequence of sandstone and mudstone, in general dips steeply to the south. This dominant rock structure is important relative to the cutting orientation. The laminated rock mass is end-on and therefore inherently stronger and less susceptible to large scale stability problems.
The Deep Cut, is at a very early stage of excavation. The general character of a steeply dipping structure is clear on the right of the photograph below. The first traces of an infrequent but persistant moderately steep joint set was noted dipping east and west at this stage.
The charts following below, show an estimate of mass and joint strengths for input into stability analysis programs. A program was developed in-house to look at a series of planar failures on the basis collected data on joint orientations. The input chart for this program is shown further down.
From the results of detailed scanline surveys, it was possible to indicate the character and orientation of the key sets of jointing. The moderately dipping joint sets which presented a potential stability problem were less frequent when dipping east rather than west, but, there were also indications that these joints were weaker than the others.
The right-hand slopes were therefore of most concern.
The planar stability program was run on each of the collected data points and a statistical analysis indicated the likelihood of failure based on these results. Both sandstone and mudstone were analysed at differing heights and with differing water conditions on a composite slope profile. One of the main concerns was given to stabilise the strong sandstone units. In the bedded sequence these rocks provided a good buttress-effect and if not in good condition then the slope would be more unstable. It was recognised that the mudstone units also required support but this was at a local, bench, scale.
Anchors were installed in the thickest bands of sandstone where they would provide the maximum benefit to the stability of the whole slope.
The Deep Cut slopes are now fully excavated and future visits will hopefully confirm the success and applicability of the analyses and data collection.
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