Monday, October 15, 2012

Slope Instability

When we look at a big river bank and see some small movements occurring, we get a feeling that this may be a progressive failure starting. So what should we do with respect to the top of bank setbacks? The City of Edmonton has a Top of Bank Policy that requires a geotechnical engineer to provide an opinion on the slope, and have created a concept of a like of stability at some point behind the top scarp.

The first way of  providing a estimate of the required setback is to use a mature slope angle concept. That is to say that the soils are uniform along the portions of length of the river, and for that portion, the slopes should be all the same. AKA, take the longest natural distance from the river and use that as a setback from the river. OK. First approximation, OK. Expect issues. I will add to that, from any daylight, a maximum of 2.5h:1v for clays, to address sharp top scarps.

Geologically, there are two methods of producing a slope, erosion or bank slumping and toe erosion. Eroded slopes have undisturbed material at there face on the upper portions, while slumped banks have disturbed material everywhere below the top scarp. It has the shape produced by heads and minor scarps, flank scarps, fault zones, and a run out toe. It is even worse it the run out toe has been eroded by a river or ran into a river, on the outside of a bend.

A bit of erosion and channelling through general slump features shows the age. Sharpe features suggests newer, round features suggest older. Existing fresh movements suggest the hill is not stable. Repairs to the hill to prevent erosion, and provide roadway support often reduce the factor of safety on the overall hill. The city does not like to to slope analysis unless necessary, as if these show an inadequate slope, it could not allow develop above the slope without liability. As a result, the City does little short term fixes, but never address the actual cause.

Instead it put the responsibility on the home owner to find a geotechnical engineer to say it is ok to build. This one gets nervious with progressive failures, as the model to to complete failure time is not good, and before failure there is sometime considerable creep, and resulting house movements.

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