Scottish Road Network Landslides Study - University of Glasgow

2 BACKGROUND TO SCOTTISH LANDSLIDES AND DEBRIS
FLOWS
by M G Winter, L Shackman, F Macgregor and I M Nettleton
2.1 LANDSLIDES
Recent extreme rainfall in Scotland has led to events that have been described in the media
using the generic term ‘landslide’. These events have intersected with the A83 (between Glen
Kinglas and to the north of Cairndow), A9 (to the north of Dunkeld) and A85 (Glen Ogle)
trunk roads.
While the recent happenings have been of both high magnitude (in terms of the amount of
material moved) and severe (in terms of their impact on the trunk road network and the
exposure of its users) it is important to understand that they are by no means unique. Similar
events have been observed in recent years by Nettleton et al. (In Press) at Invermoriston,
intersecting the A887, and at Stromeferry, intersecting the A890 local road. Other events have
been observed at A83 Rest and be Thankful, A9 Slochd, A95 Craigellachie and A84 Strathyre,
for example.
Many systems have been proposed for the classification of landslides, however, the most
commonly adopted systems are those of Varnes (1978) and Hutchinson (1988).
The International Geotechnical Societies’ UNESCO Working Party on World Landslide
Inventory (WP/WLI) was formed for the International decade for Natural Disaster Reduction
(1990 to 2000). The WP/WLI (1990) report “A Suggested Method for Reporting a Landslide”
uses Varnes’ (1978) classification and reports that it is the most widely used. The World
Road Association (PIARC) report “Landslides: Techniques for Evaluating Hazard” (Escario
et al., 1997) also presents a classification based on Varnes’.
Figure 2.1 presents the five kinematically distinct types of landslide identified by Varnes
(1978), as follows (after Escario et al., 1997):
a) Falls: A fall starts with the detachment of soil or rock from a steep slope along a surface
on which little or no shear displacement takes place. The material then descends largely by
falling, bouncing or rolling.
b) Topples: A topple is the forward rotation, out of the slope, of a mass of soil and rock about
a point or axis below the centre of gravity of the displaced mass.
c) Slides: A slide is the downslope movement of a soil or rock mass occurring dominantly on
the surface of rupture or relatively thin zones of intense shear strain.
d) Flows: A flow is a spatially continuous movement in which shear surfaces are short lived,
closely spaced and usually not preserved after the event. The distribution of velocities in
the displacing mass resembles that in a viscous fluid.
e) Spreads: A spread is an extension of a cohesive soil or rock mass combined with a general
subsidence of the fractured mass of cohesive material into softer underlying material. The
rupture surface is not a surface of intense shear. Spreads may result from liquefaction or
flow (and extrusion) of the softer material.
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