Dawn T. Nicholson,
Alastair C.
Lumsden and Steve R. Hencher
Engineered and quarried rockslopes
are subject to excavation-induced deterioration over engineering time, leading
to unplanned maintenance and constituting a safety hazard. Whilst it is standard
practice to evaluate deep-seated instability for excavated slopes, scant attention
is given to shallow surface processes at the design stage and subsequently.
This is because deterioration is not perceived as a significant risk, it is
difficult to quantify, and its mechanisms are poorly understood. Deterioration
is defined here as the shallow, progressive alteration, detachment and removal
of material from a parent mass by mechanical and chemical processes. It occurs
because excavation releases confining pressures leading to expansive recovery
of the rock mass, and because the newly-created slope is exposed to the external
environment.
The primary aim of ongoing research
at the University of Leeds is to develop a systematic approach to the evaluation
of rockslope deterioration potential. Such an evaluation should be applicable
both to existing and proposed rockslopes, and should lead ultimately to appropriate
design modifications or to the selection of remedial treatments. In pursuit
of this aim, a selective field investigation of the extent, nature and consequences
of deterioration has been undertaken for excavated rockslopes in the UK. Nearly
100 sites have been characterised in terms of rock mass and material properties
(eg fracture characteristics, rock mass structure, lithological properties and
weathering grade) and external factors. These have included environmental conditions
(eg hydrology and climate), stress conditions (eg stress release and quarry
blasting), slope and engineering factors (eg geometry, remedial measures and
excavation method), and time since exposure. Records have also been made of
(i) slope morphology (particularly forms attributable to deterioration), (ii)
evidence of weathering activity and (iii) the products of deterioration.
This data has led to the identification
of several distinct types, or modes, of deterioration, which, collectively,
result characteristically from shallow surface weathering and erosion acting
on slopes excavated in rock. Each mode is distinct in its frequency of occurrence,
its velocity of movement, the size of constituent material, and event magnitude.
In addition, each mode of deterioration is unique in terms of its implications
for slope maintenance, safety hazard and appropriate remedial treatments.
The deterioration modes recognised
are grouped according to their frequency of occurrence: Isolated modes such
as rockfall, debris flow, debris slide and rock slide tend to be large magnitude
events which occur infrequently and were rarely in evidence. Conversely, slabfall,
toppling, blockfall, scaling, stonefall and grainfall can be described as sporadic,
and occur frequently even in otherwise intact rock masses. Evidence for sporadic
modes was recorded almost universally on the slopes investigated, commonly being
controlled by the removal, reinforcement or support of individual blocks. Deterioration
modes such as ravelling (block, stone and grain), flaking, wash, rock creep
(flexural toppling) and solution occur on a semi-continuous basis and are best
controlled by containment or surface protection. These mechanisms tend to occur
in highly fractured, fissile, or weak rock masses and create the greatest maintenance
burden.
For each of the slopes investigated,
deterioration modes were compared with external factors, and rock mass and material
properties of the host rock(s). This has shown that there is no one intrinsic
slope characteristic or external factor which controls the mode of deterioration,
but that each mode of deterioration responds to the presence of one or more
specific conditions. Though the relationships are complex, the nature of the
fracture network and rock material strength are commonly of overriding importance
in determining deterioration mode. Various distinct types of rock mass structure
(eg regular or irregular blocky, massive, weak massive, fissile, composite and
layered) which rely heavily upon these two parameters, have also been identified,
and correlate well with deterioration modes identified. As such, many excavated
rockslopes may have a built-in propensity to deteriorate in a certain fashion,
a factor which can be exploited in any evaluation of deterioration potential.
A new, systematic, three stage engineering
evaluation of deterioration, called Rockslope Deterioration Assessment (RDA),
is currently under development. In the first stage of RDA, a range of potential
rock and environmental controls on deterioration are given a rating to determine
the deterioration susceptibility class. Stage two of RDA comprises the assessment
of deterioration mode using the classification discussed herein. The final stage
utilises the results of stages one and two to provide guidelines for slope design
modifications and remedial treatments.