Description
Loose or potentially unstable vegetation, blocks and localized bulges or overhangs remaining on a rock slope as a result of previous falls or excavations, progressive loosening and weathering of discontinuities as a result of freeze-thaw cycles, growing tree roots, weathering and/or washing out of clayey infill in rock discontinuities can be removed to sound rock by a variety of means and techniques, collectively referred to as “trimming” and “scaling”. Larger scale reprofiling of rock slopes falls within the scope of sheet 2.1 – “Removal of (actually or potentially) unstable soil/rock mass”. In particular, “trimming” refers mainly to the removal of potentially unstable overhangs, bulges and other geometric anomalies protruding above the general lay of the slope, while “scaling” refers to the removal of individual blocks or boulders which may be or readily become detached from the slope, even if they do not represent a geometric anomaly.
Trimming and scaling may be carried out by a variety of methods and techniques, depending on the size of anomaly to be removed and, even more importantly, access conditions. While scaling can be carried out, to some extent, by conventional hand held tools, such as pry bars, shovels, etc., this may need to be supplemented by controlled blasting or other significant mechanical assistance, especially when trimming involves the removal of blocks which are not yet detached.
Where small scale blasting is used, blast mats may need to be used to prevent flying debris, since typically there will be insufficient overburden to provide confinement. To minimize the risk of blasting causing uncontrolled fracturing of the rock, requiring additional trimming and scaling, cotrolled blasting is typically carried out by drilling one or more series of closely spaced (typically at 10 to 12 times the diameter), parallel holes along the intended breakline to evenly distribute the explosive on the face. Holes drilled by hand-held equipment are normally up to 40mm diameter and up to 3 m in length. Typically, low-velocity explosive is used, with a decoupling ratio (the ratio between the the hole diameter and that of the explosive) of about 2 to limit the pressure on the side of the hole to limit uncontrolled fracturing, stemming the holes to minimize venting and detonating each hole on a single delay. Where the thickness of material to be removed is significant, multiple breaking lines are detonated in sequence, starting from that nearest the free face, to the final line.
Blasting is often precluded by regulations in or near urban areas. Blanket bans on blasting may be in force in some countries or it may be so cumbersome to obtain permission for and to actually carry out blasting that to all effects this option is not available. In this case, alternative methods of demolition may be considered, depending on circumstances, including:
-
hydraulic hammers (rock breakers), either hand held or mounted on the boom of an excavator;
-
hydraulic rock splitters, which are inserted in a line of drilled holes and expanded hydraulically to create or open cracks;
-
expansive grouts (soundless chemical demolition agents), which expand slowly as a result of chemical reactions.
Both trimming and scaling can be highly dangerous and need to be carried out by specialist personnel operating under a strict safety regime. Typically the work is carried out proceeding from the po of the slope downwards, so that the workers are not unnecessarily exposed to the hazard of material falling from above and to avoid that the debris from the operation accumulates on previously completed portions of the slope.
Workers and equipment are typically suspended from ropes anchored in a safe area above the slope. On smaller slopes, access can be provided by self elevating platforms, with heavier equipment suspended from cranes, but this arrangement tends to be cumbersome and does not afford workers the same freedom of movement in case of need.
Since the debris from these operations will fall to the base of the slope, access to this area must be restricted during this type of work and the exclusion zone must extend sufficiently to cover for all possible run-out trajectories. Vulnerable structures within the exclusion zone may need to be temporarily protected.
Trimming and scaling may need to be repeated at regular intervals, especially if the rock is susceptible to rapid weathering, for example in mountain areas subjected to repeated freeze-thaw cycles, or where the rock face is overlain by debris.
Design methods
The design of scaling and trimming does not typically involve calculation. Rather, the design involves the identification and mapping of the main unstable blocks, bulges and overhangs that need to be removed, delegating to some extent to the workers on the face the task to determine whether a specific block needs to be removed, preferably to pre-defined criteria.
In relation to the need to define a safety exclusion zone and to protect vulnerable structures from falling debris, computer programs may be used to simulate the trajectories of falling rocks as they bouce down the slope (Piteau, 1980; Wu, 1984; Descoeudress and Zimmerman, 1987; Spang, 1987; Hungr and evans, 1988; Pfeiffer and Bowen, 1989, Pfeiffer et al., 1990). These programmes require information on the geometry and roughness of the rock face, the attenuation characteristics of the materials and details of the size and shape of the blocks. The statistical analysis of the results of a large number of simulations may be used to estimate the optimum position and dimensions of ditches and the height and capacity of fences and barriers.
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 9 | Only suitable to prevent/anticipate falls and, to a lesser extent topples, of individual blocks. |
| Topple | 8 | |
| Slide | 3 | |
| Spread | 1 | |
| Flow | 1 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 4 | Applicable to rock slopes and, to a much lesser extent, to cemented soils. |
| Debris | 3 | |
| Rock | 9 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | Applicable to superficial or very shallow movement. Large scale reprofiling to be considered separately. |
| Shallow (0.5 to 3 m) | 5 | |
| Medium (3 to 8 m) | 2 | |
| Deep (8 to 15 m) | 1 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 2 | Rock face must be stable; conditions should not be conductive to falls occurring whilst the work is being carried out. |
| Slow | 2 | |
| Very slow | 7 | |
| Extremely slow | 7 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 2 | Generally most suitable in dry conditions or minor seepage from the face; in other conditions it needs to be supllemented and preceeded by drainage. |
| High | 3 | |
| Low | 8 | |
| Absent | 9 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 8 | Suitable to reduce hazard associated with rainfall, snowmelt and freeze-thaw cycles and intermittent localized flows over the face. |
| Snowmelt | 9 | |
| Localized | 7 | |
| Stream | 0 | |
| Torrent | 5 | |
| River | 4 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | High, provided parent material not susceptible to rapid weathering, in which case it may need to be repeated on a regular basis. |
| Feasibility and Manageability | 8 | Widespread experience. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 4 | Difficult and hzardous. |
| Environmental suitability | 6 | will be updated |
| Economic suitability (cost) | 8 | Relatively low. |
References
- Descoeudres F., Zimmeman T. (1987).“Three-dimensional calculation of rock falls”. In: Proc. 6th Int. Congr. On Rock Mechanics, Montreal, Canada, International Society for Rock Mechanics, Lisbon, Portugal, 337-342.
- Highland L.M., Bobrowsky P. (2008). “The landslide handbook – a guide to understanding landslides”. Circular 1325, U.S. Geological Survey.
- Hungr O., Evans S.G. (1988). “Engineering evaluation of fragmental rock fall hazards”. In: Proc. 5th Int. Symposium on Laandslides, Lausanne, Balkema, 685-690.
- Pfeiffer T.J., Bowen T.D. (1989). “Computer simulation of rockfalls”. Bulletin of the Association of Engineering Geology, Vol. 26 (1), 135-146.
- Pfeiffer T.J., Higgins J.D., Turner A.K. (1990). “Computer aided rock fall hazard analysis”. In: Proc. 6th Int. Congr. Of the International Association of Engineering Geology, Amsterdam, Balkema, 93-103.
- Piteau D.R. (1980) “Slope stability analysis for rock fall problems: the computer rock fall model for simulating rock fall distributions”. In Rock Slope Engineering, Part D, Reference Manual of FHWA-TS-79-208, FHWA, U.S. Department of Transportation, 62-68.
- Spang R.M. (1987) “Protection against rock falls – Stepchild in the design of rock slopes”. Proc. 6th Int. Congr. On Rock Mechanics, Montreal, Canada, Intermnational Society for Rock Mechanics, Lisbon, Portugal, 551-557.
- Washington State Department of Transport (2010). “WSDOT’s Unstable Slope Management program”. http://www.wsdot.wa.gov/biz/mats/folios/unstableslopes.pdf.
- Wiyllie D.C., Norrish N.I.. (1996). “Stabilization of rock slopes”. In: Landslides: Investigation and Mitigation, Special Report 247, Chapter 18, A.K. Turner and R.L. Shuster (eds.), Transportation Research Board, Washington.
- Wu S.S. (1984). “Rockfall evaluation by computer simulation”. Transportation Research Record 1031, TRN, National Research Council, Washington, D.C., 1-5.