The Short Range Perspective
Only in the short range view are the texture and fine features of the cut slope important. Serving the short range view means incorporating textural enhancements at the rock fabric scale, such as ledges, slope roughening, planting pockets, and seeding or revegetation. Although these textural enhancements are noteworthy parts of most aesthetic enhancement programs, they are disadvantageous in some ways, and should only be employed where the visual prioritization shows that their advantages outweigh their disadvantages.
The duration of view, and the perspective of the viewer, should be kept in mind when
considering what and where textural enhancements might be advantageous. It has been widely observed by construction contractors, and not without justification, that the driver passing by a road cut at 65 mph is not as likely to appreciate intricate enhancements to rock cut texture as is the construction inspector standing at center line viewing the freshly excavated cut. A driver is much more likely to be affected by the appearance of the road cut ends, which are visible for a much longer period of time. This should be kept in mind when developing the aesthetic treatment criteria. Slopes incorporating ledges cannot be as steep as slopes without them. Therefore, textural enhancements commonly imply flatter slopes than the maximum that may be justified based on geotechnical criteria alone. This affects economy, safety, and slope performance.
The geologic setting governs the feasibility of textural enhancements. These features are most appropriate in masses of hard rock with moderately spaced (1 to 3 ft) fractures (Figure 1). In poorly fractured, massive rocks with fracture spacings more than 10 ft, there are few opportunities for ledging and pocketing along natural joints, so the contractor will have to resort to carving artificial ledges and pockets into the rock, leading to exactly the artificiality that the treatment is intended to remedy. In such cases a smooth surface would be preferable. In heavily fractured or strongly weathered, soft rock, ledges can be formed with equipment, but may not be stable or will develop a rounded or humped appearance. In those cases the ledge dimensions should be reduced except where the slope can be flattened enough to exaggerate the features.
Figure 1 This cut slope in massive, unfractuired conglomerate was smooth blasted. Treatment consisted of only drill trace removal on the main slope, but ledges were cut in natural boulder lenses at the cut end where similar features are in the adjoining natural terrain.
It is ordinarily inappropriate to show the locations of ledges, pockets, and other textural enhancements on the construction drawings, except where clearly defined stratification, faults, or other geologic features can be accurately projected to the finished slope location. Generally, this is not the case, and it is necessary to specify a general range of ledge widths, ledge areas, or pocket quantities. The highway construction plans may show conceptual details, sketches, or photographs of similar features, as guidance to the contractor, and should define the quantity of ledges expected between roadway stations. (As one might expect, if planting pockets or ledges are shown as pay items, contractors are much more enthusiastic about developing them.) During construction, the specific locations and extents of ledges are chosen by the construction contractor, approved by the Resident Engineer, and checked by the design geotechnical engineer.
The ability to develop ledges and roughened slopes implies that some degree of rock mass disturbance and overbreak must occur during initial excavation. Unless the excavation process is controlled, the disturbance and overbreak can lead to long-term slope deterioration. The objective should be to remove rock selectively, reducing the overbreak so that only the closest fractures to the nominal slope (in blasting, this is the last row of drill holes) is affected (Figure 2).
Figure 2 Rock breakage concept with controlled blasting.
What should be avoided is to employ more general (and cheaper) production blasting techniques along the desired slope line, and then to form ledges by excavating selectively with in the general zone of production blasting disturbance. Instead, specialized controlled blasting techniques and machine excavation should be employed, along with a program of continuous review and improvement of the blasting process and the finished slopes.
Roughened slopes with ledges and pocketing are more likely to project rock fall beyond the ditch and onto the roadway. This is because the ledges represent launching surfaces, and the slope flattening necessary to create ledges tends to promote the horizontal rock deflection component. Furthermore, when textural enhancements are employed, it is generally on projects where rock fall control elements like catch benches and rock fences are out of the question. It is very important to thoroughly scale rock slopes that incorporate ledges for planting pockets, and to aggressively round the slope crests.
One way to limit traffic exposure to rock fall, while still providing effective aesthetic enhancements, is to specify a graduated schedule of desired ledge widths indexed to station, cut height, and slope angle. The idea is that ledges can be wider, and slopes can be flatter, where rock cuts are lower. To reduce rock launching and rock fall, ledges should be narrower or absent in the higher portions of rock cuts. This approach provides effective visual enhancement, because the oncoming driver holds the view of the cut end longer than he does the middle. Providing additional ledge width at the end of a cut presents opportunities for revegetation, and also helps warp the cut slope smoothly into the natural terrain. The slope layback associated with additional ledge width at a cut end is not as costly, because the portion of the slope affected by the layback is not as tall. Overall, this presents a good combination of economy, visual enhancement, and safety.
Often, along with textural enhancements, the textural enhancement criteria specify that evidence of the construction technique be eliminated or prevented. Evidence of construction usually means blast hole traces and the marks left by construction machinery.
In order for blast hole traces to be omitted from the final cut slope, it is necessary that the rock containing the blast hole traces be removed along with the rest of the production rock. To permit this, overbreak from blasting must extend behind the last row of blast holes, and/or the angle of the finished slope must be steeper than the last row of blast holes. In massive, sparsely fractured rock, the blast hole spacing may be close to or less than the fracture spacing, in which case blast hole traces are almost unavoidable. In these cases it may be necessary to remove portions of blast hole traces by chipping with a hoe ram, excavator bucket, or pneumatic hammer (Figure 3). With respect to construction machinery, the most common sources of undesirable marks come from the corner bits on bulldozer blades, teeth on excavator or loader buckets, and ripper shanks. These marks are fairly easy to avoid except in soft/massive rock. Where the rock is soft and massive, it may be necessary to remove the machine scars by a rubbing with a plate bucket attached to an excavator or loader, or (preferably) using a high-pressure water spray.
Figure 3 15,000 ft-lb hoe-ram used for sculpting and hole trace elimination.
The visual significance of blast hole traces and machine scars must be considered in terms of the degree of slope roughness attained. Often, the importance of blast hole traces is overstated; not all blast hole traces are deleterious. Blast hole traces that may be evident when the cut is viewed from a stationary position out in the roadway, may disappear from the perspective of the oncoming driver who sees the same rock cut slope in profile (Figure 4). The bid documents should specify a level of blast hole trace reduction that is in accordance with the visual prioritization, taking into account the overall slope roughness, the rock mass fracture spacing and blockiness, and the perspective of the viewer. In most cases blast hole traces need not be completely eliminated or removed. Complete elimination of blast hole traces is commonly associated with a level of rock mass disturbance that is not desirable for safety, economy, or slope performance.
Figure 4 This slope in basalt was blasted using techniques that reduced rock mass disturbance but left behind some drill traces. However the drill traces are not apparent from the driverâ€™s perspective.