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In many cases, engineered slopes require stabilization to ensure their long-term viability and reduce localized slope failure (which includes erosion and rockfall). Generally speaking, the most effective strategy is to prevent the failure at the source through stabilization, not to install structures to protect against them in the future.
There are many methods that can be used to stabilize a rock slope. These include altering the slope geometry, installing drainage, adding reinforcement, or a using combinations of these methods. Table 8 provides an overview of common stabilization procedures. A more detailed discussion of each is included in this section.
These methods change the configuration of a slope by removing rock and/or soil.
Scaling is the process of removing loose or potentially unstable material (or a small section of slope) that might dislodge or affect the trajectory of falling rock by creating a launching point for materials falling from above. It is accomplished by hand or mechanical scaling, or by small blasting operations called trim blasting. Scaling is effective on natural and newly excavated slopes, and is done as periodic maintenance for any slopes that pose a potential rockfall hazard to roadways.
Scaling is used to reshape slopes and to stabilize existing slopes and mitigate rockfall. For new construction, scaling should be completed immediately after the initial slope construction and periodically thereafter to remove any loosened rocks. Hand scaling on existing slopes may be required on a more regular basis, depending on the construction and condition of the rock face.
As a stabilization or mitigation measure, scaling is typically effective for a period of two to ten years, depending on site conditions, so it is not considered a permanent mitigation measure. However, it is relatively inexpensive and serves as an effective short-term strategy. Because it enhances site safety, it is routinely included with other mitigation efforts such as new rock excavation, rock reinforcement, or draped mesh.
Because of the obvious danger from falling debris, complete road closures are generally employed during scaling operations. In some cases, temporary measures such as draped netting suspended from a crane can be employed while traffic is flowing, but such cases are rare. Temporary barriers (including concrete Jersey barriers, cable net fences, bound or confined hay bales, and earthen berms) are often used to protect the roadway surface, bodies of water, buildings, or other critical features from rockfall.
In most cases, engineers will indicate areas that require scaling in the roadway layout plans. In all cases, scaling operations should be observed and carefully controlled to prevent the creation of unsupported or overly steep slope areas. This is particularly true when using heavy excavation equipment.
Table 8. Overview of stabilization procedures and their limitations.
| Mitigation Measure | Description/purpose | Limitations |
|---|---|---|
| Slope Geometry Modification | ||
| Hand/Mechanical Scaling | Used to remove loose rock from slope via hand tools and/or mechanical equipment. Commonly used in conjunction with other stabilization methods. | A temporary measure that usually needs to be repeated every 2 to 10 years, as the slope face continues to degrade. |
| Trim Blasting | Used to remove overhanging faces and protruding knobs and to modify the slope angle to improve rockfall trajectory and slope stability. | Possible right-of-way issues, debris containment, difficulty with drilling, and undermining or loss of support by key block removal (blocks which exert major control the stability of other blocks). |
| Reinforcement | ||
| Internal Stabilization | ||
| Rock Bolts | Tensioned steel bars used to increase the normal- force friction and shear resistance along discontinuities and potential failure surfaces. Applied in a pattern or in a specific block. | Less suitable on slopes comprising small blocks. Requires good access to slope. Visible bolt ends and hex nuts may need to be covered with shotcrete to improve aesthetics. |
| Rock Dowels | Untensioned steel bars installed to increase shear resistance and reinforce a block. Increase normal- force friction once block movement occurs. Less visible than rock bolts. | Passive support system requires block movement to develop bolt tension. Requires good access to slope. Visible bolt ends may need to be covered. |
| Shear Pins | Provide shear support at the leading edge of a dipping rock block or slab using grouted steel bars. Can easily be blended with surrounding rock by colored concrete. | Cast-in-place concrete needed around bars to contact leading edge of block. Requires good access to slope. |
| Injectable Resin/Epoxy | Resin/epoxy injected into the rock mass through a borehole; travels along joints to add cohesion to discontinuities. Decreases the number of rock bolts or dowels needed in a rock slope. Great for aesthetics as it cannot be seen. | Joint apertures must be greater than 2 mm (1/16 in) for migration of product. In slopes with excessive moisture, product will expand and provide little increase in cohesion. Should not be used as the only mitigative measure on a rock slope. |
| External Stabilization | ||
| Shotcrete | Pneumatically applied concrete requiring high velocity and proper application to consolidate. Primarily used to halt the ongoing loss of support caused by erosion and raveling. Adds small amount of structural support for small blocks. Sculpted and/or colored shotcrete can be used for improved aesthetics and to cover rock bolts and dowels. Drainage must be installed. | Reduces slope drainage. Can be unsightly unless sculpted or colored. Wire mesh or fiber reinforcement required to prevent cracking. Must be applied in a minimum thickness of 50 mm (2 in) to resist freeze/thaw. Quality and durability are very dependent on nozzleman skills. |
| Drainage | ||
| Weep Drains | Reduce water pressures within a slope using horizontal drains or adits. Commonly used in conjunction with other design elements. Good for aesthetics because drains are rarely visible. | Difficult to quantify the need and verify the improvements achieved. Will need periodic cleaning to maintain water drainage. |
Hand scaling is the most common and inexpensive form of scaling. Workers rappel from the top of the slope or work out of a crane or man lift basket and use steel pry bars or air bags (also known as pneumatic pillows) to remove any loosened rocks. In most cases, several workers are scaling a slope at one time. Hand scaling is effective on small areas that are accessible by workers and that have rocks that are not too big to be removed manually.
Scaling companies typically provide their own equipment, including rappelling ropes, harnesses, pry bars, air bags, air compressors, and safety equipment. If access from the roadway is not feasible, a helicopter may be used to transport the scalers to an area above the slope. Figure 39 shows a typical hand-scaling operation.
Mechanical scaling is used on larger slope areas or to augment hand-scaling efforts. This process uses hydraulic hammers, long-reach excavators as shown in Figure 40, or cranes that drag a heavy object, such as a blasting mat or old "Caterpillar" track, across the slope (contractors have developed many ingenious scaling implements, including bundled cables, large steel rakes, and a used tread from a bulldozer, although not all methods have been equally successful). For removing very large rocks, power-assisted mechanical equipment such as pneumatic pillows or splitters can be inserted into open cracks, and then expanded to dislodge the rocks.
Mechanical scaling can also be performed by placing explosives into cracks and drilled holes (a process known as crack blasting) or using heavy construction equipment such as a trackhoe. It should be noted that without confinement, crack blasting can be relatively ineffective and can also produce loud explosions and flyrock.
The most important aspect of designing a rock scaling operation is ensuring the selected method is capable of handling the rock (or sections of rock) that need to be removed. Once scaling has begun on a feature, it will become unstable, and it cannot be left and the area re-opened to traffic restored until it is removed.
Most mechanical scaling operations use a crane or excavator, plus a front-end loader and dump truck to haul rock from the site.
Trim blasting, or trimming, is used to remove sections of rock that are too large for conventional scaling operations. Trimming typically uses cushion or smooth blasting techniques, as described earlier. After the rock section is blasted, the area should be hand scaled to remove smaller material. As with all blasting procedures, trimming can produce flyrock and loud air-blast.
Trim blasting requires drilling equipment and explosives. Spires of rock or large single rocks will require minimal blasting and drilling (which in many cases can be accomplished with hand drills), while bigger rocks, rock overhangs, or unstable rock faces may require more extensive drill and blast techniques.
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An introductory webinar to assist local and tribal agencies with applying for transportation funding under the Bipartisan Infrastructure Law (BIL).
