◆ Security interception technology
Landanggou
Wider trenches will be very effective in catching falling rocks, but the geometry of cliffs must be considered when designing trenches, and it is best to design the detailed dimensions of trenches after consulting relevant experts. The bottom of the ditch must be covered with loose soil to prevent falling rocks from bouncing up or breaking into pieces and splashing around. If there is not enough space to build grooves that meet the design size, they can also be combined, such as setting multiple rows of small grooves, followed by rock filling or rock walls.
Cables, nets, fences and rock curtains
Cable and barbed wire are simple and economical preventive measures to protect roads or highways from rock collapse. For large unstable rock blocks, metal cables can be wound around the rock blocks and then fixed on the slope with anchor cables. If the rock mass is too broken to be restrained by a single anchor cable, metal mesh can be considered. Barbed wire mesh (with small spacing) can be used to prevent rocks smaller than 75 cm from caving (see the example photo of barbed wire in figure C2 1). The standard wire mesh is double wire mesh. When the rock surface is relatively flat, the steel wire mesh can be easily hung on the rock wall; When the rock surface is uneven, it is necessary to fix the net on the rock wall with rock anchors to ensure the close contact between the net and the rock wall. Fixing the net on the rock wall can prevent the displacement of the rock block and strengthen the overall stability of the slope or rock wall. Steel wire mesh is also effective in steep soil cutting slope, especially in gravel soil slope.
Catching nets made of cables and wire mesh can be built at the bottom of slopes and valleys to catch falling rocks. After the barbed wire is suspended and fixed by the anchor cable, it forms a flexible barrier, which can dissipate the energy and impact of falling rocks. If properly fixed, it can be used to catch rolling stones with a diameter of about 1 m. In addition, the catching net can be used in conjunction with the retaining ditch set by the roadside.
Rock fences as shown in figure C22 are easier to build and can prevent small-scale rocks from falling on the road, but they can't control falling rocks that bounce over the top of the fence. As shown in figure C23, the rock curtain can guide the falling rocks into intercepting ditches or other intercepting structures more effectively, and prevent the falling rocks from bouncing onto the roads or other facilities outside.
earth-retaining wall
Like the retaining wall used to reinforce the soil slope mentioned above, the rockfall retaining wall has the same function to prevent rockfall and debris from entering a certain area. This kind of retaining wall is somewhat similar to a rockfall fence, but it is stronger. The retaining wall can be made of reinforced concrete, wood or other materials, but it needs to be properly fixed with anchor cables to prevent the rock from overturning when it collapses.
Rock shielding and masking
Rock barriers and bunkers refer to facilities built on highways, railways and sometimes buildings to protect against rock landslides and large-scale landslides. These facilities use concrete or steel bars, or other materials, and their ends can be open or completely surround the rockfall area. They can guide the falling rocks on roads, railways or structures to other places, so that they will not affect the safe use of roads, railways and structures. Some examples of this are shown in Figures C24 ~ C27.
Rock and rock frame reinforcement
This kind of structure is not commonly used, because it only works under special circumstances, and it must be constructed with great care and requires high structural strength. Figure C27 is an example.
◆ Rock excavation
series
Building horizontal steps on the rock wall is one of the most effective methods to prevent rockfall disasters. In order to prevent falling rocks, steps can reduce the tensile stress of surface rocks and slow down the erosion rate of rock surfaces, thus reducing the incidence of falling rocks. However, it must be noted that this method has little effect on the potential deep rock failure.
In the process of construction, the wall of stepped steps can be steeper than the whole slope, so that falling stones can stay on the steps (Figure C28). However, the wall inclination of nearly 90 degrees must be avoided, because it will lead to tensile cracks, dangerous suspension and even steep rock collapse. The step surface shall not be placed at the bottom of weak rock stratum, rock fracture zone or water-bearing zone. The minimum width of the steps shall not be less than 4m, and all steps must have drainage ditches to lead water out of the slope.
Remove turquoise and repair slopes.
Loose, unstable and/or suspended rocks that are dangerous to traffic and pedestrians can be removed by removing turquoise and repairing slopes. Scaling refers to removing loose rocks with hand-held probes or light explosives. Dressing refers to removing potentially dangerous rocks in a large area by means including drilling and light blasting after removing turquoise. Controlled blasting can reduce the necessity of repair work, but blasting work is not always feasible. Suspended rocks can be removed or fixed on stable slopes. The work of removing turquoise is carried out by workers using probes, jacks or explosives, and workers must hang and fix it with ropes or other means. These jobs are sometimes time-consuming and expensive (and sometimes dangerous). On those active slopes, it may need to be repeated every few years or when necessary. Removing turquoise requires high technology and is accompanied by danger; Workers who maintain slopes need special training, and professionals must be present during the maintenance work.
◆ Reinforce areas with potential danger of falling rocks.
Sprayed concrete and pressure sprayed mortar
Shotcrete and pressure mortar are reinforced by directly spraying concrete on the unstable rock surface by air pressure. Spraying concrete refers to the process of spraying dry or wet concrete or mortar. Pressure-sprayed mortar only refers to the process of spraying dry mortar. The dried concrete mixture is conveyed to the sprinkler through the empty pipe, where it is contacted with the injected water. This method is fast and easy to operate, which can strengthen the connection between rocks and reduce the weathering and looseness of rock surfaces. The cumulative thickness of shotcrete can reach 2 cm, which is more common than pressure shotcrete mortar (the cumulative thickness is smaller). Because these two substances can be implemented quickly by air pressure, even large-scale work can be completed quickly in a short time. Figure C3 1 shows a scene of spraying concrete on an expressway.
Anchor bolts, anchor bolts and pins
These steel members or cables improve their stability by strengthening and connecting rocks. Anchors support large rock blocks by applying prestress, while anchors are relatively short and mainly used to support rocks on slopes. The dowel pin is similar to the anchor bolt, but it is not prestressed beforehand. Strengthening rock slope with steel structure requires professional knowledge of rock slope stability analysis, anchoring technology and test steps. For a successful anchoring system, it is very important to correctly determine the orientation of the potential sliding surface, which requires a lot of engineering practice experience. Figs. C32 and C33 show anchor bolts and examples of installing anchor bolts on expressways.
Figure C32 Close-up photo of rock anchor on the rock surface is fixed on the rock surface, which can be connected with barbed wire to play a greater protective role. The anchor depth here is about 5 meters.
Figure C33 Rock bolt operation. Pay attention to the nature that rocks are easily broken into thin slices (Source: Washington State Department of Transportation, USA)