Image Source (Walcoom)
We all are familiar with the devastating implications of a disaster. The outrage of landslides, rockfalls, floods, earthquakes brutally affect our daily lives and yet somehow, we manage to ignore the precautions, that would have reduced the destruction. The crown of steep slopes accumulated with rocks, debris, and soil can be deadly, if unstable. Therefore, a proper slope stability technique comes in handy for reducing the damage. Analysis of the slope to evaluate the stability of rocks and soil is the first step to lay out the plan. Although, considering the consistency of the material and trigger potentiality is also essential.
There are many techniques for rockfall mitigation such as adding steel pins, rock bolts, tiebacks etc. but only a few last long enough to reduce the risk of up to 90%. One such technique involves ‘slope stabilization using Tecco mesh’. A Tecco mesh is a two-layer system composed of a 50nm chain-link mesh and a woven wire-rope net constructed typically with 8 mm diameter wire rope in a diagonal pattern on 100 to 200 mm centers. The stability process requires a series of steps for the strategic implementation of the double-layer woven net.
A. Site Evaluation (Slope Characteristics and Loads): The suitability of mesh/cable net systems is an essential step towards executing stability process. There are numerous examples where systems have poorly installed on slopes according to the site conditions. Let us see how to evaluate slope and loading conditions:
The trajectory of the potential rockfall can be predicted by looking at the slope condition. A near-vertical slope is dominantly governed by the trajectory of freefall. TheTecco mesh
in the case of vertical slopes imparts little stabilization effect through its weight, and there is no hindrance for the rocks to pass between the mesh and the slope. Whereas for flatter surface, where a translational movement takes place, mesh contact is often more, and its weight can impart a significant resistance force on individual blocks. The uniformity of the slope along the mesh can result in the displacement of the rockfall trajectory and, a considerable decrease in the frequency of the rockfall. It is also necessary to define slope coverage and the ongoing slope degradation to extend the mesh for the expected long-term configuration of the slope.
The size of rock and debris in the anticipated rockfall must be thoroughly examined before the execution process. A rockfall consisting of different single or sever blocks must be addressed by a draped mesh system which is intended to stabilize the trajectory of the rockfall. The threshold of the mesh and the size of the block should be taken into consideration. In the case of threshold failure, mitigation measures such as removal or reinforcement with shotcrete/anchor.
The macro and micro-roughness regulate the surface friction of the slopes. As the name suggests, the macro roughness is defined as the abrupt and highly irregular surface slope, whereas micro-roughness is more planar and smooth side of the slope. The surface friction is the component that provides resistance for the stability of the system. Minimal surface friction may occur when in contact with the smoother surface that leads to large mobilized force on the anchors. For macro roughness, the surface friction is comparatively very high with little to no mobilized force, on the anchors.
The accumulation of debris at the toe of the slope is a quite common phenomenon. But, is the mesh stable enough to withstand the force exerted by the accumulated debris? The assessment should be coupled, with how and where the debris collects, after mesh installation. Many other critical factors like type, size, volume and frequency of the debris must be considered. It is important to remember that to impart little load on the system, the debris must accumulate beneath the mesh.
B. Design Methodology:
After evaluating the type of slope, the next parameter is the determination of the wire mesh/cable suitable for the slope stabilizing system. The selection of wire mesh, anchors capacity and spacing for load conditions are assessed under the design methodology.
The expected block size, flexibility of the wire and puncture resistance are some of the key points to be considered while choosing the wire mesh. Mesh selection is primarily done to sustain the dynamic load and retain slope stabilization. The types of wire mesh include chain link wire mesh, double twisted hexagonal wire mesh, high tensile steel wire mesh (Tecco), cable nets, ring nets and a hybrid fabric that combines both mesh and cable nets.Tecco mesh
has more strength comparable to that of cable nets and is extraordinarily flexible compared to chain link and cable net. It has a rhomboid mesh structure and is quite suitable for both micro and macro roughness. The knotted ends provide better puncture resistance and tensile strength. A combination of hexagonal mesh, cable nets and Tecco mesh maximize the flexibility.
Steel anchors made of steel bars are fixed inside the soil or rock slopes and provide primary support for mesh systems. Although surface friction generates sufficient resistance to hold the mesh systems sometimes, smoother surface with minimal surface friction requires anchors for resistance contribution. An appropriate factor of safety should also be applied to these anchors loads and spacings.
The already assessed debris load will now be used for the anchor spacing. To maximize anchor spacing hexagonal mesh, Tecco G65 mesh and 12-inch square grid cable nets backed with either hexagonal or chain link mesh is recommended. If the mesh is significantly heavy than the cable nets use narrow spacing approach.
C. Design Specifications:
The design specifications are those little details that are necessary to incorporate for the better functioning of the system. These include slope coverage, anchor location, support ropes and increasing mesh contact.
The potentiality of the rockfall coverage area can be gauged during the evaluation of the slope condition. We can incorporateTecco Mesh in Slope Stabilization
using the possibility of extending the mesh beyond the current slope brow in the final design. If little catchment area is available, it may be advisable to lower the mesh to near the ditch line; however, these installations will require more frequent inspection and maintenance if debris accumulates and loads the system.
To avoid obstructions and for ease of installation, allow latitudinal siting of anchors. The location of the anchor can be either along the slope or upslope of the top horizontal cable. Anchors for the mesh systems are generally loaded perpendicular to the surface. Note that the load frame is sufficiently broad and is not influenced by the stress within the soil.
A moderately and steeply inclined slope maximizing mesh contact can lead to functional benefits like reduced slope erosion and rockfall. Increasing mesh contact can increase interface friction and eliminates the gap between the mesh and the slope, which is often discernable when viewed from the side. Moreover, the rhomboid mesh structure of Tecco mesh allows plants to grow regularly and is an eco-friendly choice.
After thorough evaluation and inspection of the layout and design, the last step includes installation of the mesh. There are some considerations to be followed for the better functioning of the mesh:
The installation of the specified stabilization work like anchors and scaling should be done before the placement of the mesh. Although in some cases it is preferred to install rock anchor after the placing of the mesh.
An inspection by the contractor, construction inspector and designer should take place to verify the field and measure the coverage area.
A minimum number of anchor tests should be conducted depending on the complexity of the slope stabilization process.
The main three methods of colouring involved, painting, powder coating and polyvinylchloride (PWC).
A careful inspection of wire rope clips should take place for proper clamping orientation, spacing and torque.
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