Landslide mitigation refers to human construction activities on slopes undertaken with the goal of lessening the effect of landslides. Landslides can be triggered by many, sometimes concomitant causes. In addition to shallow erosion or reduction of shear strength caused by seasonal rainfall, landslides may be triggered by anthropic activities, such as adding excessive weight above the slope or digging at either the mid-slope or foot of the slope. Often, individual phenomena join to generate instability over time, which often does not allow a reconstruction of the evolution of a particular landslide. Therefore, landslide hazard mitigation measures are not generally classified according to the phenomenon that might cause a landslide. Instead, they are classified by the sort of slope stabilization method used:
Geometric methods, in which the geometry of the hillside is changed (in general the slope); Hydrogeological methods, in which an attempt is made to lower the groundwater level or to reduce the water content of the material Chemical and mechanical methods, in which attempts are made to increase the shear strength of the unstable mass or to introduce active external forces (e.g. anchors, rock or ground nailing) or passive (e.g. structural wells, piles or reinforced ground) to counteract the destabilizing forces. Each of these methods varies somewhat with the type of material that makes up the slope.
Rock slopes
Reinforcement measures
Reinforcement measures generally introduce metal elements, which increase the shear strength of the rock and reduce the stress release created when the rock is cut. Reinforcement measures are made up of metal rock nails or anchors. Anchorage subjected to pretensioning is classified as active anchorage. Passive anchorage is not subjected to pretensioning and can be used both to nail single unstable blocks and to reinforce large portions of rock. Anchorage can also be used for pre-reinforcement on a scarp to limit hillside decompression associated with cutting. Parts of an anchorage include:
the header: the set of elements (anchor plate, blocking device, etc.) that transmit the traction strength of the anchor to the anchored structure or to the rock the reinforcement: part of the anchor, concreted and otherwise, placed under traction; can be constituted by a metal rod, a metal cable, a strand, etc. the length of the foundation: the deepest portion of the anchor, fixed to the rock with chemical bonds or mechanical devices, which transfer the load to the rock itself the free length: the non-concreted length.
When the anchorage acts over a short length it is defined as a bolt, which is not structurally connected to the free length, made up of an element resistant to traction (normally a steel bar of less than 12 m protected against corrosion by a concrete sheath). The anchorage device may be connected to the ground by chemical means, mechanical expansion or concreting. In the first case, polyester resin cartridges are placed in a perforation to fill the ring space around the end part of the bolt. The main advantage of this type of anchorage lies in its simplicity and in the speed of installation. The main disadvantage is in its limited strength. In the second case, the anchorage is composed of steel wedges driven into the sides of the hole. The advantage of this type of anchorage lies in the speed of installation and in the fact that the tensioning can be achieved immediately. The main disadvantage with this type of anchorage is that it can only be used with hard rock, and the maximum traction force is limited. In the third case, the anchorage is achieved by concreting the whole metal bar. This is the most-used method since the materials are cheap and installation is simple. Injected concrete mixes can be used in many different rocks and grounds, and the concrete sheath protects the bar from corrosion. The concrete mixture is generally made up of water and cement in the ratio W/C = 0.40-0.45, producing a sufficiently fluid mixture to allow pumping into the hole, while at the same time, providing high mechanical strength when set. As far as the working mechanism of a rock nail is concerned, the strains of the rock induce a stress state in the nail composed of shear and traction stress, due to the roughness of the joints, to their opening and to the direction of the nail, generally non-orthogonal to the joint itself. The execution phases of setting up the nail provides for:
formation of any header niche and perforation setting up of a reinforcement bar (e.g. a 4–6 m long FeB44k bar) concrete injection of the bar sealing of the header or of the top part of the hole Closing up and cementing any cracks in the rock can prevent pressure caused by water during the freeze-thaw cycles from producing progressive breaking in the reinforcement system set up. The procedure for this usually consists of:
cleaning out and washing of the cracks; plastering of the crack; predisposition of the injection tubes at suitable inter-axes, parallel to the crack, through which the concrete mix is injected; sequential injection of the mixture from bottom to top and at low pressure (1-3 atm.) until refusal or until no flow back of the mixture is noted from the tubes placed higher up. The injection mixtures have approximately the following composition:
cement 10 kg; water 65 l fluidity and anti-shrinkage additive or bentonite 1-5 kg.
Shotcrete As defined by the American Concrete Institute, shotcrete is mortar or concrete conveyed through a hose and pneumatically projected at high velocity onto a surface. Shotcrete is also called spray-concrete, or spritzbeton (German).
Drainage The presence of water within a rocky hillside is one of the major factors leading to instability. Knowledge of the water pressure and of the runoff mode is important to stability analysis, and to planning measures to improve hillside stability. Hoek and Bray (1981) provide a scheme of possible measures to reduce not only the amount of water, which is itself negligible as a cause of instability, but also the pressure applied by the water. The proposed scheme was elaborated taking three principles into account:
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