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• Application of external forces to increase the resistant of the slope to movements will increase the stability of the slope. Such resisting forces aze most often applied to the toe of the moving mass by a variety of methods, including: • Buttresses and tce berms • Structural retention systems such as cantilever and gravity retaining walls, externally braced walls supported by anchors or tie backs, soil nailing, micropiles, or conventional piles, and; • A variety of reinforced-soil systems The principal behind the use of buttress and tce berms is to provide sufficient dead weight or resistance near the tce of the unstable mass to prevent slope movement. Buttresses and tce berms that would be considered in Sylvester Gulch would consist of material developed from cuts in the sandstone on the project and from oversized boulders and wbbles encountered in the constmction excavations. Structural retention systems such as crib walls, gabion walls, and various soil reinforcement systems may be considered as mitigation measures, The tce of the mapped landslide in stability section D-D' is buttressed with a reinforced soil system (MSE wall). One of the more effective measures for stabilizing slope failures is the use of anchor systems such as tieback walls to increase the resisting forces by applying restraint to a moving mass. Techniques that are used to increase the internal strength of unstable masses include subsurface drainage, soil reinforcement, and miscellaneous techniques such as chemical, electrical, and thermal stabilization. • In-situ reinforcement systems include soil nailing and micropiles. Soil nails are steel bars, rods, or cables that are drilled and grouted into natural soil and soft rock slopes. Together with the in-situ soil, the nails form coherent reinforced-soil structures capable of stopping the movement of unstable slopes. Nailing differs from tieback support systems because the nails are passive elements that are not post-tensioned; also, the nails are usually more closely spaced than tiebacks. Stability of the surface between the nails is provided by a thin layer of shotcrete reinforced with wire mesh and by intermittent large steel washers or panels. Soil nailing is most effective in dense granular and low-plasticity stiff silty clay soils because a top down sequential construction procedure is commonly used. This top down constmction method is particularly well suited with the construction sequence for the cuts along the Sylvester Gulch. Reticulated micropiles aze cast- in-place reinforced concrete piles with diameters ranging from 3 inches to 12 inches. The insertions aze provided with a central reinforcing rod or steel pipe for the smaller diameters, whereas those of lazger diameters may be provided with areinforcing-bar cage bound with spiral reinforcement. A micropile systems forms a monolithic block of reinforced soil that extends below the critical failure surface. In contrast to soil nailing, the reinforcement provided by the micropiles is strongly influenced by their three dimensional geometric arrangement. General Construction Approach and Schedule for the SGFA As described in Exhibit 69, Sylvester Gulch Facilities Project Description, the general scope of construction activities includes a network of access and light use roads, installation of a new electrical substation and power distribution system, and development of a site on which two large diameter mine ventilation shafts will be constructed. High priority construction tasks include: • 1) Roadwork and site development for the electric power transmission and distribution lines,