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Soil Nailing and Shotcrete
Soil nailing is a methodof retaining wall forexcavation or as a slopeprotection system,by reinforcing in-situ soilusing grouted nail inclusions(reinforcing bar) andthesurface between nails is stabilized by shotcrete. Shotcrete is a specialized method of applying concrete (reinforced with wiremesh) to a soil face using a high pressure spray. Soil nails provide passive resistance, as the soil mass deforms the nails become tensioned to arrest the deformations and stabilize the soil. Water can be accumulated behind the wall, care must be taken to release the water pressurecommonly by using strip drains installed behind the wall from top down to drain the excess water or alternatively with weep or discharges pipes installed at intervalson the wall.
The main advantages of soil nail system are rapid and flexible construction methods, accommodating any variations in soil and rock conditions and work progress, can be used for wide applications, and relatively more economical compares with other retaining wall systems.
Ground anchor or tied back anchorage functions as load carrying element, consisting essentially of a steel tendon inserted and grouted into suitable ground formations in almost any direction to obtain a specially formed bond length beyond zones of potential yielding of the soil mass. Its load-carrying capacity is generated as resisting reaction mobilized by stressing the ground (pre-stress) along the bond length, providing direct or active tension resistance to the anchored wall.
A ground anchor is commonly applied as tied back anchorage for retaining walls such as sheet pile, contiguous and secant piles, diaphragm wall and others. The use of ground anchors instead of strutting or raking shores to support retaining walls has the considerable advantage of providing a completely clear working area. In some cases, ground anchors are also often applied as pure tension piles such as tied back for raft foundation subjected to uplift.
Rock bolt is a steel bolt inserted and anchored in a hole drilled into a rock mass to reinforce the surface or near surface stability of rock cliffs and slope excavations by restraint the rock mass and wedges together from falling or collapsing, and tunnel excavation from cave in. There are three methods of anchoring rock bolts; mechanical, grouted and friction. The first uses an expansion shell bolt which preload can be applied (active), grouted anchored is a fully grouted reinforcing bar, and friction type is generated from radial force over the whole length of bolt.
Rock bolt reinforcements have advantages that can be used in any face geometry and ground condition, simple and fast to install, and relatively economical. In areas of highly jointed or fractured rock, steel wiremesh can be used in conjunction with rock bolts to hold the small blocks of rock (rock fall).
Dewatering is a technique to decrease water inflow and ground water table during construction excavation, thereby providing working space and increasing soil stability in the side and base of excavation. There are various methods; open sumps, well points, deep wells, horizontal wells and electro-osmosis (vacuum). Open sumps are necessary in any excavations, well points are multiple closely spaced wells generally suitable for large area of excavations, deep wells are more power full than well points, requiring only few holes thereby ease in excavation work and very commonly applied in deep excavations. The latter methods are more to remedy a difficult situation where other methods unsuccessful.
The choice of method depends to great extent of equipments, site conditions and soil characteristic, and often a combination method is inevitable. The amount dewatering works can be reduced by combining with seepage cut off such as impermeable retaining walls, groutings and freezing.