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Ground improvement and stabilization techniques-Ground reinforcement

Ground improvement and stabilization can be achieved by different methods such as ground freezing, lowering the water table, ground grouting, and ground reinforcement. Three distinct methods used to reinforce ground.

  • Rock dowels: these reinforcing elements consist of dowel, faceplate, and nut, as shown in figure 1. those elements are installed without tension. The materials of dowel will vary depends on the purpose of use, whether it is temporary or permanent. Deformed bar, glass fiber, and plastic can be used. The dowel of the reinforcing element usually embedded in mortar filled or grouted pipe, also resin capsules are used. Rock dowels can be used to stabilize the ground or hard rock. Dowels are placed at a discrete location to prevent unstable parts of ground or hard rock from falling during excavation.

Another form of rock dowels is inflatable dowels. Steel at this type of dowels is folded, as shown in figure no:2. The water is injected at one end. The folded steel will expand under the water pressure and press against the surface of the bore. Using these dowels will cancel the need for grout or mortar. 
Figure 2

  • Rock bolts: in these reinforcement elements, tension is applied during installation. They consist of a rod and mechanical or grouted anchorage (resin capsules or cement) coupled with some means of applying and retaining the rod tension. Expansion shell anchorage is suitable for hard rock, and it can be used for a length 3 to 20 m. end-grouted is ideal for all types of rock and can be used for a length of 2 to 8 m. 

Figure 3

  • Rock anchors: these anchors are tensioned after the installation, and they used for higher capacity and greater length than other reinforcing elements. They consist of high strength steel tendons usually in the form of cables. These cables are fitted to stressing anchorage at one end, and the tensile transferred to cable other ends. Cables should be protected against corrosion for permanent anchorage. Load test should be conducted to verify the capacity of rock anchors. Mechanical anchors slacken with time and hence could allow movement of the ground, fully bonded anchors should be used.

mechanisms by which rock reinforcement can improve the stability of the ground:

  • Stabilizing and prevent blocks of materials from separating and falling due to gravity for well-jointed rocks. The capacity of anchorage should be more than the materials block weight.
  • ground reinforcement will enhance and improve shear strength along the fractures and discontinuities at the ground. 
  • By using fully grouted un-tensioned rock bolts in laminated or stratified rocks to preserve the inter-strata shear strength.
  • By using tensioned rock bolts installed relatively quickly after excavation to improve the degree of confinement or the minor principal stress (this usually is perpendicular to the tunnel wall) in overstressed rocks.

Rock reinforcement alone is unlikely to be appropriate if (Woodward 2005):
  • the support pressure required is greater than 600 kN/m2;
  • the spacing of dominant discontinuities is greater than 600 mm;
  •  the rock strength is inadequate for anchorages;
  • the RQD( is a measure of the quality of rock core taken from a borehole.) is low, or there are infilled joints or high water flow.


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