Accurate geostress material is the vital precondition to make geotechnical engineering quantitative calculation and realize the engineering design optimization. Only by comprehensively mastering the geostress situation around the project, can the arrangement be ensured reasonably. Therefore, it would reduce the excavation and supporting costs to improve the economic benefit of the project on the premise that the surrounding rock was ensured stable.
Considering the general arrangement of the project, two points should be payed attention to:
- In the weak rock mass, the direction of the axis should keep consistent with the direction of the biggest principal stress in the arrangement of the underground construction such as tunnels. Then pass through the fault or other weak rock mass zone.
- The principle to choose the best section in the underground construction is that its geometrical shape and dimension should be adapt to the functioning method of the geostress.
For the majority of geotechnical applications except when drilling through clay, a maximum pressure of 7 – 20 bar is normally sufficient to ensure full placement of grout when using simultaneous drilling and grouting. Where pressures do not exceed these pressure, standard grout pumps with direct mixing and screw pumps (e.g. Metis MAI grout pump M400 NT) are sufficient.
For larger pressures of 20 bar (300 psi), as sometimes needed in clay or larger circulation of grout volumes for dill bit diameter larger then 100 mm, or where grouting is performed as a subsequent operation following drilling, e.g. mining and tunneling, higher pressures are also required. Typically pressures ranging up to 28 bar (400 psi) are sufficient, although pressures upt to 60 bar (860 psi) have been used in some underground applications. For these applications a double action piston pump with volume control should be selected .
As a general rule all grouting equipment, components and hoses must meet hydraulic specifications.
Drilling of self-drilling anchors can be performed with the use of rotary or rotary-percussion top hammer systems. Rotary-percussion is the preferred and the main method of installation as this is nowadays very common and also offers higher rates of production.
Top Hammer Rotary-Percussion drilling is featuring a Rotary/Impact Mechanism on the Drill Feed/Mast which is refers as Rock Drill or Drifter or Top Drive depending on the drilling machine and its manufacturer.
Drilling system can be either pneumaticly or hydraulicly powered. Generally a higher performance can be expected when using hydraulic drilling rigs. Typical drilling machines used around the world are Surface Crawlers like Geotechical Drill Rigs equipped with Drifter or a Rotary/Impact Mechanism.
Selection of the appropriate rig depends on:
• Scale of works
• Access to work site/face
• Ground conditions
• Size of self-drilling anchor and depth of drilling
Smaller sizes like R25 and up to some extend R32 with short lengths can be installed using Hand-held rock drills. This method of installation is selected for works in areas with restricted access or limited headroom application. Hand-held equipment work best when used in conjunction with an air pusher leg or within a slide frame arrangement as high operator fatigue limits production rates. Sliding frames are easily fabricated locally. The air leg is placed within the frame and normally reacts off the rear cross brace.