The problem arises with the grouting operation is that the quality of the borehole filling can not be controlled since unstable borehole walls can collapse and prevent a complete filling of the annulus resulting in a reduced function of the stress transfer to the SDA bar and furthermore reducing the grout to act as a layer of corrosion protection.
A more advanced installation method which can only be utilized with self-drilling anchors is the installation by “Simultaneous Drilling and Grouting”, which although not new is being adopted by many contractors today in the industry. This method requires a grouting swivel providing the drilling operator the option to use the grout mix directly as a flushing media.
The percussive force of the hammer should be balanced with rotation of the drill bit, as advancing the drill string with excessive percussion and feed pressure, but limited rotation, will restrict the formation of a true hole resulting in inconsistent grout cover. The flushing and grouting efficiency of the drill bit will also be reduced. Normal rotation is between 40 and 100 RPM.
Grouting pressure should be sufficient to maintain circulation at all times, with a small amount of grout return visible at the collar of the hole. A constant rotation should be maintained.
Grouting Swivel consists of a body with inlet for flushing medium/grout and a female-female Swivel shaft which connects the Rock drill/Drifter shank adapter with the bar at the other end.
Mounting of the grout swivel shall be in a way to keep the Body and the Shaft always aligned and also to prevent the body from rotating during drilling. For this a steel bracket is being used, which is fabricated in accordance with drill rig and drifter specifications.
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.
A flushing medium is needed to remove the cuttings from the advancing borehole produced by the drill bit to enable an efficient drilling performance and avoid jamming, as well as cooling the drill bit and ultimately reduce the wear of the bit.Three methods of flushing can be used:
Mainly for underground drilling in soil/rock formations or wherever there is an increased risk of soil mass instability. It avoids dust development in ventilated large cuttings and cooling of the drill bit.
Mainly for surface applications in all types of ground condition including swelling type clay and all other applications where excessive water spillage is to be avoided.
Simultaneous Drilling and Grouting:
Apply for both underground and surface applications in all unconsolidated soil conditions. Precautions are to be taken if large cracks or fissures are encountered through which the grout could be lost in the grounds thus not filling the annular space back to the surface.
Simultaneous drilling and grouting is today the most effective method for flushing and grouting in one single operation, it stabilizes the borehole during the drilling advance, provides for a high quality grout cover along the full anchor length, has a good penetration into the surrounding soil, thus improving the external interface between soil and grout and as such the friction values.
It is important to use a grout pump system that can regulate and control the flow and the pressure to minimize the waste and to maintain a constant flow during penetration.
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.