The grout mix is an essential part of the self-drilling anchor system providing the interface between the bar and the ground formation. Placement of the grout mix can either be subsequent to the drilling which is referred to as Post Grouting or combined with the drilling which is called Simultaneous Drilling and Grouting. Depending on the method to be used, the appropriate grouting equipment needs to be planned and set up prior to installation.
The basic grouting equipment consist of:
A mixing unit in which all ingredients are brought together always in the same sequence. First the water, then cement and finally the defined admixtures ( eg. Filler such as flyash or ultra fine sand, chemical additives such as plasticers and/ or non shrink additives). The mixing unit shall be capable of producing a grout of uniform consistency with the workability and strength characteristics required by the design.
A pump unit with sufficient pressure capacity to transport the homogeneously mixed grout paste through the grout hose and the hollow system of the MAI self drilling anchor bar, exiting at the drill bit through the flush holes and filling the annulus of the borehole until it exits at the borehole mouth.
Suitable grouting equipment available and recommended are:
a)Separate mixing and pumping unit such as the Uni-grout grouting platforms
b) A single pass mixing and pumping unit like the MAI grout pump M400 NT
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.