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3.2 Injection techniqueFor investigation of the groundwater conditions, especially the hydrogeochemical characteristics,of the cored part of a telescopic borehole, it is essential that the deeper groundwater isnot mixed with surface water or groundwater from shallow parts of the bedrock. Therefore, iflarge inflows of groundwater are met with during percussion drilling of a telescopic borehole,it is essential to prevent it from permeating into deeper parts of the bedrock. This is achieved bycement grouting of water-yielding fractures or fracture zones, as they come across. The simplestmethod is to fill part of the borehole with cement and to continue drilling after setting of thecement. This is also an effective method to stabilize the borehole wall as well, e.g. if a highlyfractured and unstable section is penetrated.If the percussion drilled part of a telescopic borehole is fractured and water-yielding, it isnormally cased to the full drilling length. The gap between the borehole wall and the casing isthen cement grouted, which further decreases or, often, completely prevents, inflow of shallowgroundwater to the borehole. Application of cement in the gap between the borehole wall andthe casing pipe can be performed according to different techniques. Two variants are illustratedin Figure 3-2.Borehole KFM06C was grouted at two occasions: 1) the entire borehole after pilot drillingto 61.20 m, and 2) after installation of the Ø i 200 mm, 100 m long casing (C2 in Figure 3-1).After installation of the casing, gap injection through a packer was applied, see Figure 3-2.Gap injectionCementGap injectionthrough packerPressureCementOverburdenFragmented rockLow fractured rock> 10 mCement pluggFigure 3‐2. Gap injection techniques. In order to fill the gap between the borehole wall and the casing,different techniques may be applied. To the left, a flexible hose is lowered between the casing andthe borehole wall, and to the right the grouting is performed through a borehole packer.14