Abstract: The viscosity of polyurethane resins is 5 to 30 times higher than of cementitious

suspensions, and it increases due to a hockey-stick model. Thus, contrary to cementitious slurries, the

process of viscosity growth may take only several dozen seconds up to as much as few minutes. On the

other hand, viscosity determines the rate at which a grout travels from the injection hole into the rock

crack system at a specific pressure and with a certain thickness of an open joint. In other words,

a certain pressure at the pump outlet must be maintained to keep a resin flowing, where the pressure

is in proportion to the viscosity of the fluid and the flow is constrained by substantial resistance

of delivery lines with small internal diameters ranging about 10 mm. Even if the pumping rate is as

low as a few litres/min, the pressure drop is high, which is evidenced by a difference between

indications of the manometer mounted at the pump outlet and the one at the far end of the extension

pipes inserted into the borehole where the system of rock cracks begins.

This paper compares flow resistance and pressure drops for various accessories commonly used

for resin injection, like packers, static mixers, hoses and extension pipes. Furthermore, measurements

were taken over broad temperature ranges to consider influence of ambient temperature on the

injection process. Finally, the pressure loss is reflected in the function of the resin flow rate i.e.,

the pumping rate. Data generated during the experiments enabled a fresh insight into the injection

pressure loss to understand better how grout properties affect the safe execution of grouting operations.