Pressure behaviour: an important monitoring tool in grouting

The relationship between grout pressure and injection rate is almost linear, all other things being equal; i.e. grout pressure increases with increasing injection rate, and decreases as the injection rate is reduced. At a uniform injection rate, an increase in the rate of pressure build-up indicates increased restriction within the formation or injection system, whereas a drop in pressure build-up indicates a reduction of restraint to injection. A sudden pressure drop nearly always accompanies uplift or hydraulic fracturing of the formation, and will almost always occur with leakage within the system. Conventional rock grouting has traditionally been controlled by maintaining a predetermined maximum pressure, resulting in ever variable injection rates, whereas much structural and compaction grouting is done using uniform injection-rate increments. This enables precise monitoring of the otherwise invisible grout movement, through continuous observation of the pressure behaviour. Injection control based solely on the pressure behaviour at a constant injection rate, and the advantages and limitations thereof are addressed. Use of such control in a wider range of grouting applications is discussed.