Acceptable water flows for rock anchor grouting : 1F, 2T, 14R. GROUND ENGNG, V8, 2, MAR. 1975, P46–48

Abstract

This technical note assesses the reasonable threshold values for water loss or gain which, when exceeded, dictate the need for waterproofing. The determination is described of the minimum width of fissure which will permit flow of cement at low pressure. An equation is presented which may be used in calculating the fluid loss or gain when a hole fully intersects a fissure. A graph illustrates the nomogram relating geometry of radial fissures and excess head to rate of flow of water into a cylindrical hole. Examples of limiting flow rates used in various countries are tabulated. It is estimated that a single 160 micron fissure under an excess head of one atmosphere gives rise to a flow rate of 0.7 gal/min (3.2 Litres/min). It is suggested that this value be considered as a threshold for water loss when Ordinary Portland Cements are employed. Lower rates are indicated with fine-grained cements. In conventional rock grouting applications the best known acceptable water loss is 1 litre/m/min at 10 atm. Assuming laminar flow the leakage rate is reduced and the Lugeon coefficient (L) is equivalent to 0.066 gal/min/atm. In the grouting of fissured rock beneath dams a low threshold for flow rate of L to 3L is logical since it is necessary to minimize water leakage. Recommendations are made relating to: the application of back pressure where there is a measured outflow or water gain (under artesian conditions); calculation of driving or excess head inducing flow at the section being investigated; interpretation of acceptable leakages quoted in terms of unit length of borehole or stage; and the minimum value of flow rates.