The hydromechanical behaviour of a fracture: an in situ experimental case study

F. H. CORNET, L. LI, J-P. HULIN, I. IPPOLITO AND P. KUROWSKI

INTERNATIONAL JOURNAL OF ROCK MECHANICS AND MINING SCIENCES

Año: 2003 vol. 40 p. 1257 - 1270

1365-1609

Relationships between flow distributions and mechanical opening in a single natural fracture are investigated in situ through fieldexperiments, at a scale of about 1m, in a granitic quarry. Experiments have been conducted at various injection flow-rates while thenormal stress applied to the fracture was controlled by hydraulic flat jacks. Variations of the collected flow-rate (monitored throughcontiguous flow collectors distributed along the fracture periphery) with the injection pressure are fully reproducible. They showthat the fracture opens only above a threshold pressure which increases with the externally applied stress. This threshold is non-zerowith no applied flat jacks pressure which raises questions on the reliability of hydraulic jacking techniques for the measurement ofthe normal stress on preexisting fractures. It is shown that equivalent hydraulic aperture and mechanical opening are comparableonly above a critical mean fracture opening estimated to be around 15–20 mm for the tested granite. For mean fracture openingssmaller than this value, the standard time scale used for stress measurements distorts the results. It is also shown that channelingeffects may control flow away from the injection hole so that the hydraulic jacking stress measurement technique may overestimatethe mean normal stress acting on the fracture plane by as much as 4 MPa. It is concluded that hydraulic testing techniques fornormal stress measurements should not include results from the fracture opening phase. Moreover, criteria should be established forvalidating results from the closing phase in order to demonstrate the absence of channelling effects. Finally, it is shown that, becauseof the elastic response of the rock, water injection in a fracture system decreases the interstitial pressure ahead of the increasingpressure front associated with the water flow.