ABSTRACT:

This article describes a simplified method of determining the in situ stress by hydraulic fracturing to evaluate the safety of pressurized tunnels and shafts. Test data from four hydropower plants in South Norway are presented, the interpretation of the stresses discussed and the results compared with those from overcoring tests.

RESUME:

Cet article fait une description de la méthode de la fracture hydraulique pour evaluer la sécurité des puits et tunnels pressurisés relative à la fracture hydraulique. Les résultats d''éssais de quatre centrales hydroélectriques du sud de la Norvége sont présentés avec les résultats de mesures de pression roche use dans un forage sur dimensionné.

ZUSAMMENFASSUNG:

Dieser Artikel beschreibt eine vereinfache Form der "hydraulic fracturing" Methode um die Sicherheit von Druckschächten gegen "hydraulic fracturing" abschätzen zu können. Messungen von vier Wasserkraftanlagen in Sud-Norwegen wirt presentiert, und mit andere Bergdrucsmessungen vergleichert.

1. INTRODUCTION

Recent Norwegian hydro-electric power projects have been utilizing high water heads in unlined pressure tunnels and shafts. In situ stress measurements were required to determine the risk of hydraulic fracturing occurring in these excavations. The Norwegian Geotechnical Institute (NGI) in collaboration with the Norwegian consultants Serdal A/S have developed a simplified hydraulic fracturing technique to determine the minimum in situ stress. The results from this method are directly applicable to determining the likelihood of hydraulic fracturing occuring in the excavations during operation. The tests can be carried out in a relatively short time. The preparation work can be done by a contractor and the actual testing of the hole accomplished in a couple of hours. The cost of such tests can be as low as 10 - 15000 NOK (1 US$ 7.5 NOK, June 1986).

2. TEST METHOD

The tests are conducted in boreholes drilled in the wall of an underground excavation. High pressure steel tubes are grouted in the test holes using a rubber disc to separate the grout from a test section of about 5 m. The length to the test section is generally about twice the tunnel diameter. A sketch of the test set-up is presented on Figure 1. A high pressure pump is used to pressurise this test section with water. Pressure in the test section is measured by a transducer at the mouth of the borehole and recorded on a strip chart. The test section is first pressurized until rupture, indicated by a sudden drop in pressure, when the pump is shut off and the pressure in the borehole observed to determine the shut-in pressure. The test zone is later vented and then repressurised to determine the refracture pressure and further values of the shut-in pressure. A typical record of the pressure variations in the test section during a test is shown on Figure 2. The pressure-time diagrams generally indicate a clear shut-in pressure as shown on this example. In other cases, however, leakage of water from the system may cause the pressure-time curve after shut-in to look smooth and the shut-in pressure is ambiguous (Figure 3).

3. INTERPRETATION

The breakdown pressure is determined from the highest pressure attained in a test before rupture. The shutin pressure is determined at the point of inflection on the pressure time curve following shut off of the pump. In the cases where there is no obvious inflection in the curve a graphical method must be used to determine the shut-in pressure. Many such methods have been proposed. The use of log-log and semi log plots of pressure and time, and squareroot time and time ratio plots have been described (1). Similarily the Muskat method (2) and tangent intersection and tangent divergence analyses (3) may be used. However, a simple method involving the plotting of rate of pressure decay against pressure (4) was found to give unambiguous and consistent results. A typical plot taken from the shut-in curve in Figure 3 is shown on Figure 4. The most relevant parameter to the design of pressure tunnels and shafts is the minimum stress determined directly from the shut-in pressure. The values of the minimum stress reported, are the mean values of 3 (occasionally 2) shut-in values determined from breakdown and refracture tests in a borehole. Generally 2 to 3 holes in the same area are tested, and the results combined to give a mean minimum principal stress for the area. The method does not permit inspection of the borehole after fracturing to determine the orientation of the fracture and thereby orientation of the principal stresses.

4. SITES

The sites tested by these methods are shown on the map, Figure 5. Stress orientations determined by overcoring tests at each site are shown on the appropriate figures in upper hemispherical polar diagrams. The overcoring tests reported here have been carried out by The Foundation for Scientific and Industrial Research of the Norwegian Technical University, Trondheim, Norway (SINTEF).

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