This paper presents basic principles and required instrumentation for rock stress measurements with U.S. Department of the Interior, Bureau of Mines, hydraulic borehole pressure cells. Existing stresses and stress changes can be measured with the same instrumentation. Based on the elastic theories of plates and thick-walled cylinders, the, magnitude of biaxial rock stresses existing in a rock mass can be determined by pressure convergence-divergence tests using a combination of one cylindrical and two encapsulated flat hydraulic borehole pressure cells installed in a single drill hole. Case studies are included to demonstrate the validity of the technique.
ce papier décrit les principes de base ainsi que l''instrumentation nécessarire pour mesurer les efforts dans les roches au moyen des cellules de pression du Bureau des Mines du ministère de l''interieur. Les efforts présents et les changements de ces efforts peuvent être mesurés, par les mêmes instruments. La détermination des efforts est fondée sur la théorie de l'élasticité des plaques et des cylindres de paroi épaisse. La valeur des efforts biaxiaux qui existent dans une masse rocheuse est déterminée par des essais sous pression. On utilise a cette fin trois cellules de pressign dont une est cylindrique, et les geux autres plates. Toutes les trois sont placées dans un seul trou perforé. Des études de cas particuliers sont traitées à fin de démontrer la validité de cette technique.
Diese Arbeit beschraibt die Grundlagen und die notwendige Ausrustung fur die Messung von Spannungen in Gestein mittels der hydraulischen Bohrlochdruckzellen des U.S. Department of the Interior, Bureau of Hines. Bestehende Spannungen sowie Spannungsänderungen können mit denselben Instrumenten gemessen werden. Nach dem elastizitätstheoretischen Verhalten von Platten und dickwandigen zylindern kann der Betrag von zwe1achsigen Spannungen in einer Gesteinsmasse durch Druckkonvergenz und -divergenz Tests bestimmt werden. Dazu wird eine Kombination von einer zylindrischen und zwei etngekapselten, flaenen, hydraulischen Bohrlochdruckzellen in ein einziges Bohrloch eingebaut. Zur Demonstration des Verfahrens werden Fall studien angeführt.
The U.S. Bureau of Mines'' hydraulic borehole pressure cells flat (BPC) and cylindrical (CPC) were initially developed in the early 1960''s. Since then, the BPC has been used primarily for measuring uniaxial rock stress and relative changes in rock stress (Panek 1961). whereas the CPC, has been used mainly for in situ determination of the modulus of rigidity of rocks (Panek et a1 1964). The new technique of using a combination of BPC and CPC to measure biaxial and triaxial rock stresses both static and dynamic was developed during the last several years (Lu 1981. 1984). In this paper, the basic principles and necessary instrumentation to measure static and dynamic rock stresses are described. The validity and practicality of the technique are demonstrated with three typical case studies.
Based on the elastic theories of plates and thick-walled cylinders, the magnitude of biaxial rock stresses existing in a rock mass can be determined by pressure convergence,tests using a combination of one CPC and two pre-encapsulated BPC''s. as shown in figure 1. With this instrumentation the cylindrical cell provides the sum. whereas the two flat cells give the ratio of the biaxial stresses provided the rock mass is homogeneous and isotropic and the stresses are uniform within that portion of the rock mass tested. Then by drilling a pair of orthogonal holes for such instrumentation the magnitude of the triaxial rock stresses can also be measured (Lu 1981). The state of stress of an in situ intact rock mass whether homogeneous or heterogeneous isotropic or anisotropic. elastic or inelastic is initially equilibrium or inactive. However, once a ho1e is drilled into such a rock mass. the rock stress existing in the vicinity of the hole will become active owing to stress relief of the portion of that hole. Some types of rocks may be considered elastic but most are viscoelastic and show time effects such as creep under an intermediate (compared to ultimate strength) constant stress. The general problem of stress-strain Analysis, however is the same for elastic and visco-elastic rocks. Therefore, the viscoelastic solutions can be derived from those of the elastic analysis problem by replacing the elastic modulus with the viscoelastic modulus on the basis of the elastic-viscoelastic correspondence principle (F1ugge 1967. Lu 1981). Thus the problem of determining biaxial or triaxta1 rock stresses reduced to finding the equilibrium pressures, of the hydraulic borehole pressure cells; The cell equilibrium pressure can be determined from the long-term (1 to 6 months) pressure convergence of a single cell by taking the stabilized cell pressure as the equilibrium pressure as shown in figure 2A. However, the cell equilibrium pressure can also be determined from the short term (10 to 15 days) pressure convergence of several identical cells by using the rate of cell pressure change as shown in figures 2B and 2C.
