How to determine elastic and inelastic components in a load-displacement curve of jointed rock is discussed through the borehole jack test. We can establish the relations among elastic modulus of intact rock, deformation coefficient and joint stiffness in a brief equation with the introduction of ‘permanent deformation Coefficient’.
La presente communication concerne la methode de determiner les composants elastique et Inelastique à partir de la courbe charge-deplacement obtenue par l'experimentation sur verin en trou sondage du rocher jontoye. La relation entre le module d elasticite, le coefficient de deformation et la rigidite du joint peut être decrite par une equation briève en introduisant le 'coefficient de deformation permanente'.
Anhand des Bohrloch-Preβluft-tests wird diskutiert, wie sich dis elastischen und unelastischen Komponenten in ciner Last-Verdrangungs-Kurve Von zerklueftetem Gestein bestimmen lassen. Mit dem neu vorgestellten ‘permanenten Verformungskoeffizienten’ gelingt es, die Zusammenhange zwischen dem elastischem Modulus von unbeschadigtem Gestein, dem Verformungskoeffizienten und der Kluftsteifheit in einer kurzen Gleichung ausdruecken.
In designing and constructing rock structures it is very essential to know the realistic deformability of rock mass. It is not sufficient to characterize rock deformability by elastic constants alone, for many rocks are inelastic. Elasticity refers to the property of reversibility of deformation in response to load. Many fresh, hard rocks are elastic when considered as laboratory specimens. But on the field scale, where the rock can be expected to contain fissures, fractures, bedding planes, contacts, and zones of altered rock and clays with plastic properties, most rocks do not exhibit perfect elasticity. The extent of irrecoverability of strain in response to load cycles is as important for the design as the slope of the load/deformation curve. The most widely used testing procedures for deformability measurements are laboratory compression and bending tests, wave velocity measurements in laboratory and field, field loading tests using flat jacks or plate bearing apparatus, and borehole expansion tests. The borehole expansion tests are classified into two types: one is a dilatometer test in which expanding rubber sleeve provides uniform inner pressure onto the entire circumference of the borehole wall, and another is a borehole jack test in which the loads are applied unidirectionally across a diameter. Goodman [1980] suggested a concept for "modulus of permanent deformation (M)", whose definition is shown in Figure 1. The authors carried out the borehole jack test in three drill holes with the length of 70 m, respectively, and clarified the relationship among elastic modulus of intact rock, deformation coefficient of rock mass, joint stiffness, and modulus of permanent deformation.
The deformability of jointed rock is obtained conveniently through the plate bearing, flat jack or borehole expansion test, and the modulus of deformability (or deformation coefficient), which is given by the entire gradient of a load-displacement curve, has been employed as an index parameter of rock mass apart from Young's modulus of intact rock. It is widely known that the deformation of rock mass occurs mostly at joints when the strength of intact rock is comparatively high. Also, in many cases a certain amount of nonrecoverable (inelastic) deformation is recognized as the hysteresis in the load-displacement curve. We can know a deformation coefficient (D), elastic modulus of rock mass (E), and modulus of permanent deformation (M) as shown in Figure 2. Deformation coefficient (D) is defined as the gradient of enveloping straight line onto the hysteresis of several cyclic loading-unloading curves with different peak stress levels, and elastic modulus (E) is defined as the gradient of a reloading or unloading curve.