The special features and interpretation of the results of tests carried out on weathered and fissured rocks are described, particularly the wide variations between results. Other factors which lead to variations in the properties of the rocks are classified, including time effects. Criteria are established which enable the engineer to select the appropriate designs, and to assess a range of coefficients of the elasticity and deformability of the rock which will apply during the life of the structure. There is no single value which will apply throughout the site and at all times if the rock is weathered or fissured. The design of the structure should allow a reasonable factor of safety for all values of deformability within the established limits.
On decrit les methodes d'essai et l'interpretation des resultats concernant des roches alterees ou diaclasees, en particulier la grande dispersion des resultats. Les autres causes qui modifient les caracteristiques mecaniques des roches sont classees, y compris les effets du temps. Des critères sont etablis pour permettre à l'ingenieur de choisir le projet le mieux adapte, et d'evaluer l'ordre de grandeur des modules d'elasticite et de deformation des roches à prendre en compte pour toute la duree des ouvrages. II n'est pas possible d'appliquer une valeur unique de ces coefficients à tout l'ensemble du site et à tout instant, si la roche est alteree ou fissure. Le projet doit assurer un coefficient de securite raisonnable pour toutes les valeurs des coefficients de deformabilite prises dans la fourchette etablie.
Die Ausfuehrung und die Auswertung der Ergebnisse der Versucbe fuer verwittertes und rissiges Gestein sind beschrieben, insbesondere die weiten Streuungen dieser Ergebnisse. Andere Ursachen, fuer Veranderung der Felseigenschaften einschliesslich Zeiteinwirkung, sind klassifiziert. Regeln sind ausgearbeitet, die es dem Ingenieur ermöglichen, die passende Konstruktion zu wahlen, und eine Grössenordnung der Elastizitats- und Formveranderungskoef'fizienten des Gesteins abzuschatzen, die wiihrend der Lebensdauer des Bauwerkes anzunehmen sind. Es ist nicht richtig, einen einzelnen Wert fuer die ganze Baustelle und fuer jede Zeit anzunehmen, wenn das Gestein verwittert und rissig ist. Die Konstruktion und Berechnung des Bauwerkes sollte einen angemessenen Sicherheitsfactor fuer aile Werte des Formveranderungsvermögens in den festgelegten Granzen gewahren.
This paper deals primarily with the interpretation of rock test results in order to establish the parameters necessary to enable a structural design to proceed. A brief resume of rock testing methods, of which there are fundamentally four, is given:
Laboratory testing on samples which will determine the physical properties of small unbroken pieces of rock.
Insitu pressure tests, by conventional plate bearing and jacking tests. The measured deformation of the rock can theoretically enable a modulus of elasticity or deformability to be calculated.
Insitu rock testing by seismic methods which again theoretically allows a modulus of elasticity to be calculated
Insitu shear tests in which the resistance of a selected area of rock to lateral forces can be measured while normal forces are applied. This will give a measure of the shear strength of the rock mass, or the angle of friction at the joints of the rock. Over the last few years, new methods have been evolved, but they do not depart in principle from those enumerated above, although they may include valuable improvements or refinements of a practical nature. The study of large and increasing numbers of site test results shows that there are wide differences in the values of the moduli of deformation calculated from the test results, using the various methods. The laboratory results naturally give high values associated with the strength of the rock itself, independent of its insitu fractures. The seismic methods give instantaneous and relatively high values of the modulus of elasticity; the poorer the rock, the greater is the discrepancy between this high value and the results obtained from pressure tests [1]. The information which is most directly useful, will come from the plate-bearing or jacking tests, provided these are carried out by a standard method which includes the measurement of movement during waiting periods following each increment or reduction of load.
There are many factors which influence the form of the graph, the principal being:
An elastic movement which is wholly recoverable when the load is released.
A plastic, non-recoverable movement which is related to both stress and time. This movement assumes great importance when the rock is weathered and/or fissured. The relative amount is shown by the continued deformation at each waiting period during the incremental loading; but clearly, such movement is taking place all the time. A series of long-term tests would be necessary to find out how the strain varies with time. It is analogous to time consolidation of clay and may continue indefinitely.
