Scale effects in the determination of rock mass strength and deformability

Abstract

Summary --Z u s a m m e n f a s s u n g --Rdsum~ ,Scale Effects in the Determination o[ Rock Mass Strength and Deformability.An extensive literature search was conducted, to investigate the effect of the scale of testing on the measurement of strength and stiffness of rock masses. The results are presented for bearing capacity tests, compression tests, and several types of deformability tests.Field strength values are generally several times smaller than laboratory values. Field moduli generally appear to be between 20 per cent to 60 per cent of laboratory measured moduli.All strength and deformability results reviewed point to the great need for more field tests. Specific suggestions are given. Maflstabeffekte bei der Bestimmung mechanischer Eigenscha[ten yon Fels.Ein eingehendes Literamrstudium wurde durchgef/ihrt, um den Maf~stabeffekt auf die Festigkeits-und Verformbarkeitsmessungen im Fels zu untersuchen. Es werden Resultate von Plattendruckversuchen, Druckversuchen und mehreren Verformungsversuchen aufgeffihrt. Die in-situ gemessene Festigkeit ist im allgemeinen ein Bruchteil derjenigen, die bei Laborversuchen festgestellt wird. Die Verformbarkeiten in-situ betragen meistens 20 bis 60 Prozent der im Labor erhaltenen Werte. Alle Resultate zeigen, daf~ mehr In-situ-Versuche notwendig sind. Der Autor macht dazu einige Vorschlfige. L'effet d'dchelle dans les mesures de propridt& mdcaniques des massi[s rocheux.On a fait une recherche bibliographique d&ai!lfie, pour &udier l'effet d'&helle sur les mesurcs de r~sistance et de d~formabilitd des masses rocheuses. On pr6sente des rdsultats concernant les essais de r&istance 5 la plaque, les essais de compression, et plusieurs types d'essais de ddformabilit6 en place.Les resistances mesur6es in-situ sont g8n6ralement une fraction de celles mesur6es au laboratoire. Les d6formabilitds mesurdes en place sont gdn&alement entre 20 et 60 pour cent des valeurs au laboratoire.Tous les r&ultats que l'on a examin~ indiquent la n6cessitfi d'effectuer plus d'essais en place. L'auteur offre un certain nombre de suggestions.

Effects of mechanical layering on hydrofracture emplacement and fluid transport in reservoirs

“…Also, in situ elastic properties are normally different from those measured in the laboratory. In particular, in situ static Young's moduli tend to be as much as 1.5-5 times lower than that measured of the same rock type in the laboratory (Heuze, 1980). This is mainly because fractures in in-situ rock masses lower their stiffnesses whereas the rock samples measured in the laboratory are essentially free of fractures (Goodman, 1989;Priest, 1992;Hudson and Harrison, 1997).…”

Subsurface Geotechnical Parameters Report

“…The results from the borehole jack tests show that most of the estimated in situ rock modulus values range from about 6 to 12 GPa (with extremes of 4 to 23 GPa) as shown in The estimated volume of rock mass (Heuze 1980, Table 1) involved in a NX-borehole jack test is about 0.13 m 3 (4.6 ft 3 ). Since this is not large enough to incorporate discontinuities on the drift scale, properly conducted borehole jack tests in better rock may overestimate actual rock mass modulus values.…”

Proposta para a estimativa da resistência à compressão uniaxial in situ de camadas de carvão com a utilização de geofísica

“…Adicionalmente, um conjunto de amostras de uma mesma rocha ou de um mesmo maciço rochoso quando submetida a carregamento em condições similares, apresenta características não constantes, mas dependentes da dimensão da amostra. Essa variação é denominada de efeito escala (CUNHA, 1990(CUNHA, e 1992 O efeito de escala na resistência de materiais rochosos tem sido observado em diferentes modalidades de ensaios, além da compressão uniaxial, como na compressão diametral, carga pontual, cisalhamento direto, ensaios de deformabilidade e capacidade de carga (HEUZE, 1980;BRACE, 1981;TSUR-LAVIE & DENEKAMP, 1982b;CUNHA, 1990;BANDIS, 1990). WEILBULL (1952) observou o mesmo fenômeno em outros materiais diferentes de rocha.…”