Post-Mining 2005, November 16-17, Nancy, France 1 A STUDY OF THE EFFECT OF STRAIN RATE ON STIFFNESS AND STRENGTH IN SILTY MUDSTONE USING MULTISTAGE TRIAXIAL TESTING HAMZA Omar 1 , STACE Rod 1 , REDDISH David 1 1 NOTTINGHAM CENTRE FOR GEOMECHANICS, School of Civil Engineering, the University of Nottingham, University Park, Nottingham – NG7 2RD, UK; omar.hamza@nottingham.ac.uk, rod.stace@nottingham.ac.uk, david.reddish@nottingham.ac.uk ABSTRACT: The effect of strain rate on mechanical properties has been reported in literature for some types of rock using uniaxial compression tests. In this paper, the authors investigated the phenomena of strain rate dependency of strength and stiffness in a silty mudstone, from the coal measure rocks of the UK. Using multistage triaxial testing, a series of experiments were conducted on both intact and fractured rock samples to cover the two ultimate structural conditions of the rock mass. The results showed that the rate-dependency of strength and stiffness is particularly significant in the fractured samples and this dependency varies according to the level of confinement. KEYWORDS: Time-dependent rock properties, Strain-rate, Abandoned mine, Multistage-triaxial test. RESUME : L'effet du taux de déformation sur les propriétés mécaniques a été rapporté en littérature pour quelques types de roche en utilisant les essais de compressibilité uniaxiaux. Dans cet article, les auteurs ont étudié le phénomène de dépendance de la résistance et la raideur d’une argile schisteuse au taux de déformation. Des séries d'essais triaxiaux ont été conduites à la fois sur des échantillons de roche intacte et sur des échantillons de roche fracturée afin de couvrir les deux états limites caractérisant la structure du milieu rocheux. Les résultats ont prouvé que la taux de dépendance de la résistance et la raideur de la roche à la déformation est particulièrement significative dans les échantillons rompus et cette dépendance change selon la valeur du confinement. MOTS-CLEFS : Effet du temps, Taux de déformation, Essais triaxiaux. 1. Introduction In common with any rock mass, the response of coal measure rocks may be time-independent or time-dependent. Within these two main categorized responses, the material may behave elastically or yield according to the confining pressure and the applied stress level (Kaiser and Morgenstein, 1981-a). Figure 1 illustrates all these possible behaviors in rocks. An understanding of the time-dependent properties is considered essential for further development in the field of underground mine design and long-term strata control (e.g. Singh, 1975). The effect of time has been manifested in rock specimens in laboratory tests as well as at larger scale during insitu observations. To investigate the time-dependent behaviour of rocks experimentally, researchers have used three main testing methods: (i) creep, (ii) relaxation and (iii) loading tests at different stress or strain rates. Using the third method, many experimental studies in literature have shown that stiffness and/or strength of some rocks (under uniaxial stress) can be highly dependent on strain rate and this dependency varies according to rock type and its condition (e.g. level of damage or fracture). Figure 2 and 3 show a copy of mechanical data from uniaxial compression