It is important to note that creep is only one aspect of the
time-dependent behavior of rocks. In Fig. 1, three cases are
illustrated with respect to the complete stress–strain curve:
creep, i.e., increasing strain when the stress is held constant;
stress relaxation, i.e., decreasing stress when the
strain is held constant; and a combination of both, when the
rock unloads along a chosen unloading path. This ISRM
suggested method deals only with the case of creep, which
is particularly relevant for cases where the applied load or
stress is kept constant.
Creep tests have also been carried out on soft rocks such as
tuff, shale, lignite, and sandstone, medium-hard rocks such as marble, limestone, and rock salt, and hard rocks such as
granite and andesite (i.e., Akagi 1976; Akai et al. 1979,
1984; Ito and Akagi 2001; Berest et al. 2005; Doktan 1983;
Passaris 1979; Serata et al. 1968; Wawersik 1983; Okubo
et al. 1991, 1993; Masuda et al. 1987, 1988; Ishizuka et al.
1993; Lockner and Byerlee 1977; Boukharov et al. 1995;
Fabre and Pellet 2006; Aydan et al. 1995; Chan 1997;
Cristescu and Hunsche 1998; Hunsche 1992; Hunsche and
Hampel 1999; Ito et al. 1999; Mottahed and Szeki 1982;
Perzyna 1966; Slizowski and Lankof 2003; Yang et al.
1999). These experiments were mostly carried out under
compressive loading conditions.
There are few studies on rocks using creep tests under a
tensile loading regime (Ito and Sasajima 1980, 1987; Ito
et al. 2008; Aydan et al. 2011). In particular, shallow
underground openings may be subjected to a sustained
tensile stress regime, which requires the creep behavior of
rocks under such conditions.