Silt mudstone lied in deep field underground is the immediate roof of roadway and coal mining face, which mechanical properties may be important to corresponding engineering. By uniaxial mechanical test in laboratory for rock specimens, average instantaneous ultimate strength of the silt mudstone is 9.841MPa. Plastic-viscous constitutive model (Pwipp) has been constructed to simulate uniaxial creep properties of the silt mudstone. Basic creep parameters have been got by numerical simulation with six steps loading methods separately. The ratio of long-term strength to instantaneous strength is 96.79 percent and that of creep strength to instantaneous strength is 97.05 percent, which have special differences with traditional viewpoints. Results have given extrude rheological features of soft rock in deep site to be used in supporting design of underground engineering. That is, instantaneous strength, creep strength, and long-term strength of unstable soft rock with strong rheological properties will decrease in turn, but their values are closed one another.
Universal creep features of rock masses with increasing depth have been recognized in coal mining engineering. That rock mass whose ultimate strength is beyond 25MPa can bring large plastical deformation is typical creep properties of general rock in deep site (Gasc et al. 2004, He et al. 2002, Sun. 1999, Zhang et al. 2004). Under the condition of higher initial stresses, rock masses have distinct volume enlarged creep features and enlarging degree increases with higher stress grade or smaller enclosed pressure. Influenced by complicated geological environment, high stresses, rock cracks and water, rock masses surrounding roadways in deep field may present creep properties. That is, stable state and standard of rock masses in engineering will change with development of time or controlling factors (Deng et al. 2002, Karato et al. l993, Sun. 2005,Wang et al. 2001).