In order to study the long-term creep law of extremely soft coal rock, the uniaxial and triaxial creep tests of extremely soft coal rock subjected to single-stage loads were carried out by using the self-developed triaxial creep test system. The following results and conclusions are obtained. 1) In the uniaxial long-term creep test, transient creep stage, steady-state creep stage, and accelerated creep stage occur successively during 232 hours, and the cumulative creep strain is as high as 3.45%, which is 10.5 times of the instantaneous deformation. The strain rate for the total steady-state creep stage is as high as 8×10-5-10×10-5 /h, the maximum accelerated creep rate reaches up to 0.043/h, and the strain rate of total creep process is distributed in a U shape. 2) In the triaxial long-term creep tests subjected to the same axial pressure(0.96 MPa), the ability of resisting long-term deformation of extremely soft coal rock increases continuously with the increase of confining pressure from 0 to 0.6 MPa, which is shown as follows: the creep strain decreases significantly; the strain rate of steady-state creep stage decreases by order of magnitudes; the duration before creep failure increases obviously; the ratio of creep to instantaneous strain decreases dramatically; the intensity of accelerated creep failure decreases markedly. The creep strain and deformation rate are especially sensitive to the confining pressure change in the range of 0-0.2 MPa. 3) The accelerated creep stage of extremely soft coal rock is characterized by “gradual” time dependent instability, which is significantly different from the “abrupt” accelerated fracture instability of ordinary rock. 4) The nonlinear viscoelastic plastic creep model is established by connecting a new nonlinear viscoelastic model considering the concept of accelerated creep start-up time with Burgers model in series to describe the three creep stages of extremely soft coal rock subjected to single-stage load. Then, the creep parameters were identified by employing the Levenberg-Marquardt optimization algorithm. The fitting curves are highly consistent with the experimental curves, verifying the validity of the proposed model. These findings can provide reference for theoretical analysis of nonlinear large creep deformation and support design of extremely soft rock.
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