A rheological model for peat that accounts for creep

The rheological model of Gibson and Lo is revisited for peat behavior including creep. A top linear spring is used to account for macro-compressibility of the peat skeleton, combined with a linear spring and dashpot that simulate the compressibility and viscous behavior of porous peat structure.

The equation of consolidation of Terzaghi for a layer of peat, drained on the top, is combined with the rheological model. Finite difference method is used to solve the system of equations, based on initial condition of uniform pore pressure distribution equal to the total applied pressure on top and a variation of pore pressure (U) equal to zero at the undrained bottom of the layer. A constant load is applied to simulate both consolidation and creep. The results are compared with an empirical creep equation, giving the strain as a function of log(t). The physical mechanisms involved in the compression of peat are discussed with a focus on creep effect. Peat provides both viscous and micro-pore mechanisms of consolidation. Constant Load tests are performed on samples of saturated peat (10cm diameter, 25cm height), drained on the top and undrained on the bottom. Vertical and lateral pressures are measured as well as pore pressure. The results are compared to the predictions of the rheological model and to the empirical relation. Oxidation in peat due to variations of water level and temperature is taken into account using a model proposed by Stephens et al [10]. Combining creep and oxidation gives significant vertical deformations in time.