Modified Hyperbolic Model for Dynamic Properties of Peaty Organic Soils

Modulus reduction and damping versus shear strain relationships (hereafter MRD) are essential for nonlinear ground response analyses of soil sites. For sites containing thick deposits of peat, large shear strains that produce pronounced nonlinearity are expected in seismically active regions. Currently, available MRD models for peat have strongly divergent characteristics, particularly with regard to the influence of confining stress. We assembled a large database of test data from literature and proposed a hyperbolic model, similar in form to widely used models for non-organic soils, for application to peat. We related the critical model parameters to stress history-modified effective stress, finding no strong dependence on organic content (OC) and peat composition (fibrous structure) over the parametric range of the database (OC >30%). The new model captures both the weak and strong dependencies of MRD behaviors with effective stress from literature, with weak dependence occurring in normally consolidated peats for ${\sigma }_v^{\prime }/p_a>0.2$ (where ${\sigma }_v^{\prime }$ is vertical effective stress and $p_a$ is atmospheric pressure) and stronger dependence at smaller stresses. We present here a model for OC >30% conditions and its associated aleatory variability, which was established from residuals derived from test results. Compared to previous peat MRD models, the proposed model has more easily interpretable parameters and reduces the model bias and uncertainty.