public double GetTangentModulus(double ratio)
{
double stress = ratio * SoilModel.GetFailureStress();
var solver = new NewtonSolver(x => SoilModel.GetDeviatoricStress(x) - stress, 0);
double eps = solver.Solve();
var derivative = new Derivative(x => SoilModel.GetDeviatoricStress(x), DerivativeAccuracy);
return(derivative.GetFirstDerivative(eps));
}