where \(\firstlame_{u} = \firstlame_{d} + \alpha^2 M\) is undrained first Lamé parameter while \(\firstlame_{d}, \secondlame_{d}\) are Lamé parameters measured in drained condition, \(\alpha, M\) are the Biot effective stress coefficient and Biot modulus defined by

where , \(\bm\sigma' = \firstlame_{d} (\trace \bm\varepsilon) \bm{I} + 2 \, \secondlame_{d} \, \bm{\varepsilon}\) is effective stress, \(p\) is pore pressure, and the variation of fluid content \(\zeta\) is

(104)¶\[
\zeta = \frac{p}{M} + \alpha \nabla\cdot \bm u.
\]