Issue 61

A. Kostina et alii, Frattura ed Integrità Strutturale, 61 (2022) 1-19; DOI: 10.3221/IGF-ESIS.61.01

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Figure 10: (a) Effect of  (a) and  (b) on pore pressure variation.

Except for  , Eqn. (6) includes one more parameter k 0 . This parameter describes the hydraulic conductivity of the unfrozen soil. The effect of k 0 on pore pressure variation is presented in Fig. 11. Higher values of k 0 lead to the increase in pore pressure value compared to the lower ones. Therefore, the less permeable soils can provide a significant delay in pore pressure rise. The first reason for this delay is similar to the delay induced by high magnitudes of  . Higher values of k 0 lead to a rise of amount water migrated into the frozen zone. As a result, a more amount of water transforms into ice, so the compressive loadings acting on the unfrozen soil increases and, as a consequence, the pore pressure rises. The second reason is that in low permeable soils water flows from the compressive zone of the unfrozen soil to a feeding area of a hydro observation well with lower velocity leading to a delay in pore pressure rise. From Figs.8, 11 we can conclude that even though k 0 in the second simulation case was higher it could not provide a significant increase in pore pressure. Fig. 11 (b) shows the effect of initial porosity on pore pressure evolution. We can observe that higher initial porosity induces higher pore pressure at the hydro-observation well. High-porous soils contain large amount of water, so during freezing, an intensive frost heave occurs. This situation is similar to the first simulation case.

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Figure 11: Effect of k 0 (a) and n 0 (b) on pore pressure variation. Fig. 12 demonstrates influence of the Biot tangent modulus N un of the unfrozen soil on pore pressure at the hydro observation well. According to Eqn. (21) the pore pressure is directly proportional to this parameter. Therefore, higher values of Biot tangent modulus of soil lead to higher pore pressure value as it was obtained in numerical simulation. Presented results have shown that an abrupt rise in pore pressure inside the frozen wall depends on the effect of frost heave in the frozen zone on the unfrozen soil inside the wall. The intensity of the effect rises with the thickness of the frozen wall. Therefore, the pressure rise can occur with a perceptible delay relative to the closure of the frozen wall. Also, it should be noted, that water migration into the frozen zone contributes to the frost heave of the soil. However, significant water migration can also lead to pore pressure reduction. Simulation results show that in the presence of a strong cryogenic suction, which defines by N un =200 MPa , k 0 =1.5·10 -9 m/s and  = -2.53 parameters we can obtain the situation when pore pressure decreases with an increase in thickness of the frozen wall as illustrated in Fig. 13.

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