Effects of trench drain systems on pore water pressures in slow, deep, clayey landslides: Influence of hydraulic properties of the slip zone

Trench drain systems are widely used as remedial measures for slow landslides in saturated fine-grained soils. Among the factors that influence their effectiveness, the hydraulic peculiarities of the slip zone have not been sufficiently investigated. This paper presents the results of numerical analyses of the effects of trench drain systems on clay slope models characterised by very low hydraulic conductivities of the landslide body (kl) and stable formation (kf), with the conductivity of the slip zone (ksz) being several orders of magnitude higher. The hydraulic models reproduced the conditions of a real landslide. Analyses were performed using the code SEEP3D. SEEP/W 2D and PLAXIS 2D were used for comparison. The 3D model shows that, as the ksz/kl ratio increases, the effectiveness of a drain system shallower than the slip surface significantly decreases. As an example, in the case of 12-m-deep trenches, a 25-m-deep slip surface, ksz = kl = 10-9 m/s, and kf = 10-10 m/s, the drains reduce the pore water pressure in the deepest points of the slip zone by approximately 100 kPa. Conversely, if ksz = 10-6 m/s, the pore pressure reduction is only about 10 kPa. Therefore, a drain system designed without considering the hydraulic peculiarities of the slip zone may not be effective. As the trench depth increases, drainage reduces the pore water pressure with a highly non-linear trend, exerting significant effects when the trenches reach the slip surface. Furthermore, 2D models may significantly overestimate the pore water pressure. The differences between the results of 2D and 3D models depend on the trench depth, hydraulic conductivity, and hydraulic boundary conditions.