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 = 10-10m/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.

Understanding the rheological behavior of marine clay is crucial to analyzing submarine landslides and their impact on marine resource exploitation. Dispersed bubbles in marine clay (gassy clay) and electrolytes in seawater (e.g., NaCl concentration of 0.47 M) significantly impacts rheological properties. Under low ionic strength and low pore water pressure conditions, dispersed bubbles have a strengthening effect on the yield stress and the viscosity of clays. This effect turns into a weakening effect when the pore water pressure reaches 300 kPa or the ionic strength exceeds 0.18 M. It was proposed that the effect of bubbles, whether strengthening or weakening, was determined by the size of bubbles with respect to the characteristic size of the particle structure formed by clay particles. A theoretical model was developed, which reasonably captures rheological behaviors of gassy clays.

Climate change with extreme hydrological conditions, such as extreme rainfall, poses new challenges to earth dam safety. Reliability analysis helps to reduce the uncertainty of the real behavior of a dam, and provides one more tool to improve dam safety control. Reliability analysis is very important for unsaturated soil dams under rainfall conditions. This paper investigates the safety of an earth dam over time, considering different initial conditions, rainfall intensities, and normal operating conditions (NOC). A direct coupling (DC) method is used to integrate different software. Coupling enables us to use the deterministic software packages Seepage/W and Slope/W with the StRAnD reliability software to perform the numerical investigation. The reliability analyses are performed employing the first-order reliability method (FORM) using the improved Hasofer-Lind Rackwitz-Fiesler (iHLRF) algorithm of optimization. The contribution to failure probabilities of random parameters was analyzed with sensitivity analysis. The saturated hydraulic conductivity (ks) contribution increases when the rainfall intensity increases and when NOC increases or decreases the reservoir. Finally, a critical deterministic slip surface is shown to be very close to the probabilistic one, but a high difference in terms of pore water pressures is reported.