Understanding the Effect of Unsaturated Hydraulic Conductivity of Surface Soils on Landslide Triggering: A Case Study in Yahalabedda Landslide Risky Area, Sri Lanka

Abstract

Hydraulic conductivity (K) is one of the most important soil properties for rainfallinduced landslide triggering. Yahalabedda receives higher rainfalls in Northeast and Southwest monsoon seasons and considered a potential landslide risky area in Sri Lanka. The objective of this study was to evaluate the unsaturated K (Kunsat) of surface colluvium soils using a minidisk tension infiltrometer together with relevant basic soil properties. Field investigations were conducted at three soil depths; 0, 30, and 60 cm of a prepared soil profile in September 2019. The infiltration test was conducted at each soil depth using three tension levels of -0.03, -0.02, and -0.01 m. The Kunsat has been calculated using the Kunsat = C1/A, where C1 is the slope of the curve of the cumulative infiltration versus the square root of time, and A is a value relating the van Genuchten parameters for a given soil type to the suction rate and radius of the infiltrometer disk. Kunsat increased with the soil profile depth for each tension value. Kunsat values for -0.03 m tension, are 1.35 × 10-6, 2.62 × 10-6 and 7.77 × 10-6 m s-1 ; for -0.02 m tension, are 2.13 × 10-6, 3.91 × 10-6 and 1.02 × 10-5 m s -1 and for -0.01 m tension, are 4.23 × 10- 6, 7.86 × 10-6 and 1.42 × 10-5 m s-1 for 0, 30 and 60 cm depths, respectively. Kunsat increases with decreasing the tension values at each depth as expected. 0 and 30 cm depths show nearly the same bulk density (1.0 g cm-3 ), while the 60 cm depth shows higher bulk density (1.1 g cm-3 ). The soil texture is clay loam for the entire soil profile while the uniformity coefficient (Cu) is higher (6.8) in upper soil resulting in a lower chance to interlock between soil particles and higher pore spaces. As the Kunsat increases with the depth, more water will be percolated to deeper soils resulting in the increasing soil weight in deeper soils and the landslide risk. The study reveals that the Yahalabedda area has a threat to landslide triggering in rainy seasons. Keywords: Minidisk tension infiltrometer, Hydraulic conductivity, Infiltration, Landslide risky soils