Soil suction model for grass evapotranspiration

Abstract

This study investigated the soil matric suction distribution in a field covered by grass Axonopus Compressus and free from the effect of the tree. The research employed several approaches such as field monitoring, laboratory experimental, model proposal, coding program and slope stability. A field monitoring program was carried out from August to December 2015 to collect the data of matric suction by jet-filled tensiometer, accounting for less than 10 times of continuous drying of longer than 5 days over 5 months of measurement. The suction profiles show that the variation was greater in the root zone (< 30 cm) and less effect in deeper depth. The grass field failed to retain the soil suction, which dropped to a minimum magnitude at all depths after some rainfall event. Besides, the rate of evapotranspiration of grass was investigated by measuring the daily total weight loss of grass samples. The water loss from soil continuously even on full cloud rainy day. Generally, the water lost from soil to air every day even during the rainy day with the lower evaporation of 0.4-0.9 mm/day. The rate of evapotranspiration could reach almost 8.0 mm/day and around 5-6 mm/day on normal sunny day. A mathematical equation was proposed as the suction model by considered rooting depth and evapotranspiration to estimate the suction profile of soil after specific drying period. Proposed suction model and some existing water uptake models have been coded into a program by MATLAB graphical user interface. The code in the program was verified with a set of test plan to ensure the program works as planned and designed. The suction model has been validated with the site measurement data. The shallow slope stability was analysed by program SLIP4EX in saturated and unsaturated conditions. The enhancement due to the influence of grass induced suction and root tensile strength were provided in this research. The factor of safety against slope failure has improved 0.6-4.8% at various depths when the effect of suction included. The comparison between the effect of induced suction and root tensile strength showed better enhancement from mechanical effect since suction was not high. The contribution of suction was not affected by changes of soil cohesion, however, the effect is higher when friction angle of soil is high and angle of slope is low. This research developed mathematical equation for soil water uptake to deliver a better understanding of grass suction distribution and effect to the slope stability.