Effects of vegetation types and characteristics on induced soil suction

Abstract

Plant induced suction reduces hydraulic conductivity (k) and hence rainfall infiltration. This is vital for stability of slopes and landfill covers. Vegetation is also known to influence soil water characteristic curve (SWCC). This effect is seldom considered while interpretation of plant induced suction. In addition, influence of variability in plant characteristics (root area index (RAI), leaf area index (LAI)) on plant induced suction is rarely investigated. The principal objective of this research was to investigate vegetation induced suction and its understanding with plant characteristics and soil hydraulic properties. In addition, influence of vegetation type (i.e., tree and grass) on suction distribution is also studied and compared with bare soil. A laboratory investigation, field monitoring programme and a numerical parametric study were carried out in this study. Completely decomposed granite (silty sand) is considered for investigation. Schefflera heptaphylla and Cynodon Dactylon are selected as tree and grass species, respectively. They are commonly found in Asia and often used for ecological restoration of slopes. In laboratory, comprehensive test programme was conducted in a test box (flat ground) and sloping ground (flume) to quantify suction under wetting. Suction retained under dark condition was investigated to study effects of vegetation on SWCC on suction retained. In field, slopes (including bare slope) were constructed with same vegetation species (trees and grass) and soil with that in laboratory. These were instrumented to monitor suction for period of 10 months so as to study effect of seasonal variation (dry and wet). Plant root depth, RAI and LAI were measured during and after testing/monitoring for interpretation of suction. In addition, infiltration testing was conducted on ground with grass and tree species. In order to interpret suction induced by grass and tree species, transpiration reduction function (T_rf) which represents total root water uptake at a given suction is identified in laboratory. Numerical parametric study was conducted to investigate three different scenarios of soil density effects (i.e., change in only soil hydraulic properties; change in only root characteristics and changes in both soil and root properties). As compared to the bare soil, vegetated soil has higher adsorption rate and lower permeability. This is likely because of water retention capability of roots present inside soil. At this depth, suction retained after ponding in the treed soil can be 100% – 300% (1 kPa to 104 kPa) higher than that in bare soil. This was caused by lowering of k due to higher initial ETr induced suction and also higher adsorption rate for vegetated soil. Unlike near surface for bare soil, maximum suction in treed soil is found to occur at deeper depth, where maximum RAI is found. The effect of roots on SWCC is more dominant mechanism in retaining suction during wetting under dark condition. Whereas, under light conditions, the effect of transpiration before wetting also play significant role in retaining higher suction in vegetated soils. When compared with treed ground, grassed slope is found to induce and retain lower suctions. This is because of shorter root depth, lower root biomass and also actual transpiration rate than tree. Under rainfall event corresponding to 20 years of return period, suction within root zone of tree and grass is completely destroyed, whereas, suction higher by 20 kPa was found to be retained in treed slope below its root depth. This is expected due to lower infiltration rates measured in both vegetated covers (trees and grass) as compared to bare soil. Based on suction retained, it can be implied that shear strength in treed slope can be up to higher than bare slope and grassed slope. Suction increases when effects of soil density on soil hydraulic properties are considered, whereas, it decreases when effects on root characteristics are considered. This is because reduction in root depth and actual transpiration rate counters the effects of decrease in desorption rate due to increase in soil density.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology