Abstract
The shear strength increases. Soil water removal can therefore be viewed as a technique to strengthen the ground and enhance stability.A natural, low-cost, and low-carbon approach to remove soil water consists in exploiting the evaporative demand of the atmosphere. Evaporation from bare soil is well studied, but it is difficult to ‘engineer’ the process to enhance soil water removal. The problem of plant transpiration is complex as it involves the coupling between soil, plant and atmosphere. However, it offers the chance to control actively the process of water removal if adequate species are selected to cover the ground surface. As a result, vegetation can potentially be ‘engineered’ to stabilise geotechnical structures.This work has developed an experimental framework to investigate the effectiveness of vegetation in removing soil water by transpiration and the overall methodology has been designed around the comparison between transpiration (from vegetated soil) and evaporation (from bare soil), both in the laboratory and in the field. Experimental investigation into transpiration processes therefore requires the monitoring of the water flow through the Soil-Plant-Atmosphere Continuum.A novel technique was developed to monitor xylem water pressure. The High-Capacity Tensiometer (HCT), developed by geotechnical researchers, was tested on the plant to measure the xylem water pressure. This technique was validated via comparison with techniques routinely used in plant science. The novel procedure for the measurement of negative xylem water pressure is a step change in the study of continuous flow along the soil-plant system, especially in the geotechnical field. This allows the use of a single instrument to monitor the entire soil-plant continuum.The transpiration process was then first investigated in the laboratory. Two soil columns were developed, one vegetated and one left bare to compare the transpiration and evaporation respectively under the same atmospheric conditions. The columns were instrumented to monitor the water content, (negative) pore-water pressure in the soil respectively and the transpiration rate.The straightforward outcome from these laboratory tests is that vegetation does not have necessarily a beneficial effect. In the energy limited regime, the combination of the aerodynamic and canopy resistances can play either in favour of the bare or the vegetated soils depending on the vegetation type. In the water-limited regime, the effect of vegetation is always beneficial due to the different mode of water extraction. This is reflected in the time at which the transpiration enters the water limited regime, which is definitely longer in the vegetated soil than the bare one. Both aspects have been clearly demonstrated in the laboratory experiments.The hydrological effects of vegetation where finally investigated in the field in a poplar plantation in Montpellier, France. The water content profile was monitored throughout the dry season and the following rainy period in a poplar vegetated area as well as in the adjacent ploughed (virtually bare) field. The conceptual framework developed on the basis of the laboratory experiments was therefore key to interpret the field data and show in which regime the vegetation has beneficial effects in this specific case.In conclusion, this dissertation has shown how the effects of plant transpiration in removing soils water and, hence, enhancing the stability of slopes and earth structures can be assessed based on quantitative measurements.