Measurement of water menisci contact angles in bulk soil samples

The increase in the frequency and intensity of extreme weather events in the future such as heat waves and droughts is likely to induce long lasting changes of soil wettability. This is important because pore water pressure, which controls water flow, the deformation and shear strength characteristics of soils, depends on the spreading of small water menisci at the surface of the soil particles. Our current understanding is for conditions where the water spreads continuously (i.e. the soils are wettable), but little is known when water has limited spreading (i.e. the water menisci have contact angles greater than zero degrees). To predict the engineering behaviour of soils with variable wettability (frequently those with high organic carbon content), the relation between soil wettability and pore water pressure has to be determined. There are several methods for pore water pressure from unsaturated soil mechanics. However, the existing soil wettability methods have been developed for soil science applications (e.g. agriculture) and are not directly suitable for ground engineering. For instance, several methods use a single layer of particles to measure the water menisci contact angles but this is not appropriate to soil mechanics where testing is conducted in bulk samples. Funds are requested to travel to Prof. Jörg Bachmann laboratory (world leader on the measurement of soil wettability) at the Institute of Soil Science, Leibniz University Hannover, Germany to learn and adapt the existing soil science methods to ground engineering. This will provide the basis for future studies where the interaction of liquids with the surface of soil particles is important. Applications are in geoenvironmental and geotechnical engineering, earth surface processes (erosional and landsliding in slopes subjected to wildfires), soil carbon sequestration, development of new materials (granular, water repellent) and particle surface processes (long term effects).

As mentioned in the Academic Beneficiaries section the immediate impacts are within academia. In the mid to long term I expected this research to have applications in industry, government departments and public bodies.

Geoenvironmental and geotechnical engineering
In the mid to long term this research could have an impact on the re-use of waste materials (not recycling). For instance, if the mechanics of water repellent soils is understood, the costly common practice in the construction industry of removing the shallow contaminated soils could be avoided. An estimated 800.000 m3 of contaminated soil (with oil and other pollutants) have been removed and treated at the London 2012 Olympic Park site. Chevron USA has recently proposed using these contaminated materials in embankments or fills. The impact of wettability in the tar sands in Alberta (Canada) has recently been studied. Since organic carbon occurs at shallow depths in natural and manmade soils, there is a possibility that soil wettability would influence soil behaviour and consequently play a role in the interaction with shallow buried structures in the ground, such as pavements, pipelines and shallow foundations.

Engineered granular materials
I have also explored the possibility of developing water repellent granular materials with Aggregates Industries. There is the potential to use the water repellent effect as a method to keep the aggregates dry before mixing with bitumen. This could improve adhesion with the bitumen and avoid cracking of the pavement. Impregnating gravel with water repellent substances may also accelerate drainage of the subgrade since water does not adhere to the surface of the grains.

Ground improvement / soil stabilization
The interaction of liquids with soil particles depends on the characteristics of the surface of the particles, as well as the properties of the water. For instance, the surface tension of water (~72mNm-1 at 20 degC) can be decreased by adding surfactants. Exploratory work recently conducted at Cardiff showed that pore water pressure increases (becomes less negative) with the addition of surfactants. While this was expected, the change in pore water pressure was surprisingly large. A 1% surfactant-water solution produced an increase of pore water pressure from -1100 kPa (no surfactant) to -200 kPa (with surfactant) measured with a high suction tensiometer in a compacted kaolin-sand mixture at 10% water content. These results confirm that manipulating the interface solid-water-air at the particle scale can produce dramatic changes in certain aspects of the soil behaviour which could lead to a series of new opportunities.

Other topics
Debris flows are a natural hazard known to occur in water repellent, recently burned slopes, and constitute a hazard to people and the built environment. Post-wildfire debris flows occur in the western U.S. states, southern Europe and South Australia. In a 2003 rainstorm on burnt slopes in California, 16 people died and rebuilding activities had a cost of US$ 9.5 million. An improved understanding is needed. Contact has recently been established with Dr Peter Cleall (Engineering, Cardiff) to simulate slope processes (infiltration, surface runoff, and failure) in burned slopes.
One the main issues in soil carbon sequestration is the protection or stabilization of the organic carbon in the soil. The decomposition by microorganisms, the availability of water and nutrients has a detrimental effect. Prof. Bachmann has extensively studied these processes in a Priority Program 1090 of the German Research Foundation (DFG): "Soils as Sources and Sinks for CO2". One of the main conclusions is that reduced soil wettability (among other factors) contributes towards the protection of the organic carbon.