Soil-biochar-water interactions: Role of biochar from Eichhornia crassipes in influencing crack propagation and suction in unsaturated soils

Meticulous research has been done to observe and measure the impact of biochar on soil-water retention properties. On the contrary, very little research is conducted on quantifying cracking in soil biochar composites. Investigation of crack intensity factor is essential as it affects permeability and hence, overall water balance in agricultural fields. It is well known that cracking of soil is influenced by suction and water content but their interactive effects are not very well understood. This technical note aims to investigate the cracking of soil amended with biochar made out of water hyacinth, an invasive weed, with focus on analysing interactive effects of water content and soil suction. An integrated reduced scale monitoring and modelling using artificial neural networks have been adopted. A series of tests were conducted to monitor suction, water content and crack intensity factor in soils mixed with four different compositions i.e. 0%, 2%, 5% and 10% by weight of biochar for around 3 months. The measured results have been evaluated first, and eventually modelled using artificial neural networks to predict crack intensity factor as function of water content and suction for all composites. In most of the cases it has been found that suction is more influential than water content. However, the relative significance of suction to water content in determining crack intensity factor decreases with increase in biochar content of soil. It is concluded that the optimum biochar content is 10% for most of the agricultural purposes based on the water retention capacity and cracking. The probability distribution of crack intensity factor is found to be normal for case with 5% biochar and while for others, it is skewed negatively. A simple cost based analysis taking in context of a small developing urban city of India, showcased a cost savings in irrigation of urban infrastructure by around 63,000 US dollars for a particular summer.