Experimental investigation of induced suction distribution in a grass-covered soil

► We quantify the suction induced by grass in relation to grass characteristics. ► Suction induced in grassed soil is nearly two times higher than that in bare soil. ► Grass-induced suction influence zone is up to 4 times the root depth. ► We investigate suction distribution upon wetting under different soil suctions. ► Under wetting, suction retained in grassed soil is higher than that in bare soil. Evapotranspiration from a grass-covered ground is known to induce suction by soil evaporation and grass transpiration. However, grass-induced suction in the ground when it is subjected to wetting and drying are not yet well understood. In this study, a laboratory test program was conducted to investigate the magnitude and distribution of suction induced by Bermuda grass growing in silty sand. In total, four test boxes compacted with silty sand were prepared, three of which covered with Bermuda grass while one test box was left bare as control. All the four test boxes were subjected to wetting and drying in a plant room with temperature and humidity controlled. Under identical atmospheric conditions and initial soil density and water content, peak suction induced within the root zone in grassed soil was 1.5 times higher than that in bare soil after 20 days of drying. A vertical suction influence zone was identified to be up to four times the root depth while the lateral suction influence zone was one diameter of ring collar away from the centre of the plot. Upon wetting, suction retained at depth right below the root zone in grassed soil was found to be 40% higher than that in bare soil. For three grass replicates that were germinated under identical atmospheric conditions, they produced different shoot lengths and induced different magnitudes of suction. No direct correlation between grass shoot length and grass-induced suction could be found.