Strength-deformation characteristics of rooted soil

Abstract

Embankment protection in Bangladesh is an important issue. Conventional methods for embankment protection are expensive and most of them are not environmentfriendly. To this context, bioengineering method has been selected in this study. At first the growth of selected plants has been studied for different soil conditions. To evaluate the effectiveness of different plants in improving the stability of the slopes, strength-deformation characteristics of rooted soil has been illustrated conducting both laboratory and field tests. For the evaluation of effectiveness of plants as bioengineering solution, four plants namely hardy sugarcane, wild cane, tiger grass and vetiver grass were selected. Growth of these plants in different soils (dredge fill sand, red clay, nursery soil, contaminated soil and saline soil) had been studied in BUET premises in plain land and slope ground. It was found that hardy sugarcane and wild cane grew well in nursery soil and sand whereas vetiver grew better in sandy, clayey, saline and contaminated soil. Among the selected plants, vetiver is widely available in the country and its root morphology is most effective for slope protection. Strength-deformation characteristics had been evaluated using both laboratory and field tests. Direct shear tests were conducted on twenty different types of specimens with four types of soil and roots. Tests were conducted on samples prepared with 20- 25% water contents under normal loads of 10, 15 and 20 kPa. By analyzing the results, it was observed that shear strength increased slightly due to the addition of root while horizontal deformation increased 1.5-2.0 times. Peak shear stress due to the addition of hardy sugarcane, wild cane, tiger grass and vetiver grass root increased up to 12%, 4%, 13% and 7%, respectively. Apparent angle of internal friction, φ́ increased due to the addition of hardy sugarcane and tiger grass root up to 8% and 19%. Again due to the addition of hardy sugarcane, wild cane and vetiver grass root apparent cohesion, ć increased by up to 50%, 25% and 30%, respectively. From the stress-strain behaviour, it is understood that root is effective in taking load after the failure of the soil. In addition to these, effectiveness of vetiver grass in remediation of heavy metal from soil had also been studied. Vetiver grass was planted in industrial dump contaminated soil collected from Buriganga river bank. It was found that the concentrations of heavy metals (Pb, Cu, Cr, Ni and Zn) in this soil are above tolerance level. From the analysis, it was found that heavy metal uptake through vetiver was very significant. Uptake of Pb, Cu, Cr, Ni and Zn after a time period of 50 week were 110, 53, 33, 53 and 2389 (gm per sq.m area), respectively. It is observed that plant root mechanically increase soil shear strength by transferring soil shear stress from soil into tensile forces of the root themselves, via interface friction along the root surface. Orientation and geometry of the root also influenced the effectiveness in reinforcing.