Bioengineering properties and reinforcing mechanism of vegetation root-soil matrix in soil slope / Noorasyikin Mohammad Noh

The green vegetation for cut soil slope stability is widely used despite not many studies
on its bioengineering benefit, thus there is still frequent soil slope failure with this
vegetation. This research has recently studied the bioengineering properties of soil-root
matrix toward the stability of shallow soil slope with depth from 1 to 1.2 m. The vast
scope of work from in-situ pull out test on Bermuda grass and Vetiver grass at 11
selected sites, i.e Karak, Lancang, Maran, Gambang, Dengkil, Batang Kali, Kuala Pilah,
Ulu Yam, Temerloh, Kuantan, Gombak, and Sabak Bernam. Soil-root samples from the
sites were examined for their physical properties, microorganism, root texture and
physicochemical content. The bioengineering properties of the roots were determined
from uniaxial tensile strength, direct shear strength and triaxial tests. The respective
results, data analysis and comparative study were done by converging on the
significance and influence of the roots system as the mechanism of root reinforcement
in soil slope stability. The Bermuda and Vetiver grass roots have fibrous root type. Both
grass roots have higher pull-out resistance in dry sandy soil. In wet sandy soil, the
fibrous root of Bermuda grass easily to fracture. In dry and wet clay soil, the fibrous
root of Bermuda grass has higher pull out resistance compared to Vetiver grass. The
fibrous root of Vetiver grass not able to survive well in dry clay soil. From root uniaxial
tensile strength test results, the single root for both grasses has a higher tensile strength
in sandy soil compared to clay soil. While for the group root system, the tensile strength
increases as the no of root system increases especially dry primary root. Under the view
of the Scanning Electron Microscope test, the root shrinks and enlarge as the day
increases due to the process of evapotranspiration occurred thus release moisture from
the root micropores. From direct shear tests results, the soil obtained greater cohesion
and friction angle with the appearance of the root. Similar to triaxial strength test results,
the root appearance increases the shear strength of the soil. The appearance of Plant
Growth Promoting Rhizobacteria (PGPR) and Plant Growth Promoting Fungi (PGPF)
colonize in root thus enhance the root strength in soil. Based on the physicochemical
test results, the nutrient concentration and pH not much affects the growth of both
grasses. Factor of Safety (FOS) obtained values above 1 with the presence of root in
soil compared to soil only itself. The ultimate pull out bond strength resulting in the
effectiveness of interaction between root and soil similar to the grip between steel and
concrete. The root system of vegetation plays important roles by enhancing the shear
strength of soil thus prevent shallow soil slope failure.