Root Tensile Behaviour and Soil Shear Reinforcement of Tropical Shrubs for Soil Bioengineering Applications

Siti Norhafizah Ahmad Tarmidzi; Zulfahmi Ali  Rahman; Wan Mohd Razi Idris; Normaniza Osman; Mohamad Edri Aznan; Tukimat Lihan

doi:10.55230/mabjournal.v55i3.3935

Authors

Siti Norhafizah Ahmad Tarmidzi
sitinorhafizah@ukm.edu.my
Department of Earth Science and Environment, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia; Bangi Botanic Gardens (TBB, FST-UKM), Faculty of Science and Technology, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia https://orcid.org/0009-0005-2761-3039
Zulfahmi Ali Rahman Department of Earth Science and Environment, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia
Wan Mohd Razi Idris Department of Earth Science and Environment, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia
Normaniza Osman Institute of Biological Sciences, Faculty of Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
Mohamad Edri Aznan Department of Earth Science and Environment, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia
Tukimat Lihan Department of Earth Science and Environment, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia

Keywords:

FESEM microstructural analysis, Pseuderanthemum carruthersii, root biomechanics, Strobilanthes crispa, triaxial shear test, tropical residual soil

Abstract

The use of tropical shrub species in soil bioengineering has gained increasing attention for slope stabilization; however, information on their biomechanical contribution to tropical residual soils remains limited. This study evaluated the root tensile behaviour, soil shear reinforcement, and root–soil micromorphology of Strobilanthes crispa (SC) and Pseuderanthemum carruthersii (PC) cultivated for six months. Root tensile tests were conducted across diameter classes, while unconsolidated-undrained (UU) triaxial tests and Field Emission Scanning Electron Microscopy (FESEM) were used to assess soil reinforcement mechanisms and root–soil interactions. Root tensile strength followed an inverse power-law relationship, ranging from 0.72–9.79 MPa for PC and 0.51–21.67 MPa for SC, highlighting the importance of fine roots. In triaxial testing, root-permeated soils exhibited significantly higher maximum shear stress (τ_max) than non-vegetated soil. At 200 kPa confining pressure, SC recorded the highest τ_max (248.9 ± 16.67 kPa), followed by PC (231.1 ± 0.01 kPa), and control soil (214.8 ± 2.30 kPa). Strong linear relationships between τ_max and confining pressure were observed (R² = 0.9689-0.9943), accompanied by slight reductions in apparent cohesion (100.30 kPa in the control soil, 91.80 kPa in SC, and 90.05 kPa in PC) and substantial increases in internal friction angle (25.52°, 35.86°, and 34.24°, respectively). FESEM analysis revealed denser aggregate packing, finer root fibrils (~232.9 µm) and stronger root-soil interlocking in SC than in PC with larger fibrils (~251.9 µm). These findings demonstrate that both species, particularly Strobilanthes crispa, possess favourable biomechanical characteristics for sustainable shallow slope stabilization in tropical environments.

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