Role of roots in the shear strength of root-reinforced soils with high moisture content
Introduction
The role of vegetation in the stability of slopes has gained increasing recognition in the functions of mechanical and hydrological mechanisms (Greenway, 1987). Although some of the mechanisms, e.g. wind-induced dynamic forces on slopes due to vegetation, are adverse to the stability of slopes, vegetation has been known as a natural and helpful bioengineering method to protect slopes from erosion and shallow landslides and has also been used in practice throughout the world. The main beneficial effects of vegetation on the stability of slopes can be classified as: (1) root reinforcement, (2) soil moisture depletion, (3) buttressing and arching, and (4) surcharge, etc. The most conspicuous source that vegetation enhances the stability of slopes is via root reinforcement (Gray and Sotir, 1996). The effect of root reinforcement on the stability of slopes can be evaluated directly in terms of the additional shear strength provided by roots in root-reinforced soils. Simple force equilibrium models for evaluating the additional shear strength that roots can provide in soils have been developed (Wu, 1976, Wu et al., 1979, Gray and Lieser, 1982) and can provide useful insights into the mechanism of soil–root interactions. In addition, analytical models for soil–root interactions have also been developed (Waldron, 1977, Waldron and Dakessian, 1981, Wu et al., 1988a) and were employed to analyze in situ shear test results (Wu et al., 1988b, Wu and Watson, 1998). Furthermore, laboratory shear tests and in situ shear tests on root-reinforced soil blocks have been conducted by several researchers (Endo and Tsuruta, 1969, Wu et al., 1988b, Tobias, 1995, Wu and Watson, 1998, Operstein and Frydman, 2000). These studies provide a substantial insight into the role of roots in the shear strength of soils.
Rainfall is considered the major cause for most of the landslides. Shallow landslides are commonly seen on steep residual slopes during or after intense rainfall event. Decisive factors controlling shallow landslides are morphology of slopes, geology, mechanical and hydrological properties of the soils at shallow depth, and rainfall event (Matsushi et al., 2006). Additionally, Normaniza et al. (2008) indicated that vegetation and the selection of plant species are important in stabilizing slopes and protecting against soil erosion in terms of its capacity of the root reinforcement and water absorption capacity. Precipitation, however, is known as the most important factor for triggering shallow landslides. Infiltration of rainfall in slopes lead to an increase in soil moisture content, especially in the near-surface. The role of vegetation in the shear strength of root-reinforced soils with possibly a significant increase in the soil moisture content during or after heavy rainfall may be crucial in governing the occurrence of shallow landslides. Normaniza and Barakbah (2006) suggested that both the soil moisture content and the root length density (RLD) could be used as indicators of slope stability.
To investigate the role of vegetation in providing the additional shear strength to root-reinforced soils during or after rainfall event, in situ shear tests on root-reinforced soils at various soil moisture contents were conducted in this research. The plant used in the shear tests is Prickly Sesban (Sesbania cannabina Merr.). The experimental results obtained can give a better understanding on the effect of soil moisture content on the additional shear strength provided by roots for root-reinforced soils.
Section snippets
Test site and soil properties
The test site was located at the campus of the National Kaohsiung First University of Science and Technology, Kaohsiung City, Taiwan. Area of the test site is about 50 m × 15 m. The soil at the test site consists primarily of sands mixed with silts and a small amount of gravels, silts, and clayey material. Particle distribution curve of the soil at the test site is shown in Fig. 1. Plastic index (PI) of the soil ranges from 4 to 6 based on the test procedures of ASTM Test Designation D-4318. Unit
Effect of soil moisture content on the shear strength of root-reinforced soils
The effect of the soil moisture content on the additional shear strength contributed by roots was investigated by conducting in situ shear tests on root-reinforced and root-free soils. All experimental data of the additional shear strength provided by roots were obtained by conducting a pair of in situ shear tests, one on root-reinforced soil and another on contiguous root-free soils. The measured peak and residual shear strengths for root-reinforced soils and root-free soils at various
Discussions
The experimental results conducted herein show that the role of roots in the contribution of shearing resistance to soils becomes increasingly important as surface runoff induced by rainfall infiltrates the soil in slopes and results in a high-moisture content at shallow depth. The shear strength of root-reinforced soils may reach up to 100% greater than that of root-free soils at shallow depth during or after heavy rainfall events.
Establishment of the relationship between the additional shear
Conclusions
In situ shear tests on root-reinforced soils were conducted in this research to investigate the influence of the soil moisture content on the additional shear strength provided by roots in soils. The plant used in the shear tests was Prickly Sesban (S. cannabina Merr.). Its root morphology is categorized as the heart root system. The role of roots in providing shearing resistance against shallow landslide during or after heavy rainfall event was identified through the experimental results
Acknowledgment
This research work was sponsored by the National Science Council in Taiwan under grant number NSC 95-2221-E-327-014. This support is gratefully acknowledged.
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