Elsevier

Ecological Engineering

Volume 33, Issue 2, 3 June 2008, Pages 157-166
Ecological Engineering

Role of roots in the shear strength of root-reinforced soils with high moisture content

https://doi.org/10.1016/j.ecoleng.2008.02.013Get rights and content

Abstract

This paper presents an experimental study on the effect of soil moisture content on the shear strength of root-reinforced soils by conducting in situ shear tests. The plant used in the shear tests was Prickly Sesban (Sesbania cannabina Merr.). Performance of plants in stabilizing slopes against shallow landslides in a rainfall event is closely relevant to the variation of the shear strength of root-reinforced soils with changes in the moisture content, which may take place due to seasonal effects (e.g. rainfall and temperature changes) as well as the influence of vegetation. The shear strength of soils may decrease noticeably with increasing soil moisture content. The role of roots in providing the shear strength to root-reinforced soils turns out to be important if soils are subjected to an increase in the moisture content. The experimental results show that root efficiency, defined as the ratio of the additional shear strength provided by roots over the shear strength of soils, may reach more than 1.0 if soils are at a high degree of saturation. In addition, an approximately linear relationship between the additional shear strength (ΔS) provided by roots and the tensile strength of roots per unit area of soils (tR) was obtained in this study, and the ratios of ΔS/tR are about 0.39 and 0.42 in terms of the peak and the residual shear strength, respectively. These results are considerably less than the theoretic value (=1.2), based on a simple force equilibrium model derived by Wu et al. [Wu, T.H., McKinell, W.P., Swanston, D.N., 1979. Strength of tree roots and landslides on Prince of Wales Island, Alaska. Can. Geotech. J. 16 (1), 19–33].

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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