The effect of root architecture on the shearing resistance of root-permeated soils

doi:10.1016/j.ecoleng.2010.03.003

Ecological Engineering

Volume 36, Issue 6, June 2010, Pages 813-826

https://doi.org/10.1016/j.ecoleng.2010.03.003 Get rights and content

Abstract

This paper aims to investigate the effect of the root architecture on the shear strength increment provided by plant roots in the soil. In situ shear tests were conducted for this purpose. Five plant species – Hibiscus tiliaceus L. (Linden hibiscus), Mallotus japonicus (Thunb.) Muell.-Arg. (Japanese Mallotus), Sapium sebiferum (L.) Roxb. (Chinese tallow tree), Casuarina equisetifolia L. (ironwood), and Leucaena leucocephala (Lam.) (white popinac) – were used in this study. Tensile tests on roots of various diameters and surveys on the root system structure were carried out for each of the plant species tested. The shear strength increments (ΔS) provided by the roots of Linden hibiscus, Japanese Mallotus, Chinese tallow tree, ironwood, and white popinac were 0.34t_R, 0.462t_R, 0.688t_R, 0.3t_R, and 0.87t_R, respectively, when t_R was estimated on the basis of the tensile root strength crossing through the shear plane. The shear strength increment provided by plant roots with conspicuous oblique and vertical roots was greater than that of root structures in which lateral roots were dominant. In comparison with other types of root architecture, the R-type root architecture was found to be the most effective root system against shear failure in the soil. Its shear strength increment was slightly greater than that with the V-type root architecture, followed by the VH-type root architecture. The shear strength increment provided by plants with the H-type root architecture was less effective than that contributed by plant species with other types of root architecture.

Introduction

The use of plants and their roots to protect slopes from erosion and shallow landslides is a useful and well-known natural bioengineering method that has been applied extensively worldwide. The most conspicuous vegetation source that enhances the stability of slopes is root reinforcement (Gray and Sotir, 1996). Gray and Sotir (1996) and Reubens et al. (2007) showed that the shear strength increment provided by plant roots in the soil relied not only on the properties of the roots (root strength and soil–root interface properties) but also on the concentration, branching characteristics, and spatial distribution of the root system in the soil. Reubens et al. (2007) summarized the structure-related root characteristics that could affect the process of soil erosion and soil mass movement, namely, root density, root length density, number of roots, root area ratio (RAR), root taper, basal diameter, inclination, percentage of bare soil with fine roots, maximum root depth, branching pattern, angle between lateral roots, and total length after the intersection point. Based on this, it is evident that root architecture plays an important role in the mechanical behavior of soils permeated with roots.

The mechanism of the soil–root interaction and the contribution of plant roots to the shear strength of the soil have been studied both analytically (Waldron, 1977, Wu et al., 1979, Wu et al., 1988, Waldron and Dakessian, 1981) and experimentally (Waldron, 1977, Tobias, 1995, Operstein and Frydman, 2000, Docker and Hubble, 2008, Fan and Su, 2008). A simple root reinforcement model based on the force equilibrium principle has also been developed to evaluate the shear strength increment that can be provided by roots. This model has been applied to both vertical roots (Waldron, 1977, Wu et al., 1979) and inclined roots (Gray and Leiser, 1982). However, experimental studies showed that the shear strength increment provided by plant roots was considerably less than that estimated using the simple root reinforcement model (Operstein and Frydman, 2000, Docker and Hubble, 2008, Fan and Su, 2008). The mechanism by which plant roots contribute to shearing resistance inherently involves an underground 3-D soil–root interaction. The architecture or branching characteristics of the root system play an important role in mobilizing the increase in shearing resistance and in protecting the soil mass from erosion or shearing failure. The root system geometry and root topology determine the force transmission in the entire root system, which in turn affects the extent of soil reinforcement.

Docker and Hubble (2008) used in situ shear tests and reported RAR-based estimates of the increased shear resistance of soils due to the presence of four common Australian riparian tree species—Casuarina glauca, Eucalyptus amplifolia, Eucalyptus elata, and Acacia floribunda. At equivalent RAR values, the roots of A. floribunda were the greatest contributors of shear strength increment to soil blocks of these four plant species. However, these results may be influenced by the difference in both the geometry of the root system and the tensile strength of the roots across different plant species. Docker and Hubble (2009) further linked the RAR-based shear strength increment provided by plant roots to the root architecture system of these four plant species. The RAR values of the root systems were measured in terms of the spatial distribution (vertical and lateral extent) below the ground surface. E. elata exhibited the highest RARs in soil zones beneath it, while E. amplifolia reinforced a greater volume of soil than the three other species. When the spatial distribution of RARs in the root system was taken into account, E. elata showed the highest values of increased soil shear strength followed by A. floribunda, E. amplifolia, and C. glauca. Norris et al. (2008) showed that root systems composed of deep taproots and sinker roots crossing the slip surface would be ideal for reinforcing the soil against shallow slope failure. Numerous roots of small diameters would develop into a root-permeated soil matrix that had a better shearing resistance. At the top or toe of a slope, the heart root system, which has deep sinkers and wide-spreading lateral roots, would be the ideal root architecture to protect the soil from slope failure. In the middle of the slope, the tap or heart root system would have sufficient depth to interact with the slip surface and provide better shearing resistance in soils. Stokes et al. (2009) discussed how plant root traits affect the protection of slopes from shallow mass movement. This research indicated that root architecture (branching pattern) can significantly change the distribution of stresses and plastic strains within the soil medium, and affecting the resistance to pull-out. However, the aspects of the root architecture are not yet taken into account in the root reinforcement model. Thomas and Pollen-Bankhead (2010) demonstrated that root reinforcement in soils may vary considerably across plant species that differed in terms of root architecture and growing location (sloping versus horizontal surfaces). Selection of plant species for enhancing slope stability should take into consideration the ability of the plant to provide root reinforcement.

Yen (1987) proposed a classification system for root structures based on the branching pattern. He classified the plant root system architecture into five branching patterns (Fig. 1), namely, VH-type, H-type, V-type, R-type, and M-type. The H- and VH-types are considered to be beneficial for slope stabilization and wind resistance. The H- and M-types are regarded to be beneficial for soil reinforcement, and the V-type is considered to be wind resistant. Reubens et al. (2007) classified the root architecture in terms of three characteristics, i.e., size characteristics, branching pattern, and number of roots per soil area or volume. They concluded that a dense rooting pattern of fine roots, which are important for providing tensile resistance, in the top layer in combination with coarse, deeply penetrating roots, which provide good bending and shearing resistance, was most effective against shallow slope failure. The importance of the root architecture in the mechanical mechanism of roots against shearing failure is well acknowledged. The effect of the orientation of reinforcing fibers on the increase in the shearing resistance of the soil is well known in theoretical analyses (Gray and Leiser, 1982) and laboratory studies (Gray and Ohashi, 1983). However, there has been limited quantitative research on the effect of the root architecture on the contribution of roots to the shear strength of the soil or soil fixation.

In this study, to investigate the role of the root architecture in providing shearing resistance to root-permeated soils, in situ shear tests were carried out on soil blocks permeated with various root architectures. Five plant species – Hibiscus tiliaceus L. (Linden hibiscus), Mallotus japonicus (Thunb.) Muell.-Arg. (Japanese Mallotus), Sapium sebiferum (L.) Roxb. (Chinese tallow tree), Casuarina equisetifolia L. (ironwood), and Leucaena leucocephala (Lam.) (white popinac) – were studied here. Root characteristics, including root length, root diameter, and root orientation with respect to the horizontal (shear) plane, were measured and recorded after each in situ shear test. Correlations between the shear strength increment and tensile force in roots per unit area of soil (t_R) were determined for plant species having different root architectures. In this paper, the contribution of the root system architecture to the shear strength increment provided by plant roots in the soil is discussed.

Section snippets

Test site and soil properties

The test site (approximately 50 m × 20 m) was located on the campus of the National Kaohsiung First University of Science and Technology, Kaohsiung City, Taiwan. Two types of soils, i.e., sandy and clayey soils, were used for planting at the test site depending on the source of the plant shoots. The particle distribution curve of the soils used for planting is shown in Fig. 2. The properties of these soils are described in Table 1. To minimize the influence of the soil moisture content on the

In situ shear tests

Fig. 6 shows typical shear stress–shear displacement relationships for each of the plant species tested in this study. The relationship between shear stress and shear displacement for root-permeated soils subjected to shear is affected by the pattern of root growth, and it can be used to evaluate the capability of vegetated slopes to withstand large soil displacement before collapse. The test results obtained in this study suggest that the presence of strong vertical roots and the penetrating

Discussion

The ratios of the shear strength increment (ΔS) to the tensile force of roots per unit area of the soil (t_R), shown in Table 4, Table 5, vary substantially for the plant species tested here. Additionally, the ratios of ΔS to t_R for the plant species tested here are considerably lower than those obtained on the basis of the simple root reinforcement model where ΔS ≅ 1.15–1.2t_R (Wu et al., 1979, Gray and Leiser, 1982). Literatures on the relationship between the shear strength increment (ΔS) and t_R

Conclusions

Five plant species – Linden hibiscus, Japanese Mallotus, Chinese tallow tree, ironwood, and white popinac – were used in this study to investigate the effect of root architecture on the shear strength increment provided by roots. Correlations between the shear strength increment (ΔS) and the tensile force of roots per unit area of the soil (t_R) were established for each of the plant species tested. Typical root architectures were quantitatively described in terms of the distribution of the

Acknowledgments

This study was sponsored by the National Science Council in Taiwan (grant numbers NSC 96-2221-E-327-023 and 97-2221-E327-030), and the support is gratefully acknowledged. The authors would also like to thank Mr. Chia-Bin Hsu for his help in carrying out some of the experimental studies.

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Citation Excerpt :
Thus, no additional normal force was applied. The selected shearing speed, 0.01 m min−1, was within the range of values reported in literature: 0.0005 to 0.018 m min−1 (e.g., Docker and Hubble, 2008; Fan and Chen, 2010; Veylon et al., 2015; Fan and Tsai, 2016). We performed undrained direct shear tests on both rooted and root-free samples.
Plant roots can protect soil against shallow landslides. The mechanics of rooted soil failure under varying soil water content remain poorly understood. The aim of our study was to characterize the impact of soil water content on root mechanical traits and reinforcement to soil. One-year-old Symplocos setchuensis trees were replanted in shear boxes (0.4 × 0.4 × 0.5 m³), which were placed in a canopy gap on Jinyun Mountain, Chongqing, China. After three years of vegetation growth, root mechanical measurements and soil direct shear tests were performed at five levels of volumetric soil water content varying from 14.9 to 44.4 %, by either wetting or drying the soil. For each shear box, we measured soil shear strength, root area ratio, root tensile force, root water content, and root failure mode (slippage versus breakage) during or after the shearing process. Root area ratio and root failure modes were examined for different root diameter classes. With increasing volumetric soil water content, we found a persistent decrease in root tensile strength (ca. 15 %) and an increase in root water content (ca. 11 %), although the dependence on volumetric soil water content was not statistically significant for certain soil water content levels. We found that both additional shear strength of rooted soil and proportion of root failure in breakage increased first and then decreased, with peak values occurring at a soil water content of ca. 22 %. Altered root tensile force (68.0 % contribution to root reinforcement variation) and the change in proportion of root failure modes (breakage or slippage; 8.4 % contribution to root reinforcement variation) jointly contribute to the negative impact of soil water on observed root reinforcement. Soil shear strength was positively correlated with root area ratio and their relationship was linear. At high soil water content (>28 %), the slope of the linear relationship decreased rapidly with increasing soil water content, suggesting that increasing root biomass is not effective for enhancing root reinforcement in very wet soil conditions. Regarding hysteresis, root reinforcement magnitude and efficiency were higher in the drying process than in the wetting process when soil moisture was low. Our results highlight the vital role of soil water features in affecting soil-root mechanical interactions and such effects should be taken into account in future root reinforcement modelling and assessment

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The effect of root architecture on the shearing resistance of root-permeated soils

Abstract

Introduction

Section snippets

Test site and soil properties

In situ shear tests

Discussion

Conclusions

Acknowledgments

Geomorphology

Ecol. Eng.

Ecol. Eng.

Ecol. Eng.

Ecol. Eng.

The distribution and strength of riparian tree roots in relation to riverbank reinforcement

Hydrological Processes

Contribution of vegetation to slope stability: an overview of experimental studies carried out on different types of plants

Geo-Frontiers

Use of Vegetation in Civil Engineering

Nutrient supply and the growth of the seminal root system in barley I: The effect of nitrate concentration on the growth of root axes and laterals

J. Exper. Bot.

Biotechnical Slope Protection and Erosion Control

Mechanics of fiber reinforcement in sand

J. Geotech. Eng., ASCE

Biotechnical and Soil Bioengineering Slope Stabilization