Triaxial compression test of soil–root composites to evaluate influence of roots on soil shear strength
Introduction
It has been widely recognized and accepted that plant roots can improve soil shear strength (e.g. Waldron and Dakessian, 1981, Abe and Ziemer, 1991, Zhou et al., 1997, Operstein and Frydman, 2000, Campbell and Hawkins, 2003) and stabilize slopes of surface soil (Ziemer, 1981, Marden et al., 1991, Ekanayake et al., 1997, Nilaweera and Nutalaya, 1999). Such functions turn vegetation into an effective ecological engineering tool to achieve harmony between humans and nature. At present, evaluation of the reinforcing effect of roots is mainly focused on comparisons of the shear resistance of soil with and without roots. Since the slope stability is highly dependent on the shear strength of soil, an increase in soil shear strength can effectively improve slope stability (Burroughs and Thomas, 1977, Wu et al., 1979, Coutts, 1983, Coutts, 1987, Gray, 1995, Coutts et al., 1999, Watson et al., 1999, Davoudi et al., 2004, Pollen, 2007). It is especially necessary to study how to increase soil shear strength.
Many empirical models have been developed to predict soil shear strength to save time and labor from direct measurement. Those models include the multiple regression model by Hirata in 1990, the hyperbolic model by Miao and Yin in 1999, the Fredlund model by Fredlund in 1996, the linear regression analysis model and non-linear regression analysis model by Goktepe et al. in 2008, etc. However, differences in physical properties of different soils may result in big changes of soil shear strength, and applications of empirical models may have some limitations.
To quantify the contribution of roots to soil shear strength, several shear tests have been used for measurement of soil shear strength. Day (1993) conducted a direct shear test and concluded that the shear strength of soil with roots is greater than that of soil without. Wu and Watson (1998), Zhang et al. (2006), and Deng et al. (2007) carried out in situ direct shear tests on plain soil and soil–root systems and their results demonstrated that the shear strength of plain soil is distinctly lower than that of soil–root systems. Campbell and Hawkins (2003) used a direct shear test in laboratory to show soil permeated with roots of paper birch and lodgepole pine can significantly increase soil shear strength, although the degree of increased shear strength is different for different types of soil. Similar results have also been reported by Ali and Osman (2007) in a direct shear test on four types of soil–root systems, and Fan and Su (2008) in in situ direct shear tests on soil with roots of Prickly Sesban. In summary, plant roots make important contributions to an increase in soil shear strength.
Modified direct shear tests have been widely adopted in situ or in laboratory studies on the influence of roots on soil shear strength. However, the research on using a triaxial compression test to study the same topic is relatively new. The application of a triaxial compression test for the study of reinforcing material in soil was first used in civil engineering-related fields. Schlosser and Long (1974) used a triaxial compression test to study sand reinforced by metal strips in 1974. They suggested that metal strips could be helpful in increasing the shear strength of sand. Gray and Al-Refeai (1986) reported that the shear strength of reinforced earth could be determined by the tensile strength of reinforcing materials in his research on shear strength of fiber-reinforced soil using a triaxial compression test. Moreover, a large number of triaxial compression tests were carried out to study effects of geotextile reinforcement on the mechanical behavior of earth materials (Futaki et al., 1990, Athanasopoulos, 1993, Al-Omari et al., 1995, Krishnaswamy and Isaac, 1995, Ashmany and Bourdeau, 1988, Haeri et al., 2000). These reports discussed effects of shear strength of reinforced earth materials from different angles by using different sizes of samples, types and arrangements of reinforcing materials, types of earth materials, etc. and offered important advice for the design of reinforced systems. Liu et al. (2006) were among the first to use a triaxial compression test to study shear strength of forest roots–loess composites. In their consolidated undrained (CU) triaxial compression tests, the relationship between shear strength and stress was studied and factors that affect shear strength of roots–loess composites were also discussed. This paper studies, using a consolidated-drained (CD) triaxial compression test, the shear strength of composites comprised of representative loess from the Loess Plateau in Northwest China and roots of Robinia pseucdoacacia dominating in reforestation of the plateau. This study also tests impacts of soil water contents on shear strength of loess and roots–loess composites. The triaxial compression test is expected to be applied in future study of soil reinforcement by roots.
Section snippets
Tests apparatus
Triaxial Compression tests were conducted with a KTG Automatic Triaxial Compression System (Beijing Huakan Technology Co. Ltd., China) in the Soil Mechanics laboratory at the College of Architectural Engineering, North China University of Technology. The system, similar to ELE Triaxial System (ELE International, UK) and Material Test System (MTS Systems Corporation, USA), is one of strain-controlled triaxial apparatus. It consists of four main components (Fig. 1): (1) a triaxial chamber for
Soil property
The test soil was dark, uniform, stiff, and of little macropores. It represented typical Q3 (Liu and Wang, 1997) Longxi loess. To reduce differences between samples and ensure that samples were taken from the same horizon, the field sampling depth was limited to 50–100 cm below ground surface. Physical properties (Table 2) and grain size grading curve (Fig. 3) of loess were analyzed in laboratory.
Relationship between principal stress difference (σ1 − σ3) and axial strain
Stress–strain curves of plain soil and composites (Fig. 4) were produced from triaxial compression
Test methods
High tensile strength materials have been often used in soil to produce reinforced earth (Vidal, 1969). Friction, which is attributed to displacement between soil particles and materials, integrates soil with reinforcing materials together. Moreover, it diminishes the displacement and enhances the stabilization of composite. Materials in reinforced earth, such as metal, geotextile, geomembrane, geocomposites, etc., are widely studied by triaxial compression tests. However, roots hardly have had
Conclusion
Triaxial compression tests have been carried out to study shear strength of root reinforced soil by using plain soil samples and composites of roots of R. pseucdoacacia and soil from the Loess Plateau in Northwest China. Test results show that roots can effectively improve soil shear strength by significantly increasing the cohesion of soil. Among three forms of root distribution (HR, VR and CR), effect of the CR reinforced soil is the most considerable, and plants or trees with CR root pattern
Acknowledgements
This study was financially supported by Projects of the National Natural Science Foundation of China (Grant no. 30872067, no. 30571531) and Project of the National Major Fundamental Research Program of China (grant no. 2002CB111502). The authors would like to especially thank Tianshui Soil and Water Conservation Experimental Station of Yellow River Conservancy Commission and Soil Mechanics Laboratory of College of Architectural Engineering, North China University of Technology for providing
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