Graduate Thesis Or Dissertation
 

Shear stress transfer between roots and soil

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/dn39x419g

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  • Mass wasting events are a principal form of erosion that occurs on steep forest land throughout the Pacific Northwest. Numerous studies have reported that tree roots help stabilize steep forest land. A complete understanding of the mechanism by which roots strengthen soil would enable modeling to more closely approximate field conditions, and would provide an improved basis for formulating forest management guidelines directed at minimizing slope failure incidents. Laboratory investigations were conducted to determine the material properties of green Douglas-fir roots which include the modulus of elasticity and the shear stress transfer at the soil/root interface. Two distinct types of moduli of elasticity were observed for the roots sampled in the study: form modulus (average = 2.7 x 10⁴ psi) which occurs as a root straightens out, and material modulus (average = 7.3 X 10⁴ psi) which occurs once the wood fibers fully resist elongation. For the cohesionless, dry sand (uniform 30 grade) used in the pull-out tests, the average ultimate shear stress transfer between roots and soil was observed to increase from 0.73 psi to 1.79 psi as a function of increasing soil density and confining pressure. Apparent friction coefficient (f*) values were calculated and f* was shown to decrease with increasing confining pressure and to increase with increasing density. These findings are in agreement with those reported in the literature. Two mechanisms that may account for the high f* values are discussed. Reduced f* values (based on these two mechanisms) are demonstrated to approach the tangent of the angle of internal friction (ø'), which is the upper bound of the actual friction coefficient as reported in the literature. Preliminary results indicate an increase (from 13 to 34 %) in shear stress transfer for roots with rootlets left attached over that obtained for the same roots with rootlets removed. This is considered to be a conservative estimate of the shear stress at the soil/root interface for in situ conditions. A comparison of the findings in this study with values assumed in soil/root (fiber) system models, suggests that the coefficient of friction between roots and soil may be significantly greater than values used in the models.
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