Electronic Theses and Dissertations

Date of Award

2016

Document Type

Dissertation

Degree Name

M.S. in Engineering Science

Department

Geology and Geological Engineering

First Advisor

Craig J. Hickey

Second Advisor

Zhen Guo

Third Advisor

Louis Zachos

Abstract

Civil engineering practice has shown that vegetative roots on slopes and streambanks can substantially increase shear strength of soil and reduce erosion. Research has been done to understand and quantify the effect. Most studies have been conducted on slopes and streambanks with woody vegetation. Past research has used a perpendicular root model to predict increase in cohesion, or shear strength, due to the mobilization of roots' tensile strength. Acoustics can be used to monitor internal changes of soil by interacting with soil particles and interstitial fluids. Compressional wave, or p-wave, velocity can be used to predict changes in effective stress and bulk density which can be related to geotechnical parameters such as cohesion and porosity. A literature review has concluded that an acoustic based apparatus capable of predicting increased cohesion due to grass root growth would be beneficial to geotechnical engineers and soil scientists. The goals of this study include: measuring changes in the acoustic response of soil reinforced with grass roots, modeling and measuring the effect of grass root reinforcement on the soil cohesion, and relate soil cohesion to p-wave velocity in soil reinforced with grass roots. For a laboratory experiment, two types of bermuda grass (cynodon dactylon) and bahia grass (paspalum notatum) were planted with a bare soil quadrant for control. Acoustic measurements during a year of grass root growth showed a 90% increase in p-wave velocity. A comparison of the modeled acoustic response using independently measured root density and cohesion (from direct shear tests) to in-situ measured p-wave velocity was explored. Two models were presented to explain the increase in p-wave velocity: increase in cohesion with no settlement and increase in cohesion including settlement. For the case with no settlement, the required root cohesion to explain the velocity increase was from 500 to 40,000 kpa. With the inclusion of settlement the range of required root cohesion was from 50 to 25,000 kpa.

Concentration/Emphasis

Emphasis: Geological Engineering

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