Erodibility and Scour by a Vertical Submerged Circular Turbulent Impinging Jet in Cohesive Soils
Date
2017-04-20
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Degree Level
Masters
Abstract
The erodibility of cohesive soils is commonly represented by the critical shear stress, which is
the applied shear stress for which erosion is initiated, and the erodibility coefficient, which is a
measure of the rate of erosion once the critical shear stress threshold is exceeded. These
erodibility parameters are typically considered properties of the soil and can be measured using a
number of laboratory or field testing devices. In this study, a submerged, turbulent jet with a
circular nozzle, impinging vertically on a soil surface, was used for testing 18 natural and 10
manufactured cohesive samples. There are a number of different methods available for analyzing
the test data and the first objective of this study was to compare the resulting erodibility
parameters calculated using multiple methods. The second objective was to use the calculated
erodibility parameters to develop a relationship that can be used for estimating the ultimate
dimensions of a scour hole formed using a vertical jet.
Four data analysis methods (Hanson and Cook (2004), Visual, Equilibrium, and Thomas (Personal Communication, 2010))were used in determining the critical shear stress for the tested samples. While it was expected
that the results from these four methods should provide similar results, this was not found to be
the case. Differences in the calculated values were largely attributed to the disparity between
theoretical estimates for the equilibrium centre line scour depths and the equilibrium scour
depths observed from testing. Results were also found to be impacted by the duration of test data
used in the analysis. The relative ranking of the critical shear stress values for the soils tested are
very close to being the same between analysis methods.
Two data analysis methods (Hanson and Cook (2004) and Thomas (Personal Communication,
2010)) were used in determining the erodibility coefficient for the tested samples and were found
to have a significant discrepancy in results. Each analysis method is a theoretical model of how
the scour hole depth grows with time and both are based on a form of the excess shear stress
equation. This equation is generally assumed to be linear; however, results from this study
support the inclusion of an exponential term. This term was found to vary between 0.5 and 2.0
for Hanson and Cook’s (2004) analysis method and between 0.6 and 6.1 for Thomas’ Method.
Adopting nonlinear time development of scour equations affects how the erodibility coefficient
can be compared between samples since its units depend on the value of the excess shear stress
exponent. It is recommended that a nondimensional form of the erodibility coefficient be
considered when using a nonlinear expression of the excess shear stress equation for data
analysis.
A number of scour analysis methods from the literature were highlighted in this study and
Mazurek’s (2001) approach was chosen. When using the critical shear stress from Hanson and
Cook’s (2004) method and the equilibrium method, reasonable relationships were developed for
estimating the centre line scour depth and scour hole radius at the equilibrium state. As expected,
data from the natural samples showed more variability than the manufactured samples.
Description
Keywords
erodibility, scour, jet erosion test, critical shear stress, cohesive soil
Citation
Degree
Master of Science (M.Sc.)
Department
Civil and Geological Engineering
Program
Civil Engineering