This study focuses on flow-induced bank erosion on the Red River. The study includes field measurements, experimental testing, and numerical simulation. Soil samples from the riverbank were collected at seven sites and their erodibility parameters were estimated through laboratory testing. The hydraulic shear stresses applied to the river reach were obtained by developing a 2D numerical model. Erosion rates for these sites were modeled using a linear excess shear stress equation. A bank monitoring and total suspended sediment investigation were also conducted to assess the erosion and deposition rates and patterns. The locations susceptible to erosion were determined and the periods during which these processes are likely to occur were estimated. The numerical modeling and soil testing results show that most of the time, the magnitude of flow shear stresses exerted on the bank are less than the soil sample critical shear stresses. Therefore, without considering other bank widening mechanisms as well as their interactions, the fluvial bank erosion (in isolation) should not be a significant process. However, bank monitoring shows significant bank erosion. It is recommended that the effect of subaerial processes (especially freeze-thaw) be investigated further to determine their effects on flow-induced erosion. The monitoring results convincingly show that climate-related phenomena influences cohesive soil structures and consequently, a soil’s cohesive resistance forces are significantly reduced. Therefore it can be concluded that subaerial mechanisms play a significant role in widening the banks of the Red River. / February 2016

No description available.

A set of solutions to the cohesive soil erosion problem were developed through this study. A first device, the Erosionometer, was developed to perform a quick and reliable test to determine the critical shear stress of soils. The Erosionometer is based on physical shearing of the soil surface and has been calibrated through comparison with piston flume measurements of critical shear stress for entrainment of various fluvial bed sediments. This device is portable, easy to deploy in the field and in the laboratory and allows engineers and researchers to cover a sizeable terrain by performing many tests in a short timeframe, with immediate results. A modification to the Erosionometer was made to allow for subjecting the soil sample to a pressure differential while testing for critical shear stress. The added functionality is intended for investigating the effect of pressure gradient on the erodibility of cohesive soils by allowing for the erosion test to be conducted under a high pressure head while the other face of the sample (away from the flow) is maintained at zero head. Testing demonstrated that a positive pressure gradient on the eroding side (high pressure on flow side) can significantly increase the critical shear stress of cohesive soils, which is in line with other research available in the literature. The results show a simple linear relation between pressure differential and critical shear stress. Practical implications of these results are discussed. A second device, the Erosion Rate Meter, or ERM, was developed to test cohesive soil samples to determine the rate of erosion under various levels of bed shear stress. This device, while being portable and fast to setup and run, is a very realistic simulation of the flow-bed interaction and allows for a direct measurement of bed shear stress on the soil sample and a precise measurement of the erosion rate. An obvious outcome of using the ERM is the easy development of erosion rate vs. bed shear stress relationships or models to characterize the different soils for design projects or further research. Of the 16 tested cohesive soils, all but two demonstrated a linear relation between erosion rate and bed shear stress. The testing systems and methods developed in this research provide a comprehensive solution to the erodibility of cohesive soils from investigation to design. Significant improvements are achieved over existing systems in the speed, reliability, accuracy, and cost of estimating the erodibility of cohesive soils.

Thesis advisor: Gail C. Kineke / To determine the role of cohesive suspended particle characteristics on sediment transport patterns in an energetic estuary floc size, density, and settling velocity were investigated in the Connecticut River estuary over three years spanning varying fluvial discharge regimes. Concurrent measurements of in-situ floc size, flow, bed stress, salinity and suspended-sediment concentration (SSC) were used to identify primary influences on floc size variability. Water discharge ranged from 202 to 910 m³/s between the three sampling campaigns, and the timing of major sediment discharge events preceding measurement periods from 23 to 162 days. Two distinct particle populations were observed under high and low sediment discharge regimes. With abundant fluvial sediment input, flocculation occurred resulting in large, loosely-packed flocs dominating the suspended signal (median sizes of 194-209 µm; median excess densities of 13-17 kg/m³). Following an extended period of low sediment discharge, small, dense aggregates resuspended from the bed were observed throughout the water column (median size of 171 µm and excess density of 60 kg/m³). The timing of and partial decoupling of water and sediment discharge led to inter-annual patterns of cohesive particle characteristics controlled by fresh sediment supply. The large, light flocs with lower settling velocities characteristic of high sediment supply regimes likely bypass the estuary. Smaller compact aggregates dominated the low-sediment discharge regimes. However, the similar disaggregated size distribution of the two regimes suggests the same fine source material is reintroduced to the estuary with the intrusion of the salt wedge, which extends farther up-estuary during low discharge regimes and ultimately supplies the off-channel bays and coves. / Thesis (MS) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Thermocompression bonding of gold is a promising technique for the fabrication and packaging microelectronic and MEMS devices. The use of a gold interlayer and moderate temperatures and pressures results in a hermetic, electrically conductive bond. This paper documents work conducted to model the effect of patterning in causing pressure non-uniformities across the wafer and its effect on the subsequent fracture response. A finite element model was created that revealed pattern-dependent local pressure variations of more than a factor of three. This variation is consistent with experimental observations of bond quality across individual wafers A cohesive zone model was used to investigate the resulting effect of non-uniform bond quality on the fracture behavior. A good, qualitative agreement was obtained with experimental observations of the load-displacement response of bonds in fracture tests. / Singapore-MIT Alliance (SMA)

Bridge scour, which is the removal of bed materials from near the bridge foundations, is observed to be the most predominant cause of bridge failures in the United States. Scour in cohesive soils is greatly different from scour in cohesionless soils owing to the differences in critical shear stresses, scour extents and the time taken to reach the maximum scour depth in the scour process. The present solutions available for the cohesionless soils cannot be applied to cohesive soils because of the above crucial reasons. Also, a compound channel model with main channel and flood plain arrangement represents more closely the field stream conditions rather than a simple rectangular prismatic model. In this study, a systematic investigation of the scour process due to flow contractions in a compound channel with cohesive soil bed is made by conducting a series of flume tests representing typical field conditions. The effect of the most crucial factors causing contraction scour namely flow velocity, depth of flow and the shape of the abutment is examined. Correction factors are developed for changes in flow geometries incorporating simulation results from the one dimensional flow simulation model HEC RAS. Most importantly, a methodology to predict the depth of the deepest scour hole and its location in the vicinity of the contraction structure is developed for compound channels through an extension of the presently available methodology to predict maximum scour depths in simple rectangular channels. A prediction method to identify the extent of the uniform scour depth is also developed. Finally, an investigation of precision of the proposed methodology has been carried out on the field data from a number of real life contraction scour cases. The results obtained from this study indicate that depth of flow and geometry of the contraction section significantly influence final scour depth in cohesive soils with deeper flows and harsh contractions resulting in increased scour depths. However, corrections for different contraction inlet skew angles and long contractions need to be further explored in future studies.

Bridge scour, which is the removal of bed materials from near the bridge foundations, is observed to be the most predominant cause of bridge failures in the United States. Scour in cohesive soils is greatly different from scour in cohesionless soils owing to the differences in critical shear stresses, scour extents and the time taken to reach the maximum scour depth in the scour process. The present solutions available for the cohesionless soils cannot be applied to cohesive soils because of the above crucial reasons. Also, a compound channel model with main channel and flood plain arrangement represents more closely the field stream conditions rather than a simple rectangular prismatic model. In this study, a systematic investigation of the scour process due to flow contractions in a compound channel with cohesive soil bed is made by conducting a series of flume tests representing typical field conditions. The effect of the most crucial factors causing contraction scour namely flow velocity, depth of flow and the shape of the abutment is examined. Correction factors are developed for changes in flow geometries incorporating simulation results from the one dimensional flow simulation model HEC RAS. Most importantly, a methodology to predict the depth of the deepest scour hole and its location in the vicinity of the contraction structure is developed for compound channels through an extension of the presently available methodology to predict maximum scour depths in simple rectangular channels. A prediction method to identify the extent of the uniform scour depth is also developed. Finally, an investigation of precision of the proposed methodology has been carried out on the field data from a number of real life contraction scour cases. The results obtained from this study indicate that depth of flow and geometry of the contraction section significantly influence final scour depth in cohesive soils with deeper flows and harsh contractions resulting in increased scour depths. However, corrections for different contraction inlet skew angles and long contractions need to be further explored in future studies.

No description available.

A multiple scale theory is developed for the prediction of damage evolution in heterogeneous viscoelastic media. Asymptotic expansions of the field variables are used to derive a global scale viscoelastic constitutive equation that includes the effects of local scale damage. Damage, in the form discrete cracks, is allowed to grow according to a micromechanically-based viscoelastic traction-displacement law. Finite element formulations have been developed for both the global and local scale problems. These formulations have been implemented into a two-scale computational model Numerical results are given for several example problems in order to demonstrate the effectiveness of the technique.

The peel test is a commonly used testing method for adhesive strength evaluation. The test involves peeling a pressure sensitive tape away from a substrate and measuring the peel force that is applied to rupture the adhesive bond. In the present study, the mechanics of the peel test is analyzed based on a cohesive zone model. Cohesive failure is assumed to prevail in the vicinity of the peel front, that is, the adhesive fails not by debonding from the adherends but by splitting of the adhesive itself. Generally, the failure of the adhesive is accompanied with a process of cavitation and fibrillation. Therefore, the cohesive zone is modeled as a continuous fibrillated region. A Maxwell model is employed to characterize the viscoelastic behavior of the adhesive. The governing equation and boundary conditions that describe the mechanics of the peel test are derived. Numerical results are obtained under steady state conditions. The model predicts the peel force in terms of the peel rate, the peel angle, the nature of the adhesive, and the properties of the backing and the substrate. The traction distribution on the substrate surface is found to depend on various test parameters. Finally, finite element analysis is performed using the commercial software package ABAQUS. The results from FEA are compared with those from the mathematical method to evaluate the validity of the present model. The effective range of the present model is found to be related to the ratio of the critical fibril length to the extent of the cohesive zone. Given the nature of the adhesive as well as the properties of the backing and the substrate, the proposed model is able to predict the peel force and the traction distribution in terms of the peel rate and the peel angle, and thus provides a measure of the strength of the adhesive bond.