Mandar M. Dewoolkar

A Substitute Pore Fluid for Seismic Centrifuge Modeling

A time-scaling conflict exists between dynamic and dissipative phenomena in seismic centrifuge experiments. Thus, the interpretation of the test results, such as the extrapolation of model performance to prototype scale, is difficult unless the conflicts in the time-scaling relations are first resolved. One way to accomplish this is by slowing the diffusion event by employing a substitute pore fluid, which is more viscous than water (the assumed prototype pore fluid). In this paper, a substitute pore fluid consisting of powdered methylcellulose (known commercially as Metolose) was studied. The suitability of the methylcellulose-water mixtures (metolose) as a substitute pore fluid was examined in an experimental program using water- and metolose-saturated sand specimens. The program included triaxial compression tests, permeability tests, and a seismic centrifuge experiment on level ground models. In addition, modeling of models type experiments were conducted on metolose-saturated embankment and retaining wall models. Results form the triaxial tests indicated that the constitutive behavior of the saturated sand specimens was not significantly altered with metolose as the pore fluid. Results from the permeability tests showed that the scaling requirements of the centrifuge environment were satisfied. The centrifuge experiments demonstrated clearly that the conflict between the dynamic and consolidation time scales exists and reinforced the need for a substitute pore fluid in tests designed to model prototype behavior. Based on this experimental program, metolose was found to be an acceptable substitute pore fluid.

Experimental developments for studying static and seismic behavior of retaining walls with liquefiable backfills

The effects of earthquakes on cantilever retaining walls with liquefiable backfills were studied. The experimental techniques utilized in this study are discussed here. A series of centrifuge tests was conducted on aluminum, fixed-base, cantilever wall models retaining saturated, cohesionless backfills. Accelerations on the walls and in the backfill, static and excess pore pressures in the soil, and deflections and bending strains in the wall were measured. In addition, direct measurements of static and dynamic lateral earth pressures were made. In some tests, sand backfills were saturated with the substitute pore fluid metolose. Modeling of model type experiments were conducted. The experimental measurements were found internally consistent and repeatable. Both static and dynamic earth pressure measurements were determined to be reliable. It was also observed that for the test configuration adopted, a special boundary treatment such as the use of duxseal is optional. Static and seismic modeling of models were also successful, which indicated that the assumed scaling relations were essentially correct.

Porous Concrete Pavements: Mechanical and Hydraulic Properties

A study evaluating the mechanical and hydraulic properties of several porous concrete pavement mix designs is presented. Objectives of the study were to examine various mix designs with constituents available in Vermont, evaluate compressive strength and hydraulic conductivity of laboratory and field-cured specimens, compare results with those found in the literature, and characterize the effects of specimen size on measured parameters. To evaluate the role of sample size on these testing procedures, experiments were performed on specimens of three diameters: 76.2 mm (3 in.), 101.6 mm (4 in.), and 152.4 mm (6 in.). Multiple specimens were tested for a particular size. A specimen size of 101.6 mm (4 in.) was found to be optimal for the experiments performed and is therefore recommended. The measured compressive strength and hydraulic conductivity for the various mix designs showed a clear linear dependence on sample density. Also, the measured values fall within the expected range obtained from a review of the literature. Parametric studies included effects of the water-to-cement ratio and admixtures. Generally, increased water content yielded a higher density, higher compressive strength, and reduced hydraulic conductivity. Admixtures such as a high-range water reducer and viscosity modifying admixture had insignificant effects on the compressive strength, hydraulic conductivity, and workability of the porous concrete mixes examined. Field cores displayed a much greater variability in hydraulic conductivity than that of laboratory-prepared specimens, largely because of differences in compaction effort that are inherent to porous concrete placement in the field.

Centrifuge modeling for undergraduate geotechnical engineering instruction

A small, simple, and economical instructional centrifuge has been developed at the University of Colorado at Boulder to assist in undergraduate geotechnical engineering education. Centrifuge experiments on stability of slopes and retaining walls have been developed. These experiments are conceptually simple, yet fundamental, and do not require elaborate instrumentation and data acquisition. Classical failure patterns discussed in the class can be reproduced in the models. Experimental results can be used to verify such theories as undrained slope stability analysis and Rankines or Coulombs lateral earth pressure theories. Each of the tests can easily be conducted up to four to five times in a 2-h laboratory session. Comprehensive laboratory reports can be generated by students discussing both qualitative and quantitative aspects of the tests in relation to the theoretical concepts taught in the classroom. In addition to the experiments on slope stability and lateral earth pressures, demonstration experiments on footings and reinforced earth slopes have also been conducted using the instructional centrifuge.

Service-Learning in Engineering Education and Heritage Preservation

Many communities and nonprofit organizations in the United States grapple with the reuse of publicly owned buildings and infrastructure from the 19th and early 20th centuries that have become landmarks in their communities. Often these communities or organizations do not have adequate resources (technical expertise, funding) for even preliminary engineering services. Engineering programs at universities can assist these communities by providing engineering expertise through service-learning (SL) projects, at the same time providing engineering students exposure to the field of heritage preservation and to important preservation engineering issues. This article presents SL case studies at the University of Vermont (UVM) as examples of this type of collaboration. Benefits, challenges, and SL assessment results are included. For the past 4 years, as part of a National Science Foundation Department Level Reform (NSF DLR) grant, civil and environmental engineering students at UVM have worked on multiple SL projects with local communities throughout their 4-year program. In many cases, these SL projects have involved historic structures. Although not an original emphasis of the DLR grant, heritage preservation is becoming a component of the UVM reform efforts, such that in 2010 this topic began to be formally included in both the first-year introductory engineering course and senior capstone design course. The SL approach was found to be effective in meeting education goals of the civil and environmental engineering programs and their accreditation requirements as well as providing meaningful service to the local communities caring for historic structures and sites. Both students and faculty gained exposure to and understanding of preservation engineering topics, as well as networking with the heritage preservation community in Vermont and elsewhere. Incorporating heritage preservation issues into engineering programs through SL projects may prove beneficial at other institutions.

Long-Term Field Monitoring and Evaluation of Maintenance Practices for Pervious Concrete Pavement in Vermont

Pervious concrete pavement (PCP) is used because of its unique properties that allow water to infiltrate into the surface. The objectives of the study reported in this paper were to observe the performance of PCP in the field, determine the effectiveness of cleaning methods to restore infiltration rates, and compare field observations with laboratory results when possible. Two PCP sites in Vermont were monitored over a year through the measurement of infiltration rates at several locations. Facility-wide cleaning operations, such as street sweeping and vacuum-truck cleaning, were tested for their capability to restore infiltration rates. Spot-cleaning methods, such as hand vacuuming, pressure washing, and a combination of the two, were also tested. Infiltration rates decreased gradually during the monitoring period, with average reductions of 59% at the first facility and 26% at the second. Street sweeping and vacuum-truck cleaning restored infiltration rates by 21% and 30%, respectively, but could not restore severely clogged areas. Spot-cleaning methods increased infiltration rates by 85% after pressure washing, 10% after vacuuming, and 100% after pressure washing followed by vacuuming. Vacuum-truck cleaning was recommended. Either method, however, should be used for maintenance operations and should be started after construction. Spot-cleaning methods, with the exception of vacuuming, restored infiltration rates of severely clogged areas and were recommended for localized cleaning. Long-term monitoring compared reasonably well with that of earlier studies, whereas the results associated with cleaning were substantially lower than those found in the literature.

Laboratory Freezing-and-Thawing Durability of Fly Ash Pervious Concrete in a Simulated Field Environment

This laboratory study investigated the durability of pervious concrete containing fly ash to freezing and thawing and salt exposure in a field-representative environment. Pervious concrete was prepared by replacing cement with 0, 10, 20, and 30% fly ash. The specimens were subjected to one slow freezing-and-thawing cycle per day up to 100 days in a drained condition with sodium chloride solution with concentrations of 0, 2, 4, 8, and 12%. The void content, compressive strength, and hydraulic conductivity of the mixtures were all within the range of typical pervious concrete applications. Freezing-and-thawing testing suggested that for all concentrations of salt solution, 10 and 20% fly ash replacement improved freezing-and-thawing durability. Specimens with 30% fly ash showed more damage than that of the 0% control. The greatest damage from salt solutions was seen in 8%, 4%, and 2% concentrations, respectively. Water and 12% salt solution showed little damage across all mixture designs.

Commonly Used Porous Building Materials: Geomorphic Pore Structure and Fluid Transport

Knowledge of microscopic geomorphic structures is critical to understanding transport processes in porous building materials. X-ray scans were obtained of a variety of commonly used porous building materials to both qualitatively and quantitatively evaluate their pore structures. The specimens included natural materials (two sandstones and a limestone) and engineered materials (three types of concretes and a brick). Scanned images were processed to reconstruct the geomorphic structures of these materials. Random walk analyses were performed on the reconstructed pore structures to estimate macroscopic transport properties (including tortuosity, specific surface, and permeability). The effective porosity and permeability of these materials were also experimentally determined and compared to computed values. Calibration of the threshold pixel value used in the postprocessing of X-ray images against measured effective porosity appears to be a more appropriate method of selecting this value than the typical approach, which employs selection based solely on observed histograms. The resulting permeabilities computed by using a calibrated threshold pixel value compare better with the measured permeabilities. This study also demonstrates that the relatively homogeneous and heterogeneous pore structures associated with the natural and engineered building materials under investigation can be captured by X-ray tomography.

Evaluating Spatial Variability in Sediment and Phosphorus Concentration‐Discharge Relationships Using Bayesian Inference and Self‐Organizing Maps

Given the variable biogeochemical, physical, and hydrological processes driving fluvial sediment and nutrient export, the water science and management communities need data-driven methods to identify regions prone to production and transport under variable hydrometeorological conditions. We use Bayesian analysis to segment concentration-discharge linear regression models for total suspended solids (TSS) and particulate and dissolved phosphorus (PP, DP) using 22 years of monitoring data from 18 Lake Champlain watersheds. Bayesian inference was leveraged to estimate segmented regression model parameters and identify threshold position. The identified threshold positions demonstrated a considerable range below and above the median discharge—which has been used previously as the default breakpoint in segmented regression models to discern differences between pre and post-threshold export regimes. We then applied a Self-Organizing Map (SOM), which partitioned the watersheds into clusters of TSS, PP, and DP export regimes using watershed characteristics, as well as Bayesian regression intercepts and slopes. A SOM defined two clusters of high-flux basins, one where PP flux was predominantly episodic and hydrologically driven; and another in which the sediment and nutrient sourcing and mobilization were more bimodal, resulting from both hydrologic processes at post-threshold discharges and reactive processes (e.g., nutrient cycling or lateral/vertical exchanges of fine sediment) at prethreshold discharges. A separate DP SOM defined two high-flux clusters exhibiting a bimodal concentration-discharge response, but driven by differing land use. Our novel framework shows promise as a tool with broad management application that provides insights into landscape drivers of riverine solute and sediment export.

Analysis of bridge and stream conditions of over 300 Vermont bridges damaged in Tropical Storm Irene

Abstract The 2011 Tropical Storm Irene deposited 100–200 mm of rain in Vermont with a rainfall recurrence interval for a 12-hour storm exceeding 500 years in some areas. This single hurricane-related event damaged over 300 bridges. The wide range of damage prompted a networkwide analysis of flood, scour, stream and structural conditions. A first step was the assembly of a unique data set containing information on 326 damaged bridges, 1936 undamaged bridges and the surrounding stream conditions. Descriptions of the damage appear as case studies that include pre-storm bridge and stream geomorphology conditions. The assembled and georeferenced data include rainfall, damage type and extent, estimated and actual repair costs, bridge characteristics, bridge ratings and stream geomorphic assessments from a number of sources. The analyses identified significant features of bridge vulnerability under extreme floods. The bridge age and rating assessment characteristics, such as substructure, channel and structural adequacy ratings, followed by scour, waterway adequacy and sufficiency ratings, correlated strongly to damage. The stream geomorphic features have promise to supplement future bridge rating systems and in identifying hydraulic vulnerability of bridges. Empirical fragility curves relating probability of meeting or exceeding different bridge damage levels based on channel and waterway adequacy ratings are also presented.