ASSESSMENT OF STRESS-STRAIN BEHAVIOR OF EPS GEOFOAM BY USING NEW SENSORS

Date of Award

May 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Dawit Negussey

Second Advisor

Jianshun Zhang

Keywords

EPS, Geofoam, Sensors, Strain, Stress

Subject Categories

Engineering

Abstract

Expanded Polystyrene (EPS) geofoam has been used for lightweight fill purposes, for settlement and load reduction, energy absorption, and insulation functions in geotechnical engineering for many years. Engineering properties of EPS geofoam such as density, Young’s modulus, Poisson’s ratio, compressive and flexural strengths are necessary for design and modeling. Pressure mapping and innovative sensing technologies now enable observation of physical behavior in ways that previously could only be imagined or theoretically predicted. This study examined alternative methods for assessing the stress-strain properties, effects of unintended mixing of densities and improper installation of EPS geofoam blocks by using innovative sensors. Sensing of interface contact pressures and internal stresses in EPS geofoam should be valuable for calibrating computer models and improving design practice.

Conventional unconfined compression tests on small samples were previously found to significantly underestimate Young’s moduli for EPS geofoam. Back-calculation from field observations, large sample test results, and non-destructive tests have consistently indicated much higher values for Young’s modulus. A simpler and less expensive alternative method for evaluating moduli and strengths was investigated. A series of three-point load bending tests were conducted to determine Young’s moduli and flexural strengths for EPS geofoam of different densities. Cases of EPS samples with geofabric laminates on the tension side were also investigated to evaluate possible benefits of geosynthetic reinforcement. The results strongly confirm flexure tests to be a simple and practical alternative method for assessing Young’s moduli of EPS geofoam. In addition, the results of flexure tests indicate geofabric reinforcement on the tension side of the outer surface, increase both moduli and strengths.

Observations of lateral deformation made with proximity transducers indicated Poisson’s ratio of geofoam is positive at low strain levels and transition to negative at high strain levels.

Effects of unintended mixing of densities were investigated with FlexiForce sensors. Lower grade (density) blocks in the same layer resulted in degraded strengths and deformations. Good quality assurance during field delivery is important to guard against installation of mixed density blocks in the same layer.

Small-scale I-Scan results from testing traditional 50mm cube samples of different densities indicate the stress-strain behaviors of small samples are affected by end effects. Moduli determined on the bases of global measurements depend on the uniformity of the cut end surfaces. Interface pressures between different mating materials (metal/geofoam) developed greater variations than interface pressures of the same material (geofoam/geofoam). The large-scale test results with I-Scan sensors embedded internally and placed at geofoam interfaces provided stress bulbs under the applied circular loads that attenuate significantly below a depth equal to the diameter of contact pressure at surface. The highest pressure was observed under the centerline of the pressure and reduced with depth and radial distance from centerline. The normal load distribution spread within the geofoam material is about 16o relative to vertical based on the research findings.

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