Internship Study Report (supported by JGP Program)

Internship Study Report

Name of student

Aoi Seo, Master student, Graduate School of Global Environmental Studies

Title of internship program

Deformation of reinforcing geosynthetics in thermally-active geotechnical systems

Institution

Department of Structural Engineering, University of California, San Diego

Supervisor

John McCartney, Ph.D., P.E.

Period of internship

6/9/2015 ~ 4/12/2015

 

BACKGROUND & OBJECTIVES

Mechanically-stabilized earth (MSE) walls consist of alternating layers of compacted soil backfill and geosynthetic reinforcements. Although the mobilized resistance of these structures depends on the shear strength of the soil, the tensile strength of the geosynthetic, and soil-geosynthetic interaction, the deformation response of MSE walls is closely linked to the effective stress state within the backfill. Interaction with the environment or changes in the groundwater level may lead to changes in pore water pressure, which will affect the effective stress state. Increases in pore water pressure or degree of saturation that may occur in poorly draining backfills will lead to decreases in effective stress. A method that is being investigated for control of the stress state in poorly draining backfills is the incorporation of geothermal heat exchangers within the reinforced soil mass (Stewart and McCartney 2013). In this case, thermally induced water flow is expected to occur in the unsaturated soil away from the heat exchangers, leading to a lower degree of saturation, increased suction, and increased effective stress in the backfill at the locations of the heat exchangers (Coccia and McCartney 2013). This soil improvement technique (Figure 1) has the added advantage that MSE walls could be used to dissipate excess heat from power plants or buildings, making these systems more environmentally friendly and potentially less expensive due to the cost offsets associated with the cooling system requirements.

Despite the potential positive effects of thermally induced water flow on the shear strength and stiffness of the reinforced soil mass, studies on geosynthetics indicate that there may be negative effects of temperature on the stress-strain response of reinforcing geosynthetics. Thermal softening (i.e., a decrease in tensile modulus) may be encountered when geothermal heat exchange elements are incorporated into mechanically stabilized earth (MSE) walls. This project aims to characterize the effects of thermal softening on geosynthetics confined in compacted soil from the results from a series of laboratory tests.

fig1

ACHIVEMENTS

A thermomechanical pullout device was employed in the project (Photo. 1). This incorporates standard geosynthetic testing elements and also heating elements at the top and bottom of soil layer, along with an array of dialectic sensors for measurements of soil water content and temperature. After installing a soil layer with geosynthetic reinforcement simulating the backfill of a thermally-active MSE wall in the box, the top and bottom of the box were heated to approximately 90 °C for a period of approximately 10 days at most. Next, a horizontal load was applied to the geosynthetic at a rate of 0.002 kN/m/s by pouring tungsten pellets into a hanging basket until reaching failure (defined by movement of the tell-tales). The pullout load-displacement curves for geosynthetics of four tests with various temperatures are shown in Figure 1. This clearly indicates that pullout stiffness decreased due to softening of geosynthetics caused by heat. Although decreasing of water content was also observed, it was not so significant. This indicates that longer time of heating may induce greater water flow and increase effective stress, which overcome the negative effects of thermally induced creep.

photo1 

ACKNOWLEDGEMENT

It is my extreme pleasure to put forth my sincere thanks to Prof. John McCartney (University of California, San Diego) for providing me such a wonderful opportunity and helping my research. I also would like to express my gratitude to all members of Professor McCartney’s laboratory for generous support. Prof. Katsumi, Dr. Inui and Dr. Takai (Kyoto University) gave me insightful comments and suggestions. I sincerely appreciate financial supports from “Japan Gateway : Kyoto University Top Global Program”.

 

 

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