Reducing excessive deformations of buried high density polyethylene pipes under dynamic loading using expanded polystyrene geofoam: a numerical study

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University of Delaware
Nuclear power plants are crucial structures for generating power, and play an increasingly important role as the demand for electricity increases in many parts of the world due to a growing population. The technology at the heart of the nuclear power plant is the nuclear reaction itself, which produces tremendous energy. The most visible portion of a nuclear power plant is the structure that houses the nuclear reactor, where the nuclear reactions occur and are contained. In conventional design, nuclear reactors are filled with water, and as the exothermic nuclear reactions occur, the water is heated to high temperatures, at which point it boils. The steam coming from this process is transmitted to a steam generator to generate electricity. ☐ A critical element in the design of nuclear power plants is the pipes that circulate water and steam to and from the nuclear reactor. The pipes that transmit cooling water are especially important, as this cooling water serves to control the heat that is generated during the reaction process, which prevents the reaction from becoming unstable in a way that could threaten the integrity of the nuclear power plant itself. It is consequently important in the design of nuclear power plants to select pipes that can withstand the rigors of this application, as well as any static or dynamic loads that the power plant may be subjected to over the course of its design life. ☐ During the life of a given nuclear power plant, the water and steam circulating pipes will be subjected to internal stresses from the water or steam, external geostatic stresses from the weight of the soil around the pipe, static stresses due to surface surcharge loads, and dynamic stresses due to extreme events such as earthquakes. Dynamic loads from earthquakes are of particular concern, and as such, this thesis describes the use of possible shock-absorbing barriers for reduction of the dynamic and static loads on buried pipes. ☐ In conventional design, important pipe systems are located under the nuclear power plant or buried at some depth in the nuclear plant site. These pipes can experience a significant amount of static loads, as the overlying soil is quite heavy and the containment structures for the reactor chamber are generally very substantial structures, with significant self-weight (dead load). Previous researchers have explored the use of geofoam and geogrid systems to reduce the effect of static overburden pressure on pipes. ☐ The goal of this thesis is to explore the use of an expanded polystyrene (EPS) material that is commonly referred to as “geofoam” as a barrier system to protect potentially vulnerable underground nuclear power plant piping. In this study, two different soil types (a cohesive fine-grained soil and a sandy soil) are considered to define possible field conditions. In addition, two separate EPS geofoam materials (EPS15 and EPS30) are modeled in the numerical analysis software to be able to understand the change in behavior of the buried pipes that occurs when different shock-absorbing materials are utilized. ☐ Furthermore, natural material variability of the soil can affect the dynamic and static loads that are applied, with these loads then affecting the applied pressure and deflection that occurs in the soil during a given earthquake. Thus, defining the right engineering properties for the soil such as elastic modulus, Poisson’s ratio, and unit weight are significant. Application of different loading scenarios to various simulated soil and pipe configurations is important to understand the range of possible behaviors, and this study consequently examined various locations of the buried pipe and shock absorbing material configurations, as well as soil material properties. In this study, three different depths under the soil layer (2m, 3m, and 4m) were examined for the HDPE pipe. Results showed that EPS15 is a better option than EPS30 for reducing deflection and stress occurring on the pipe. It was also observed that as the pipe burial depth increases, the vertical deformation contribution of both EPS15 and EPS30 also increases. For a pipe buried at a 4 m depth, 6 different EPS geofoam configurations were analyzed to protect the pipe from dynamic loading. Analyses of model results indicated that the dynamic response of all of the geofoam configurations was relatively similar, with roughly the same amount of deformation and applied stress occurring at the top of the pipe for all of the models that were analyzed. A small difference in deflection and applied stress was observed for some of the model configurations at the end of the static loading, prior to the dynamic load being applied.
EPS geofoam, HDPE pipe, Seismic analyses, Soil-buried structure interaction, Soil-structure interaction