Modeling the Resilience of Power Distribution Systems Subjected to Extreme Winds Considering Tree Failures: An Integrated Framework

Abstract Overhead electrical power distribution systems (PDS) are very susceptible to extreme wind hazards. Power outages can cause catastrophic consequences, including economic losses, loss of critical services, and disruption to daily life. Therefore, it is very important to model the resilience of PDS against extreme winds to support disaster planning. While several frameworks currently exist to assess the resilience of PDS subjected to extreme winds, these frameworks do not systematically consider the tree-failure risk. In other words, there is no integrated framework that can simultaneously consider tree failures, PDS component failures induced by falling trees, resilience assessment, and evaluation of resilience enhancement with vegetation management. Therefore, this study proposed an integrated simulation framework to model the resilience of PDS against extreme winds, which includes tree fragility modeling, PDS fragility modeling, PDS component failure estimation, system performance evaluation, system restoration modeling, and resilience enhancement evaluation. The framework is demonstrated with a power distribution network in Oklahoma. The results show that the estimated system resilience will reduce if tree failures are considered. Crown thinning can effectively enhance the system’s resilience, but the effectiveness is affected by both wind speed and direction

Seismic behavior of pile foundations in unsaturated soils

Earthquakes have caused significant damage to civil engineering structures worldwide due to inadequate lateral load capacity and excessive deformation of pile foundations supporting these structures. The seismic performance of pile foundations interacting with unsaturated soils could be affected by changes in matric suction due to the moisture content variation induced by seasonal weather changes or water table fluctuations. Hence, the main objective of this study is to investigate the effects of unsaturated soil conditions on the seismic response of a pile-soil system in silty clay soils. This study utilized a stand-alone finite element computer code called DYPAC (Dynamic Piles Analysis Code) developed using the Beams on Nonlinear Winkler Foundation (BNWF) approach. Free field soil displacements and p-y curve parameters, inputs needed for DYPAC analyses, were updated based on the soil suction variations. This study found that soil suction can significantly influence the seismic performance of piles interacting with unsaturated silty clay soils, especially as the soil becomes drier in the transition zone. The best seismic performance of the pile, which is the minimum lateral pile displacement, happened in the transition zone between fully saturated and nearly dry conditions

FULLY COUPLED DYNAMIC SOIL-STRUCTURE INTERACTION ANALYSES OF PILE FOUNDATIONS IN WEAK SOILS

ABSTRACT Dynamic soil-structure interaction analyses of several single pile foundations in a soft clay during seismic loading are presented. While one foundation was in the unimproved soil, the soil surrounding three other foundations is improved to varying extents. The analyses are performed using a framework-based finite element computer code TeraDysac. TeraDysac solves the coupled differential equations governing the behavior of the solid skeleton, pore water, and the structural elements using the TeraScale finite element framework. A framework represents a collection of common software components for building different computer codes and is an efficient way to build large parallel codes. The analyses reveal that lateral displacements of the piles can indeed be controlled by improving the ground around the piles to a sufficient extent

Seismic behavior of pile foundations in unsaturated soils

Earthquakes have caused significant damage to civil engineering structures worldwide due to inadequate lateral load capacity and excessive deformation of pile foundations supporting these structures. The seismic performance of pile foundations interacting with unsaturated soils could be affected by changes in matric suction due to the moisture content variation induced by seasonal weather changes or water table fluctuations. Hence, the main objective of this study is to investigate the effects of unsaturated soil conditions on the seismic response of a pile-soil system in silty clay soils. This study utilized a stand-alone finite element computer code called DYPAC (Dynamic Piles Analysis Code) developed using the Beams on Nonlinear Winkler Foundation (BNWF) approach. Free field soil displacements and p-y curve parameters, inputs needed for DYPAC analyses, were updated based on the soil suction variations. This study found that soil suction can significantly influence the seismic performance of piles interacting with unsaturated silty clay soils, especially as the soil becomes drier in the transition zone. The best seismic performance of the pile, which is the minimum lateral pile displacement, happened in the transition zone between fully saturated and nearly dry conditions

SETTLEMENT BEHAVIOR OF COMPACTED OKLAHOMA SOILS (FHWA-OK 02(03) 2152)

Numerous highway embankments experience post-construction settlement problems, such as bridge approach settlement that results in the "bump at the end of the bridge." One of the causes may be wetting-induced collapse settlement or simply, collapse settlement. Collapse settlement is a time-dependent process resulting from post-construction increases in moisture content. The post-construction settlement of numerous Oklahoma highway embankments raised questions as to whether the current Oklahoma Department of Transportation embankment specifications and construction practices are adequate in addressing collapse settlement, and prompted the current study to examine the influence of soil type on collapse potential of Oklahoma soils. One-dimensional oedometer tests were conducted to study the potential for collapse settlement of22 Oklahoma soils and shales under conditions typically encountered in compacted fills. Results show that factors related to fine composition, such as clay-size fraction, plasticity index, liquid limit, activity, and AASHTO group index can be used for preliminary estimation of collapse index. Statistical analysis of the oedometer test data indicates that variables having the most impact on collapse index are moisture content, dry unit weight, plasticity index, and clay-size fraction. Settlement charts were developed to facilitate the estimation of collapse settlement of fills for different conditions, including fill height, moisture content, and soil type. Three scale centrifuge models compacted at different conditions and a case history of an embankment that has experienced significant collapse settlement are presented. Predictions based on one-dimensional oedometer-based method and settlement charts are compared to measured collapse settlements at the embankment centerlines. Given the uncertainty with field estimates of settlement, the comparison showed a reasonable agreement between predictions and field estimates of collapse settlement at the embankment centerlines; the limited evidence suggests that predictions based on one-dimensional assumptions tend to underestimate actual settlements possibly due to the two-dimensional nature of embankments. The review of literature regarding settlement of compacted fills, the laboratory test results obtained, and the field study of an actual embankment suggest the need for embankment design and specifications that will account for collapse susceptibility of different soil types. Specifications should demand for exceptional quality control and more stringent compaction requirements during embankment construction, particularly for large embankments, collapse-susceptible soils, and embankments susceptible to flooding.Final Report April 1999-November 2001N

Interpretation of In Situ Tests as Affected by Soil Suction / Gerald A. Miller, Rodney W. Collins, Kanthasamy K. Muraleetharan, Amy B. Cerato, Roy Doumet.

In situ testing of soil with invasive methods such as the Cone Penetration Test (CPT) are increasingly used in geotechnical engineering practice. However, there has been very little work to develop methods for interpreting results of these tests when performed in unsaturated soil. It is important to develop such methods because the in situ test results in unsaturated soil will depend on the moisture conditions at the time of testing and may not reflect the soil behavior corresponding to the moisture conditions during the life of supported structures. Research described in this report involved conducting in situ tests at two test sites, containing lean to fat clayey soils, at various times of the year. The purpose was to investigate the influence of changes in moisture conditions and soil suction on the test response. In situ testing included the Cone Penetration Test (CPT), Standard Penetration Test (SPT) and Pre-bored Pressuremeter Test (PMT). Test sites were characterized by sampling and laboratory testing to determine important soil properties and moisture content profiles. Additionally, test sites were instrumented with weather monitoring equipment and sensors to measure temporal variations in soil moisture with depth. Another goal was to evaluate two commercially available computer programs to evaluate their predictive ability with respect to soil moisture change. The PMT, CPT and SPT parameters determined from standard interpretation of the results were compared to total suction measured at corresponding depths obtained from measurements on samples obtained in the field using a chilled mirror hygrometer. At both sites, the suction had a profound impact on the in situ test parameters, especially at total suction in excess of approximately 150 psi, which generally corresponded to shallow test depths. At both sites, for the shallow depths there was a noticeable trend of increasing magnitude of in situ test parameters with increasing suction, as expected. Plots of normalized in situ test parameters against suction show expected, albeit weak, correlations for the two test sites. These correlations give a sense of the variation of in situ parameters that might be expected for similar soils under changing suction conditions. It was demonstrated how correlations for PMT, CPT and SPT parameters with suction can be used to roughly predict possible changes in these parameters as a result of suction changes. Unsaturated seepage modeling was found to provide reasonable comparisons to measured soil moisture profiles; however, significant effort was required to define the various input parameters through calibration procedures.Final report, Oct. 2011-Dec. 2015N