Doctor of Philosophy
Civil and Environmental Engineering
El Naggar, M. Hesham
Foundations supporting electrical transmission line towers are subjected to oblique loading due to tower own weight along with wind loading. Due to land limitation and linear alignment of transmission lines, many power transmission towers are situated in sand near sloping grounds and proper evaluation of their foundation performance is essential for the safety and reliability of the critical power transmission lines. There is currently no guidance available on the behaviour of transmission towers foundations constructed near slopes. Thus, the primary objectives of this study are to evaluate the response of transmission tower foundations in cohesionless sloping grounds and investigate the effects of various parameters on their ultimate capacity.
The research methodology consisted of three main components: i) conducting a series of laboratory tests on two types of overhead transmission tower foundations, namely: pad and inclined chimney footing and vertical and batter pier foundations, situated in sand near sloped grounds to evaluate their response under combined loads and comprehensive soil characterization so as to attain experimental results and necessary soil parameters for numerical modeling; ii) developing, calibrating, and validating nonlinear three dimensional (3D) finite element models (FEMs) employing the results of laboratory tests; and iii) performing a comprehensive parametric analysis on the response of both foundations under oblique loads considering a range of ground slope configurations and soil properties using the validated FE models. The investigated parameters include: slope height and inclination, foundation setback distance from the slope edge, load inclination, pier batter inclination, as well as soil strength and relative density. The results of laboratory tests and numerical simulations were analyzed to establish design guidelines for transmission line foundations in sloping grounds. Seven laboratory tests were carried out on pad and inclined chimney footings: three tests under 2 oblique compression loads in sloping grounds, one test under 2 oblique compression loads in a flat ground, and three tests under 2 oblique pullout loads in sloping grounds. In addition, thirteen laboratory tests were conducted on pier foundations: one test under axial compression loads in a flat ground, one test under pure lateral loads in a flat ground, one test under 1 oblique compression loads in a flat ground, five tests under 1 oblique compression loads in sloping grounds, one test under 1 oblique pullout loads in a flat ground, and four tests under 1 oblique pullout loads in sloping grounds.
For pad and inclined chimney footings, both experimental and numerical results revealed that an increase in the slope height and slope inclination and a decrease in the setback distance of the footing from the slope edge result in significant decrease in the footing capacity. It was also found that, as expected, load inclination and lower soil relative density resulted in further reduction of footing ultimate capacity. Finally, the results demonstrated that a setback distance of 2 to 5B eliminated the effect of the slope on the footing ultimate bearing capacity.
The experimental and numerical results of pier foundations revealed that the peak soil pressure and the rotation point occur at depths of and , respectively, below the ground surface in case of pure lateral loading. It was also found that, in case of pure vertical loading, less than 30% of the applied load was transmitted to the soil by the pier shaft resistance, indicating that the pier toe resistance had a significant contribution. The results also indicated that the ultimate capacity of an obliquely loaded pier buried in a flat ground was greater by 17% than that of a vertically loaded pier in a flat ground. Furthermore, the results indicated that the load carrying capacity of vertical and battered piers increase significantly as and increased and decreased. The ultimate capacity of vertical pier () increased as increased due to the significant increase in shaft resistance. For example, the shaft resistance increased from 15% to 30% of pile capacity as increased from 5° to 25°. In addition, the ultimate capacity of battered pier () increased as θ decreased. Finally, the effect of slope on the capacity of both vertical and batter piers vanished at a setback distance of 3 to 10 times the pier diameter, depending on the slope, load, and batter inclinations as well as the soil relative density . This means that the pier capacity reaches that of a pier buried in a flat ground beyond these distances.
Summary for Lay Audience
Transmission towers play an important role to operate and convey electrical power systems, which is known as a lifeline system. Their foundations are critical components since they are responsible for transmitting the loads on conductors and on support structures to the underlain soil. These foundations are occasionally placed near an existing earth slope or near a proposed excavation for the basement construction of other structures due to land limitation and linear alignment of transmission lines. They are also subjected to oblique loads due to tower own weight along with wind loading. In these cases, design requirements state clearly that in addition to transmitting the load safely to the surrounding soil, the slope stability after incorporating the foundation's load must remain intact. There is currently no guidance available on the behaviour of transmission towers foundations constructed in sloped grounds under oblique loads. Accordingly, a series of laboratory tests were carried out to investigate the response of transmission tower foundations built in sloping grounds as well as investigate the effects of various parameters on their ultimate capacity. After that, an engineering software was employed to verify the results of laboratory tests. The results obtained from the engineering software simulation were similar to those obtained from laboratory tests. Consequently, many models were simulated using the engineering software to better understand the performance of transmission towers foundations near slopes under various load conditions. A clear understanding of the performance of such foundations is presented based on the results of the laboratory tests and engineering software simulations. Furthermore, design charts for transmission tower foundations placed near slopes are provided to facilitate design in engineering practice.
Alkahtani, Meshel Q., "Guidelines for designing the overhead transmission tower foundations adjacent to cohesionless slopes" (2021). Electronic Thesis and Dissertation Repository. 8256.
Available for download on Monday, December 25, 2023