Experimental, Numerical and Probabilistic Analysis of Reinforced Anchors for Transmission Tower Foundations

Abstract

The foundations of tall structures such as transmission towers are subjected to loads in three directions coming from wind and other environmental events. The use of anchors for application in transmission tower foundations to resist pullout forces has been investigated in this thesis. To this end, experimental and numerical analyses were conducted to investigate the influence of geogrid reinforcement and other design factors on the pullout capacity of inclined and horizontal anchors. In addition, probabilistic study is conducted to analyze the influence of soil, load, and seismic variability on the performance of the anchors. Laboratory experiments were conducted on inclined anchors placed in geogrid-reinforced sand. The results indicate that the presence of geogrid improves the pullout response of the anchors. The other design parameters, such as reinforcement width, embedment depth, and inclination angle, were also found to have a considerable effect on the anchor response. Next, a three-dimensional numerical analysis was performed to study the influence of other parameters, such as the geogrid stiffness, anchor width, higher embedment depths and friction angle, on the response of reinforced anchors under pullout forces. An increase in the width and the embedment depth of the anchor had a diminishing impact on the improvement factor. An illustrative design example of reinforced anchors for a transmission tower foundation under wind loads is presented to bring out the benefits of geogrid reinforcement to minimize the foundation’s displacements and reduce construction costs. Next, numerical analysis on horizontal anchor foundations under combined loads was carried out using a novel three-dimensional geogrid reinforcement model. A parametric study was then carried out to investigate the influence of combined uplift-lateral loads on the failure mechanisms and the displacements of the foundation. Two distinct failure modes, vertical wedge failure and rotational failure, were observed under pure uplift and lateral forces, respectively. Failure locus was proposed for unreinforced anchor foundations under combined uplift-lateral loading conditions. The use of geogrid reinforcement was found to reduce the anchor displacements significantly by distributing the pressure over a wider region. Displacement reduction factors were introduced to assess the performance improvement of the foundation in reinforced soil. Maximum reduction in the foundation displacement in presence of reinforcement was found at 75° inclination. Reliability analysis was carried out to assess the influence of the inherent soil and load variabilities on the performance of anchors under a probabilistic framework by employing the conventional random variable and the advanced random field approaches. The influence of COV values of the soil and wind load properties and the correlation length of the spatially varying soil was investigated, and a sensitivity analysis was performed. The stochastic seismic analysis employed in the thesis explored the response of anchor foundations under combined loads in spatially varying soils and variable ground motions. The displacements of the foundation significantly increased with applied loads, while higher relative density reduced the displacements in both horizontal and vertical directions. The results indicated a considerable impact of soil spatial variability and variable ground motions on the foundation’s performance, with vertical displacements primarily influenced by soil variability and horizontal displacements more affected by the random seismic excitations.