TABLE OF CONTENTS The foundation is the name given to the system which transfers to the ground the various steady state (dead) and variable (live) loads developed by the tower and conductors. Foundations may be variously subjected to compressive or bearing forces, uplift and shear forces, either singly or as a result of any combination of two or three of the forces.
Usually, the limiting design load with transmission line foundations is the uplift load. In this respect, there is a major difference between the design of foundations for transmission lines compared to the design of foundations for most normal civil engineering structures. Accordingly, the amount of literature describing design techniques for overhead line foundations is relatively small compared to the literature available for more traditional civil engineering foundation design practice.
The selected foundation design for a particular tower must provide an economical, reliable support for the life of the line. The foundation must be compatible with the soil and must not lose strength with age. With the progressive increase in transmission system voltages there has been a related increase in foundation sizes and it is worth noting that with a typical quad conductor 500 kV line, single leg uplift and ultimate compression loads of 70 or 80 tonnes are usual for suspension towers. With tension towers, ultimate loads of 200 or 300 tonnes are often developed. In ground of poor load-bearing capacity the dimensions of foundations become considerable.
In the past, it was often acceptable to 'over-design' foundations...
