TABLE OF CONTENTS The design of overhead line tower or substation gantry structure foundations must be such as to safely sustain and transmit to the ground the combined dead load, imposed load and wind load in such a manner as not to cause any settlement or movement which would impair the stability of the structure or cause damage. The settlement is a result of the transfer of load from the structure to the soil layers. Essentially settlement must be minimized to an acceptable level for the design life of the structure and adequate factors of safety applied to ensure this. Foundation design requires information on the properties of the soil and in particular its compressibility, moisture content, plasticity characteristics, friction between soil particles and for fine soils its undrained shear strength. This chapter describes typical soil investigations and foundation design. Such design is the responsibility of the civil engineer. The details described in this chapter are intended to give the transmission and distribution electrical engineer an appreciation of the factors involved.
Ground investigations are carried out by geotechnical experts using boreholes, trial pits and penetrometer tests.
Investigations take the form of in situ and laboratory tests. In situ tests include standard penetration tests to provide data on the relative density to sand for the more coarse-grained soils.
Laboratory investigations on soil samples taken from boreholes or trial pits will measure grain size, density, shear strength, compressibility, chemical composition and moisture content such that the soil can...
