Design of substructures for transmission towers

A study was made of the influence of underream diameter, depth of underream and angle of pull on the ultimate capacity of drilled and underreamed footings subjected to uplift. Discussion of present methods of design recommended by other investigators in the field are presented and reviewed. Field uplift tests were performed on underreamed footings constructed in sand and clay soils in the north central area of Texas. Seven footings were constructed in the sand area while in the clay area a total of seventeen footings were constructed and tested to failure. Laboratory determinations of the unit weights and strength characteristics of the soils were made to determine their engineering properties thought to be of value in the interpretation of the uplift tests. Photoelastic model tests were performed to delineate the nature of failure as well as the magnitude of the stresses involved. Evaluation of the field results and laboratory test data indicates that the uplift resistance of underreamed footings in stiff to hard clays is the sum of the following: (a) The upward bearing on the horizontal projection of the underream. (b) Adhesion along the shaft. (c) A quantity which is a function of soil overburden on the underream, lateral pressure, coefficient of earth pressure at rest. (d) Weight of the footing. The uplift bearing capacity and shaft resistance are both functions of adhesion and strength. The depth factor considers primarily the weight of the in-place soil above the bottom of the footing. The weight of the soil affects confinement and bearing, but it is not fully lifted before failure