TABLE OF CONTENTS The design and use of helical anchors was briefly discussed in Chapter 1 (Section 1.4). Figures 1-8 and 1-9 show photographs of helical anchors with single and dual helices, which are generally used for light to medium loads. However, at the present time, tapered multi-helix anchors (3 to 4 helices) are commonly used to carry uplift loads up to about 550 kN. Figure 5-1 shows the typical dimensions of a multi-helix anchor used in the United States for construction of foundations of electrical transmission towers. These anchors are fairly easy to install and, hence, are cost effective.
At the present time only a few studies are available, the results of which can be used to estimate the ultimate uplift capacity of helical anchors. In many instances the ultimate load estimate is based on the rule of thumb. This chapter summarizes the existing theories relating to the prediction of the net ultimate uplift capacity of tapered multi-helix anchors embedded in sandy and clayey soils
Figure 5-2 shows a tapered multi-helix anchor embedded in soil subjected to a vertical uplifting force. The diameter of the top helix is D 1 and that of the bottom helix is D n. The distance between the ground surface and the top helix is H 1 and, similarly, the distance between the bottom helix and the ground surface is H n The...
