Abstract: Base enlargement is a widely used method to improve uplift performance of digged foundations of transmission towers. The portion of concrete amount of enlarges base increases with the increasing of embeddment and bell diameter of tower foundation. Furthermore, base enlargement is difficult and dangerous during construction. Taguchi method is applied to investigate effect of structural parameters of enlarged base on uplift load bearing capacity of the digged foundations. The selected structural parameters are embedment to bell diameter ratio, angle of the conical surface against the vertical and shaft diamter. A orthogonal array is selected and 9 digged foundations are designed according to Taguchi method with those three factors and corresponding three levels. Thirty-six field tests are performed at four sites in Gansu province including three Gobi gravel sites and one loess site. Analysis of signal-to-noise(S/N) ratio and analysis of variance(ANOV) methods are used to evaluate testing results. The results show that soil strength is the decisive factors for ultimate uplift capactiy of digged foundations for transmission towers. The uplift performance improves when each of three structural parameters mentioned above increases. However, sensitivities of these factors are different for the foundations embedded in different grounds. In Gobi gravel, the senstivities in ascending order are embedment to bell diameter ratio, shaft diamter, and angle of the conical surface against the vertical. In loess soil, senstivities for the same sequence are in descending order.

Key words: transmission line, tower foundation, digged foundation, uplift, enlarged base, Taguchi method, orthogonal experimental test, load-displacment curve, analysis of variance