Model Tests And Theory Analysis Of Enlarged Piers For Transmission Line Foundations In Unsaturated Soils

Enlarged base piles are widely used as the foundation of transmission towers in hilly areas because of their easy construction and high ultimate uplift capacity. Although they are usually built in unsaturated soil, the effect of soil saturation on their uplift behavior is still unknown. In this study, two series of large-scale model tests, one in saturated silt and the other in unsaturated silt, were conducted in a big steel model box. Each series consisted of 4 up-lifting tests with 4 different embedded depths. From the test results, it was found that the ultimate uplift capacity decreased with soil saturation and embedded depth and the failure pattern of the soil around the piles changed from inverted-truncated-cone slip surface to local failure around the enlarged base with the increase of embedded depth.For the belled piers constructed in the soil beds with variation of degree of saturation, their uplift behavior was not only influenced by the mechanical parameters of the soil but also influenced by the changing of environment including raining, varying of ground water level etc. PLAXIS was used to calculate the results of large-scale tests of belled piers. Then the influence of mechanical parameters of the soil and degree of saturation on the uplift behavior of belled piers was investigated. The variation of the uplift behavior caused by raining and the change of ground water level was also studied. It is found that different influences of mechanical parameters of the soil were found on the belled piers with different embedment depth. Degree of saturation has a significant effect on the uplift behavior of belled piers. The influence of raining was more obvious on the belled piers with shallow embedment, while the variation of ground water level has a more obvious influence on the belled piers with deep embedment.Finally, a half empirical formula which can consider the influence of environment changing on the uplift capacity was formed and charts for Hand calculation was given.