Field test on uplift behavior of micropiles in soft ground

Micropiles have the virtue of flexible layout, economical construction, environmental protection and machinery miniaturization. These piles are especially suitable for transmission tower foundation due to inconvenient traffic, higher environmental requirement and complex geological condition. Being subjected to pullout is one of main functions of transmission tower foundation, but there are few researches on the characteristics of micropiles pullout behavior in soft ground. By the pullout tests of single micropile and group piles in soft ground, the shaft and lateral friction force of single piles are measured, and the influences of construction technology on uplift capacity of single micropile, the characteristics of load-displacement of micropiles and the coefficient of group effect and so on are studied. The test results indicate that the compaction of soil from twice pressure groutings can raise the ultimate uplift capacity of micropiles obviously and reduce the displacement effectively. For twice pressure groutings can reinforce the soil around the piles, the load-displacement curve of group piles appears “gradually varying type”, the measured coefficient of group effect is relatively small, and the group effect is obvious. These findings have reference value for designing transmission tower foundation in soft ground.