Behavior Of Laterally Loaded Micropile Foundation

Micropiles, which are characterized by light construction equipment,excellent performance in bearing capacity and flexible layout, are gradually adopted as transmission tower foundation in soft soil area, but there is no technical design method for reference in the current. Based on field test in Tianjin typical soft soil area, behaviors of laterally loaded micropiles are summarized, the applicability of theory in general pile engineering to micropiles is discussed, finite difference solution is derived and the factors influencing the behaviors of laterally loaded micropiles are studied in all aspects.Adopting Tcchnical Code for Building Pile Foundations (JGJ 94-2008) to calculate the lateral bearing capacity of the foundation pile, it is founded that the results agree with data from micro-pile lateral load tests, but the ratio of the code rusult to experimental result is reversely related to the pile number and decreases to 0.81 as the pile number increases to 4×4. Experimental data shows that the effect coefficient of goup pile varies from 1.02 to 1.37, which is a good proof of the excellent bearing capacity. Backstepping results show that the p-y curve in the test area is a softening type and is similar to Duncan-Chang model in which the initial tangent stiffness and ultimate subgrade reaction is positively correlated to depth which is related to confining pressure.Finite Differential Method (FDM) written in MatLab languag is used to study the response of pile under lateral load, which makes it easy to take into consideration of the difference in subgrade response model, layered soil and the variation of flexural rigidity and diameter along the pile. Pile length, flexural rigidity, pile head constraint condition and the property of the subgrade soil are all nonliearly related to the response of pile under lateral load and there is always a critical state value. Pile length is inversely correlated to pile head deflection and positively correlated to the maximum bending moment, and the response is the same as infinite long pile when the pile length reaches 17.1 times of pile diameter which is the critical pile length. The length of pile locally strengthened with flexural rigidity are positively correlated to the maximum bending moment and negatively to pile head deflection, and they stabilize when the length reaches 10 times of pile diameter. The pile head deflection and maximum bending moment are positively correlated to the thickness of the hard shell, and they stabilize when the thickness of the hard shell increases to 5 times of pile diameter. Pile head constraint conditions have a maked effect on response of pile under lateral load. There is a 61.80% decrease in pile head deflection, a 20.54% increase in maximum bending moment in absolute value when the pile head constraint condition changes from free to full constrained. The intercalated layer mainly has effect on pile response when its depth is in 8 tiems of pile diameter. There is a single-valley function relationship between the pile head lateral deflection and intercalated layer depth, in which the pile head deflection reaches the valley value at the intercalated layer depth of 2 times of pile diameter; There is a sin or cos relationship between the maximum bending moment and the intercalated layer depth, in which the maximum bending moment reaches extremum values at the intercalated layer depth of 2 and 5 times of pile diameter respectively.