Performance of micropiles under axial tensile loading

Abstract

This thesis focuses on the behaviour of micropiles under axial tensile loading and modelling of the pile head displacement of micropiles. In the behaviour of micropiles, defined in load and corresponding pile head displacement, the development of the shear stress along the pile shaft is found to be of leading influence. This development of the shear stresses determines not only the lengthening of the pile during the whole loading process, but also the displacement of the pile tip. Due to the compressibility of the slender foundation pile the shear stress acting at the pile tip will only develop with high loads on top. Also a closer look is given to the pile stiffness. The contribution of the grout to the pile stiffness is found to be only of small influence on the pile head displacement and corresponding axial spring stiffness. However it is found that the crack width in the grout due to the tension load on top is relatively big and this might give problems with corrosion. The model to calculate the pile head displacement and correspond axial spring stiffness of the micropile developed in this thesis is called the Revised FOREVER model. It is based on the current Dutch method to determine shear stresses and maximum bearing capacity. The development of the shear stresses and displacement along the pile shaft is modelled using load transfer curves. Because the soil stress-strain relation is assumed to behave linear elastic-perfectly plastic along the pile shaft, the calculations can be done analytically. Due to a scaling factor which implements pile stiffness, shaft area, soil behaviour and bond length the model can be adapted different local situations. The model is compared with measurements from field tests and with other numerical models and the results correspondence with each other. Due to the scaling factor implementing all local parameters it was possible to simplify the calculation method. Therefore the Simplified Revised FOREVER model is developed. This is based on the current Dutch calculation method to determine the axial spring stiffness of tension piles.