Responses of a short column-supported highrise tower to adjacent deep excavations in water-rich sandy strata and dynamic optimization of protection plans

A highrise tower atop short columns in Nantong, China was threatened by excavation of a subway station nearby. Although an elaborate protection plan composed of isolation piles, artificial recharge and underpinning was executed throughout the excavations, the tower underwent unacceptable settlements and notable inclinations. In combination of field measurements and numerical simulations, this paper investigates the tower’s responses to the adjacent excavations, examines the effects of adopted protection plans and explores potential effective protection plans. First, the responses of the tower and the effectiveness of the three implemented measures were examined, and the contributory factors triggering intolerable tower deformations were identified; then, the effects of primary protection parameters were quantified, including the length, stiffness and layout of isolation piles, the water level surrounding recharge wells after recharging and the depth and location of wells, and the length of underpinning piles. It reveals that the underpinning plan had the best protection effect, followed by isolation piles and recharging wells. Construction timing of protection measures and termination manners of recharging are two critical factors in restraining tower deformations. Moreover, underpinning the tower with 36-m long steel pipe piles solely before implementation of adjacent excavations could be another optimal protection scheme.