Theoretical investigation of restraint effect of isolation piles on vertical ground displacements due to tunneling under the plane state

As an efficient protective measure, the isolation piles are widely used in the control of tunneling-induced environmental effects in urban areas. Based on the two-stage concept, a vertical pile-soil interaction model that can consider the relative slip between the isolation pile and the soil (simulated by the pile shaft spring and the pile tip spring) is established. Based on this model and introducing the displacement compatibility condition, using the modified Loganathan-Poulos (L&P) formula and the Melan solution of the vertical displacement in a general form, the interaction forces between the isolation pile and the soil (i.e., the internal force of the springs) are solved. Using the superposition principle, the ground vertical displacements caused by tunnel excavation under the restraint of isolation piles are further solved. The results are verified by comparing with those of the existing analytical methods and field measured data. Based on the analytical results of the proposed model, the mechanical mechanism of the effects of the isolation piles on the ground vertical displacements is investigated. The studies have shown that the restraint effect of the isolation piles on soil can be divided into two parts, upward part and downward part. The combination of the upward and downward effects together drives the tunneling-induced ground vertical displacements along the depth direction to change from non-uniform state to a relatively uniform one. The spring stiffness of the pile shaft and pile tip determines the degreeof interaction between the pile and the soil. The smaller the stiffness, the smaller the interaction force, the greater the relative displacement between the pile and the soil, and the smaller the upward and downward restraint effects. The related research may provide theoretical guidance for the design and effect evaluation of isolation piles.