3D numerical study of tunnel-soil-pile interaction

Abstract

In dense urban environments such as Singapore, tunneling activity is becoming increasingly commonplace to alleviate surface related congestion problems. However, as piled foundations and existing service utilities are frequently encountered below the ground surface in such areas, tunnel alignments are being left with few alternatives but to excavate within close proximity of these obstructions. It is therefore inevitable that some form of tunnel-soil-structure interaction will develop as the zone of influence caused by tunneling induced ground movements overlaps with the existing position of these structures or services. To date, there is limited information on how piles react to tunneling induced ground movements. Field cases where horizontal tunnel to pile centre distances of less than one tunnel diameter are becoming increasingly common without fully understanding the possible structural distress that could result from the associated tunnel-soil-pile interaction. Instrumented piles supporting viaduct pier 14 along the North East Line tunnel (Singapore) recorded significant induced bending moments and axial forces where corresponding magnitudes reached 60% and 91% of the design working load, respectively. Tunnel volume loss along the pier section was calculated to be less than 2%, thus providing an indication of the small ground movements required to induce potentially damaging stresses on piled foundations. In this study, a series of 3D finite element simulations were performed to investigate the influence of tunneling induced ground movements on existing piled foundations. Soil convergence around the tunnel excavation was modeled using a kinematic method. Results show that for the defined mesh configurations, induced bending moments are generally negligible beyond a pile horizontal offset from tunnel centre (X) greater than 2 tunnel diameters (Dt) while pile cracking moment (Mcr) is easily exceeded with small tunnel volume loss for X6 1Dt. In addition, induced pile axial force is primarily dependant on (i) position of pile tip relative to ''zone of large displacements'', (ii) soil stiffness and (iii) magnitude of volume loss. For tunnel excavations in clays, the zone of large displacements is generally enveloped by symmetrical boundaries extending upwards at an angle of 45 degrees to the horizontal from the tunnel springline to the ground surface. Back analysis of a field case history in Singapore revealed encouraging agreement between computed results and recorded data. (A) Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.