Controlling tunnel induced ground surface and pile movements using micropiles

Abstract

Tunnelling in densely populated areas is generally associated with undesirable ground movement and subsequent damage to adjacent buildings. Hence, the main concern of designers are to accurately predict ground movements and propose mitigation measures in severe cases. Nowadays, different techniques are used as a mitigation measure to reduce the impact of tunnel construction on ground settlement. Nevertheless, implementation of some of these methods is a source of unpredictable damage or undesirable effects such as the effect of installing micropiles between existing pile building foundation and tunnel which have yet to be understood. Hence, this research aims to establish a micropiles method as a mean to minimise ground surface settlement, and the settlement and lateral movement of the existing pile due to tunnelling through cohesionless soils. The study was carried out by means of laboratory physical model tests and numerical simulation using ABAQUS software. Three different relative densities of sand; 30%, 50%, and 75% were investigated while the overburden (cover to diameter) ratios used were 1, 2, and 3. A row of 3.7 mm diameter micropiles, dmp with two different lengths (11 cm and 14.5 cm) was embedded in between the tunnel (5 cm diameter, D) and the existing pile at four different locations. In model tests, settlement, bending moment and axial force of the existing pile were monitored accordingly. Generally, the results showed that increasing the value of relative density of sand reduces the ground movements. However, shallow tunnelling in loose sand produces remarkable movement on the ground surface. With the usage of micropiles, the ground surface settlement was reduced to nearly 40%. The micropiles also reduced over 85% and 75% of the piles lateral and axial movements respectively. A good compatibility was found between the experimental and numerical approaches which illustrates that the presented numerical simulation is a reliable model to predict tunnel-pile-soil and tunnel-pile-soil-micropiles interactions. Within the limitation of the study, it is recommended that the most suitable length and location of micropiles to use is 14.5 cm or about 40dmp (closest to the tunnel crown) and located at 0.5D (in the middle between tunnel and pile), based on the reduction observed on the vertical and lateral movements of pile as well as the bending moment and axial force.