The ever-increasing density of urban environments poses new challenges to tunneling for transit projects. Tunneling methods are called for that meet these challenges while minimizing the impact on every day routines. With only a few exceptions, urban tunneling is confronted with soft ground conditions resulting from two basic facts.
Historically, dwellings were established near rivers for reasons of logistics situating cities on alluvial deposits. In addition, tunnel alignments must be kept at shallow depths to optimize access and passenger circulation. This study analyzes implementation of a binocular tunnel underneath a historic building at low cover using ground freezing for tunnel pre-support and building underpinning. The shallow depth required tunneling through a large number of timber piles arranged in groups that serve as foundations for the steel frame, brick and masonry building.
Tunneling was carried out using a sequenced excavation with shotcrete for tunnel support, commonly referred to as the New Austrian Tunneling Method (NATM). During the tunneling the building remained operational for its mainly commercial purpose. The analysis focuses on the performance of the building foundations, frozen soil and tunnel opening. Observations from the construction-monitoring program are utilized in the development of a numerical model to study the performance of the support system consisting of timber piles, frozen soil and tunnel lining.
Adfreeze strength that develops between the frozen soil and wooden piles provides a significant increase in shear capacity of the pile-frozen soil interface as compared with unfrozen conditions. The analyses suggest that the interconnect between frozen soil and pile is of such structural capacity as to allow for a substantial reduction of frozen soil thickness when compared to the one implemented on this project.
This reduction provides for economies in freezing and in mitigation measures such as compensation systems for initial building heave and subsequent settlement due to freeze consolidation. It is further suggested that ground freezing in combination with NATM tunneling offers a methodical concept for tunnel construction through vertical foundation elements. A thorough testing program for adfreeze strength must support its implementation, since adfreeze strength is the primary contribution to the capacity of the hybrid tunnel-frozen ground-pile support system.
Source: University of Maryland
Author: Gall, Vojtech