Assessment of the Posttunneling Safety Factor of Piles under Drained Soil Conditions

The need to tunnel closely beneath piles is increasing due to the development of urban areas. This poses a risk to the stability and serviceability of overlying structures (e.g., buildings, piers, and piled embankments). The impact of tunneling on piles is usually assessed using a displacement threshold, yet this provides no information about the posttunneling pile safety factor. Knowledge of a pile’s safety factor under serviceability or extreme loading conditions is important, especially if future repurposing of the associated superstructure is a possibility. Tunneling can reduce the safety factor of a pile up to the point of geotechnical failure (i.e., when the pile capacity reduces to that of the applied load), yet little guidance is available to enable a straightforward means of assessing the posttunneling safety factor of a pile. This paper aims to address this shortcoming by providing design charts based on an analytical tunnel-single pile interaction approach that provides a means of determining a posttunneling pile safety factor. The methodology and design charts are applicable to drained soil conditions and include for the effects of the initial pile safety factor, the pile installation method [displacement (driven and jacked), nondisplacement (bored) with only the shaft capacity, and nondisplacement with base and shaft capacity] and varying water table depths. In the paper, as a validation exercise, analytical predictions are compared against data from geotechnical centrifuge tests designed to model both displacement and nondisplacement piles in sands, including a variety of tunnel–pile relative locations and initial pile safety factors. For a specified design value of a posttunneling pile safety factor, the design charts enable a quick assessment of the safe location of a pile or a tolerable tunnel volume loss considering ground parameters, water table position, pile installation method, and initial safety factor.