A risk assessment of downdrag induced by reconsolidation of clays after upwards pipe jacking

TL;DR

This paper assesses the risk of downdrag caused by reconsolidation of clays after upward pipe jacking, focusing on force and displacement estimation, and compares model predictions with field measurements.

Contribution

It introduces a comprehensive risk assessment methodology for downdrag effects in pipe jacking projects involving soft clays, validated by prototype model data.

Findings

Negative skin friction effects can be significant in soft clay reconsolidation.

Early installation of riser-tunnel unions may influence downdrag forces.

Model results align well with field measurements, validating the approach.

Abstract

Salini-Impregilo is building part of the largest sanitary sewer system in the history of Argentina in the suburbs of Buenos Aires City, to serve a population of almost five million people. The project is an outfall TBM tunnel 12 km long, starting from a reception shaft in the river margin, and transporting the sewage 35 meters below the \textit{Rio de la Plata} riverbed to the point of discharge. Within the final kilometer of the tunnel, a set of 36 standing pipes so-called \textit{risers} are constructed by driving steel tubes upwards and passing through dense sands, sandy clays and soft clays. Risers are linked-up with the launching lining segment using flange unions. Driving of risers upwards will generate excess pore pressure and disturbance in fine soils and, once the pipe is placed in its final position, negative skin friction due to reconsolidation and creep. A risk assessment of the downdrag is presented in this paper, based on the estimation of the force and/or displacement in the riser-tunnel union generated by this effect. The issues of whether it is desirable to instalock the riser-tunnel union at an early age after installation of the riser and the time lapse required to reduce negative skin friction effects are discussed. Results are validated by comparing the model results with field measurements in prototype models.