Deep Excavation and Improvement of Passive Earth Reaction in Weathered Chalk: Design with Deep Soil Mixing or Concrete Cross Wall Solutions-Paper

Paper

Pdf Summary

The Pont de Sèvres station, part of the Grand Paris Express, required a 35 m deep excavation supported by 45 m diaphragm walls anchored in weathered Campanian chalk. Because nearby buildings and utilities were sensitive to ground movement, the design needed to improve passive resistance below excavation level and meet strict wall-deformation limits.<br /><br />Two ground-improvement methods were used: deep soil mixing (DSM) beneath most of the station footprint, and five unreinforced concrete cross walls near a sensitive building. DSM was suitable for the future tunnel-boring machine, but possible column deviations could leave untreated gaps. A probabilistic layout model simulated these deviations and assessed the resulting stiffness. With an estimated 85% treatment coverage, the model predicted an equivalent Young’s modulus of about 1,402 MPa—above the 1,000 MPa design target.<br /><br />The cross walls offered stronger, more reliable support but required different modelling assumptions. Their concrete strength was translated into equivalent soil parameters for both subgrade-reaction and finite-element analyses. The two methods produced similar diaphragm-wall behaviour, and both predicted deformations within the contractual limits: 1.5 cm at the wall top and 3 cm near the excavation bottom.<br /><br />The study presents design approaches for accounting for DSM construction tolerances and for representing concrete cross walls in distinct calculation methods. Wall movements are to be monitored during excavation and compared with the design predictions.

Keywords

Pont de Sèvres

Grand Paris Express

deep excavation

diaphragm walls

weathered Campanian chalk

deep soil mixing

ground improvement

concrete cross walls

wall deformation limits

finite-element analysis