Soil Improvement Thanks to an Extra-Long Application of Directional Drilling

The paper deals with a soil improvement by grouting carried out through tubes a manchettes of extraordinary length, installed inside HDD boreholes. As part of the Warsaw Subway Line No. 2 project, two twin TBM tunnels were required to cross under historical buildings. In order to mitigate the risk of settlements, a preliminary soil improvement was designed. Due to the particular layout of the buildings, an extreme solution was conceived, designed and applied. Starting from the surface, boreholes from 210m to 260m long were drilled by HDD technology. Each tunnel was fitted at its crown with 24 grouting pipes around the outside of the tunnel section to be bored. Grouting was performed by selective and repeated procedure, injecting first a high penetrability cement-based grout and then a silicate-based chemical mixture.