Modification of clay adhesion to improve tunnelling excavation

Mechanical tunnel driving by means of tunnel boring machines through clayey soils often leads to clogging problems which cause delays in tunnel construction and economic issues. Taking into account the diffuse double-layer theory, two very well-known clay formations, namely the Ypresian and Boom clays were investigated. Several geotechnical tests and a new adhesion test (cone pull-out test) were performed using water and solutions with different sodium chloride concentration as pore fluids. The results showed a reduction of the liquid limit for both clays when salt solutions were used instead of water. Undrained shear strength of the Ypresian clay progressively increased as the sodium chloride concentration of the pore fluid increased, whereas that of the Boom clay, which contains less smectite, was less sensitive to the chemical composition of the pore fluids. Cone pull-out tests, developed to detect the adhesive properties of clays, showed a drastic drop of clogging for the Ypresian clay when mixed with sodium chloride solutions within the entire consistency range studied, whereas in the case of Boom clay the variation was less evident and a clear effect was only observed at low consistencies. The results are promising for improving tunnelling excavation performance by adding sodium chloride solutions to avoid problems derived from adhesion of clayey soils.