Soil Stabilization Grouting under a Railway-for-Micro-Tunneling for a Sewer Crossing

Soil stabilization grouting was performed to facilitate tunneling under an existing active railway line. The design consisted of creating a circular annulus of cemented soils around the trajectory of the future sewer tunnel by performing permeation grouting through sleeve-port pipes. Since the original soils investigation data indicated that the soils were well-graded, undisturbed native soils, grout volumes were estimated based upon 20% accessible void space. A single pass with sodium silicate grout was anticipated. The sleeve-port pipes were installed by directional drilling methods. Careful monitoring of the drilling and casing grout injection processes revealed evidence of the presence of extensive fill along the tunnel trajectory. As a result of these observations, water pressure testing was added to the scope of work in order to better define the characteristics of the material to be grouted. The water pressure testing data revealed the material to be grouted had a very high permeability (greater than of 0.1 cm/sec). In order to manage the cost associated with the grouting work, a single pass of cement based suspension grouting was performed prior to the sodium silicate grouting. During the cement based grouting, high takes at low injection pressures were noted and as a direct result of these observations two passes with the sodium silicate grouts were performed. During the tunneling process, direct verification of both the existence of extensive gravel layers and of successful full permeation and stabilization of the high and low permeability soils was provided. The grouting methodology and the volume of grout used differed considerably from the original design. The final volume of grout used corresponded to an accessible void ratio of approximately 40%. The original design was consistent with the soils investigation data, however, if the program had not been modified for the actual ground conditions, it is unlikely that satisfactory ground treatment would have been achieved. The project demonstrates that careful monitoring and analysis of drilling and grouting data and modification of the design based upon these data are necessary components of a successful geotechnical construction process.