Underground construction in areas of high groundwater inflow necessitates the installation of standpipes to provide a secure wellhead for both groundwater control and high-pressure preexcavation grouting. The common method of standpipe installation is to drill a short pilot hole into which a standpipe with centralizers is inserted and subsequently grouted in place. Once the grout is fully cured, the standpipes can be pressure tested according to project specifications. If the pressure test
is successful, drilling can be advanced through the standpipes as planned. However, if the pressure test fails, the standpipes must either be re-grouted and re-tested; or abandoned and replaced with a new standpipe and the drilling/grouting/testing procedure is repeated. The time required for grout curing results in non-productive time (NPT) that can be significant, depending on the type of grout applied.
The grouting process can be particularly challenging in horizontal and up-angled holes that are common in underground construction.
A new grout-free method of standpipe installation has been developed that can significantly reduce NPT associated with grouted standpipes. The method utilizes a steel pipe that is covered by a thin (6mm) elastomeric cover and is expanded using a compact hydraulic setting tool based on high-pressure inflatable packer technology. The expanded standpipe is self-centralizing, and the elastomeric cover provides an effective seal and anchor conforming to the borehole wall. Once placed in the pilot borehole, the expansion process takes approximately 3 minutes, after which the standpipe can be pressure tested immediately. A pilot project has been conducted in which standpipes were installed in an advancing tunnel. The grout-free expandable standpipes were pressure tested to 83 bar directly after installation and subsequently utilized for pre-excavation grouting. In contrast, in this particular project,
grouted standpipes required 24hr cure time prior to pressure testing.