Microtunnel Annular Space Grouting

Charlotte, NC

The Job

This microtunnel annular space grouting project is part of a massive interstate expansion project in Charlotte, North Carolina. Seven micro tunnels were mined under I-77 to convey various utilities below the highway. Microtunneling was used, with casing diameters of 60″, along with a hand-mined tunnel lined with an 84″ tunnel liner plate.

Due to the length of runs, volumes required and strict NCDOT mix design requirements, the tunneling contractor reached out to CJGeo to explore cellular concrete as an option for the microtunnel annular space grouting.

Concrete truck and concrete pump on a microtunnel annular space grouting project.

The Challenge

CJGeo proposed performing the annular space grouting using 30lb/cuft cellular concrete. CJGeo has approved NCDOT mix designs for cellular concrete, which helped facilitate expedited submittal processing.

Cellular concrete was also advantageous from a carrier buoyancy perspective. The carrier pipes installed in each of the tunnels were all negatively buoyant in the grout. This helps to ensure proper alignment and minimize any chances of damage during grouting.  

The Microtunnel Annular Space Grouting Solution

During the first mobilization, CJGeo placed 630CY of cellular concrete to grout the two tunnels over a period of three days. The 60″ tunnel was grouted in a single lift, the 84″ tunnel was grouted in two lifts.  

Confirmation of complete fill was through venting of uniform cellular concrete at the far ends of each tunnel.  Additionally, the 84″ tunnel liner plate structure had cellular concrete weeping out of the bulkhead on the exterior of the plate. This indicates that the cellular concrete had exfiltrated through the liner plate joints and was flowing outside of the liner plate.

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