Tampa Microtunnel Annular Space Grouting

Tunnel backfill annular space grouting with cellular concrete by CJGeo in Tampa, Florida.Challenge:

As part of a large design build utility rehabilitation and capacity improvement project, a 60″ micro tunnel was dug under an interstate.  The tunnel was approximately 40 feet below grade, and 1050LF.  Due to the length of the tunnel, traditional grouts were unlikely to be successfully placed for the annular space grouting.  Concerns with traditional grouts included flowability, carrier pipe buoyancy and high pumping pressures, compounded by a local shortage of flyash, which was a critical component of the planned grout.

The tunneling contractor reached out to CJGeo to see if cellular concrete would be a feasible product for the annular space grouting.  The project was located near Tampa, Florida.

Annular space grouting project in Tampa, Florida for backfilling a micro tunnel using cellular concrete.The Solution:

CJGeo confined that cellular concrete would be an excellent alternative to the problematic grout that was originally planned.  To meet the 1,000psi requirement at 28 days, CJGeo proposed a 60lb/cuft neat mix cellular concrete, generated using Aerlite preformed foam, and continuous generation.  

Cellular concrete provided a few key benefits for this project:

  • material supply–cellular concrete does not require fly ash in order to be highly mobile and pumpable
  • low unit weight–by being less dense than water, cellular concrete does not cause the water-filled carrier to be buoyant
  • flowability–cellular concrete is easily pumped at low pressure for long distances
  • economics–the 400CY project volume could be placed in two days instead of eight

Result:

CJGeo mobilized a cellular concrete crew to the site.  Using continuous generation and slurry from a local ready mix supplier, CJGeo grouted the annulus completely.  Completeness of fill was verified by uniform material venting from each of the vent and placement pipes at both ends of the tunnel.

Peak pumping pressure during grouting was approximately 8psi when grouted through the annulus.  Up to 400LF of 3″ sacrificial grouting pipe had been run in from each end.  These sacrificial pipes were used for both venting and placement during grouting operations.