Annular space grouting is the process of filling around a pipe installed inside another pipe or a tunnel. To ensure long term stability of the carrier pipe and the ground or any structures above, cementitious grout is used instead of leaving the space empty. Typical utility installations requiring annular grouting include sliplining, and two pass bores or tunnels.
CJGeo uses cellular concrete, a low density cementitious grout, for annular space grouting. It is batched onsite at up to 200CY/hour from bulk cement, pumped thousands of feet at very low pressures (typically less than 20psi). Because it is very lightweight, cellular concrete significantly reduces carrier buoyancy.
Whether an annulus is just an inch, or a few feet, and regardless of host pipe size, cellular concrete is an economical, reliable annular space grouting material. CJGeo has successfully performed annular grouting on hundreds of tunneling and slipline projects, ranging a few cubic yards to more than 10,000 cubic yards, and lengths ranging from 25 feet to 7 miles. Read about some of our projects below.
Typical Annular Space Grouting Mix Design
30lb/cuft wet cast density cellular concrete is CJGeo’s most common mix design. 30PCF provides a compressive strength around 21ksf (150psi). 30lb/cuft cellular grout is batched onsite using dry batch equipment, and pumped thousands of linear feet with ease.
Higher strengths, if required, come with higher density. Density of cellular concrete is infinitely variable between about 20lb/cuft and 90lb/cuft.
Carrier Buoyancy Considerations
Carrier buoyancy is a function of the grout density. Reduced uplift comes from lighter grout. Ideally, each run can be grouted in a single lift. This significantly reduces the risk of short fills or trapped air pockets.
The best place to start is to determine the actual required bearing capacity. Then, apply a reasonable factor of safety (generally 1.5 to 2). Selecting a minimum strength value that has a very high FOS and may induce significant constructibility issues, and heavier grout is more expensive.
Relative Grouting Pressures
It’s a lot easier to move a cubic foot of air than a cubic foot of water. Therefore, grouting pressure during annular space grouting with cellular grout are significantly lower than with using traditional sanded grouts.
You can use the Darcy-Weisbach equation to calculate relative differences in placement pressure between various grout densities. However, it does not take into account internal friction within the grout material, which is nominal with cellular grout, and exceptionally high with sanded grouts, so tends to understate the reduction in placement pressure that comes with using cellular grout.
In general, cellular concrete is placed at less than 20 psi for thousands of feet, at placement rates exceeding 100CY/hour.
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