Cellular Concrete Pipe Abandonment

Cellular Concrete Pipe Abandonment, Winston-Salem, North Carolina

Problem:

Collapsed sewer main at a wastewater treatment plant needed to be abandoned in place.

Constraints:

There was no way to inspect the collapsed pipe, or determine the volume of surrounding soil which had washed away as part of the collapse. The pipe was approximately 30 feet below grade, and passed under multiple structures and a street. Because the pipe was so compromised, complete dewatering was not possible, and it had to be assumed that flow from one end to the other would be severely constricted.

Cellular concrete installation for pipe abandonment of collapsed sewer main at a treatment facility.

Pipe abandonment at sewer plant, using cellular concrete.

Solution:

Cellular concrete installation to abandon the pipe. Cellular concrete density was chosen to be able to displace water out of pipe and surrounding voids during abandonment. Unlike cementitious grouts or flowable fill, cellular concrete is only comprised of cement and water. The air bubbles introduced to the mix also act to improve the flowability of the mix, making it ideal for passing through constricted spaces at high volumes without plugging.

The alternative would have been to use traditional flowable fill material.  In order to ensure complete fill using flowable fill, multiple cased well points would have to have been drilled down to intercept the pipe to grout it.  Since the extent of the collapsed area wasn’t known, this could still have resulted in leaving unfilled pockets, which wasn’t an acceptable risk.

Water was still flowing through the pipe, so there was little risk in designing the abandonment around the assumption that cellular concrete could also flow through the pipe.  This is due to cellular concrete’s 11+ inch slump.  Cellular concrete is naturally highly flowable.  To achieve the same flowability, flowable fill has to have such high water content that the solids act as a suspension, leaving them prone to segregation and loss of strength. 

Results:

CJGeo cellular concrete crew installed approximately 140 cubic yards of lightweight cellular concrete to completely abandon the pipe. As part of the cellular concrete installation, the cellular concrete displaced the water from the pipe. After the cellular concrete was placed, construction activities over the affected area were able to resume.

Completeness of fill was confirmed by uniform mix showing at the outlet end of the pipe, from a single point placement at the upstream end.