Project: Black Mountain Pipeline 3

Application: Annular Space Backfill

Owner: San Diego County Water Authority

Installed: 2,350 CY of Foam Concrete

A significant amount of corrosion and deterioration was found in the SDCWA's large diameter water transmission pipelines. Originally installed in the early 1900s, the aging infrastructure was in serious need of repair in order to extend the service life of the Water Authority's existing pre-stressed concrete cylinder pipelines and ensure the quality and availability of drinking water for San Diego residents.

SDCWA had two options -- either repair or replace the pipelines. The relining repair method was ultimately chosen because it offered substantial cost savings over the replacement option. Additionally, since most of the work would be completed underground, repairing the pipelines also minimized disruption to the community.

Repairing the pipelines involved inserting steel liner carrier pipes inside the deteriorating concrete pipelines. Unlike pipeline replacement, this eliminated the need to dig a trench. However, to protect the carrier pipes against movement and corrosion, the annular space between the new and old pipes needed to be grouted.

This annular space backfill application required a very flowable material to ensure there would be no voids left in the 1.5” annulus around the pipe. The material also had to be very pumpable since the average pumping distance was more than 1,500 linear feet and the specification required pumping pressures of less than 3 PSI at the point of placement.

MixOnSite used its dry mix equipment to produce a foam concrete material averaging 50 PCF and 500 PSI and successfully backfilled approximately 21,500' of relined pipeline through only six access shafts. The lightweight nature of the foam concrete also reduced the risk of pipe floatation and deflection/collapse.