Extensive Pipeline Field Studies and Inspections Aid in Fastpaced CMAR Project

Abstract

Cross-County Water Supply Corporation (CCWSC) was formed in 2008 to transport up to 20,000 acre-feet of water from Burleson County to the growing area northeast of Austin, Texas. The project included the design and construction of a 53-mile, 30-inch water transmission pipeline, high-service pump station and customer interconnects. In February of 2010, approval for final design, permitting, and construction of the project was approved by the Board. The project was awarded as a CMAR contract with Garney Construction. Within 15 months, the team completed final design and constructed the new water supply system which, at ultimate build-out, will have the capability of delivering 18-MGD. The project included many unique challenges during the CMAR design and construction process. This included extensive geotechnical and corrosion studies to optimize the backfill design and determine soil characteristics and cathodic protection requirements. This also included a hydraulic analysis that accomplished a maximum capacity with phased expansion approach in order to meet a phased start-up system. In addition, permits with agencies were required that spanned over five counties and sequenced with multiple design packages in order to allow for concurrent construction. The project alignment spanned over 53 miles from Burleson County to Travis County. Due to the geology in the eastern portion of the project, the project design was able to utilize native material for backfill. To locate the native material that was suitable for reuse, the geotechnical study commenced in February 2010. Fifty bores were drilled along the 53-mile pipeline alignment. The results of the bores showed that the soils on the western portion of the project are mostly clays and unsuitable for reuse. The soils on the eastern portion of the project are clayey-sands and, in many locations, suitable for use in the pipeline backfill. To determine exactly where the native soils would be used as backfill, a secondary geotechnical study was conducted in which the contractor used a backhoe to dig test pits at 26 locations. The soils in these test pits were field analyzed by a geologist who then made the final decision as to whether or not the soils met the backfill requirements. The results obtained from the initial set of borings were also used in the corrosion study. Based on these bores, it was determined that portions of the pipeline did span soils where cathodic protection would be considered. The project design included nearly 200 test stations and pipe continuity bonding being installed along the pipeline and the protocol for a future corrosion monitoring program and cathodic protection system. This project is an example of using extensive geotechnical field studies and field inspections to determine backfill requirements for a long distance transmission main project. This paper will provide details of a CMAR design and construction project, including the selection of a highly qualified contactor and schedule and review coordination efforts.