Case study 1: Boulder Creek sewer interceptor replacement
Owner: City of Redding, Calif., Public Works Department
Options: Bursting and reaming with HDPE or fusible PVC to replace 2,500 feet of 15-inch vitrified clay pipe with 25.5-inch pipe.
Engineer's estimate: $2,380,255
Winning bid: $1,493,078
Located in Northern California about 200 miles north of state capital Sacramento on I-5, the City of Redding owns and operates more than 450 miles of gravity and pressurized waste-water collection system infrastructure.
As part of the master planning process, the public works department's wastewater utility had assessed and identified capital improvements using a risk-of-failure methodology. Because the system's experienced high levels of inflow and infiltration, capacity (utilizing a calibrated hydraulic model) was the key probability criteria driving improvement prioritization.
The utility's hydraulic model indicated potential overflows and surcharging along the Boulder Creek sewer interceptor during current wet-weather conditions, which put the asset in the highest probability category. Running along sensitive environmental habitat and under four major transportation crossings, it also made the highest consequence category.
Managers made replacing the interceptor one of their highest priorities. Eventually, 4 miles of 6- to 18-inch vitrified clay (VCP) and asbestos clay (AC) pipe will be upsized with 8- to 28-inch pipe to accommodate current flows and future growth in the sewer basin. The project includes 11 creek crossings using various replacement methodologies; three Caltrans and one Union Pacific Railroad crossing using microtunneling or auger-bore-and-jack; and California Environmental Quality Act 401, 402, 404, and 1602 environmental permitting.
Because they couldn't fund the $9 million project up front, managers divided the alignment into three phases.
The first, and the one we're discussing here, is the alignment's furthest downstream reach: replacing about 1 mile of 15- and 18-inch VCP with a minimum 25.5-inch inside diameter (ID) pipe. The 4,000-foot segment includes a significant amount of open country work in what is generally a flood plain. This was conducive to cost-effective open-cut construction, but 1,500 feet traversed very near the creek in wet-lands and sensitive habitat. Given the aggressive upsizing and environmental requirements, pipe-bursting and pipe-reaming with HDPE or fusible PVC (FPVC) were bid competitively.
In addition, 1,000 feet required replacement-in-place due to property procurement and work area constraints. For these segments, open-cut was bid against both trenchless options.
Performance-based specification (PBS) bid documents specifying design criteria, goals, and objectives included HDPE, FPVC, and segmental PVC (consistent with city standards). The engineer's estimate of $2,380,255 was based on an assumption that HDPE would be used for the entire length and trenchless construction would be utilized wherever feasible, but the work was awarded for $890,000 less based on comparison of average bid to engineer's estimate for four reasons:
Familiarity. The low bidder, Contractor 1 in the table at left, is a local company that specializes in open-cut construction, was very familiar with city standards, and had done a significant amount of work in the local conditions.
Favorable bidding environment. Contractor 1 had significant staff availability and in 2010 material costs were generally lower than in previous years due to economic conditions, representing $370,000 — 42% of total savings.
Flexibility in materials. Contractor 1 maximized the use of open-cut with segmental PVC per city standards. Allowing segmental PVC at all locations except the 1,500-foot trenchless segment, instead of requiring HDPE or FPVC along the entire alignment, representing $140,000 — about 16% of total savings.
Flexibility in methodology. Letting the contractor choose between open-cut and trenchless instead of insisting in-place segments be trenchless per the engineer's estimate contributed $50,000 — about 6% of total savings.
Because Contractor 1 had limited experience with trenchless construction, the company intended to bid only with a subcontractor that would take full responsibility for the trenchless portion. Reaming was used because the contractor couldn't find a pipe-bursting subcontractor willing to sign into a performance agreement.
Contractor 1's lower unit bid price for the open-cut work enabled the company to win the job even though its unit bid price of $405/foot for reaming was $145/foot higher than the average bursting unit bid price.
Result: $330,000 in savings. Had Contractor 1 been able to find a bursting subcontractor, the utility may have saved even more.