Case study 2: Beamer Street sewer trunk line rehabilitation
Owner: City of Woodland, Calif., Public Works Department
Options: Cured-in-place pipe (CIPP), machine-spiral-wound HDPE, and sliplining with HDPE, fusible PVC, fiberglass, or segmental PVC to rehabilitate 2,500 feet of 36-inch and 4,000 feet of 48-inch reinforced concrete pipe.
Engineer's estimate: $2,169,070
Winning bid: $1,493,078
Located in California's Central Valley about 20 miles north of the state capital of Sacramento on I-5, the City of Woodland owns and operates nearly 200 miles of gravity and pressurized wastewater collection infrastructure as well as 75 miles of laterals.
The centerpiece of the public works department's sewer system management plan is customized Microsoft Visual Basic .NET software that calculates a Risk score for every pipe. Probability criteria include Closed-Circuit Television-identified structural deficiencies like cracks, off-set joints, intruding taps, voids, and holes; CCTV-identified maintenance deficiencies like roots, grease, and deposits; and maintenance history, such as number of emergency and preventive maintenance work orders. Criticality criteria include asset location (environmental, public nuisance, accessibility); date (age, material, size); and capacity: depth-of-flow-to-diameter-of-pipe (“d/D” value) at average dry and peak wet weather flows (ADWF, PWWF).
The protocol identified 2,500 feet of the 36-inch Beamer Street trunk line and 4,000 feet of the 48-inch trunk line for immediate rehabilitation. Corrosive gases had significantly deteriorated the unreinforced concrete while deteriorating joint seals, combined with the use of pea gravel for pipe bedding in high groundwater, was causing inflow and infiltration at multiple locations along the alignment.
Located just upstream of the treatment plant and below I-5, the two pipelines would produce significant public health and safety consequences if they failed. Given these high consequence and high probability factors, they ranked in the highest risk-of-failure category. The city council approved $2.17 million for rehabilitation.
Loss of flow area, especially in trunk lines, is often a limiting factor when considering potential trenchless structural lining methods. Having recently updated its hydraulic model using new growth- and sewer-use projections, the wastewater operations division confirmed the pipelines had some excess capacity. Though this indicated a sliplining method that didn't require bypass pumping would be most cost effective, managers didn't want to eliminate methods that would require bypassing because there was a parallel sewer in very close proximity. In addition, bends in the alignment supported using a flexible liner; i.e., cured-in-place pipe (CIPP). Finally, the 48-inch diameter of the downstream segments limited the viability of certain materials (HDPE), making alternatives such as CIPP or spiral-wound favorable candidates.
Based on these conditions, a wide range of structural rehabilitation methods remained viable.
To allow competitive bidding of the various solutions, plans and performance-based specifications were developed for each method — sliplining with segmental fiberglass pipe, HDPE, or fusible PVC; cured-in-place thermosetting resin pipe (CIPP); and machine spiral wound high density poly-ethylene pipe — ensuring each would provide the structural enhancement necessary to extend the life of the un-reinforced concrete pipes.
The engineer's estimate of $2,169,070 was based on an assumption that sliplining with fusible PVC would be used for 36-inch rehabilitation and structural CIPP for the 48-inch, but the work was awarded for $700,000 less based on comparison of average bid to engineer's estimate. The contributing factors:
Favorable bidding environment. Lower-than-usual material costs and other factors account for $360,000 — 51% of total savings.
Flexibility in materials and methodology. The low bidder, Contractor 1 in the table at left, maximized locations that could accept segmental pipe, which enables live-flow installation. Eliminating bypass pumping accounts for $120,000 — 17% of total savings.
In addition, the company submitted large-diameter profile wall PVC in lieu of glass-fiber-reinforced pipe for the installations. The engineer and city found the material to be consistent with bid document design criteria, goals, and objectives. Allowing that alternate material accounts for $50,000 — 7% of total savings.
Sections beneath the transportation crossing weren't approved for segmental installation because alignment irregularities could be traversed within the allowable minimum bend radius for continuous HDPE but not within the maximum joint deflection and length geometry of segmental. Allowing the contractor to slipline 36-inch segments instead of the assumed design of continuous fused pipe (PVC/HDPE) accounts for $40,000 — almost 6% of total savings.
Segmental pipe sliplining of the 48-inch segments versus the assumed design of structural CIPP accounts for $120,000 — 17% of total savings.
Abshier works for the City of Redding, Calif.; Cocke for the City of Woodland, Calif.; Fisher for Water Works Engineers in Roseville, Calif.; and Williams for Kimley-Horn and Associates in Rancho Cordova, Calif.