2014 Issue

29 Fills by future developers were also a concern. For example, the year before pipe construction began, two properties along the pipe alignment agreed to accept 10 to 15 foot deep spoil piles from nearby construction. The spoil piles were placed over the com- pressible ground and were not tapered, but had abrupt endings. This caused the ground to settle non-uniformly along the pipeline alignment. CUWCD anticipated this property would eventually be uniformly filled and developed after pipe installation. This would cause non-uniformpipe bending and strain or break welds. To avoid this problem, CUWCD proactively placed a uniform, 10-foot deep fill over these pipe easements to eliminate the potential for uneven settling of the pipe (Figure 16). Settlement monitoring plates were used to record fill settlements. The 60-inch pipe was installed with butt welded joints after the fills stopped settling. Settlement Risks at Undefined Locations. As the 7 mile Vineyard Connector Pipe was in saturated ground mostly underlain by ar- tesian aquifers, and as CUWCD could not predict future develop- ment fill locations, the challenge was to design this high pressure steel pipe to allow maximum bending without breaking or adding significant cost. Butt Joint Efficiency in Pipe Bending. In settlement-induced bend- ing of steel pipes with welded joints, the pipe yields and breaks at the weakest point, the joint welds. The ratio of the tensile strength of a welded joint to the steel is called the “joint efficiency.” For radiographically tested (RT) double lap welded joints ASME’s joint efficiency is 0.55. For RT butt welded joints it is 0.85 (for spot RT) or 1.0 (for full RT). Thus, RT- inspected butt welds are roughly 50 to 80 percent more failure- resistant in pipe bending than double lap welds. Butt Joint Design and Inspection. Because a pipe that bends with- out breaking is of great value on the Vineyard Connector Pipeline, butt joints were designed and bid as an alternative to double lap welded joints. Butt joints were designed to allow shop-installed backer plates for easy joint fit-up and welding. Butt welding speci- fications required 100% ultrasonic testing (UT) and visual testing (VT) and spot-RT inspection. Butt Joints Added Cost. Although a butt weld volume is half that of a double lap weld, its fit-up time is much longer. However, the Vineyard Connector Pipe’s deep (13 to 27 feet) trenches required Figure 17. Butt welded joints provide additional protection from settlement due to future fills likely to be placed after pipe construction. much shoring and dewatering which made for slow installation rates. Thus, the fit-up time for butt welds added little to costs. Of the 8 bids received for the project, the butt welded joints option was only 1% to 6% higher than the double lap welded joint option. CUWCD staff felt this small added cost was justified and awarded the bid with the butt welded joints option. The relatively small cost resulted in a pipe that is 50% stronger in bending in a site where bending failure is the main risk. Conclusions. During design and construction of 20 new pipeline projects result- ing in over 60miles of new aqueducts, CUWCDandCH2MHill have encountered and resolved major movable ground risks on several of those projects. Each movable ground risk was unique and each resulted in a unique solution. Movable ground solutions included: 1. A rock tunnel under a landslide 2. Two above ground pipes crossing over slowmoving landslides 3. Two miles of buried pipe with soil nail walls to stabilize steep cross slopes 4. A buried pipe with strain gauges in combination with a valve station to re-route flows 5. A 5-mile pipe realignment to avoid active landslides and faults 6. A pipe at a soft ground river crossing built away from fills, and after fills settled 7. A butt welded pipe in soft ground to better tolerate unknown future fills over the pipe’s life Major aqueducts need to address movable ground risks in design and construction to maximize system value and reliability. There are many movable ground risks. Each is unique to a particular site and project and warrants a unique solution. Solutions depend on the need for reliability, availability of a backup conduit or water supply, and the degree of risk (current and future) posed by the movable ground formation. Figure 18. Saturated ground and high water table made installation challenging.