Immediately following the rupture, city crews mobilized to shut the 60-inch valves and isolate the failed section. The city then collected informal competitive bids and expedited an emergency purchase order (EPO) to an experienced local construction contractor. While initial repairs using this EPO progressed, LAN personnel and the city worked to develop an on-call contract for emergency repairs to large-diameter water mains. That way, a contractor would be kept onboard with pre-established pricing to perform work in such instances.
As the effort to fully inspect and rehabilitate the line unfolded, the city transferred work to the on-call contract. Repairs to the 60-inch line included replacement of approximately 1900 linear feet of pipe; installation of a steel liner in three pipe sections where excavation was not possible; installation of a 60-inch isolation valve, and replacement of two isolation valve operators; installation of eight new cathodic protection test stations to monitor the water line; and other solutions.
Though the point replacements provided an immediate solution, the remaining pipe likely will continue to deteriorate. The city established a long-term maintenance program to continue monitoring rehabilitated sections and assess remaining portions of the line.SLIPLINING SOLUTIONS
The third failure occurred in a 42-inch PCCP water line, installed in 1992. In June 2004 the line suffered a failure in a section extending east from Houston's Sims Bayou Pump Station, causing several residential properties to be flooded.
“On the failed section, we saw clear signs of corrosion on portions of the wire and on the exposed parts of the steel cylinder.” said Jajoo. “This corrosion coincided with some external and internal cracking of the pipe section.” The cracking was a clear sign of structural distress, caused by repeated rapid closure of an emergency shut-off valve at the plant.
Because this line was needed to meet high water demand during the upcoming summer, another quick repair solution was needed. With only eight months to design and construct rehabilitation for 25,000 linear feet of the distressed line, the city agreed with LAN engineers' recommendation to slipline the 42-inch inside diameter with a 36-inch diameter high-density polyethylene pipe.
A bid-ready package was prepared within 30 days of this decision. This option was constructed successfully and placed in service within six months. By electing sliplining rather than open-cut installation of a new line, the city was saved more than $2 million in construction costs.CONSTRUCTION SOLUTION
The need for a system to assist in managing situations such as these catastrophic pipe failures became evident after the first incident. After meeting with the city to determine its needs, the team concluded a standard time and materials (T&M) contract would not guarantee that a contractor would work efficiently for emergency pipe repairs and would leave a great deal of uncertainty in the final cost. Another concern was that contractors on a T&M contract may use less-experienced employees. Other contracting methods needed to be set to help the city mitigate risk, ensure competitive bidding, and save time and money.
“After extensive consultation with the city, LAN developed a cost curve based on historical prices of new pipe and repairs in an urban environment as an alternative to T&M methods,” said Jajoo. “Alist of more than 250 bid items was compiled with costs assigned to each. These items are expected to account for approximately 80% of possible events. This list, accompanied by a table of pipe diameter multipliers, would be used to determine the cost of the repair once a scope is developed.”
Because much of the work likely would occur around the clock, a premium time multiplier was included to encompass work performed 7 p.m. to 7 a.m., on a Sunday, or on a legal holiday. Contractors would bid an adjustment factor, used to account for their individual overhead costs in the final price. The initially established bid item prices appeared to be fair as listed based on the low bidder's overall adjustment factor submitted.