September 19, 2014The City of Montpelier, Vt., has the distinction of being the smallest state capital in the United States, with a population of just more than 8,000 people. It’s a small town with big city amenities and a center for government, commerce, industry and services in rural Vermont.
Known as the green mountain state, Vermont is also known for the residents’ love of the land; they are committed to preserving it and sharing it. The people of Vermont are also known as being independent thinking and resourceful.
In spring and fall of 2013, multiple segments of an existing 15-in. RCP gravity sanitary sewer collapsed along a main commuter route in the downtown area causing a system failure. Numerous excavated point repairs were required to get the failed pipe flowing again. The sections of RCP had severe hydrogen sulfide (H2S) degradation at the crown, including hanging gaskets and exposed rebar that were apparent during a PACP CCTV inspection performed by Ted Berry Co. Inc. When wastewater becomes septic in a gravity flow sanitary sewer line, it is depleted of dissolved oxygen, which causes H2S gas to be released from the wastewater into the air in the sewer pipe. The gas reacts with moisture on the walls of the pipe to form sulfuric acid (H2SO4), which corrodes concrete. Reinforced concrete pipe (RCP) is one of the pipe types most susceptible to H2S corrosion.
Pipe Bursting Selected
In spring 2014, the City of Montpelier awarded the River St. Pipe Bursting Project. A 1,700-ft section of 15-in. RCP replacement and the Ted Berry Co. Inc. was hired as a sub-contractor to handle installation of the new 18-in. DR17 IPS HDPE pipe using pipe bursting. Project oversight was provided by the City of Montpelier engineering department.
City personnel’s resourceful thinking caused them to evaluate construction options and impacts, eventually choosing to specify pipe bursting in the contract documents. The trenchless method was considered and eventually selected due to the high traffic volume in the area, including numerous local businesses. Maintaining the traffic route was critical, as the highway was a critical throughway during the morning and afternoon work commute and drove the project toward a process that would allow for an expedited construction schedule to keep social disruption to a minimum. A recent similar storm drain project had taken a toll on the commuters the two previous construction seasons.
A pneumatic pipe bursting plan was developed by the Ted Berry Co. project management team, which included owner Matt Timberlake, trenchless services manager Shawn Ready and utility construction manager Isaiah Bean and presented it to the general contractor. The plan was designed to minimize site impact and construction cost while additionally managing project risk from the pipe bursting installation. The plan would include an aboveground by-pass pumping system capable of handling wet weather sewer flows that would isolate the segments being replaced and strategically placed excavations that would be used to insert the new pipe, receive the bursting tools, reconnect the sewer laterals and place the new pre-cast manholes. Pits generally consisted of 6-ft x 16-ft steel trench boxes that were excavated, dewatered and secured by the general contractor.
Pipe bursting equipment utilized was a 20-ton tracked dual capstan, constant tension, variable speed winch paired with a 14-in. pneumatic hammer (92 ft long 2,601 lbs, 220 strokes per minute at 706 cfm of air consumption), which did the lion’s share of the work coupled to a 900 cfm compressor. The bursting configuration used was a rear-mounted expander configuration, allowing the hammer itself to act as an active pilot, keeping the bursting apparatus on grade and within the degraded RCP pipe. Additional blades were field welded onto the rear-mounted expander to ensure clean cutting of the reinforcement wire in the RCP. The blades were also instrumental in cutting numerous recently installed PVC repairs. Without blades a segment of PVC in a section of RCP can sometimes want to “accordion” the expander, with the blades the PVC cut easily and stayed in place. Recent repairs can at times be problematic while bursting, if not properly compacted, as loose soil can encourage the repair to “telescope” into the next pipe segment instead of bursting creating challenges at the reception pit locations. The project would be considered a class C pipe burst, as defined by the International Pipe Bursting Association (IPBA), when degree of difficulty, depth of existing pipe, existing pipe ID, new pipe diameter in comparison to existing pipe, original trench configuration and spoil type were all taken into consideration.
The installation of the 1,700 ft was broken into five segments or “bursts,” ranging in length with the longest being 510 ft. The new high density polyethylene (HDPE) pipe was delivered to the site in 40-ft segments and thermally fused into continuous segments that would then be installed. A pre-burst CCTV inspection was performed; however, pre-cleaning was limited as the pipe could actually fail due to the water pressure from the pre-cleaning. Once an aboveground bypass pumping was setup, heavy cleaning began to remove the sediment deposits from recent breaks and allowed for a full internal CCTV inspection to show the actual condition of the pipe, location of the service laterals and location of the point repairs installed during the previous year.
New pre-cast manholes and numerous service laterals needed reconnection with Inserta-Tees and Inserta-Wyes. Intermediate services were maintained through the use of temporary pumping stations made onsite. The manhole terminations were restrained with fused on restraints poured in concrete collars to ensure the prevention of any thermal retraction from the structures.
The River Street Gravity Sewer Improvements Phase II Project was originally scheduled to begin in April 2014 and last two to three months; however, the pipe bursting was completed in approximately four weeks.
Matt Timberlake is owner and Shawn Ready is trenchless services manager at The Ted Berry Co. Inc.