CSDA Member Makes Water Treatment Retrofit Work
(Front page feature image courtesy of ALCOSAN)
A unique and challenging job was performed by a specialty contractor that involved 834 square feet of concrete wire sawing and the removal of cut sections weighing approximately 26,000 pounds.
When you flush a toilet in Pittsburgh, Pennsylvania, the waste water eventually ends up in the main intake (wet well) at the Allegheny County Sanitary Authority (ALCOSAN). The wet well is cylindrical in shape, measuring approximately 40 feet in diameter and is 120 feet deep. It is a smaller tank situated within a larger cylindrical shell that houses various electrical services, piping and an access elevator. A scheduled retrofit of equipment within the wet well involved the precise cutting of a large concrete footer using diamond tools.
At the bottom of the wet well are six separate 36-inch-diameter distribution lines, which spoke out from the bottom of the tank into the outer shell. Each line runs straight up to the top and transfers sewage to processing stations at a rate of 128,000 gallons per minute. To push the waste water up these 120-foot vertical lines at this rate, the facility has six 2,000-horsepower transfer pumps. To mitigate any movement of the pumps while in operation, the elbows at the bottom of the tanks are embedded deep in concrete. One of the pumps had reached the end of its service life and was scheduled to be replaced. This meant that sections of plumbing, including the existing 36-inch-diameter concrete-embedded elbow, also had to be removed.
For the retrofit, the new pump system required that the footer be notched large enough to remove the existing pipe elbow and provide adequate room for the new plumbing system. The notch was 10 feet long, 13 feet wide and 8 feet deep.
The replacement of this pump was scheduled to be done during a two-week plant shut down from January 28 to February 10, 2015, therefore time was of the essence. General contractor Kokosing Construction Company, Inc. of Westerville, Ohio, and lead subcontractor Noralco Corporation of Pittsburgh began the search for a skilled cutting contractor to make the necessary renovations to the concrete footer. CSDA member Matcon Diamond, Inc., also of Pittsburgh, was chosen for the work.
“This was an interesting job to work on. The method for removal was unspecified by the owner’s designer. Only the dimensions of the void for the new pump were listed. Noralco had planned on breaking the slab apart by hoe-ramming, expanding chemical compounds and hydraulic rock splitters, but they approached us to devise a quicker and more cost effective method utilizing diamond tools,” said Jon Wilson, project manager for Matcon. “The only feasible way to complete the work in such a short time period was to cut the footer into 11 large pieces—diamond sawing was the only method that could accommodate that schedule.”
Upon commencement of the pump shut down, the general contractor had a full week of selective demolition and removal tasks related to the supporting plumbing structures. As access to the area was denied until the shut down, and no wire sawing could be completed while dismantling work took place, the first shift of cutting work consisted of core drilling 12 access holes for wire and plunge cut pulleys. Operators drilled eight 9-inch-diameter holes 8 feet deep, two 4-inch-diameter holes 13 feet deep and two 2-inch-diameter holes 4 feet deep. This took two operators 50 hours to complete using two DD350 core drills from Hilti fitted with core bits supplied by K2 Diamond.
Completion of the core drilling work coincided with the general contractor completing all dismantling tasks and wire sawing commenced. A Husqvarna 2512 hydraulic wire saw was first set up to perform a vertical plunge back cut that was 10 feet long and 8 feet deep, taking eight hours to complete. The wire, supplied by K2 Diamond, was then pulled horizontally towards the wet well to create the bottom cut line before a series of pushed longitudinal and transverse cuts were made to create the 11 cut sections. Matcon made 10 cuts ranging from 48 to 130 square feet for a total of 834 square feet. The wire saw averaged 14 square feet per hour and the average weight of the 11 pieces was 26,000 pounds.
With all wire sawing work completed, Matcon began to extract the cut pieces using an on-site 20-ton gantry crane, leaving only the final piece with the embedded pipe elbow in place. As this piece was now exposed and the only one remaining, the concrete was chipped away from the pipe using a Husqvarna DXR 310 demolition robot. This was the only suitable method to remove this final section, as in its entirety it was in excess of the crane’s lifting capacity.
The removal of all cut and broken concrete pieces was a challenge for the contractor. The gantry crane was situated inside the shell, but in order to completely remove the pieces from the structure, each one had to be handled a second time. This involved picking them through a 12-foot by 12-foot access hatch in the roof using an outside crane. Moreover, the weight and dimensions of the pieces caused some extra concern on removal. Specifically, the largest pieces were roughly 5-foot by 5-foot by 8-foot columns of non-reinforced concrete weighing around 26,000 pounds. With a lack of reinforcing, deep mechanical anchoring was required to ensure that gravity would not pull the slab apart as the pieces were extracted.
Another challenge was the removal of additional concrete pieces below the bottom horizontal cut. The horizontal wire cut could only be made flush with the floor of the building. However, specifications for the new replacement pump unit required a complete encasement in concrete. Therefore, once the first 8 feet was removed Matcon operators had to essentially “spoon out” a 12-inch-thick concrete floor area measuring 13 feet by 10 feet. Rather than wire saw this area, it was determined the fastest method was to flat saw the concrete into a grid at the required depth. The operator used a 40-horsepower electric flat saw from Diamond Products fitted with a 26-inch-diameter blade for the cuts. This created sections approximately 12 inches cubed that were broken out using the DXR 310 with a breaker attachment. These smaller sections were loaded into debris hoppers and extracted from the work area.
The ALCOSAN wet well job required all units to be powered electrically. Given the 120-foot depth of the structure, it would have been almost impossible to use combustion equipment—even with the use of exhaust scrubbers. In total, the team from Matcon performed 834 square feet of concrete wire sawing, 230 linear feet of 12-inch-deep flat sawing and core drilled 98 feet in diameters ranging from 2 inches to 9 inches. The work was completed three days ahead of the specified time frame from the general contractor but precisely within Matcon’s own schedule.
By performing fast, accurate cutting techniques using diamond tools and equipment, the contractor was able to keep an extremely time-sensitive project on track while ensuring the integrity of the wet well structure. Matcon is under contract to perform further cutting work at five more pump locations.
The project was designed, managed and executed by Lester Arthur, Dan Matesic and Jon Wilson of Matcon Diamond, Inc.
Located along the Ohio River on Pittsburgh’s northside, the Allegheny County Sanitary Authority (ALCOSAN) provides wastewater treatment services to 83 communities including the City of Pittsburgh. ALCOSAN’s 59-acre treatment plant is one of the largest wastewater treatment facilities in the Ohio River Valley, processing up to 250 million gallons of wastewater daily. The Authority was created in 1946 under the Pennsylvania Municipal Authorities Act and began treating wastewater in 1959. Secondary (biological) treatment processes were added in 1972. It has 320,000 residential, commercial and industrial accounts representing a service population of 900,000.
ALCOSAN recently completed a $400 million capital improvement program which addressed odor control, treatment capacity, solids handling and wet weather planning. The organization has also embarked on the largest public works project in the region’s history through $1 billion in engineering and construction projects to address combined sewer overflows. For more information, visit www.alcosan.org.
A CSDA member since 1987, Matcon Diamond, Inc. celebrates its 30th year in business this year. The company is based in Pittsburgh, Pennsylvania, employs up to 75 operators and laborers and has a fleet of 50 trucks. Matcon offers concrete cutting services of slab sawing, wall sawing, hand sawing, wire sawing, core drilling and joint sealing.
Koksing Construction Company, Inc., Noralco Corporation
Sawing and Drilling Contractor:
Matcon Diamond, Inc.
Methods Used: Wire Sawing, Core Drilling, Flat Sawing