Pittsburgh-based geotechnical specialty contractor Nicholson Construction Co. is working on CSO and sewer jobs across the country. It most recently began work on the Missouri River Waste Water Treatment Plant (MRWWTP) in Omaha, Nebraska.
Nicholson has been a part of many of the country’s biggest remediation projects, including the Dugway Storage Tunnel in Cleveland and the Maline Creek CSO in St. Louis. Both projects included the installation of secant pile walls at multiple site locations, and both wrapped up in December of 2016.
The Missouri River Wastewater Treatment Plant is undergoing an expansion that includes the construction of a new chlorine contact basin located along the Missouri River. While the plant actually serves approximately 125,000 residents in Omaha, the industrial and domestic flows to the plant give it the equivalent population of 600,000 people.
Nicholson was awarded the contract to install a 4-ft thick reinforced concrete diaphragm wall, which will act as support of excavation and a water cut-off barrier for the new chlorine contact basin building, and will become a permanent part of the basin foundation. The wall will reach depths of approximately 82 ft. and will be excavated using a hydraulic clamshell. Additionally, augercast piles will be installed to provide increased deep foundation support for adjacent structures and as deep foundation elements for the odor control duct support.
“It’s been great to draw on Nicholson’s extensive CSO experience for this project,” said Jewels Redding, project manager for Nicholson. “Our team has worked on these types of projects across the country over the years, but we always learn something new to apply to the next one. We’re looking forward to another successful project here in Omaha.”
Nicholson’s portion of the work will wrap up in early summer 2017.
Nicholson’s work for the Northeast Ohio Regional Sewer District (NEORSD) is part of the district’s Project Clean Lake, a $3 billion, 25-year program to reduce CSO volumes into Lake Erie. For the Dugway Storage Tunnel, Nicholson was working as a subcontractor for Salini Impregilo/Healy JV on the 15,000-lf, 24-ft diameter tunnel.
Nicholson’s portion of the work included construction of shoring for one deep shaft and two near surface structures. The two near-surface structures involved the construction of secant pile boxes, 65 to 75 ft. deep, for temporary earth support and groundwater cutoff. The box structures were constructed using 900 mm primary piles and 880 mm secondary piles using a Leibherr LB 36 drilling rig. The box structures also included vertical steel I-beams for additional support.
For the deep shaft, Nicholson constructed a circular shaft using 1,200-mm secant piles drilled to depths of 122 ft. The shape of circular shaft provides hoop strength so that no reinforcement is needed. Once construction of the secant pile supports was completed, Salini Impregilo/Healy followed with excavation.
According to John Wise, Nicholson Senior Vice President of Operations, 122-ft secant piles are deeper than most. “122 ft. is not the deepest secant pile ever built, but it is approaching the limits of the technology,” he said. “With secant piles you are counting on the piles to interlock, which becomes more challenging the deeper you go. It is important to confirm your verticality as you drill to ensure you have interlock.”
Nicholson used the PRAD system from Jean Lutz to measure the verticality of the piles. The PRAD system mounts onto the drill tooling and communicates the vertical position of the pile to the operator so that any deviation is known. In the event that verticality is beyond tolerances, drilling can be stopped, the hole backfilled with lean concrete, and the process can be re-started. The location information can be used to create 3-D models to ensure that there are no gaps between the piles along the entire length.
The key, according to Wise, is ensuring that the pile starts straight. If it veers off path, it is difficult to correct. “Once you are out of alignment the best option is to re-start, so it is critically important that you start the bores straight in the first place,” he said. “Once you get the first 15 to 20 ft of pipe on line, then chances are it is going to stay straight.”
In Cleveland, the owner required the use of a polymer additive in addition to steel casing as an added guard again bottom heave. “We typically use full-length casing when drilling secant piles to keep the hole open, but in this case the owner wanted the added protection of a polymer additive to keep the ground from migrating into the bottom of the hole.”
In St. Louis, Nicholson is working as a subcontractor for SAK-Goodwin JV for the Maline Creek CSO Storage Facility. The project is part of Project Clear for the Metropolitan St. Louis Sewer District, the fourth-largest sewer collection system in the United States. Project Clear is a $4.7 billion, 23-year program to reduce pollution to the Mississippi River and its tributaries.
For the Maline Creek project, Nicholson was tasked to build secant pile structures for a large-diameter circular shaft and a large box structure. The purpose of the structures was for temporary earth support and groundwater cutoff.
For this project, Nicholson used a Bauer BG 39 rig to drill 880-mm primary and secondary piles. The circular shaft was 44 ft in diameter drilled to top of rock (~50 ft). The box structure was also drilled to top of rock (~50 ft.).For the St. Louis project, Nicholson used the Cased Continuous Flight Auger (Cased CFA) method – a first for the company in North America. Historically, secant pile drilling involved drilling the pile using segmental casing using 10- 15 ft. long pipes, followed by augering out the material inside the pipe, adding more pipe and repeating the process. With the Cased CFA method, using specialized equipment from Bauer mated to the BG 39, Nicholson was able to drill the pipe in one stroke without having to pull the auger out of the hole.
“Our sister company in the U.K. – Bachy Soletanche – has used the Cased CFA method regularly, so they and the equipment supplier assisted us in our inaugural run,” Wise said. “It worked very well. It is very fast and the shaft was well formed and watertight when the contractor dug it out.”
Why Secant Piles?
In both the Cleveland and St. Louis projects, the secant pile method of support was chosen to provide a stiff support system and provide groundwater cutoff. Wise says the decision regarding the support system comes down to site-specific criteria. Steel sheet piles are an option, but can be limited due to certain ground conditions, and do not provide as stiff of a support system that is often desired in an urban setting. Diaphragm or slurry walls are also an alternative that may be a more cost-effective option for large-scale deployment or at depths that may be beyond the capability of secant piles.
Once the decision is made to use secant piles, Wise stresses the importance of getting the piles started on line. In both the Cleveland and St. Louis projects, Nicholson drilled through guidewalls, which are a concrete template constructed 3 to 4-ft deep on the surface that ensures the piles start with the right geometry and verticality. Additionally, traditional methods like using spirit levels to plumb the casing are used.
One other critical consideration for ensuring success in constructing secant piles is the concrete itself. Having the right flowability, stability and slump characteristics are important in the final outcome. Because concrete mixtures and raw materials can vary from location to location, it is important to test materials beforehand.
Aging combined sewer overflows are a water pollution concern for more than 770 cities in the United States, per the Environmental Protection Agency, so many more projects are on the horizon. “We are bidding work from New York, through the Midwest, to the Pacific Coast,” Wise said.