Structural work recently began on two modern concrete storage tanks to replace a 140-year-old earthen reservoir currently providing Low Service Distribution Storage for the Manchester (New Hampshire) Water Works. 

Preload Inc. of Hauppauge, New York, has a contract with the city to build two 6.5-million-gallon prestresssed concrete tanks near the existing reservoir on South Mammoth Road.  The existing 20-million-gallon earthen embankment reservoir has a plastic liner on the bottom to minimize leakage and a floating plastic cover on the water surface to help protect against water supply contamination.

According to David Miller, PE, Deputy Director for MWW Water Supply, the plastic cover is now at the end of its anticipated life, requires regular maintenance and surveillance, and leaves the tap water supply vulnerable to vandalism and contamination. Miller said the new tanks will provide a safe and secure method of water storage and eliminate the possibility of contamination from waterfowl, rodents, vandals or methods related to deliberate contamination.

While the tanks are under construction the existing reservoir will remain in service for Water Works customers. Miller said the new tanks have walls approximately 25 feet tall, about the same height as the berm of the existing reservoir. He added that Water Works plans to retain as much vegetation as possible along the property boundaries and supplement lost vegetation by planting new trees. In addition, raised landscaped berms will be built to screen the view of the tanks from the street.

Public safety will also be enhanced with the replacement of the reservoir by modern tanks, since the reservoir is classified as a High Hazard dam by the Water Division of New Hampshire's Department of Environmental Services. The Water Division is responsible for inspecting all dams in the state that by reason of their physical condition, height and location may be a threat to public safety. Presently there are over 2,600 active dams throughout the state which are categorized into one of four classifications - non-menace, low hazard, significant hazard and high hazard. A dam is classified as high hazard because it is in a location and of a size that failure or incorrect or improper operation of the dam has the potential for causing loss of human life. 

The overall cost of the project, including construction, engineering and contingencies, is around $8 million.

Preparing the Site

The two new tanks are cylindrically shaped, prestressed wire-wound concrete structures spaced 258 feet apart (center to center) with spherical dome roofs and inside diameters of 206 feet, 3 inches. Tank walls are tall enough to provide a maximum water depth of 26 feet - the same as the existing reservoir - so customers of Water Works' Low Service System will experience no change in water pressure during and after construction.

Preload crews began work at the tank site early in summer 2015 under the supervision of Bruce Burke, Area Manager, and Sherman Thayer, Project Manager. Consulting engineering for the project is provided by Tighe & Bond of Portsmouth, New Hampshire, with David Cedarholm, PE, serving as Project Manager, and Craig Langton as Resident Engineer.

Subcontractor David W. White and Son of Bow, New Hampshire, performed site work, including preparation of the sub-base and floor of the new tanks. White installed a 6-inch leveling course of granular material on top of a geotextile fabric that had been placed over the prepared sub-grade. A 6-mil polyethylene film was then placed over the leveling course. Next came placement of the bottom concrete slab. 

Monolithic Slab Pour 

The floor slab for each tank is almost 210 feet in diameter, with a thickness of 4 inches that transitions to a 15-inch-thick ring footing for the outer 5-feet of slab. Concrete for the slab contained 3/4-inch reinforcing fibers and was designed to meet a 28-day compressive strength of 4,000 psi. 

Preload began placing the floor slab/ring footings in August. Each tank required an estimated 600 cubic yards of concrete, all of it specified to be placed in one continuous pour. Transit mixers from Redi-Mix Company and Persons Concrete provided a steady source of ready mix for the concrete pumps of Independent Concrete Pumping of Wakefield, Massachusetts, which in turn conveyed the material to concrete floor crews. Independent had two pumps working - a 61-meter Schwing and a 52-meter Schwing - and one 42-meter Schwing on standby to ensure an unending stream of concrete for the two placement crews.

Job specifications called for the contractor to produce a fresno finish for the concrete slab. A fresno is a large, long-handled trowel resembling a bull float but with a larger blade, typically 5 inches wide and 18 to 48 inches long and made of tempered steel. The handle is attached to the blade with adjustable or swivel brackets. A fresno trowel produces a smooth, hard surface finish and is especially useful when speed of troweling is important.

The reinforced concrete membrane floor is designed to transfer the substantial liquid load directly to the granular leveling course and sub-grade. In conjunction with a PVC water stop, this membrane floor also provides water-tightness and allows the tank to settle differentially without inducing high secondary bending stresses. 

The 15-inch-thick wall footing itself is a structural member designed to support the wall, roof, and live loads under full- and empty-tank conditions. In the case of a full tank, the weight of water, alone, is approximately 27,000 tons.

With the monolithic placement and curing of 600 cubic yards of high-performance concrete for each tank's footing/floor slabs, the stage was set for the erection of precast wall panels.

Casting Wall Panels on Site

Preload crews had previously formed and poured wall panels on casting beds placed outside and around the circumference of the tank. Panels are 27 feet, 5 inches tall with faces shaped to the curvature of the tank radius, complicating fabrication of the casting beds. The widths of the panels are equal to the lengths of arc segments of a circle with a radius of half the tank diameter, or about 103 feet.

There are 47 panels making up the tank walls, cast in two widths specified not in feet but by the size of the central angles subtended by the arc segments. Specifically, the wider panels, of which there are 42, have widths of 7 degrees-51 minutes-19 seconds, while the width of the five narrower panels are specified as 6 degrees-0 minutes-56 seconds. The sum of these panel widths equals the 360 degrees of a full circle. Measured in feet, the widths of the inside faces of the two panels are approximately 14 feet, 2 inches, and 10 feet, 10 inches, respectively, while the total inside face circumference amounts to approximately 648 feet.

Preload cast and stacked the panels in groups around the tank footprint to minimize the distance between casting beds and the final position of the panels. This also helped reduce the number of different crane setups for picks. Wall panels were cast with 26 gage inclave steel diaphragms serving as the bottom of the forms (and therefore, the outside face of the walls). Diaphragms are vertically ribbed with reentrant (closed on themselves) channels, which provide a mechanical keyway anchorage to the concrete. And they are mechanically lock-seamed together to produce a watertight wall membrane.

Workers installed side forms, vertical reinforcing and lifting inserts, then poured concrete and finished the inside face. Composite wall thickness varies from 7 inches plus 1/2-inch of shotcrete cover at the top, to 8-3/4 inches plus 1/2-inch of shotcrete cover at the bottom. The thicker portion better resists water pressure near the bottom of the walls which can exceed 1,600 pounds per square foot. 

Temporary Bracing By Dome Falsework

Prior to the erection of the precast wall panels, preparations were made to support them temporarily by bracing them to dome falsework. The falsework is a complex, custom-made engineering and construction achievement in itself. A team of Preload carpenters fashioned thousands of boardfeet of lumber into hundreds of triangles, rectangles, columns and beams that   when conjoined created towering arrays of falsework to support forms for the massive concrete tank domes. 

A 400-ton Liebherr hydraulic crane was provided by Bay Crane to hoist wall panels. The crane hooked onto lifting inserts cast into panels and tilted them up for positioning on elastomeric bearing pads on the ring footing, with dome falsework providing temporary bracing.

High-strength shotcrete was used to fill the vertical slot joints between the individual panels. The diaphragm overlap between each panel was sealed to produce a continuous, watertight membrane. A minimum of 1/2-inch of shotcrete was applied to the outside face of the diaphragm in preparation for the wire winding operation. 

Building 1,100-Ton Domes

The next step in casting tanks is the placement of concrete for the domes. Each dome is essentially a truncated hollow sphere having a surface area of approximately 18,000 square feet and a thickness of just 4 inches with the exception of the outer 3 feet of dome circumference which is thickened. The outer edge of the dome extends over the top of the walls and a 21-inch concrete dome ring around the tanks. This thickened fillet section resists the bending moments resulting from dome edge discontinuity.

Approximately 550 cubic yards of concrete weighing about 1,100 tons is required for each dome casting.

The design of the cast-in-place dome roof results in uniform compression throughout the dome shell, thus allowing the thin section of concrete to span the large tank diameters, much like the design of an arch.

Reinforcing steel is placed in perpendicular directions throughout the dome shell. Then, the concrete is pumped onto the form, finely screened and finished to the required spherical shape. A hatch and draft vent are both installed as integral parts of the concrete dome roof.

Wire-Winding Prestresses Walls

The final major operation in constructing the new water storage tanks is the wire-winding procedure.

Wire-winding induces circumferential prestressing and keeps tank walls in continuous compression regardless of service conditions, eliminating tensile cracks. This procedure is performed by a specialized machine that rolls around the exterior of the tank, wrapping the structure with extruded wire in a helical pattern. 

As each wire is applied, it is continuously stressed, ensuring no movement of the wire against the wall. Prestressing counteracts the hydrostatic load of the enclosed water, and maintains a residual compression in the tank wall. This behavior is similar to that of prestressing cables used to preload a concrete bridge beam before it undergoes traffic loads.

As the wire-winding machine wraps the tanks, each layer is covered by a 1/4-inch coating of shotcrete that meets a specified 28-day compressive strength of 4,500 psi. This process creates a bonded prestressing system and also provides corrosion protection for the wires. 

Spring 2016 Completion Targeted

As this report was prepared, concrete placement for the domes was scheduled. Once the domes were cast and cured, wire-winding would begin.

Construction on both of Manchester's two new 6.5-million-gallon water storage tanks is expected to be completed in the spring of 2016.