Pumping on the Rocks
In a winter that has had many construction companies in the U.S. fighting the cold and snow, and in some cases delaying concrete pumping, there is one pumping project that relies on sub-freezing temps to get the job done. The CapX 2020 project is a multi-phase upgrade to the power transmission grid being constructed by 11 electric utilities in the Upper Midwest. The construction of new power lines requires new pole structures between substations. In Minnesota that means following a path that traverses all sorts of terrain including low lying bogs, swamps and lakes. One power line project being constructed by Xcel Energy follows a 28-mile stretch from Monticello, MN to St. Cloud, MN. The route parallels I-94 which connects the Twin Cities and Fargo, ND. In order to place the concrete pads for the power structures, the most economical way to get into wet areas is when the ground is frozen. In a recent pour, a pole pad was situated in the middle of an ice covered marsh with 30-foot deep equipment-swallowing muck below the surface. A coordinated effort put the truck-mounted boom pump 1800-feet from shore to show the versatility of concrete pumping in extreme conditions.
“We plan the soft terrain pours for the winter months,” explains Bob Schneider, project superintendent for Xcel Energy, “We own thousands of poly mats that we lay down for the drilling rig, material handlers, pump and truck mixers to travel off-road. Sub-freezing temperatures are the safest time to do it.” Each 105 to175-foot tall power structure requires a concrete base. Most are supported by cast-foundations that are eight to eleven-feet in diameter and up to 70-feet deep. Three of the pole bases require a pile cast method of construction because of wet conditions. The mid-February pour was in the Del Claire Marsh near St. Cloud, MN. Pumping was provided by ACPA members, Apple Valley Ready-Mix (AVR), Apple Valley, MN. The company has been working since November 2010 on the project providing up to three concrete pumps as needed for the seven-day-a-week construction schedule.
AVR’s sales representative Dan Lien bid the project. “We have an affiliate AME Red-E-Mix, Inc in Monticello which was a strategic location for this project, but the concrete and pumping are bid separately. We keep three pumps at the AME location so it really was an advantage for scheduling.” According to AVR pumping sales/dispatch representative Dan Rentz, “Pours are usually scheduled a day ahead but with Minnesota weather and changing site conditions, pours are cancelled with short notice. Having both the concrete and the pump portions of the project saves on a lot of ready-mix that would be wasted without close communications.” Because of the site conditions, holes collapse unexpectedly requiring re-drilling which can set back ready-mix deliveries.
Complicating the pour in the middle of the marsh was the fact that organic matter (muck) was frozen near the surface. “Normally we can calculate the loads of pure ice, “according to Schneider, “But with the muck embedded in the ice, we asked for engineering help.” Two and one-half feet of ice was considered to be adequate to support the loads of a 56,000-lb pump and 66,000-lb truck mixer with the mats helping to spread the load over the surface area. AVR used their Schwing KVM 34-meter boom pump already rigged with extra frost-law axles that allow it to run on roads that are normally closed in spring. Schwing boom pumps are designed to minimize outrigger soil pressures, which makes the 34X an excellent choice for this project. The KVM 34 meter outrigger pressures are only 69 psi on the front and 55 psi on the rear when factory supplied outrigger pads are used. Truck mixers with bridge axles that extend their load were also used. The engineering firm specified thickening the ice by adding water which was carried to the site by ready-mix trucks and tankers. Temperatures of minus 24 degrees on the morning of the pour eliminated any fear of melting.
Added security for the pump was provided by piles driven to support each outrigger. “Once we got the pump positioned, we jacked it up on its outriggers and we didn’t worry about it disappearing,” Rentz said. Truck mixer drivers had to back up the entire 1,800-feet to feed the pump as there was no room on the mats to turn around. Pumping the piers proved a slow, steady process with only one truck mixer, limited to an eight-yard load, allowed on the ice at a time. Caution was the word of the day in the minus-twenty degree temperatures as drivers drove the 1/3-mile route out and back at a mandated 15 mph.
Pumping a 4,000 psi, ¾-inch aggregate mix was no problem for the Schwing pump’s Generation 3 2025-5 120/80 MPS 178 cu.yd./hr.pump kit. The Rock Valve equipped machine requires less water for clean-up which was an advantage in the sub zero temperatures. Preventing the concrete from freezing in the 4-section, 98-foot boom required that the operator engage the long-stroking pump occasionally between truck mixer deliveries. Hot water added at the batch plant 20-miles from the site helped keep the concrete fluid. Because of the wet conditions in the pour area, the mix carried extra cement in place of flyash.
The six-hour pour culminated in 155-yards being placed. The next day a 65-yard stem was placed on top of the base using the same methods. While the pumping was not challenging for the on-ice pour, the set-up was. In all cases, AVR operators work closely with Excel Energy to make sure that access to a pour site is safe and secure. Rentz prefers his Schwing 34-meter pumps with the extra axles to help navigate off-road. In the extreme conditions of Minnesota winters, the AVR pumps are kept in a heated shop every night. Attention to maintenance and operators trained to pump in winter has allowed the company to respond to Xcel’s changeable schedule and provide excellent service…even pumping on ice. Powerline construction requires resourceful planning and execution. Placing pumping equipment on the ice makes for an interesting day on the job. Placing the poles in these mid-marsh locales will also be interesting when they are set by a helicopter.
In its Annual Energy Outlook 2010, the U.S. Department of Energy projects electricity consumption will increase 1 percent annually from 2008 to 2035, despite energy conservation efforts. The Upper Midwest has outpaced that prediction with 80-percent growth in electricity usage since the 1980s. There is also a movement to expand transmission capacity for wind energy from southern and western MN and the Dakotas where wind farm potential is some of the highest in the nation. Despite the costs and difficulties in erecting transmission lines across the area, experts say underground high-voltage transmission lines generally cost up to 10 times more than overhead high-voltage lines.
Owner and General Contractor: Excel Energy
Pumping Contractor: Apple Valley Ready-Mix, Apple Valley, MN
Equipment: Three Schwing KVM 34 X truck-mounted concrete pumps with placing booms