In less than a split second, a series of bright flashes followed by staccato explosions cracked the quiet of an early summer morning and shattered one side of the base and supports of a 575-ft. (175 m) exhaust stack for the old, coal fired Xcel Energy electrical plant located in St. Paul, Minn.
Gravity quickly took over. Slowly, the stack, which tapered from a 62.5-ft. (19 m) diameter and 17-in. (43 cm) thick reinforced wall at the bottom to a 29.5-ft. (9 m) diameter and 8-in. (20 cm) wall at its top, began to topple over to one side.
Quickly gathering speed, it smashed into a series of raised mounds of sand built to partially offset the force of the blow of the impact and help break up the 5,770-ton (5,863 t) concrete and rebar structure.
In less than 16 seconds, the stack lay pulverized and broken into pieces. Power company officials estimate the implosion saved over a year’s time in the more labor and equipment intensive method of the traditional operation of smashing it to the ground with a ball swinging from a crane and jack-hammering the smashed pieces .
The impact sent a concussive sound wave out, startling spectators lining both sides of the river bluffs above the river. A slowly growing rolling cloud of dust looming above the 80-ft. (24 m) bluffs of the east river bank chased spectators from the north end of the nearby High Bridge
A landmark to some and an eyesore to others, the implosion of the stack is the first piece of major demolition to take place in dismantling the 85-year-old St. Paul, Minn., plant to usher in a new, natural gas fired plant.
Work crews recently completed construction of the new plant which is expected to go on-line this fall. At a cost of $350 million, the new plant eats up one third of Xcel Energy’s $1 billion plan to improve air quality and efficiency at three electrical generating plants in the St. Paul/Minneapolis area.
Xcel Energy supplies electric and natural gas energy to eight states in the Midwest.
Brandenberg Industrial Service Company, Chicago, Ill., was the prime demolition contractor for dismantling the old plant. Brandenberg brought on Controlled Demolition Inc., (CDI) of Baltimore, Md., to bring down the stack.
Brandenberg is one of the largest environmental/demolition companies in the United States with extensive experience in removing industrial complexes similar to the Xcel site.
CDI has demolished thousands of structures by explosives since the 1940s and is not a stranger to the Twin Cities area. The company imploded the fire ruins of the old Northwest Bank in Minneapolis in 1984 and the old High Bridge which sits within 500 ft. (152 m) of the Xcel Energy site.
Though construction began on the original plant in 1923, the stack was not constructed until the early 1970s as part of the environmental effort to reduce air pollution, said Jim Zyduck, Xcel Energy plant director.
Xcel Energy “added electrostatic participators to two of the units and raised the elevation of the discharge of the heat and particulates,” Zyduck explained. “The higher elevation of the stack provided more of a thermal flow to drive the exhaust higher.”
The stack also was noteworthy on the animal environmental front for being one of the first structures of a power utility to provide a safe location for the Peregrine Falcons during their spring nesting season, Zyduck said.
The nesting boxes for these birds evolved from a successful experiment 20 years ago by an employee of the King Plant, another Xcel Energy power site located on the St. Croix River, just 20 mi. (32 km) east of the High Bridge Plant, Zyduck said.
The employee spotted some Peregrine Falcons flying near the towering King stack, planted the boxes and later found the falcons nesting in them, Zyduck remarked. Now, power plants worldwide have planted nesting boxes at their own facilities.
Removal operations of the fallen stack began immediately after the micro second blasts toppled the tower. Crews recently completed removing the fallen remains of the old stack. All of the concrete will be processed and re-used on site using Liebherr equipment, Zyduck said.
Preparation for the implosion began several weeks prior to the implosion. With the stack standing near the old energy plant and sandwiched in by power lines, a road and railroad tracks, accuracy was critical.
Brandenberg crews moved in and carefully cut out 25 percent of the chimney circumference at its base in the direction of the planned fall. At the same time, crews climbed the stack internally and scrubbed the ash clinging to its inside wall, Zyduck explained.
“We cleaned everything real well,” Zyduck said. And, wet weather stepped in to help. “Mother Nature helped with a rainy spring that washed everything down the stack.”
Vibrations caused by the mechanical preparation also loosened some of the material clinging to the inside of the stack, Zyduck added.
The removal of sections of the wall at the chimney base exposed five concrete legs. Demolition crews added steel hinge plates on either side of the stack to reinforce the outside legs and assist with drop direction.
According to Brandenberg documents, demolition crews drilled 404, 1.6-in. (4 cm) holes into the five legs to place 202 lbs. (92 kg) of emulsion explosives. With a detonation velocity of 18,500 ft. (5,640 m) per second at 33 separate millisecond intervals consuming 0.5 seconds, the explosives instantly cut the five legs and sent the stack toppling away from the old plant.
To help cushion the impact and at the same time help break up the stack, crews built a series of mounds the length of the stack. Cat dump trucks dropped 15,000 cu. yd. (11,500 cu yd) of sand onto the site while Cat backhoes built the mounds.
Brandenberg also hired an independent geotechnical firm to perform pre-demolition surveys of sensitive facilities on the Xcel site, the new High Bridge and adjacent private properties. Technicians also placed nine seismographs throughout the site to ensure the anticipated vibration from the stack’s impact was well below permitted levels.
Though the stack did not hit its target dead-on; it landed within the design parameters and close enough for plant director Zyduck to call it a success.
Witnessing the implosion from the river bluffs, Zyduck said “it was exhilarating for me. It ushered in a new era of energy production that is environmentally friendly. It was a fitting way to go out with the old production system of the coal, the particulates, into something that has very little emissions for the neighborhood.” CEG