Blasting Methods
Explosives are used to remove critical structural elements of the grain elevators, allowing the remainder of the structure to collapse by gravity.
Only a small portion of the total structure is affected by the blast of the explosive. These key portions are carefully selected in order to steer the rest of the structure in the proper direction as it collapses. The majority of the damage done to the concrete actually comes from the falling elevator breaking up on it's way down or when it impacts the ground and piles up on itself.
In grain elevator work, there are two key structural elements that are
removed with blasts. These are the lower pillars which fill the first
level of the buildings and the lower portions of the silo cells themselves.
There are over 500 truckloads of concrete in each elevator. 
The elevators are roughly 110' high but are only 10' away from the edge of the dock face. The water is up to 38' deep alongside the elevators so the elevator is almost sitting on the edge of a cliff of sorts. Due to the high cost of removal, it is important that the elevator not collapse the wrong way, into the harbor. Explosives are placed to cause the elevator to drop away from the dock edge. Pillars and silo walls are blown away to make the elevator lean away from the dock, cut almost as you would cut a tree to make it fall a certain direction.
It doesn't take a lot of explosives to blow out a pillar or wall section. There are, however, hundreds of these small shots involved in a single elevator blast. Our largest shot involved two elevators, the "D" head house and the "D" west annex. In less than eight seconds, both elevators were destroyed with a total of roughly 800 separate small shots. The timing of these was controlled by a small desktop computer (an old 386 did the job, it only had to count to 800 in 8 seconds so it didn't have to think too hard about it.)
While it only takes about four seconds to detonate all of the charges to drop a building, it usually takes months of work to prepare the blast.
The first job is to carefully analyze the building drawings, noting critical structural elements, and coming up with a blast plan. Next, with flashlight and can of rat repellant spray in hand, a thorough inspection of the building is made to determine the actual construction, find weak areas, and do a final check for any hazardous materials that need to be removed.
Several test shots were conducted to determine the proper explosive loading in each of the several types of pillars and walls. Click on the picture below to see the result of a test shot on a 4 foot diameter pillar in the "C" head house.
The pillars below the "C" west annex were roughly four foot by 16 foot solid concrete. A single shot down the center was all it took to destroy it.
Further up in the structure it was necessary to eliminate the first 5-20 feet of silo wall in order to get the elevator to fall properly. Below is a photo of the results of a test shot which turned the bottom 15 feet of the silo walls to gravel. Notice how it stripped the concrete walls leaving only the bare reinforcing steel bars. The vertical column in the right of the picture is an extension of the basement pillars which was not shown on the construction drawings. It was discovered only after this test blast of the wall structure. The original blast plan was then modified to account for this internal column.
Before the explosives are loaded into the bottom pillars, holes were drilled into each one. Because of the limited space in these areas, custom drill rigs were brought in. These drilled a 1.5" diameter hole which ranged from 2' to 9' deep into each pillar depending on pillar size. The longer holes took about 15 minutes each to drill. There are between 200 and 400 separate holes drilled for explosives in each of the elevator buildings.
Often, there will be walls that are too thin to bother drilling. Usually, if the wall is less than 18" thick, a blasting technique called mud capping is used. Here, the explosive is placed against the wall and bags of mud or wet sand are placed over the top of it to give the explosive something to push against during detonation. The most effective place to place a charge on a wall section is partway up so it is usually necessary to build some sort of temporary (very temporary) shelf to hold the charges and sand bags. Here are two photos of mud capping, the left showing explosives placed atop a layer of sand bags on a shelf, the right shows sandbags covering the explosives and the mud cap shot ready to be detonated.
On most of the silo wall blasts, water was used to contain the blast, greatly cut down on flying dust, to minimize the amount of explosive used, and reduce noise from the blast.
The center elevator proved to be the most challenging of the six that were imploded. It was the oldest of the structures. It was imploded two silo cells at a time rather than the whole elevator at once. Rather than the typical 8"-10" thick silo walls, the center elevator walls were from 18" up to 40" thick. Rather than a high strength reinforcing rod, a soft mild steel strap was used for reinforcing. These straps were roughly 1/2"x2". They tended to stretch rather than yield. An early test shot resulted in the bottom 30' of the test silo bulged out but not destroyed.
By combining explosive types, a slow type, with a fast one, the walls were shattered and then pushed outward. The upper portons of the silo cell then dropped by gravity. Due to concerns for noise and the peace and quiet of the neighbors, a delicate balance had to be maintained between too much explosive (which was excessively noisy) and too little (which would blow out the bottom of the silo but leave buttress walls still standing. When the buttress walls remained after a blast, they held up the remains of the upper portion of the silo cell. In these cases, I would have to wait several days to be sure it was reasonably safe to enter. I would carefully place mud cap charges against the buttress wall and drop the silo cell by basically knocking the legs (the buttress wall) out from under it.
Loading of the explosives for dropping a whole elevator at one time usually took three or four days. In a typical elevator blast, several miles of connecting wire was used. The explosives are not all detonated at the same time. Time delays are placed in the circuits to control when each shot goes off, both to minimize ground vibration and air blast noise, but to time blasts so that the elevator twists as it falls. This twisting causes massive structural fracturing and greatly assists in assuring a successful drop.
Poking the button
When blast day arrives, there are hundreds of last minute items that must be checked and double checked. The miles of blasting circuit must be checked multiple times right up until the final moment. ( On the "D" west elevator shot, I discovered that a beaver had snagged and broken a control wire leading to a portion of the blast circuit.)
Conditions must be just right for a successful blast. If there are any storms approaching, ships docked at the nearby port cranes, tugboat activity, a bad headache or any other significant reason, the blast will be cancelled. Most of the blasts were delayed at least several times, one was delayed as many as twelve times. It has to be right. I even delayed a shot because it "just had a gut feeling about it".
As the final hour approaches, calls are made to the local 911 center, the coast guard, and all the neighboring businesses. A check is made for the passing Vista Queen tour boat and smaller watercraft. An inspection is made of the recesses of the dock face for inquisitive local photographers in kayak's. The warning whistles are sounded. Cameras are focused, the computer does its' final check of the circuits, and my heartbeat quickens.
Last minute security checks are made by two way radio with our three or four spotters around the corners of the site. The skies are searched for any approaching small aircraft. "Here we go" I usually announce over the radio. Five seconds later, the button is pushed (actually I poke "enter" on the computer). There is a delay which always seems to last far too long and without a sound, puffs of white smoke come shooting from the lower openings of the elevator.
Moments later, the sound reaches the ears. From deep within the elevator the rolling rumble of explosive destruction comes pouring out. You feel it in your ears and throughout your body. Seconds later it is suddenly quiet again except for the startled screeches of the seagulls. Then comes the creaking, crashing, banging of metal, the thuds of massive wall sections bashing into each other as the elevator slowly begins to lean, drops to one knee, then collapses into a heap. What moments before was a grand solid concrete structure a hundred feet tall, four hundred feet long, and ninety feet wide now rests in pieces, most of them smaller than a foot in size. The small crowd takes a breath and a child lets out a squeal of delight followed by hoots and hollers by the adult children.
The dust cloud is amazingly thick up close to the blast. The heavy particles quickly drop out within a few hundred feet, coating the grass, road, machines, and the unlucky video camera on its tripod. The very fine dust stays airborne and slowly rises, catches the breeze and drifts off. The gulls come back to check it out, some of them to quite a different neighborhood than they left minutes before. (I'm afraid that some of them will recognize me later, walking the lakewalk, and decide to take their revenge.)
I am nauseous. All of the preparation, all of the worries, all of the work. It looks as though the blast went well, but I won't know until the dust settles. My greatest fear is that someone will be hurt. Our security measures have paid off, everyone checks in fine. Not a scratch. I begin to relax. We stand in a small group and ponder the recent change in the Duluth skyline.
It's cleanup time. The tugboat crew needs to get through to get to work so I clear the road with the front end loader. The rest of the work will wait until morning. I take a few Advil's to fend off the tension headache that usually follows a blast and crawl into bed.