Don't be alarmed.

The sound you may hear at approximately 5 p.m. today is the explosive detonation of about 300 pounds of TNT.

It will be coming from the worksite of the Bill Emerson Memorial Bridge and will represent the first of many such explosions that will take place over the initial stage of its construction.

Flatiron Structures Co. LLC engineers will be blasting 15 feet of bedrock near the corner of William and Aquamsi streets. Traffic will be shut down for approximately 15 minutes over the Mississippi River bridge and along Aquamsi for the explosion.

"We didn't want someone to be startled by a big bang while driving over the bridge and have a head-on," Flatiron structural engineer Norm Kirk said. "You never know: Someone might be afraid the bridge is collapsing."

Kirk said the explosion should carve out one side of a 12-by-60-by-100-foot rectangle that will house the base of the bridge's second pier. The blast is as much a test as it is a construction device because once the TNT has been detonated workers will go in to discover what is beneath the bedrock.

Engineers needed to dig a 30-foot hole for the pier's base. Fifteen feet down they ran into the bedrock, among other things. Bonnie Ates, secretary for Flatiron, said the bottom of the Mississippi River has offered up some surprises.

"When we started to drill we went down a little way and hit rock," Ates said. "I figured once we hit rock there would be rock all the way through. But instead we hit mud, then rock, then mud."

Kirk said Wednesday's blast will outline the rectangle the base will fit into. Once workers clear out the debris the engineers will be better able to determine how much more blasting will be needed. The next explosion at that site would be to clear out the center of the rectangle.

The blast will consist of about 70 separate explosions that will be detonated within 28 milliseconds of each other. Kirk said it might be possible to hear the individual explosions but the entire blast, which might be heard over a half-mile area, should last only about 500 milliseconds.

"It's a relatively small blast," he said. "You should be able to see some debris in the air."

Kirk said the bedrock in the area is not very hard and should fragment easily. Other blasts may be needed throughout the early period of construction, and notification will be given before each scheduled explosion.

Horn blasts will warn of the detonation. Five ten-second horn blasts, separated by two-second pauses, will warn of an explosion in five minutes. Three five-second horn blasts, separated by two-second pauses, will give a one-minute warning to the explosion. One 20-second horn blast will signal all is clear.

Construction blasting is highly regulated. After the detonation, readings will be taken from seismographs that measure both ground vibration and air movement related to the blast. If the readings from the instruments exceed the guidelines for a safe blast -- one that does not endanger private property -- the explosives will be adjusted.

"You shouldn't be able to feel this blast," Kirk said. "Sometimes it seems like everyone in the neighborhood will blame us for cracking the brick on their home. But we're not as concerned about the people in this area reacting like that. They should be used to blasting with the quarry just out of town."

Construction of the $90 million bridge began with the groundbreaking on July 26. The Missouri Highways and Transportation Commission awarded Flatiron the $50,854,029 contract for the "cable stay" section of the bridge earlier that month.

This construction contract is the major part of the Route 74 relocation and Mississippi River bridge project, said Scott Meyer, the department's district engineer at Sikeston.