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Weeping Water, Nebraska Martin Marietta Aggregates New Mine Portal Entrance
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Omaha Nebraska is the largest city in Nebraska with a population of over 400,000 people. A growing city the size of Omaha needs crushed stone and gravel to build the roads, sidewalks and foundations on which the people depend. Weeping Water, NE is a small town just 20 miles southwest of Omaha that supplies most of the rock for the Omaha market. Limestone mining operations have been in existence for nearly 100 years in this small town. Martin Marietta Aggregates is the second largest construction aggregates producer in the United States and it operates an underground limestone operation in Weeping Water that produces over 2.3 million tons of crushed stone a year. A normal limestone operation is usually done in an open pit after the overburden has been taken off, but not in Southeast Nebraska. In this part of the country there is over 100’ of corn growing dirt on top of the limestone. To add to the problem there is another 110’ of non usable rock on top of the Kereford/Platsmouth Limestone that Martin Marietta crushes. Due to the massive amounts of overburden and junk rock economics dictate that in order to extract the good limestone it must be removed using underground mining techniques. Martin Marietta uses a room and pillar drill and blast method to extract the rock. Years of mining have left a labyrinth of rooms that spread out from the one and only entrance into the mine. Miles of roads have to be traversed by haul trucks laden with the broken rock to reach the outside world.
Engineers
were called in to design a way to get to a 20 1/2-foot layer of high quality
Kereford/Plattsmouth limestone that is over 200-feet deep. It was decided that
the most economical way to reach this layer of rock would be to ramp down to it
and remove all the material out of the way. As a safety feature the engineers
would put in catch benches. These specially designed benches would keep
material from falling from the top all the way to the bottom. By taking a giant
triangle out of the ground, haul trucks would be able to drive down the ramp and
straight into the Kereford/Plattsmouth limestone layer.
Once
an acceptable location was selected and the permits issued, construction on the
new mine portal entrance began. The future entrance was in the middle of a corn
field. Sudbeck Construction performed the site work grading and excavation for
the project. First the overburden would be striped off to get to the top of the
rock. To construct the mine entrance, 100-feet of dirt overburden had to be
removed to get to the top of the rock ( 1.6 million cubic yards of dirt). The
sides were sloped back on a 2:1 to ensure slope stability and the ramp was set
on a 10 degree decline. Once the dirt was peeled back it was time for Dykon
Blasting Corp. to blast their way down to the Kereford/Plattsmouth limestone
layer.
The final depth of the excavation was 240-feet from the top of the ground with 140-feet in solid rock. At the surface of the rock the opening is 220-feet wide and 900-feet long. The rock was excavated with 3 separate benches. The ramp followed a constant 10 degree slope down to the bottom where it leveled off for 65-feet before the back wall (Portal entrance).
Dykon
loaded the blasts with the aid of a bulk explosive truck. The bulk truck
blended ANFO (ammonium nitrate fuel oil) and emulsion before auguring the blend
into the blast holes. The amount of material that Dykon was able to blast and
the time it took to prepare a blast let Dykon stay well ahead of Sudbeck
Construction. In one week Dykon blasted enough material to keep Sudbeck digging
for a month. During that time Dykon returned home and resumed working on other
jobs in Oklahoma and Arkansas. Dykon blasted approximately 500,000 cubic yards
of rock which Sudbeck had to load and haul up and out of the hole.
Martin Marietta will now take over and begin driving drifts (tunnels) into the Kereford/Plattsmouth limestone. Martin Marietta will also be relocating their primary crusher and screening plant to the new location which is approximately two miles from their existing location. The move should keep the residents of Weeping Water, Nebraska a little happier and a lot less dusty.
Article written by Jared Redyke
Project Pictures
