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Drilling & Geotech: September 25, 2008
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From a 5,000 square foot barge floating on the river, crews are working around the clock to take samples of the river bottom and the underlying rock by drilling just off both banks of the river.
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Crews drill through about
20 feet of water, then about 50 to 70 feet
of mud and silt, then
about 85 feet into the limestone bedrock of the river. -
The drillers stop about every
five feet and take a sample of
the material at the bottom of
the hole. When they get to limestone, they exchange the drilling
head to one with a round, grinding
bit. The drill shaft has an external “sleeve” that helps guide the drill. This sleeve also rides over an internal metal shaft
that holds a core – a continuous cylindrical sample of the rock, so geologists can look at layers, or strata, in the rock.
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Crews remove a sample of limestone from the drill to package for analysis. By analyzing the data from
the drilling, the design team can accurately model the river bed of the river, and then design the river piers – the portions of the bridge’s foundations in the river
that support the driver surface.
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A sampling of Mississippi River bottom silt.
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A specialist from Geotechnology Inc. prepares
a sample of limestone for an unconfined compression test. In this test, geologists
determine how much pressure the rock can take without shattering. Design engineers will use this information to determine the strength of the bedrock in the river and how well that rock will support the bridge.
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Disclaimer: Commercial use of these photos is prohibited without permission from the Missouri Department of Transportation (contact email/phone #). All photos should be credited to the Missouri Department of Transportation.