HC-Lecture64-Excavators-for-limestone

HC-Lecture64-Excavators-for-limestone - Heavy Construction...

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Heavy Construction Heavy Construction Lecture #64 Lecture #64 Heavy Excavators Selecting a heavy excavator for hard limestones. L Prieto-Portar 2008
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Excavating in South Florida’s “Capstones”. Most geotechnical engineers that have practiced extensively in South Florida quickly become aware of the exceptional qualities of the Miami and Fort Thompson Formation freshwater limestone. This limestone is commonly referred to as a “capstone” among contractors in South Florida (an example is shown in Figure 1). Freshwater limestone strata are usually thin, ranging from 1 to 8 feet thick (0.3 to 2.5 m), and is commonly described as a very fine grained and moderately hard limestone with small freshwater gastropods (helisoma) that clearly identify the rock. The Standard Penetration Tests (SPT) usually will not differentiate between hard limestone and very hard limestone breccia and well-lithified micrite. Commonly, the limestones encountered in the Fort Thompson Formation are poorly cemented, interlayered with sand seams that permit easy excavation. The typical direct shear strengths of these limestones are 20 to 80 ksf, unit weights of about 115 pcf, porosities of up to 50%, specific gravity of about 2.23, apparent void ratios of 0.65 and electric resistivity of 11,500 ohms-cm. In comparison, the freshwater lens has average direct shear strengths of 150 ksf, but may be as high as 270 ksf, unit weights of 160 pcf, porosities as low as 13%, specific gravity of 2.74, electric resistivity of 19,300 ohms- cm, specific gravities of between 2.70 and 2.80, modulus ratios ( E / q u ) from 148 to as high as 260, Poisson ratios from 0.11 to 0.29 (the latter typical of man-made concrete), and longitudinal wave velocity from 6,000 to 6,500 m/s.
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Figure 1. Photo of a specimen of freshwater limestone found below the surface muck at US-27 and Okeelanta, Florida. This limestone is common to all of South Florida is a well-indurated medium grey micrite, and is referred to as a “capstone” by contractors. This particular stratum was only 12 inches thick, and an inexperienced contractor attempted to break it using a heavy Caterpillar D11N tractor with a single CAT-TR-70 ripper blade. The machine lost, and was badly damaged! Experienced contractors know that force alone is not effective, and that proper equipment and technique are required to penetrate and excavate freshwater limestone strata. Information courtesy of David Julian, Senior VP of Kelly Tractor.
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Ski lodges Figure 2. This sample of freshwater limestone (indurated micrite) was excavated by a marine contractor in the C-4 Canal close to FIU. It was tested with hydrochloric acid. Notice the intense reaction of a drop of acid, indicating a high content of calcium carbonate. Also notice the similarity of this freshwater limestone with the specimen from Okeelanta shown in Figure 1 (dense, low porosity, high specific gravity, very hard and concave cleavage). Photo is courtesy of Art Sengupta PE, SFWMD.
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Figure 3. This photo shows a sample of freshwater limestone excavated by a marine contractor in the C-4 Canal,
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HC-Lecture64-Excavators-for-limestone - Heavy Construction...

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