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Marble Quarry: Block Extraction, Undercut Marble Quarry: Block Extraction, Undercut From "Trade School" episode DTRS-111

Figure A

In this episode of Trade School we follow Thomas Dagnan, a marble quarryman-in-training (figure A). During his initial, 90-day probationary period, Thomas must prove proficient in operating the heavy equipment used to extract marble blocks weighing anywhere from 50,000 to 500,000 pounds. Not only that, he must also demonstrate an iron constitution as he works deep inside the mountain, where it's always winter.

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Figure B

Marble is a stone widely used in home building materials such as tile, countertops and fireplaces. Formed long ago by immense heat and pressure within the earth's crust, marble is essentially crystallized limestone and organic materials. Because marble does not split easily, it must be extracted carefully in blocks. This particular marble quarry uses a "room and pillar" style of excavation, meaning that as huge caverns of the marble deposit are removed, 50-foot-by-50-foot pillars are left alone to support the mountain (figure B).

Basic tools:
Korfmann saw
Pneumatic ratchet
Jackhammer
Hydraulic crimping tool
Wire saw
String
PVC pipe

After identifying and measuring a 20-foot-by-20-foot section of a marble wall to remove, Thomas uses a Korfmann saw to make the first horizontal undercut. The Korfmann is a huge, track-mounted hydraulic chainsaw featuring a 13-foot blade with tungsten-carbide cutting bits (figure C). Capable of cutting at a depth of 10 feet, this saw can traverse from left and right and also swivel on a pivot. A huge track hoe lowers the 14,000-pound unit in front of the marble wall (figure D).

Figure C

Figure D

With the saw mounted on tracks and leveled, Thomas sets a wood block under the blade to guide the starting cut at a slight incline (figure E). This allows the water used to cool the blade and debris to flow back out of the cut.

With the desired angle set, he pivots the blade into the one far end of the marble (figure F). After engaging the marble, he switches to automatic cutting mode, keeping a watchful eye on the saw's pressure gauges (figure G). He continues cutting by slowly fanning the blade, using the traverse and pivot simultaneously. Eventually, the blade ends up perpendicular to the wall face on the opposite side of the block.

Figure E

Figure F

Figure G

Although the tungsten-carbide bits are very strong, they tend to wear out frequently, especially when cutting through the harder quartzlike veins (figure H). To prevent damaging the saw, Thomas must frequently stop cutting and replace each bit one by one. With more than a hundred bit holders on the chain, each one containing two bits, this process is a necessary evil that can take as long as two hours to complete. After loosening each bit with a pneumatic ratchet, he secures new bits with his fingers in numbingly cold water (figure I).

Figure H

Figure I

Figure J

To finish the undercut, Thomas must make a second pass with the blade, but this time in the reverse direction. In order to do this, he pivots the blade out of the block and raises it to a vertical position to loosen the chain. Then he lowers the blade to the other side, removes the chain and reattaches it so that the bits can cut in the opposite direction (figure J). Finally, he traverses the saw to the side and repeats the cut until reaching a perpendicular position on the opposite side.

In the next segment, Thomas continues freeing the enormous block of marble from the quarry.