March 19, 2012
Knowledge of geological joints is essential in assuring the economic extraction of crushed stone from quarries, and detailed studies of joints and other planes of weakness should be included in the exploration and quarrying process.
A few years ago, I had the pleasure of being a keynote speaker at the annual meeting of the Institute of Quarrying — Australia. Before and after the conference, our hosts, Andy and Jo Stephens, took my wife, Pam, and me on tours around the Sunshine Coast of Queensland. One of the trips was to Hervey Bay for a humpback whale-watching excursion.
Early in the morning before the boat left the docks, Andy and I went to Point Vernon to look at the geology at Andy’s honors thesis study area. The rocks form a wave-cut platform that is nearly at sea level, and the water highlights the structures in the rock. One of the most obvious features we saw was the pattern created by sets of intersecting joints.
In geologic terms, a joint is a fracture in rock where there is little or no movement up, down, or sideways on one side of the fracture relative to the other. This makes joints different from a fault, which is a fracture in rock where one side slides past the other. Joints generally occur as sets and normally have a regular spacing that is controlled by the mechanical properties of the rock or the thickness of the rock layers.
Jointing can have a pronounced effect on quarrying, and geologists should document the direction and spacing between joints and whether or not the joints are open. The presence of open joints (as well as other major zones of weakness such as faults, bedding planes, and zones of less competent rock) is important to blasting. When zones of weakness are undetected, explosive energy will be vented through the weak zones, potentially resulting in poor fragmentation, excessive airblast, and flyrock. In addition, if a blasthole intersects an unidentified void, the void might be loaded with too much explosive, which has the potential of creating flyrock and excessive airblast.
Jointing also affects fragmentation and crushing. Closely spaced joints usually result in good fragmentation, which facilitates crushing. Widely spaced, pronounced joints usually result in very blocky fragmentation and more difficult crushing.
The orientation of joints affects the stability of the quarry walls. Orientation is best defined by two parameters; strike and dip. Strike is the intersection of the joint plane with a horizontal plane (like the horizontal ocean surface intersecting the joint planes at Point Vernon). Dip is a line on the joint plane drawn perpendicular to the strike.
Joints (and other zones of weakness) dipping out of quarry walls commonly are avoided wherever possible. The likelihood of wall failure (and blasthole cutoffs) increases when joints dip out of the quarry wall, especially when a joint plane daylights the wall in a down-dip direction. The stability of the slope is enhanced when joints (and other zones of weakness) dip into the quarry wall, although blasting in this situation may result in a bump at the toe of the quarry face. A nearly horizontal, pronounced joint may provide a convenient location for a smooth bench floor.
The cool thing is that most geologists love to measure strikes and dips, and asking them to do so is like asking a kid to go outside and play. They have a whale of a time doing it!
Bill Langer is a research geologist who spent 41 years with the U.S. Geological Survey. He can be reached at Bill_Langer@hotmail.com.