Mining Jerusalem Stone - Digging Jerusalem's Foundations with CAT Hydraulic Hammers

November, 2006 - Copyright by Morris Rosenthal - - contact info

Sometimes Jerusalem has the feel of one giant construction site, and just like building a suspension bridge, the tough part of the job is digging the foundation. Jerusalem could have been called the stone city, as the hills are living rock covered with a little top soil and a sprinkling of vegetation. Some of the rock is the prized Jerusalem stone, that shows pink or gold at twilight, depending on your visual acuity and imagination. This presents construction project managers with an odd sort of a choice, as removing the stone in larger, more valuable chunks, takes more time. The value of the stone can defray some of the cost of the excavation, but only at the price of slowing the timeline of the construction. Blasting in the city isn't permitted, so the stone has to be quarried or demolished with heavy equipment. The most common method on large sites is to get three or four Caterpilar excavators with a hydraulic hammer on the arm banging away in unison. The CAT hammer pounds away at somewhere between 300 and 600 blows per minute, but having lived right next to a major excavation, I can tell you that the impacts are so distinct that it sounds slower.

While little chunks of Jerusalem stone are easy to remove from the hole, they aren't that useful for sawing up and selling. The tool that looks like a gigantic chain saw in the picture to the right (they don't let anybody climb into the pit for a close-up) is a rock saw that's used to quarry some very clean slabs of stone that are ideal for resawing into tiles or cladding. As the foundation deepens, the equipment operators leave a ramp along one wall to get the equipment out, and which later serves as the ramp for the underground parking garage. All new Jerusalem construction ends up with these incredibly expensive underground parking garages, and due to strict limits on the heights of buildings, the foundations often appear to be deeper than the finished building is tall. All of this may change, as the Safdi Plan for expanding Jerusalem into the western hills has just be scrapped by the current mayor, in return (one suspects) for support from the Green parties for allowing Jerusalem to be built higher. Part of what makes me suspicious is that some of the foundations I've seen excavated look like they could take a hundred story tower, and I wonder if it's foresight.

The photo to the left really captures the CAT hydraulic hammer in action, with stone dust flying off the end like a cloud of smoke. As you can see, the result is more stone breaking than quarrying, which is why the saws are used for cutting the really prime Jerusalem stone. The excavation usually starts at 7:00 AM and stops before dark, so it's not impossible to continue life right next to a site, and the vibrations are surprisingly tame. The fragments are removed by dump truck, and probably end up as wall capping material throughout Israel. If you've ever wondered at all the beautiful stone walls in Jerusalem, the new ones are all fake. I meant that in the sense that they are formed concrete walls with rebar and steel mesh, and a facade of thin stone stuck on the face. Larger walls almost always show missing pieces, as the technique is frequently practiced by workers who either don't know what they are doing or don't care how it looks six months after they leave. Most of the large retaining walls in Jerusalem that are required to create level building plots on the hills are faced with stone, in some cases, with boulders, but a real concrete wall with drainage is always in the back.

The walls of the foundation are are reinforced concrete columns that are drilled, rebarred and poured prior to excavation. Despite the seeming solidity of the wall face, a form of steel retaining band is anchored to the wall in courses that correspond with a building story, about 3 meters. A few feet from the edge of the hole you may have a corrugated steel fence, with dwelling directly behind. I almost rented a B&B room in a place that was no more that 20 feet from an active excavation, and it was a cellar apartment at that. My memory is that hydraulic hammer noise doesn't bother me, may even lull me back to sleep in the morning by screening out all the other city sounds, but looks like I won't be finding out this year. Once the foundation is dug, it's all reinforced concrete construction, and I've been shocked to see that they still use handmade wooden form work on some most jobs, even with tower cranes dumping hoppers of concrete straight from the mixers. I've never understood why they use so many tower cranes in Israel on construction projects that rarely go over three stories, but I guess with the hole, it's fairly tall:-)

The rig to the left is drilling shafts for micro-piles along the edge of what will soon be another giant pit. It's a somewhat heavier rig than you'll see in the U.S. drilling artesian wells, and while it doesn't have to go nearly as deep, the bore is much larger, just under a half meter. The picture below shows the rotating and reciprocating shaft at what I'd call the well-head, if it were digging a well, and it gives you an idea just how beefy the rig is You can also see mud used to lubricate the drill bit overflowing from the shaft, the same as you'd see with a rig for oil or gas. It's not nearly as noisy as a hydraulic hammer, but it is a form of hammer drill, with distinct blows you can feel through your feet if you get close enough.Once the micro-piles are all poured and set along the perimeter of the excavation site, the CAT's go to work, and the operators stay a little away from the edge. Since the barrier of micro-piles has already broken the stone inside the excavation from it's natural surroundings, as the hydraulic hammers get close to the edge, the remaining chunks just fall away.

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