Monday, October 10, 2005

How we moved one very big rock

The sub-soil (that below the organic layer) of this woodlot is primarily made up of glacial till, or what the locals call “hard pan”. According to the Dictionary, till is “Glacial drift composed of an unconsolidated, heterogeneous mixture of clay, sand, pebbles, cobbles, and boulders” and I would say that pretty well describes our subsoil. It is called hard pan because undisturbed till is very hard to dig. Once disturbed though it can turn into a soup if wet enough. The till deposits or small hills left by the glaciers, around this area are commonly referred to as the “Drumlins of North Gower”. North Gower is the old township name for this area before it was grouped with Marlborough Township to become Rideau Township which in turn was amalgamated with the City of Ottawa.

The till here contains predominantly bluish grey rocks which I am led to believe is limestone. It is the same material that construction gravel is made of in these parts. There is the occasional but rare granite rock amongst them.

There was one rather large such rock that I had to move from our backyard about 20 metres (65 feet) to the fence line. The rock was large enough that I could not budge it even with a well-positioned 2 metre long pinch bar used like a lever. The rock measured roughly 1.2 M (3.93 ft) by 0.4 M (1. 3 1ft) by 0.5 M (1.64 ft) for a total volume of 0.24 cubic metres or approximately 8.4 cubic feet. These measurements are rough to around 10% but probably good enough for this purpose.

The first problem was to get some idea of the weight of the rock. I found a much smaller rock of the same material that was as large as I thought I could reasonably lift. If I knew then how much it weighed I would definitely have found something smaller. In a large olive pail from a restaurant I submersed the rock in water and made a mark on the inside of the pail at the water line. I then withdrew the rock from the water and then determined the volume displacement of the rock by carefully measuring the amount of water it took to refill the pail to the mark. I needed 13 litres plus or minus 100 ml of water – Eureka!

Using the average reading of several weigh scales I found about the house including a bathroom scale and some others, I determined that the rock weighed close to 79 lbs. plus or minus a pound. Yes the weigh scales are still in pounds. This equates to 35.8 kg. Doing the math then, the density of the rock is 2.76 metric tons per cubic metre or 172 lbs. per cubic foot.

To give some idea of how heavy that is, a rock the size of a NBA regulation size basketball would weigh around 20 kg or 44 lbs., the weight of a healthy six year old boy.

With these conversions the rock turned out to weigh around 661 kilograms or 1460 lbs. It is no wonder that it wouldn’t budge with the bar. This is the weight of a small automobile such as the VW Rabbit.

To lift the rock I erected a tripod over the rock and hung a 2 ton (4000 lbs. or 1814 kg.) rated chain hoist from the peak. A chain hoist has a gear mechanism with a built in brake, and two sets of chains and gears. One set is the lifting chain which moves very slowly, while the other set is a looped chain that the operator pulls on. The gearing has a high mechanical advantage so as the operator say pulls three feet of chain on the loop the lifting chain might move an inch or more. The operator has to pull on the chain in either direction to raise or lower the lifting chain.

I made the tripod out of three eastern white cedar polls each 5 metres (16 ft) long. I initially tried 6 metres (20 ft) but the poles would bend under the weight. If the poles were not green and had a chance to dry for six months this likely would not have happened. At the narrow end the 5 metre poles were of a 9 or 10 cm diameter. The next problem was how to securely fasten the three poles such that it would hold such a large weight. While at the Forest fair several weeks ago I observed some boy scouts erecting a tripod and borrowed that method. The scouts lash the poles by laying down the three poles next to each other on the ground with all the points together. They fasten the rope (clove hitch) to one pole and then proceeded to weave the rope over and under each adjacent pole back and forth until they have 12 rows (six each way), a bit like basket weaving. You will probably find this in any Boy Scout book. Actually I have since found an excellent description of this method on the following Boy Scout website:

Once fastened the poles are lifted up. The end poles are spread apart and the centre pole is pushed ahead and voila a sturdy tripod.

I used a 2cm, (¾ in) diameter polypropylene rope for the lashing which worked well and is quite strong.

To fasten the rock to the hook on the hoist, we used a very heavy chain with a grab hook. Placed the chain around each end of the rock and crossing over the chain in the middle and on top of the rock.

At first we needed to move the big rock up an incline for several metres. We did this by lifting the rock with the tripod centred slightly uphill. Lifting the rock would drag it up a bit, we would then lower it, reposition the tripod and repeat. This put a tremendous amount of pressure in the one down hill tripod leg, but it held. It is critical that the tripod straddles the rock. In other words the rock must sit inside the triangle created by the tripod feet. If it doesn’t the already heavy tripod could flip over with a huge and dangerous weight attached to it.

Once the rock was moved to a flat surface, we fashioned a cradle two metres long by 1.2 metres wide out of six boards – three lengthways and three crossways and evenly spaced apart. The boards were cut at the sawmill so were of various thickness’ around 10 cm by 7 cm. (4X3).

We laid down three other three-metre long boards on the ground under the rock, like railroad tracks in the direction that we wanted to move the rock.

We used as rollers the three pieces we cut off the poles we shortened. We placed the cradle on the rollers, which were on the railroad tracks, and then slowly lowered the rock onto it.

Using a chain and the ATV we slowly pulled the cradle with the rock in the desired direction. One of us drove while the other kept feeding the rear log roller to the front under the nose of the cradle. At one point the last roller slipped off from under the cradle. The tail of the cradle was dragging on the ground and the ATV even in bull-low four-way differential lock could not budge it. We were able to move it using the Dodge Ram.

Because the terrain was not always even, sometimes the nose of the cradle would dip down into the ground and we had difficulty getting the next roller under. To resolve that problem we used a car jack (scissor type) to lift the front end of the cradle and placed the roller under.

The rest was pretty easy. Once at the destination, we moved the tripod over the desired spot and lifted the rock off the cradle, removed the cradle, rollers etc. and lowered it into position. The beauty of the tripod method with the chain hoist was that we could very carefully and precisely position the rock with a bit of lateral (sideways) pressure, even for a rock that big.

No comments: