Thursday, March 27, 2014

Waterjetting 19c - Of soil removal by water

The tragedy in Washington state, where a hillside slid down, across a river and destroyed and covered the small community of Oso on the other side is a reminder of the ways in which natural ground saturation can help dislodge large volumes of material. While it is unlikely that the exact trigger for the slide will be identified the underlying cause is well known. As water from the heavy persistent rains permeated down through the hillside it penetrated deeply into the sand, silt and clays that made up the cliff, filling the voids in the material and reducing the friction that held the slope together.

Once the water is between the grains there is a second stage, where the overlying water adds hydrostatic pressure to the water below it. This pressure starts to push the grains apart, further reducing the friction holding the grains together and, since water has no shear resistance, lowering the overall resistance to the gravity that is pulling the overlying material down a potential failure plane. Once the resistance falls below that pull the slope starts to fail. It happens very quickly and large volumes can move almost as fast as though it were just water. In this case the water moved roughly 15 million cubic yards of material off the mountain. and across the valley.

The power of water to move material once dislodged was an early part of the mining process, and an earlier post referred to “hushing” where water was first trapped, and then released to erode away overlying soil, and then ore, before carrying it down to a flume where the valuable mineral could be trapped and recovered. But it does not take much pressure to dislodge weak material. Russian studies* have shown that light soil can be moved with a pressure of only about 10 psi (which would be generated in the water at the bottom of a slope only 20-ft high), while medium soil would require perhaps 30 psi, and firm clay would need a pressure of about 100 psi before the jet would mine and erode it.

That ability to move soil using a monitor was further developed in Russia, where tests showed that it would take water flows of between 3 and 10 times the amount of soil being removed depending on the strength of the material. In the gold mining regions around Lake Bykal it lowered mining costs to 40% of that for conventional mining, and gradually the tool found wider application in general soil removal, being used in the construction of several dams, and also for soil removal during construction of the Moscow Canal.

An interesting example of the speed and effectiveness with which waterjets can remove relatively soft soil formations arose during the Yom Kippur War (10th of Ramadan War, October 1973) between Egypt and Israel. The Israeli Army had built defensive positions along the edge of the Suez Canal and these were mounted behind an earthen and sand barrier known as the Bar-Lev line.

Egyptian intelligence had determined** that the Israeli Army had assumed it would take 24 hours for this barrier to be breached, and a total of 48 hours for the Egyptian tank forces to successfully penetrate the line. The response time of the military units was planned accordingly. This time estimate was based upon the time that it was expected to take to make a hole 22-ft wide through the barrier, since this had, for each breech, to move 60 cu yards of material, and a total of 60 such holes were needed to get enough troops through to be effective. Conventional methods involving explosives, artillery, and bulldozers would taken over ten to twelve hours, and required nearly ideal working conditions. For example, sixty men, 600 pounds of explosives, and one bulldozer would have needed five to six hours, uninterrupted by Israeli fire, to clear 2,000 cubic yards of largely sand from the wall. Employing a bulldozer on the east bank while protecting the congested landing site from Israeli artillery would be nearly impossible during the initial hours of the assault phase. Construction of much-needed bridges for the main army would consequently begin much too late.

Figure 1. Using waterjets to breech the Bar-Lev line during the 1973 war (Mashpedia)

To deal with these 70-ft high sand and earth barriers, the Egyptians instead used water cannons fashioned from hoses attached to dredging pumps that were floated on platforms in the canal. A study was made of the speed with which different pumps could move the sand:soil mix and the initial tests with British pumps showed that it would take around 3 hours. But by combining them with larger German pumps into six pumps per breech the army was able to get the time down to about two hours, although it took a little time after that to deal with the residual mud on the floor of the breech and give an adequate road for the tanks and other vehicles. A total of 81 holes were made, moving over 3 million cubic yards of material.

Figure 2. Egyptian forces crossing the Suez Canal, showing the size of the breech created (Wikipedia)

The bridges were then put in place, and the Egyptian Army moved on into the Sinai, well ahead of the time that they had been anticipated to be there.

The combination of high-pressure water to dislodge and separate the particles of a soil/sand layer can be combined with the active suction from a modern vacuum truck, in what is now being called hydro-excavation. By removing the soil and water as the excavation is made this stops water from entering the walls of the excavation and leaves them relatively dry – thus resolving one of the problems that the Egyptian army encountered after their holes had been made. At the same time, by using smaller jets at higher pressures and moving them much faster the excavation rates can also be increased, with lower water volumes, so that narrow trenches can be excavated without the need for support, where rapid access is needed. But I will talk about those applications in a separate post.

*Okrimenko, V.A., "Hydro-Monitor Operator in Coal Mines and Pits", State Scientific Technical Press of Literature on Mining, Moscow, 1962, pp. 264 (Translation U.S. Army Foreign Science and Technology Center, Document AD 820634, 1967).
**London Sunday Times, December 16, 1973, p. 33

1 comment:

  1. As water from the heavy persistent rains permeated down through the hillside it penetrated deeply into the sand, silt and clays that made up the cliff, filling the voids in the material and reducing the friction that held the slope together. water removal