The limit of pressure to which an underground dam may with safety be subjected, is a question that—as far as the writer knows—has not received the same attention from authorities on engineering construction, or practical mining, that has been accorded to questions on which the design and strength of other structures or machines in connection with a mine are based. That dams may, under certain conditions, be constructed to resist water under great pressure is a known fact. But the difficulty of demonstrating beforehand that any proposed structure of this kind will withstand the pressure and be effective is, owing to the indeterminate nature of some of the risks to be provided against, very great. In the present state of our knowledge it may almost be laid down as a rule, that a dam of this kind should not be undertaken where grave consequences would attend failure, until experience of structures where the physical conditions are similar, but where there are no such risks, has given assurance that such work would stand.
In Mines and Minerals, there is published a paper by Mr. JAMES MCNAUGHTON describing the construction of three underground dams in the iron mining district of Michigan.
The first of these was put in to hold back a flow of 750 gallons per minute, which when pent up created a pressure of 276 pounds per square inch, showing that the height of the sustained column was about 635 feet. The hole through which the water issued was in the bottom of the cross cut; it was enlarged by removing all loose rock for a depth of 18 ft., making it 7 ft. 6 in. long by 5 ft. wide, a heavy 10 in. pipe with valve in it was then fixed to provide for the temporary passage of the water by syphoning. A timber platform was put in 17 ft. below the bottom of the level. The cavity thus formed from the platform to the bottom of the level was filled with concrete composed of one part German Portland cement to two parts of sharp clean sand and four parts of broken limestone. The concrete was allowed to set for a week. The valve was then closed, the entire flow was held back, and in less than twelve hours the pressure had risen to 276 pounds, the same as when water was first tapped.
In the second case described, the water when first struck with the drill had a pressure of 355 pounds per square inch equal to that caused by a column 816 ft. in height. After plugging the drill-hole with a pine plug, a dam was commenced 30- ft. back from the breast to provide against any sudden inrush should the rock fail. This dam was made in the shape of a circular arch on its side, the arch being 6 ft. thick and having a radius of 7 ft. 6 in. On the crowning side of the arch, concrete to a thickness of 5 ft. was laid to act as a sealing device while the stone arch provided the necessary strength.
Local sandstone was used in the construction of the arch. A strong 3 in. pipe with gate valve on outer end extended through the dam to carry away leakage from the end of cross cut while the dam was being constructed. The mortar used was one part cement to two parts sand. After time had been allowed for setting, the valve was closed, the water filled up the space behind and the dam assumed the total load amounting to 1840 tons or 25.26 tons per square foot of surface exposed.
A new cross cut was commenced by branching off from the original one at a point some distance back from the dam, the intention being to avoid the imprisoned water. It was, however, again encountered by the drills. This time, owing to the rock being broken through, provision had to be made by means of a temporary dam and launders for the passage of the inflowing water while a permanent dam was being built.
All loose rock was picked from the bottom, sides and back for a length of 22 ft. at the site of the dam. After concrete to a thickness of 18 in. and for a length of 20 ft., had been laid an 8 in. pipe with gate valve, each capable of withstanding a pressure of 800 pounds per square inch, was placed centrally between the two sides. A heavy wrought iron yoke was used to anchor the pipe into the concrete. When the concrete had attained a height of 5 ft. and had sufficiently set, the temporary dam and launders were removed, and the flow of water began through the 8 in. pipe. The concrete was built up to within 9 in. of the back and then discontinued, the remaining opening being, filled with hard brick and cement mortar. The total load being in excess of 2500 tons, as an extra precaution against the dam moving bodily, lengths of 70 pound rails were placed horizontally in the concrete with the flanges facing out and flush with the face. The ends of these rails were cemented in hitches 6 ins. deep in each side wall. Two steel girders 13 ft. long and 32 ins. deep, with 12 in. flanges were placed vertically, so as to give a perfect bearing to the face of the concrete and flanges of the rails. These girders were so placed as to divide the face of the dam into three spaces of 3 ft. 4 ins. each. The girders were cemented into hitches 18 ins. deep in the back and bottom.
In less than four weeks from the time the inside face of this dam had been completed, the valve was closed, provision having been made for the air between the dam and breast to escape through a small pipe leading to the highest point in the back near the dam.
After all the air had escaped, the dam seemed to assume the load suddenly, the hand on the gauge moving from o to 220 lbs. instantly. In six hours the pressure had reached 340 lbs.
It will be observed that each of the dams may be said to belong to a separate type. The first which had a face area of about 28 square feet may be classed as a concrete plug, and as such would depend for its stability, upon the adhesion of the concrete to the portion of the surface of the surrounding rock with which it was in contact, and also, of course, upon the resistance to shearing of the rock itself.
The second dam is of the arch type, although additional strength was no doubt given by the concrete on the crown, which is stated as having been used as a sealing device. Its face area was apparently about 62 square feet.
The third dam was a concrete plug reinforced with steel girders, but as the areas of bearing for the ends of the main girders amount to only about 6 square feet, it seems certain that the bulk of the resistance is performed by the concrete plug.
It is noteworthy that in all of the cases described, although the volume of flow held back was considerable, there was no storing up in abandoned workings of any large body of water. And it is to be presumed that the worst that would ensue from the failure of any one of the dams, would be the stoppage of work until the pumping power, if not already sufficient, could be increased enough to cope with the additional inflow.
Coming to Victoria, the information as to what has been accomplished in the way of underground dams is very meagre. It is on record from an official source that a dam 4 ft. 6 ins. thick, had in 1893 been in existence in the 500 ft. cross cut of Mr. George Lansell’s No. 180 Mine, at Bendigo; but no particulars are given as to the materials used, form of structure, or pressure sustained.
Some few years ago there was considerable discussion over a proposal to build an underground dam in one of the Walhalla mines for the purpose of holding back an inflow, that was small in itself, but which if stopped by the dam, would have been backed up into old workings of considerable extent, and would have risen to a height of 1000 feet, presuming the rock to have been tight enough to hold it.
The cross cut in which it was proposed to place the dam was about 8 ft. high by 7 ft. wide, and the proposed thickness for the dam was 10 ft.
Two practical mining experts were requested to state whether a dam sufficiently strong to withstand the pressure due to a column of water 1000 ft. high could be built, and also to investigate and report whether the proposed work could, with safety, be constructed. As to the practicality of building a dam of the kind described, the experts appear to have had no doubt, nor had they any fears as to the construction of the work proposed, provided that it was in the form of a curved wall, let into chambers 2 ft. deep in sides, bottom and back of cross cut, and that the material used was brick in cement of the best quality. The dam was afterwards built, but not strictly in accordance with the recommendations. The thickness from back to front was made 11 ft, 6 in., the chambering was made about 1 ft. deep at the water face and tapered off to nothing at the back—giving the dam the form of a wedge¬shaped plug-and only the two faces were built of brick work, the body of the dam between these being of slate set in cement mortar. Fig. 1 shows in more detail the dimensions and style of construction.
On completion, the question of safety again arose and the case was referred to a civil engineer of experience in the construction of hydraulic works. The conclusions he arrived at were:
That the dam as constructed was unsafe. That it would be a matter of extreme difficulty to build a masonry dam that would be safe under the conditions. That the inflow which it was desired to keep back would probably, under such a great head, find its way by percolation past the dam which would thus become ineffective for its purpose.
The correctness of the latter conclusion was eventually verified, for on water being allowed to rise to a height varying from 130 to 180 feet only behind the dam, it was found that considerable leakage occurred through the body of the wall, and also in the strata for a few feet in advance of the dam. The structure was finally condemned and not used.
This dam does not differ materially from the concrete plug type employed apparently with successful results in Michigan; but there is one feature of the design which is decidedly bad. The angle that the slant sides of the plug or dam make with the general direction of the cross cut is about 7½ degrees and the total area of the bearing surface on the sides or skewback is about 340 square feet. The total pressure on the face of the plug, if the water rose to a height of 1000 feet above it, would be 1700 tons or 28 tons per square foot. Hence, the bearing surface or skewbacks would be compressed with a force, per square foot, equal to:
1700 cosec. 7½ degrees/340 = 38 tons.
It is evident that the angle of the slant sides, or the thickness of dam, or both, should have been increased, so that the compression on the skewbacks would not be greater than that on the face, 28 tons per square foot. There would be no advantage in reducing it beyond that point.
To bring about this condition the design should have been, on plan, somewhat as shown in Fig. 2. The dam about 15 feet from intrados to extrados ; constructed entirely in concentric rings of 4 in. brick work, set in the best cement mortar, well bedded, and thoroughly flushed and grouted every course; the angle of the skewbacks should be about 10 degrees with the line of the wall, which would make the thrust carried by them the same as the pressure on the face or extrados, provided that the work were so closely and homogeneously built that all its parts should act in concert and the strains be perfectly transmitted. The skewback seats in the walls would have to be carefully formed to the precise angle with true and perfect face. As it would be hopeless to expect to attain this in schist rock, which would cut in the lines of its natural cleavage, the faces would have to be brought up and formed to a template with masonry in cement mortar. It is not to be understood that a dam so constructed is recommended as safe; what is intended to be conveyed is, simply, that if a brick dam were adopted, the foregoing would be the proper mode of construction so as to attain the maximum of strength. But it would not be safe unless built of bricks capable of resisting with safety a crushing force of 28 tons on the superficial foot; which according to modern engineering practice, would mean, with factors of safety from 4 to 8, an ultimate resistance to crushing of 112 to 224 tons per foot. It is interesting to note in this connection, that at a conference of civil engineers in America called together to consider the plans of the Quaker Bridge Dam, the limit of pressure on a dam was affirmed to be 16 tons.
The failure of the dam mentioned to serve its purpose, must be taken as showing that extreme caution should be used in applying what has been effective in one case, in another where the conditions may only have a superficial resemblance.
This paper has been prepared in the hope that it may elicit particulars of any other local efforts in the way of underground dam construction. The discussion of which may help in demonstrating where, and under what conditions, underground dams may with safety be constructed.