Table of Contents
While the installation of mine bulkheads to retain water under high pressure is by no means a rarity, the following points which arose in the designing and placing of two of these bulkheads may be of interest:
The Hibernia magnetite mine, located about 40 miles west of New York, in the State of New Jersey, is partly filled with water. This mine is located on an ore lens, the outcrop of which is over a mile long. The ore has been mined from this lens from the surface to a depth of more than 1,500 ft. It was held desirable to separate the old workings from the new, and to allow the former to fill to the 850-ft. level. Two weak places existed between the new and old workings below this level: namely, a temporary bulkhead on the 10th level and a rock bulkhead of indeterminate thickness on the 16th level. When the matter of allowing the old workings to fill was investigated, the fact was developed that if these old workings were filled, the bulkhead on the 10th level would be subjected to a water pressure of about 50 lb. per square inch, and that 200 lb. per square inch would act on the 16th-level bulkhead. As the above- mentioned barriers were not deemed of sufficient strength to permit of these pressures, it became necessary to design and install new bulkheads.
Design
As the 16th-level bulkhead was required to withstand a pressure of 200 lb. per square inch, it presented some difficulties. A careful consideration of the various types of mine dams now in use led to the adoption of a design of the form of a truncated wedge. In this, the pressure side of the dam is of greater area than the back, so that the resultant action is similar to driving the wedge. By cutting generous skewbacks in the walls, roof, and floor, this type in reality becomes an invisible arch. The wedge feature tends to compress the materials in the bulkhead, thereby adding to its imperviousness.
Concrete was chosen as the material. In order to lessen the labor and simplify the construction of the forms, straight forms were placed on both the front and back of the dam; the arch in this bulkhead is therefore invisible.
As ordinary concrete is by no means impervious to water under the head of 200lb. per square inch, it was decided to water-proof this bulkhead. The most economic method, consistent with good construction, was to face the entire pressure side with a 3-in. layer of a water-proofing compound. As described in detail under “Construction,” this facing was carried up with the concrete, thus insuring a perfect bond. The subsequent test proved the efficacy of this facing.
To provide for future contingencies, involving a possible further waterproofing of the pressure side, it was decided to place a manway through the dam, permitting inspection or repairs.
The relative positions of the bulkheads on the 16th level are shown in
Fig. 1. Attention is called to the fact that the old bulkhead leaked to the extent of about 6 gal. per minute, and that five pipes pass through it for the purpose of draining the water in the old workings by pumping it to the surface from the 16th level of the new workings after it had passed through the old bulkhead. The new bulkhead was designed to continue this function of drainage in case it should be so desired, and for this reason prolongations of the pipes pass through it. Due to the afore-mentioned leak, when the new bulkhead is completed and its manway sealed, the water which passes through crevices in the old bulkhead will soon fill the space between the old and the new dams. The new bulkhead will then be assuming the entire load.
The details of the steel reinforcement placed in the dam are shown in Fig. 2, the horizontal rails being curved to the radius of the invisible arch. Great care was taken to thoroughly coat all metal surfaces with mortar.
The design of the manway is shown in detail in Fig. 3.
It was decided to use a 1:2:4 mixture of concrete for this bulkhead. Atlas Portland cement was used, and a local sand, carrying less than 3 per cent, of foreign matter, was obtained. The broken stone employed
was a gneiss. This was the result of former milling operations, and was screened and washed before being used.
Construction
The construction of the 10th-level bulkhead presented no great difficulties. Ample storage space was available on this level and all the necessary, materials were lowered and stored near the site. After thoroughly cleaning and washing the mushroom-shaped cavity, Fig. 4, the forms were placed, and braced from behind with 6-in. and 8-in. round timbers. The inside of the forms was covered with tar paper, and a 3-in. drain pipe, for possible future use, was run through the forms. This pipe was fitted with a gate valve on the working side. Two-thirds of a cubic yard comprised a batch of concrete, which was mixed rather wet, so that after a batch had been placed water rose slightly above the level mass. Three iron rails were placed across the mouth of the cavity for reinforcement. The 12 yd. of concrete were placed in 10½ hr.
In drilling the recesses for the 16th-level bulkhead, care was taken to
so point the holes that the excavation would coincide in form and dimensions to the design. At a distance of 25 ft. from the old dam, holes 36 to 39 in. in length and spaced 1 ft. apart were drilled in the sides, roof, and floor, at right angles to the course of the drift. Stoping and hand drills were used in this work and four men constituted the gang in this as well as the subsequent drilling with, column drills. Thirty-five feet from the old bulkhead a series of holes 4 ft. in length and from 1 to 1½ ft. apart were placed slanting to conform approximately with the inclinations of the skewbacks. These holes were only burdened with about 1 ft. of ground. Under ordinary conditions longer holes would have been drilled, but the proximity of operating pumps made extraordinary precautions necessary for their protection during the shooting, and so heavy blasting was not attempted. A round of six holes was shot at a time. A third series of holes was drilled slanting to conform with the deeper portions of the recesses. When blasted, this series broke evenly at the line of the 3-ft. holes first drilled and the resultant recess conformed almost exactly to the figure determined upon, and the total excavation agreed with the original estimate of 60 yd. The muck was economically disposed of in a nearby chute.
The materials required for the work were unloaded and stored close to the mouth of the shaft. Due to the lack of space in which to store the materials on the 16th level, the matter of lowering and delivering the required materials without interrupting the work was one of the most troublesome obstacles encountered. Eight to twelve men were employed on the surface in sacking sand and stone while the excavation was in progress on the 16th level. The empty bags produced as the cement was used augmented the 200 old cement bags purchased for the sacking. While enough sand could be stored on the 16th level for this entire bulkhead, there remained insufficient room for the storage of the daily requirements of stone and cement. It was found advantageous, therefore, to employ a small night crew, who lowered much of the material required for the next day’s work.
The ten curved rails were bent to a 14-ft. radius over a form in half a day. This was done on the surface.
The forms on the 16th level bulkhead were built of 2-in. undressed lumber and 6 to 10 in. round posts were used for studding and braces. The forms were thoroughly braced and were wired to stiffen them further. The interior faces of the forms were covered with tar paper, and the junction of the forms with the rock was plastered with a 1:1 cement mortar on all sides. The pressure-side forms were carried to the roof of the level at once, but did not extend into the recess.
The recess was thoroughly cleaned of loose rock and washed down, and all the reinforcing material, pipes, and the manway were placed in position before the concreting was started. Furthermore, the floor and sides of the recess were plastered with a 1:1 cement mortar before placing the concrete.
The concrete was made of one part cement, two parts sand and four parts stone, these proportions being determined by actual measurement. A batch of concrete contained 2/3 cu. yd. The sand was first placed on the mixing platform and the heaps flattened down. On this was emptied the cement, and these two materials were thoroughly mixed and flattened out before receiving the stone. This mixing took place about 12 ft. from the front form of the bulkhead. Enough water was used to make a wet mixture; Two men did the first mixing and turned the mass, then passed it on to the next two, who again turned it, passing the finished concrete to the last two men at the mixing board. These men shoveled directly into the form. In this manner, while each two men received a short rest of a few minutes between batches, fresh material was being placed on the starting end of the mixing platform while the men nearest the form were still disposing of the concrete mixture. This also insured a thorough mixing. One man remained in the form to level off each batch. The best day’s work consisted in the placing of 12 yd. of concrete.
The water-proofing compound, “Impervite,” was carried up as a 3-in. facing, its level being kept the same as that of the concrete. An even thickness of the water-proof layer was maintained by the use of three forms of 3/16-in. plate, 6 ft. long by 6 in. wide, fitted at the upper corner with 3-in. spreading bolts. These forms, placed across the entire width of the face, were raised 3 to 4 in. at a time, and enough concrete was then shoveled against them to keep them in place. The almost semi-liquid water-proofing compound was mixed on the level and was carried to the forms in buckets.
Before leaving at night, sharp man-size (about 100-lb.) stones were set at least 6 in. apart in the concrete mass. This made a strong bond, and before concreting the next day this rough surface was freshly plastered with a thin 1:1 mortar.
As the roof was reached, false forms were placed, and the work was finally finished in tightly bonded dovetailed blocks.
Throughout the work, the leakage from the old dam, 6 gal. per minute, passed through the 2-in. drain pipe of the bulkhead.
Seven 2-in. grout pipes, four on the pressure side and three on the opposite side, were placed in the concrete as the work neared completion. They were all located near the roof and were directed to such places as were most difficult to fill with concrete. As the work had to be hurried, but a day and a half elapsed after completion of the cement work before grouting was begun. The grout mixture was a mortar consisting of one and one-half parts of sand to one part of cement made fluid with water-dissolved “Impervite.” A mine-made grout “gun” was used, and the grout was forced successively into the several pipes by means of air under the pressure of 85 lb. per square inch. As the grout was forced through the different pipes the ejection of some of this material through the other pipes indicated that the greater voids. were filled. As the “gun” connections were changed those pipes giving the greatest discharge were plugged, and the discharge was finally limited to one pipe. This too, was filled and plugged. The first day’s grouting was allowed to set over night, and the following day all the pipes were again tested. This time there was no communication between the pipes, and as little or no grout could be forced into any one of the pipes the grouting was considered most satisfactory.
Three weeks were determined upon as the period which should elapse before the new bulkhead should receive any load. During this time the 2-in. drain pipe was left open.
At the expiration of this time the completed 16th-level bulkhead was tested by pumping water up to the pressure of 160 lb. per square inch into the space between the old and new bulkheads through the 2-in. drain pipe. The results were entirely satisfactory, as the total seepage amounted to only ½ gal. per minute at first. This small leakage subsequently stopped almost completely.
Cost
A cheap class of labor was employed exclusively, the men receiving $2 per 10-hr. shift.
Following are tables showing the cost of the work. The interference caused by the necessity of keeping two large pumps in operation within 50 ft. of the 16th-level bulkhead was perhaps the greatest cause for the apparent high cost. The labor cost of lowering materials was also very high for the amounts handled, which, in the case of the 16th-level bulk-head, had to be lowered 1,350 ft. in one skip.
