THE NEW WATERWORKS RESERVOIR AT TRENTON, N. J.
THE reservoir at Trenton, N. J., has a capacity of 110,000,000 gallons. It is situated on rather valuable land near the northwest edge of the city. The water on the flat portion of the bottom has a depth of forty-five feet, and only thirteen feet of this is below the field level. The remaining thirty-two feet is inclosed by a masonry wall and earth embankments (figs. 1 and 2), built above the original level—that is to say, twenty-nine per cent. of the basin is in excavation and seventy-one per cent. is in embankment. Besides an increased storage capacity, it was desired to raise the flow level to the city as much as possible. After due consideration, an increased head of twenty-five feet over and above the old, or No. I basin was decided upon, this being all that was easily obtainable within the cost decided upon. Trenton has grown upon,and is rapidly spreading over a slightly elevated plateau, and as there is no great elevation to the surrounding country thereabout, the problem l>ecame one of deciding how high into the air safety and the means at command would permit the work to be carried. There was another possible site about three miles farther away from town, but as there was only seven feet additional head to be gained, and about ail of this would be needed to overcome the friction of the water flowing the extra distance, to say nothing of $100,000 or more needed for additional water mains, the meagre apparent gain in elevation was abandoned and the site near the city decided upon.
The field selected contains about fourteen acres in one convenient, nearly level parcel of land. The first work was the sinking of several wells about four feet in diameter, so that the character aud strata of the subsoil could be clearly determined. The material was found to consist of a very superior quality of hardpan, overlying a hard, close sandstone; it is a dense stratum of ocheroua clay, solidly compacted with gravel and some sand. In some places the clay predominates, although a goodly proportion of gravel was nearly always present; iu other portions the sand would seem to be the greater part of the mixture, but even in such sections the argillaceous binding element was sufficiently in evidence to insure good material. A very considerable portion of the stratum, with slight tempering, would make a nearly ideal puddle; in fact, some portions were carefully selected and worked moist into a perfect puddle for ramming into the trench at the base of the walls. The inner embankment, extending from the field level up to the masonry wall, or withiu twenty-one feet of the high water line, is practically composed of puddle. The stripped surface of the ground was well rolled with a heavy grooved roller, and then the embankment built thereon, thoroughly rolled as it went up. After the inner embankment and the inner slopes in the solid earth were formed, as well as the flat bottom of the basin, the whole surface of these was thoroughly rolled, thus forming a BOlid mass of hardpan mixture, compacted and practically impervious. Over the bottom and inner slopes there was placed twelve inches of concrete, made of American Portland cement, crushed trap rock, and clean, washed Delaware river sand. This concrete was laid in two thicknesses of . six inches each; the first layer, next the earth, was made with one and one-half-inch stone, and the top or finishing with fine stone siftings. The inner slope is 3 to 1, and the slope of the outside embankment is 2 to 1. The latter was built against the outside of the wall, and was partly composed of the same materials as the inner portions, although not so critically selected. The entire surface of the outer embankment was covered with six inches of top soil.
The peculiar design employed, consisting of a heavy stone wall, a low inner embankment and a full outer embankment, was brought about by the balance of the equation having the least cost and greatest capacity on one side, with the most available land for the increased head on the other side. To make regular full banks inside and out, with a core wall, would either produce a basin below the desired head or compel such deep excavation to secure the necessary material that the cost would be greatly increased and a great dead water space be formed lowor than the possible bottom flow level. Besides this, the available storage would be less, both actually and in proportion to the cost of the property bought for the site. If material were “borrowed” or brought from elsewhere, the value of the land or right necessary to be bought for such material near enough for cartage, or railroad freight on material from a more distant a and cheaper source of supply, would increase the cost materially. Excellent stone was to be had within economical distances, and preliminary sketches and calculations showed that the amount of excavation down to the bottom flow line would make a full embankment outside the wall, and an embankment within twenty-one feet of the high water line inside the wall, the toe of the outer bank coming within ten feet of the boundaries of the property. So a combination was made with a stone wall of a section giving a fairly low centre of gravity, safe from overthrow against the pressure, regardless of any outer bank; then a full outer bank for insurance, and a low water bank thoroughly protecting the base of the wall. The desired increase of twenty-five feet in the water head having been obtained within walls secure against overthrow, all that remained was to provide a tight bottom for the basin, which, being composed of gravely clay, practically puddle, throughly rolled and covered with a foot of strong, dense concrete, was not difficult of accomplishment.
A peculiar and interesting feature of the construction of the masonry of this reservoir was the employment of three traveling derricks operated by steam power, specially designed and built for the work by the Lambert Hoisting Engine company, of Newark, N. J. Each derrick was mounted upon a low, broad car, which traveled on a track of about twelve feet gauge, and so arranged that the power could be applied to moving the car, swinging the boom, and raising or lowering either the load or the boom and load together. A railroad switch track was laid parallel to the inner line of the wall and far enough away to permit of the derrick track being placed between the switch track and the wall, turntables for the derricks being at the corners, as the curves of the wall were too sharp to permit the curving of the derrick track. The result of this arrangement was that several cars of stone could be switched in and most of the stone taken directly from the cars and laid upon the wall without loss of time or labor, most of the dressing being done on the cars. When the heavy coping for the top of the wall began to arrive, and could not be at once used, it was piled at convenient places near the base of the wall by means of these derricks, and then, when wanted,could be easily handled and laid. The cope stones averaged four and one-half feet wide, three to six feet long, and fourteen inches thick; and some of the stones used in the wall were as heavy as 8,000 pounds, which was the limit of weight allowed in a single stone.
The water is brought to the reservoir, as shown in fig. 3, through a thirty-six-inch force-main, which terminates in a vertical, bell-mouthed pipe, from whence it falls down a series of steps By this means the water is aerated to a very considerable extent, and enters the basin with little disturbance of the water already in storage, thereby insuring, as far as possible, the absence of special currents, and magnifying the chances of sedimentation before being taken out at the opposite end. The water leaves the basin about 700 feet from where it enters, and, during atmospheric conditions most likely to produce anchor ice, at the protected end of the reservoir. There are two fortyeight-inch outlet pipes, as shown by figs. 4, 5, and 6, supported on stone piers, and surrounded by concrete and puddle. These pipes, one at an extreme low drainage level and the other twenty-feet above the bottom, are united in the gatehouse and provided with valves, so that either or both may be used in connection with either or both of the street service mains shown. The water is usually taken from the upper pipe, thus accentuating the benefits of sedimentation as the water passes from one end of the reservoir to the other. The advantage of twenty-five depth of water below the working outlet is thus combined with facilities for “blowing out” the sediment at the lower pipe whenever desired.
At present, at least, this reservoir is fairly large in proportion to the system it supplies, and one of the practical benefits of this fact is the chance for sedimentation afforded, and also cessation of pumping when the river is turbid. These features, combined with the points already noted in connection with the inlet and outlets, result in a very high quality of effluent as to clearness, as was at once noticed all over the city after the new reservoir had been placed in commission. A more specific statement relating to the improvement due to sedimentation is with the gauge for turbidity showing 0.12 at the pumping station, the effluent from the basin at the end opposite the inflow has shown only 0.02.
In continuing Mr. Hague, in the Engineering News, goes to say that the water was turned into the reservoir on September 20, 1899, and in the course of thirty days it was run up to within ten feet of the high level. Then it became apparent that seepage through t he uew concrete was taking place, although not to any serious extent, and on account of some saud streaks in the hardpan bottom, water slowly found its way to the outside, showing on the surface of the ground at different places. However, as the water level was easily maintained and the pumphouse records showed no material increase, the matter was not considered more than a natural and temporary leakage for new concrete and masonry. When the excavation for the basin was commenced and down for about ten feet, the “ puddle ” material continued all over the field, but below the ten-feet spots, pockets, and streaks of sand commenced to develop, and, finally, at the complete depth, there were three or four places along the Pennington avenue side which looked as though they might give trouble under forty-five feet head of water Also, when the two thirty-inch service mains were laid along the avenue a little below the level of the bottom of the reservoir, sand pockets corresponding to those within the basin about 200 feet away were revealed. But, as this sand stratum was very dense and hard, it was not considered at all dangerous, but rather presented a proposition as to whether it justified the necessary expenditure for shutting it off. Finally, for the sake of the chance of saving several thousand dollars, it was decided to go ahead to completion and then judge upon the matter of remedy, after the fine silt which tne water carried had an opportunity of asserting itself on the surface of the concrete. The matter of asphalt lining was also considered, and some interesting experiments were tried with a slab of asphalt concrete. The slab was made of the thickness aud consistency projiosed to be used, and was two feet long by one foot broad. It was placed under water in a tank ahd supported at each end, so it could be bent in the middle about four inches below level; it was left so several weeks and then turned upside down and bent the other way about an equal length of time. Then it was immersed and kept in water, in which cakes of ice were constantly floating, so as to represent possible conditions in winter, the bending and rebeuding being continued. It acted in a very satisfactory and promising manner as a reservoir lining, especially in a case like this one, where the asphalt concrete would never be exposed to the high temperatures of the summer months; and it showed that any settlement or loss of alignment in a reservoir bottom could be met by the adjustability, perfectly automatic, at any probable water temperatures, without crack or rupture.
After water had been held in the basin at a depth of about thirty feet for six weeks, it was emptied aud examined : but everything was found to be sound and in good order, so it was again filled and put into service in December, 1899, continuing to supply the city to the present time. A blind drain of crushed stone was laid all round the reservoir, at the toe of the outer embankment. This drain was three feet deep and covered with a foot of earth, so the ground surface presented a normal appearance. The drain was included in the original design, but was omitted to save cost,although the sequel indicated it to be desirable. This encircling drain was tapped by two tile drains at opposite ends of the reservoir, and the seepage, together with the surface drainage, was carried away from the neighborhood of the basin about 1,500 feet in each direction, emptying Into a running brook. The surroundings of the reservoir immediately began to dry up, and in fact, the fields thereabouts, in the spring nnd fall, were drained as they never were before. At first the drains showed an outflow of a volume which half filled the six-inch outlet at one place, and with the water running about two inches deep in a ten-inch drain at the other; so, on considering that 110,000,000 gallons of water was In storage under a head of forty-five feet, the seepage could not be said to be excessive. Part of the escaping water was, of course, the underdrniuings of two extensive, rather fiat fields, lo tho course of seven or eight months the (low nearly ceased; in fact, at times entirely so at the six-inch drain, and probably dropped down to au amount representing mostly land drainage As no other flowing water is in evidence, and the pnmpage and effluent seem to be normal in quantity, there is evidently no further appreciable leakage from the basin.
When the saud streaks were first discovered, and their potency for mischief recognized, the idea of additional lining, of course, evolved itself; but, when the basin was emptied after six weeks from the first water, the wet, Impalpable silt, spread like a velvet carpet over the entire bottom, gave promise of remedy which was finally realized; the drawing of the effluent so far above the bottom as twenty-five feet no doubt Insuring such a quietness of the body of water as very materially to affect the seepage for the better.
[The designer and engineer of the reservoir was Charles A. Hugue, of New York city, member of the American Society of Civil Engineers, the American Warerworks association, and the American Society of Mechanical Engineers, with K. Albert as assistant engineer. The contractor was Lewis Lawton. The board of water commissioners of Trenton,is as follows: Chas. A. May, president; Cbas. C. Engel, Chas. H. Young, Wm. H. Baker, Henry J. Nicklio, Harry E. Fisher. Chas. H. Skirm is secretary and treasurer, and John H. Hell is the able and intelligent superintendent. ED. F. & W.J