A SMALL CONCRETE LINED RESERVOIR

A SMALL CONCRETE LINED RESERVOIR

In the design of any system of water works intended to furnish water to a municipality, a reservoir of some sort is usually included. The introduction of this part of the system has been found to be desirable from the standpoint of economy and safety. This article will describe the design and construction of a small concrete lined reservoir having a capacity of one million gallons. A reservoir of this type was lately completed for the Town of South Boston, Va., in connection with improvements and extension to their water works system. Concrete was chosen as the material to be used on account of its cheapness while at the same time giving a practically water-tight structure. The section of the reservoir selected was chosen in order that the structure might be as compact as possible and at the same time keep the cost factor low by limiting the volume of concrete and the amount of form material required. The slope of the side walls was made one to one and one-half, this being as steep a slope as would allow the side walls to be built without any face forms and permit the earth banks to serve as the forms for the back. In the construction of the reservoir proper no steel reinforcement was required. This reservoir is situated on a hill on the outskirts of the town and was constructed partly in cut and partly in fill. This method reduces to a minimum the charge for excavation and embankment as the material in cut was used in making the embankment on which the sloping side walls were constructed. The excavated part was carefully made and trimmed to line and surface, the embankment being made full and the inside trimmed down after being compacted and allowed to stand for several weeks. The excavation in the bottom was carried six inches deeper than the bottom of the concrete lining and this space refilled with puddle composed of clay, sand and fine gravel. In making the embankment the material was deposited evenly in layers about six inches thick and thoroughly solidified by rolling. After being rolled each layer was wet by sprinkling just ahead of the new layer being deposited, which was immediately rolled. A small horsedrawn roller was used, weighted down with stone and pigs of lead. After completion of the grading and the trimming of the sloping banks, the concrete work was begun. A one, three and five mixture of Portland cement, sand and broken stone was used throughout. The six-inch bottom was first laid in a monolithic slab, trapezoidal pieces of timber being inserted in the green concrete near the outside edges of the bottom to provide a joint for the side walls. The concrete bottom having become sufficiently set, the nine-inch lining for the sloping side walls was started. Planks to serve as guides for screeding were placed on the side slopes and securely held to the proper line and grade by fastening to stakes driven into the earth slopes. These planks were so placed as to leave sections approximately eight feet wide between them, this width proving to be a desirable one for placing the concrete and screeding and smoothing the surface. Pieces of timber similar to those used in the bottom were spiked to the face of the planks to form a key for the adjacent section. It was first intended to construct alternate slabs of eight-foot width leaving the intermediate slabs to be completed later. It was decided, however, to change this method of procedure by decreasing the width of the sections omitted to about eighteen inches and the remainder of the construction was done in this manner. In this way it was thought to eliminate almost entirely the formation of cracks between the adjacent slabs due to contraction in setting, since the wide slabs were allowed to set for some time before the narrow ones were laid and the small width of these latter gave very little contraction. On account of the numerous construction joints alonjj the side slope no expansion joints were provided. It was necessary to have the mortar quite stiff in order to keep it in place, as a wet batch was sure to land in the bottom due to the rather steep slope of the blanks. The mortar was thoroughly tamped in place, and screcdcd and given a rough finish true to grade. Each section was completed before stopping work on same for the day. The curved corners were divided into small wedge-shaped sections, each being finished with flat surface instead of curved. At the top, stakes for the curved corners were set with a transit by turning off deflection angles. The curb was not poured with the sloping side sections. These latter were cut off at a point near the bottom of the fifteen inch radius curve through which the side lining is continued to form the vertical wall above the beam. Forms were used in muolding the curb which was constructed in sections corresponding to the ones on the side slopes. The curb at the corners was made curved, however, instead of being divided into several small tangents. After the concrete in all of the sections had been poured, the entire surface was given two brush coats of cement grout. This grout was mixed to about the consistency of whitewash and applied in thick coats as possible.

Gate House.

The small gate house is constructed of reinforced concrete with a brick superstructure. It houses a ten-inch electrically operated gate valve and two ten-inch sluice valves. A ten-inch line of cast iron pipe leading into the gate house from the town serves both as an inlet and an outlet. A single ten-inch line leads from the gate house to the reservoir. Making one pipe serve as inlet and outlet to the reservoir decreased by about one mile the length of pipe that would otherwise have been required. A twelve-inch pipe to serve as an overflow and drain leads from the house. The gate valve on the inlet and outlet line is operated electrically from the pumping station, being left open ordinarily but capable of being shut immediately when extra pressure is needed as in case of fire. The stopping of the ten-inch pipe to the reservoir at the bottom of the sloping side wall would appear to the writer to be a feature which might perhaps have been improved upon. Continuing this pipe, alternate sections to have small Tee outlets opening upward, and protecting this pipe with a concrete casing would perhaps effect a better mixing of the water in the reservoir and prevent any tendency to stagnation. This reservoir was designed by Anderson & Christie, Inc., of Charlotte, N. C., and was built under their supervision.

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