English Service Reservoir Construction
A rather interesting departure in the construction of service reservoirs is seen in the system instituted by the chief engineer of the Metropolitan Water Board, London. England, in connection with several new works of this class in progress and contemplation, economy being, of course, the main consideration.
It had previously been the practice of several water works engineers in Britain to build service reservoirs on the circular plan with the concentric walls so arranged that the water entering the outermost section flowed in a spiral path to the central draw-off well, the object being to keep the water in constant circulation.
The construction as outlined being costly and the benefit of the circulation of the water having been proved doubtful it has now been abandoned in favor of the more obvious rectangular plan, making use of ferro-concrete.
The standard units adopted is a rectangle 11′-6 3/4” by 17′-6 3/4″ and throughout the interior this is the spacing of the columns and all the future reservoirs of the hoard are intended to be constructed as suitable multiples of this unit.
One of them is now near completion having 11 rows of columns each way which with the surround to the walls gives a floor area of 128′-9″ by 200′-10 1/2″ and a top area of 136′-9″ by 208′-10 1/2″ the depth of water being 16′. The main spring from a point 3. feet above this giving a total height from floor to underside of roof of 18′-5″.
The standard wall section is seen in Fig. 1 the section shown being adopted for foundations on sand, but the base width can be varied according to the class of backing to the wall and other considerations. Generally, the footing is carried to a depth of four feet below floor level.
Fig. 2 shows the standard roof construction. The columns are of plain concrete 15″ square with 1 1/2 chambers at the corners. Columns resting on normal ground have bases 2 feet square extending 1 foot below floor tapering to 4 feet square at underside of floor slab. The normal span from beams is 17′-6 3/4″ reduced in the span from outside walls to 16‘-7 1/2″.
The main beams, shown in Fig. 2a, are 19″ deep by 12″ wide deepened at the columns to 30″. The standard reinforcement shown consists of 7/8″ bars and 3/8″ stirrups for the most part. At the ends, these beams rest on the side walls for a distance of 12″. The secondary beams, Fig. 2B are 11” deep and 9″ wide, the depth being increased to 19″ where these beams run into the main beams either at the piers or intermediate points, and they are spaced 5′-10 1/4″ apart. The roof slab is 4″ thick with two rows of mesh reinforced placed 1″ from top and under sides.
The construction, it will be seen, follows the accepted lines of ferro-concrete construction, and the layout of the work permits it being carried out with the minimum of false work, and moreover much of this can be used again, giving a method of construction both simple and strictly economical.