CHAPTER XI SEWERAGE (Part 2)

their strength. The inner ring of brickwork should be of blue Staffordshire bricks, which form a non-absorbent lining. The bricks should preferably be specially made and moulded to the proper radius, so as to permit of thin joints of uni- form thickness, instead of wedge-shaped joints as they would otherwise be, though this involves considerable additional expense. Between the inner and outer rings of brickwork a thick line is shown. This represents a collar joint, consisting of a layer of cement mortar, about 4 inch in thickness, the object being to give a second line of defence against leakage. Sometimes the sewer is only lined with Staffordshire bricks up to the commencement of the covering arch, A—B, the arch being of ordinary pressed, or engineering, bricks. The outer ring of brickwork should be of pressed bricks in all cases, and it is not important that the bricks should be moulded to the proper radius. Good cement joints are quite sufficient. Invert Blocks for Brick Sewers. If the sewer is very large, the invert blocks may be built up of three pieces in the width. The sketch (Fig. 887) shows the centres from which the ares are struck, and from which the joints of the brickwork radiate. Concrete would, in any case, be put under the invert blocks, and up the sides of the sewer also, to form a support on which the brickwork can be laid. 858 SEWERAGE EGG-SHAPE AND CIRCULAR SEWERS COMPARED UNDER DRY WEATHER FLOW COND/T/IONS JOINTS FOR SEWERAGE 859 An alternative method of construction is shown in Fig. 888, in which the joints of the brickwork are not indicated. The sewer is partly in two rings of brickwork, and partly in one. In- vert blocks are used, and the lowermost third of the height is lined with blue bricks, the remainder being of pressed bricks. Dealing with Subsoil Water. Difficulty sometimes occurs by the rise of subsoil water as the work is being carried out. Sometimes special invert blocks are used, having holes to admit the subsoil water to the spaces left in the hollow blocks, which form channels conducting the water to the lowermost end of the system. Another good method is that shown in Fig. 889. Below the concrete a small trench is cut to take a land drain of from 8 to 6 inches in diameter. The pipes are unglazed and are merely butt-jointed, without sockets. The trench is filled with broken stone. A third method, adopted where springs occur, is to place a pair of drain pipes on end, one on either side of the sewer, and fill them with gravel, leaving small openings in the sewer to allow the water to escape laterally to the top of the pipes and so down through them. The openings in the sewer are closed as the work proceeds. The methods of setting out the course, and gradients, of a sewer are similar to those adopted for a drain and described in the preceding chapter. ““Grips”’ for Pipe Sewers. In the case of pipe sewers, grips should be cut under the sockets to permit the joint to be properly made and to permit the pipe to be supported throughout its length. Timbering for Sewer Trenches. The timbering of the trenches also follows the same lines as given in Figs. 359 and 362, but for very bad ground the sizes of the timbers must be increased. Should the ground be very treacherous it may be necessary to leave in permanently the lowermost set of timbering, as otherwise the sides may fall in as the timber is removed, but it is seldom that this is necessary. Sewers Laid in Running Sand. In the case of running sand, small sewers should be of iron pipes supported similatly to that shown in Fig. 365. So also should they be treated where they pass above ground, with additional strengthening given by a cross timber above the pipe, and longitudinal timbers connecting the heads of the piles. Open Cutting Versus Tunnelling for Sewers. Where the sewer is very deep below the surface, it is not economical to lay it by means of an open cutting. In such a case tunnelling should be resorted to. For a simple case, the tunnel might be formed by 860 SEWERAGE TERRA-COTTA INVERT BLOCK USED IN BRICK EGG-SHAPE SEWER Petes Pees TPRPer eS yy " FOR SEWER IN WY vy / a We DRAINAGE FOR SUB-SO/L WATER ff) U SECTION A-B BRICK MANHOLE FOR 30" 45" EGG-SHAPE SEWER SEWERAGE 861 excavation, timbering the work on lines similar to those shown in Fig. 390. The timbers shown would be of about the following sizes: longitudinal members 9 x 8 inches or 9 x 4 inches, other members 9 x 9 inches, at horizontal intervals of about 5 feet. If the tunnel is large, it is often constructed by means of a com- pressed-air shield of the type used in the construction of tube railways, the tunnel being lined by cast-iron sections built up to form a circular lining, bolted together and well grouted, between and behind, with Portland cement grout. Such a tube could be completed to form the sewer by lining it with fine concrete washed with neat cement, or the sewer might be constructed inside it in the ordinary way. Sewer Manholes. Sewers should be laid in straight lines between points of inspection, manholes being placed at all changes of direc- tion and gradient. Manholes should also be put at the junctions of main and branch sewers, and also at storm overflows, and, on straight lengths, at intervals of about 100 yards. Bends in small sewers should be formed entirely in manholes, but in sewers of large size, this is, of course, impossible. The forms of manhole vary with the size and type of sewer, and also with individual preference. Thus, with a small sewer, there is an opportunity of forming a reasonable benching at either side of the open channel. Manholes may be built of brickwork or formed of concrete tubes, the latter being much the cheaper. Figs. 893-895 show details of a brick manhole on a 12-inch pipe sewer. The plan of the floor is shown in Fig. 895, while Figs. 893 and 394 show longitudinal and cross-sections respectively. An inspection of these details will show that the lower part of the manhole is 4 feet 6 inches by 3 feet internally, the upper part being reduced, by arching over, to a shaft of considerably smaller size. The benching is formed by means of Staffordshire blue bricks. Over the man- hole shaft is a heavy iron frame with removable cover. In all manholes step-irons must be built in as shown in Figs. 891 and 893. They should be protected against corrosion by a thick bituminous coating or by heavy galvanising. The bench- ings should be at, or slightly above, the level of the top of the pipes and should have enough crossfall for condensed water to run off them into the sewer, but not so much as to make it difficult for a man to stand on them. The brick arch at C in Figs 393 and 394 may well be replaced with a reinforced concrete slab. To prevent the weight of the walls cracking the pipes small arches are formed over the latter as shown in Fig. 894, 12° 862 SEWERAGE Ve BWW ww PLAN ON A-B 395 BRICK MANHOLE FOR 12 INCH C/RCULAR PIPE SEWER - SEWERAGE 868 Manholes of Circular Plan. Some engineers prefer manholes of circular plan, on the ground that they are stronger, but they are rather more costly to construct, unless of precast concrete. If circular they should be made tapering upwards, so as to give adequate working room at the bottom and not too large a cover at the top. A disadvantage of this type of manhole is that it is difficult to get up and down the step-irons at the tapered part. Manholes for Large Sewers. In the case of a larger sewer, the base of the manhole is of different construction. An example of this is shown in Figs. 891 and 892. The latter shows the plan, and the former a cross-section, of a manhole on an egg-shaped sewer. It will be seen that, without making the base of the manhole of exceptional size, there is no appreciable benching, but merely enough to give a foothold. The curved shape of the walls on plan is to give greater strength. Manholes for Streets with Heavy Traffic. In a street in which the traffic is very heavy, manhole openings in the roadway are apt to cause considerable interference with the traffic; and it is then a common practice to provide manholes with side entrances, the latter being accessible from the pavement. Figs. 396 and 897 show details of such a manhole. An access shaft leads down from the pavement to a short tunnel leading to the side of the sewer. The tunnel is best formed by arching over, or using reinforeed- concrete construction. The whole manhole can be formed of either brickwork or concrete, or partly the one and partly the other. A vent shaft is carried up to the roadway, in the form of a pipe, preferably to a ventilating column, but if it is not per- missible to erect such a column, there will have to be a ventilating grating at street level. Ventilation in some way is very desirable in the case of a side entrance manhole, however much one may object to sewer openings at street level, as the large size of the manhole makes it a considerable receptacle for the accumulation of foul air. Spacing of Manholes. Manholes should be from 75 to 120 yards apart; if for any reason two manholes must be placed at a rather greater distance apart than this, a lamphole should be provided between them. Lampholes. A lamphole can be formed of 12-inch or 15-inch pipes leading vertically down from the street to the crown of the sewer, cased round in concrete and surrounded with a block of the same material, or a stone slab at the top, on which is placed a removable cover. A lamp can be lowered down the shaft thus SEWERAGE 364 LNIWINYd OL ~~ LIVHS SSIIIY SHAPED SEWER B 9 % aw i 8 KR 1S) ly y PLAN ON LINE A- 397 SIDE ENTRANCE MANHOLE ON EGG SEWERAGE 865 formed, and anyone in the next manhole on either side should be able to see the light and the condition of the inside of the sewer. Sewer Gradients. While a sewer should, if possible, be laid with sufficient gradient to ensure the flow through it being a self-cleans- ing one, it should not be given an excessive fall; it is often said that excessive fall causes undue wear and tear on the pipes, but there does not seem to be any real evidence of this. Ramps for Sewers. Where the fall is considered too great, the difficulty can be overcome by the use of a ramp similar to that shown in Fig. 398, but it is more usual, in the case of a sewer, to form it somewhat as in Fig. 419. The incoming sewer, at the higher level, is fitted with a specially made vertical junction bend of easy radius, and is discharged into a vertical pipe, which should be of smaller diameter because the velocity in it will be so high. The energy of velocity is dissipated by eddying in its fall and its flow around the bend leading into the manhole. The junction bend at the top of the vertical pipe is also extended through the manhole wall at A; this is to give access and ventilation to the upper pipe. No stopper should be put at A, as this would impede ventilation. It will be noted that the vertical pipe is cased in concrete. Disadvantages of Ramps unless Essential. Ramps should be avoided as far as possible, as they add considerably to the cost of the sewer; for the manholes must be deeper than if the sewer were laid at a uniform gradient; they also tend to churn up the sewage and liberate gas. They can in most cases be avoided by the judicious use of intercepting sewers. Prefabricated Manholes in Concrete. Figs. 400 and 401 show sections of a concrete tube manhole, whilst Fig. 899 shows a sectional plan. It will be seen that whilst the shaft tapers, so as to secure good working room at the bottom with a cover of reason- able size at the top, yet the difficulty of access, which usually occurs with a tapered shaft, is avoided by making the upper and lower shafts tangential to one another on one vertical line; the step-irons are, of course, fixed on this straight side. The shafts are built up from precast rings, from 2 to 3 feet long, having “ogee” joints, as described for concrete tube sewers. The sewer inlet and outlet can be formed in one with the bottom ring, with either ogee or socketed joints, or, if preferred, the bottom ring will be supplied with stoneware pipes inset. The channel and benchings are formed in cement concrete and cement mortar after the bottom ring has been set in position. Increased Gradients at Bends. Where a bend occurs in a sewer, 366 SEWERAGE pe Ee PLAN ON A-B 399 PLAN AND SECTIONS OF PRE-FABRICATED CONCRETE MANHOLE FOR 12” SEWER SEWERAGE 867 it should have a rather greater fall in its length than the uniform fall of the straight lengths it connects, to allow for increased friction. Where a small sewer discharges into a larger one, their inverts should not be at the same level, that of the smaller being above that of the larger, in order to prevent sewage standing in the smaller, and the consequent check on the velocity of its flow. No Right-angled Junctions. There must be no right-angled junctions on a sewer, just as there should be none on a drain. All branch sewers should join the main with a bend pointing in the direction of flow. In the case of pipe sewers, this is easily effected inside the manhole by the use of proper stoneware channel junc- tions. In the case of an egg-shaped or other form of brick or concrete sewer, the junction is effected by the use of specially made stoneware junction blocks, built into the sewer at the required positions. Bell-mouth Junctions. Where large main sewers join, a special form of construction becomes necessary, the form depending on the special condition to be met in each case. Figs. 402-406 give details of what is known as a bell-mouth junction between three egg-shaped sewers. The construction will be fairly evident from the illustrations. The walls of the sewer are thickened around the junction, as shown in Figs. 402 and 403. The former shows a plan and the latter a longitudinal section. The remaining illus- trations show cross-sections of the junction at three different points. This example will give an idea of the method of con- struction in any somewhat similar case. Road Gullies. Road gullies should be placed at intervals of from 40 to 75 yards, according to the longitudinal fall of the road. They can be of iron, stoneware, artificial stone, concrete or brick- work, They can also be either circular, square or rectangular on plan, but the general principles given for the design of any sanitary fitting will apply; that is to say, there should be a minimum of angles and corners. Fig. 407 shows a section through a well- known type of stoneware gully. The trap is formed in one piece with the main body of the gully, and an access stopper is provided to the drain leading to the sewer. In the example shown, the stopper is of iron, fitting into an iron frame with an airtight joint and secured by means of gunmetal thumb screws. The part below the trap is for the purpose of intercepting the mud, which is removed at more or less frequent intervals. One of entirely different construction is shown in Figs, 408 and 409. It is built of brickwork, lined inside with cement mortar, covered by a stone slab, and surmounted by the usual heavy iron 868 SEWERAGE SSS N SECTION E-F (> = may SECTION A-B Ome 403. BELLMOUTH JUNCTION SHOWING TWO 18" SEWERS CONVERG/NG JO SECTION G-H JOIN A 27" MAIN SEWER STONEWARE . ROAD GULLEY NEEDS SETTING ON CONCRETE AND SUR- 8 ROUND OF SAME TO SECTION A-8 SECTION C-D TAKE HEAVY /RON 408 &409. BRICK ROAD GULLEY WITH GRATING AT GUTTER STONEWARE TRAPPING BLOCK SEWERAGE 869 grating. Fig. 408 shows a longitudinal section and Fig. 409 a section at right angles to the kerb. The trap is formed by means of a stoneware trapping block, T.B., in two pieces, a light flap valve being added at F.V. if desired. There is no direct access to the drain from the gully, but an access stopper exists just under the roadway surface as shown. Flushing Sewers. Flushing is often necessary for sewers. For example, if a sewer is designed for prospective requirements and will, for some time, receive much less than its ultimate quantity, it should be provided with means of flushing. So also, if it is of a very flat gradient, insufficient to cause it to be self-cleansing. There are many means of flushing. Automatic flushing tanks have been dealt with under the heading of drainage, and large tanks of that kind, built of brickwork and of course placed under- ground, are often used. Such a tank should always be provided at the upper or dead end of each section of any sewers that need flushing. Any flushing arrangements should have for their object the maintenance of an effective velocity through the sewers, for a sufficient time to remove any deposits and cleanse the sewer. One of the simplest ways of flushing is by