(3.) Laying the pipes at too great a depth without protection by concrete or otherwise to resist the pressure of the superincumbent earth, or by not sufficiently punning the filling-in, when a sudden settlement will often crack or crush a pipe. (4.) Accidental or wilful injuries to pipes which are not noticed before the trench is filled in. (5.) Laying the pipes at too shallow a depth without protection, when heavy traffic or a falling weight upon the surface will crush or crack a pipe. (6.) Defective or weak pipes.[184] The following plates give the different forms of sewerage pipes that are now manufactured in this country, in addition to those of common shapes: [Illustration: BROOKE’S PATENT SUBSOIL DRAINS AND PIPE-RESTS.] [Illustration: CREEKE’S PATENT CAPPED PIPES.] [Illustration: MAWBEY’S PATENT GROOVED SOCKET-PIPES.] [Illustration: HENRY SHARP, JONES, & CO.’S ROCK-CONCRETE PIPES.] [Illustration: JENNING’S PATENT CHAIR AND SADDLE PIPES.] The chokage in pipe sewers generally arises from one or more of the following causes:-- (1.) Improper gradients. (2.) Insufficient flush.[185] (3.) Foreign articles finding their way into and choking the sewer. (4.) Defective joints through which the liquid runs leaving solid matters behind. (5.) An excess of road detritus or of ashes, through the house closets of the poor, finding their way into the sewer. (6.) Improper bends in the line of sewer. (7.) Right-angle or improper junctions being formed with the sewer. (8.) A collapse of the sewer. A temporary chokage in a small-sized sewer which does not arise from any structural defect can be speedily and effectually remedied by the use of Ben Reed’s patent drain-cleaning rods, which are probably too well known to every town surveyor to need any description, but in conjuction with man- or lamp-holes and straight lines of sewers they are very valuable. Before closing this chapter, a few words upon what is called the “separate system of sewerage” may be of use. The mistake hitherto made has been to try to absolutely separate all rainfall from the sewers, and there is no doubt that a partial separation of the rain-water from the sewage proper has many advantages; but it must be understood that a great deal of the rain-water that falls upon roofs of buildings and in back yards and small back streets must of necessity be carried into the sewers, as well as liquid refuse from manufactories. A partial separation thus carried out has the following advantages: (1.) It is not necessary to have sewers of enormous diameter. (2.) The depth of the surface water conduits need not be so great as that which is necessary for sewers. (3.) The avoidance of road detritus being washed into the sewers. (4.) Where the sewage has to be pumped or treated chemically or put on the land, the combined system causes an immense unwieldy bulk of liquid to be at times dealt with.[186]