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Text 17

DRAINS

Drains and sewers must be formed of good sound pipes of glazed stoneware, heavy cast-iron or other equally suitable material. The size will vary according to the volume of drainage, but it is essential that the pipes should be of adequate size and laid to a proper gradient, with suitable watertight joints. The minimum internal diameter for a sewage drain is 4 in., and it is the usual practice to use this size for most work of a domestic nature where the work consists of separate units with separate sewer connections. House drains should be laid in a similar manner, as already described for sewers. The work, however, is usually less involved, owing to small sizes and shallow depths. It is best to use the sight-rail method for gradients, although frequently a tapered board is used for setting out the fall of the drain. This method, although not so accurate, is satisfactory for the construction of short lengths of drain. A suitable tapered board cut to the required fall is used with a spirit level for grading the trench bottom.The following are some general principles:

(1) A good foundation is essential; therefore the best method is to lay a 6-in. bed of concrete, even if the soil is firm.

(2) Avoid, if possible, passing drains under a building. Where no other means are practicable, then surround the drain with 6 in. of concrete or lay in cast-iron pipes and provide suitable access at each end of any straight length which passes underneath a building. The access, of course, must be provided outside and convenient for rodding purposes.

(3) Keep the drain about 4 ft. away at least from the external walls. Where circumstances will not permit this and the drain is adjoining the wall, make sure that concrete is carried up to underneath the lootings of the wall.

(4) Where a drain passes through a wall it must be protected against the weight of the wall by constructing a relieving arch or other similar support.

(5) If the levels are such that part of the drain is to be above ground, then use cast-iron pipes and construct suitable pier supports adjoining the joints.

COMBINED DRAINAGE-Usually a number of houses may be connected to the drain, which in turn is connected to the sewer. This method saves tapping the sewer for each house, and is quite satisfactory. Not more than six hours should be on a combined drain, and the size of the pipes should be 6-in. after the first two houses. All sewer connections should be 6-in. from the last house inspection chamber to the sewer. The fall or gradient is designed to provide a self-cleansing velocity so as to keep the drain free from deposits. Where a sufficient fall cannot be obtained, the solids will accumulate and eventually choke the drain; on the other hand, if an excessive fall is provided, the effluent will run off quickly, leaving the solid matter behind to accumulate and putrefy. A satisfactory gradient is essential, and for a 4-in. pipe the fall should be in 40; for a 6-in. pipe provide a fall of 1 in 60; for a 9-in. pipe, 1 in 90, and so on; the rule being to multiply the internal diameter of the pipe in inches by ten.

Stoneware pipes are usually jointed with cement mortar composed of one part of Portland cement to two of sand. A good method is to insert first a few strands of tarred gasket or hemp before finally completing the joint, as reviously described. The gasket prevents the jointing material finding its way inside the pipe. Cast-iron pipes are jointed with gasket and metallic lead. The joints are not so numerous, owing to the longer length of such pipes.

HOISTING AND FIXING-During recent years, methods which were used in the department of masonry have changed considerably. This change is due to the different types of structures now being erected, and the general use of derrick or jib, cranes. This applies chiefly to the larger buildings, but as the old methods are still in use on buildings other than steel-framed structures, a general survey of both methods will be of interest to the student.

The mason, or fixer, as he is termed, doesn’t usually suggest the type of scaffolding, as the erection of the necessary hoist is usually left to those who are specialists in this work. It is only necessary for him to give the approximate weight of the heaviest piece of stone and state his requirements.

In towns a staging, known as a gantry, of baulk timbers, should be erected over the footway, the scaffolding being erected from this.

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