One of the first pieces of work which a plumber is called upon to do, when building operations commence, is to run in the terra-cotta sewer from the street sewer into the foundation wall.
Fig. 35. - Connection of house sewer to main sewer.
When the street sewer is laid, Y-branches are left every few feet. A record of the branches and their distance from the manhole is kept generally in the Department of Sewers or Public Works. Therefore, the exact measurement of any branch can be obtained and the branch found by digging down to the depth of the sewer. A branch should be chosen so that the pipe can be laid with a pitch, the same way as the main sewer pitches. This can be done by getting the measurements of two of these branches and choosing the one that will serve best. When there is a brick sewer in the street and no branches left out, the sewer must be tapped wherever the house sewer requires it (see Fig. 35).
After the measurements and location of the house sewer and sewer branches are properly located, the digging of the trench is started. The methods employed to dig the trench vary according to the nature of the ground, that is, whether it is sand, rock, or wet ground. A line should be struck from sewer to foundation wall to insure a straight trench.
Fig. 36. - Laying of plank for trench dug in sandy ground.
If the ground is sandy, the sides of the trench will have to be sheathed or planked and the planks braced so as to prevent the bank caving in. As the trench is dug deeper, the planks are driven down. When the trench is very deep, a second row of planking is necessary. The planks must be kept well down to the bottom of the trench and close together, otherwise the sand will run in. It is well to test the planking as progress is made by tamping the sand on the bank side of the planks.
Where the ground is mostly gravel and well packed, the above method of planking is unnecessary. The bank should have a few stringers and braces to support it. When only a few planks are used the term "corduroy the bank" is used (see Fig. 37).
Fig. 37. - Arrangement of plank for gravel.
Where rock is encountered, blasting is resorted to. The plumber should not attempt to handle a job requiring the use of powder. It is dangerous in the hands of a person not used to handling it and the work should be sublet.
A sketch of the two methods above for planking trenches is given and a little study will make them clear.