Grand │ 25│ 0.10│ │ 0.80│ 50│ used to Trunk │ │ │ │ │ │ spring │ │ │ │ │ │ holes Clay, rock │ │ │ │ │ │ and │Tunnel│ │ │ 1.00│ │ Gypsum │ │ │ │ │ │ │ │ │ │ │Grade │ │ │ │ │ │ varied │ │ │ │ │ │ ⅗ at │ Hard shale │Tunnel│ │ │ 2.07│ 45%, ⅕ │ │ │ │ │ │ at 60%,│ │ │ │ │ │ some at│ │ │ │ │ │ 100% │ Hard rocky │Tunnel│ 1.60│ │ 3.57│ │ slate │ │ │ │ │ │ Hard rocky │Tunnel│ 1.46│ │ 3.57│ │ slate │ │ │ │ │ │ Mill Creek │ │ │ │ │ │Mun. sewer, │Tunnel│ │ │ 4.00│ 60│ Eng’g. St. Louis│ │ │ │ │ │ Vol. 52, │ │ │ │ │ │ p. 14 ───────────┴──────┴────────┴─────────┴─────────────┴─────────┴────────── PIPE SEWERS =179. The Trench Bottom.=—It is customary to dig the bottom of the trench to conform to the shape of the lower 45 degrees to 90 degrees of the sewer if the character of the material will allow such construction. In soft material which will not hold its shape the sewer may be encased in concrete or a concrete cradle may be prepared for the pipe. In rock the trench is excavated to about 6 inches below grade and refilled with well-tamped earth so as to form a cradle giving bearing to 60 to 90 degrees of the pipe circumference. For large sewers to be constructed in the trench special foundations are sometimes built. =180. Laying Pipe.=—Before the pipe is lowered into the trench the sections which are to be adjacent should be fitted together on the surface and the relative positions marked by chalk so that the same position can be obtained in the trench.