CONSTRUCTION METHODS 185 



"In installing covered drains either hand labor or trench- 

 ing machinery may be used. Frequently, on small proj- 

 ects, hand trenching is cheaper, but usually on larger 

 projects machines can do the work more rapidly, economi- 

 cally, and satisfactorily. It is preferable to let a contract 

 for the work to an experienced and capable contractor. 



"If hand labor is used it usually is necessary to operate 

 with small gangs, ordinarily about a half dozen men to 

 the line, as the trench must be opened from the top to 

 the bottom as rapidly as possible and the tile laid and 

 blinded before caving takes place. The men should 

 work as closely together as practicable and not even the 

 first spading should be taken more than a rod in advance 

 of the tile laying. Each man should remove a spading, 

 moving backward at the same time. The man removing 

 the last spading should also grade the bottom. He should 

 not step on the finished bottom and no one should stand 

 near the edge of the trench, nor should wagons or material 

 of any sort be permitted near the trench. The soil removed 

 from the trench should be placed as far back as it con- 

 veniently may be. The tile should be laid at once and 

 blinded by means of a few inches of earth caved from the 

 edges of the trench. If the banks tend to cave off in 

 large chunks or slabs it will be necessary to brace them 

 apart with planks separated by stout cross-pieces or trench 

 jacks. 



"A very troublesome condition is that in which the 

 presence of a wet, pervious stratum near the bottom of 

 the trench causes a lateral and upward movement of 

 the soil in the bottom of the trench. In such a case it is 

 necessary to provide a tight cribbing to shut out the oozing 

 material. It consists of two heavy timbers held apart 

 by trench jacks, behind which is driven lumber sheeting 



