DRAINAGE 



287 



the trench. On deep trenches there is usually a man at 

 work on each spading depth, the trench being carried along 

 in steps. In hand trenching the cost per rod or 100 feet 



After University of Minnesota 



Fig. 235. Excavating a 3-foot tile trench 



includes the digging of the trench, laying of the tile, and 

 blinding, and is customarily a fixed price for all trenches 

 averaging 3 feet in depth or under, then a given amount 

 is added to that sum for each inch in depth the trench 

 averages over 3 feet. The average is found by dividing 

 the sum of the cuts at all the stakes by the total number 

 of stakes. 



Trenching machines. Several forms of trenching machines 

 have been designed for sewer work. Any of these can be 

 used for tile work where the ground is solid. By the appli- 

 cation of caterpillar traction or by the use of very wide 

 wheels it is possible to use these machines on soft ground. 

 Some of them operate by a revolving wheel carrying 

 buckets on the rim which cuts slices of earth as the machine 

 moves ahead and deposits it on an apron which throws it 

 to one side. Others use a ladder arrangement around which 

 operates an endless chain carrying buckets which cut slices 

 of earth as they come up out of the trench. These machines 

 are economical for laying large-size tile and greatly reduce 



