CONSTRUCTION METHODS 187 



"Tile should be laid with extreme care. The joints 

 should be as close as possible, and if the soil is semi-fluid 

 and contains much fine sand and silt, it will be necessary 

 to provide some means of keeping the oozing material 

 from entering the tile joints. Almost all the water enter- 

 ing tile lines makes its way through the joints, practically 

 none entering through the walls of even the more porous 



Fig. 29. Drainage Machine with the Digging Wheel in 



the Trench. 



tile, so the covering for the joints must provide for the 

 ready passage of water. Straw makes a very good filter 

 when new, but it is likely to decompose and form a sticky, 

 impervious mass over the joints. Brush and willows are 

 not satisfactory and render any subsequent removal of 

 the tile very difficult. Graded gravel, ranging in size 

 from sand to pebbles an inch in diameter, makes an ex- 

 cellent filter, but it is not always available. Cinders also 

 are satisfactory. Strips of burlap wrapped about the 

 joints give good service. The custom of laying strips of 

 building paper over the joints cannot be commended, 



