Attack by Drainage 133 



obtained. After the tile is wedged securely in 

 place the trench is filled with stone to the height 

 of a few inches above the original ground surface. 

 Earth excavated is placed only on the downhill side 

 of the excavation. Small stone is preferable for 

 the top layer of cover stone at the ground surface. 

 This scheme of drainage has given excellent 

 results. 



There are conditions under which intercepting 

 tile drainage should not be used. If unforeseen 

 future changes of topography produce these con- 

 ditions, the tile line will probably operate in an 

 unsatisfactory manner. The soil over the greater 

 part of the Isthmus contains a large percentage of 

 clay. If the lands above the tile line are kept 

 covered with vegetation or left in their natural 

 condition, the surface water running down the 

 hillsides is not heavily charged with clay and silt, 

 and passes through the cover stone above the tile 

 line and then off through the tile. Should the soil 

 be bared or excavated above the tile line, or a hillside 

 road located across the tile line, large quantities of 

 impervious material will be washed onto the cover 

 stone, and fill up all the spaces between the stones, 

 and no water can reach the tile. Tile drains are 

 not planned to meet these conditions, and cannot 

 operate under them without costly maintenance. 



