642 SOILS: PROPERTIES AND MANAGEMENT 



ing until a good joint is formed. An earlier type was 

 the U-shaped tile laid on a board. These tiles are easily 

 broken by the pressure of the earth. They are no more 

 efBcient than the ordinary round tile. 



542. Protection of joints. — Soil water should enter 

 the tile at the lower side of the joint. Any unusual 

 opening in the joint should be on the lower side. If the 

 soil has low coherence, such as may be the case with fine 

 sand and silt, the upper half of the joint should be pro- 

 tected against the entrance of sediment. A cap of paper 

 or of burlap cloth, two or three inches wide and long enough 

 to cover the upper half of the joint, may be used. 



Other methods of protecting joints are to cover them 

 with clay, thick cement mortar, or the sod and granular 

 soil from the surface. The last named is most commonly 

 employed. Filters may be constructed by placing around 

 the tile a layer of coarse sand or gravel, cinders, straw, 

 or leaves. Where the soil is of a serious quicksand nature 

 (clean, fine sand or silt filled with water), it may be desir- 

 able to place a bed of gravel or cinders under the tiles 

 as well as around them. The entrance of water from the 

 lower side of the joint in small trickles will generally 

 prevent any difficulty from sediment. Water should 

 flow from a drain approximately clear, and any other 

 condition usually indicates a too rapid entrance of water. 

 Where the soil is a fine clay with high cohesion, the ends 

 of tiles should not be so close together as in loose soil. 

 The tops may sometimes be separated an eighth of an 

 inch with entire safety. In such cases it is especially im- 

 portant to return the soil to the trench in a dry condition 

 and to place the topsoil next to the tile. 



543. Entrance of roots into tile. — The entrance of 

 roots into the joints of tile drain sometimes causes an 



