194 ENGINEERING FOR LAND DRAINAGE. 



other loose material which does not become thoroughly 

 compacted. When a load pasess over them, the ma- 

 terial conveys a vibration to the covered pipes which 

 subjects them to a series of shocks which in time shat- 

 ters the brittle and rigid material of which sewer-pipe 

 is composed. When the pipes are full of frost or sur- 

 rounded by ice, as they are at times in cold climates, 

 the ware is especially subject to fracture by shocks. 

 That this is the cause of failure rather than the super- 

 incumbent weight or compression is evidenced by the 

 shattered condition of the sections, whereas if they 

 failed by pressure the breakage would be in cracks par- 

 allel to the length of each piece. The double-strength 

 sewer-pipe has given but little better results than the 

 regular thickness for the obvious reasons already noted. 

 Railroad companies have discarded sewer-pipe and are 

 substituting cast-iron pipe for small culverts, since it 

 was found that the tremor of embankments caused by 

 passing trains destroyed the value of the sewer-pipe 

 culvert. 



Sewer-pipe may, however, be used for road cross- 

 drains and culverts in good clay soil in which the joints 

 can be firmly embedded and the same material thor- 

 oughly compacted about the entire length and with the 

 top line of the pipe not less than 2 feet below the sur- 

 face of the road. The well-known property of clay 

 soil to form a bridge or crust surface under continuous 

 travel does much to carry the load, while the elastic 

 material about the pipe relieves it from injurious vi- 

 brations. The superiority of soil or sand over gravel, 

 stone, or other loose material for bedding or covering 

 culvert-pipe has been proven by experience- 



