OTHER CONDITIONS AND PROBLEMS 155 



In Michigan and Minnesota, drainage outlets are secured 

 by way of wells drilled into sandstone ; in Georgia, Indiana, 

 Kentucky, Tennessee and Virginia, and other states, 

 by the use of drilled wells in limestone. 



The efficiency of such a drainage well is usually depend- 

 ent upon the depth of the well. It is dependent also upon 

 the nature of the material ; for it varies with the same 

 material. In gravelly sand and sandstone the size of 

 the openings (pore space) rather than the percentage of 

 pore space is important. In limestone the size of crevices 

 is the important item. 



205. Quicksand. It sometimes happens that quick- 

 sand is encountered at or near the bottom of the ditch in 



FIG. 66. Steel shield used to hold back quicksand ; 28 inches long, 

 12 inches high, width governed by size of ditch. 



laying the tile. In such case it will usually be necessary 

 to use some kind of guard to hold back the quicksand while 

 the bottom of the ditch is being completed to receive the 

 tile. Figure 66 shows a shield of iron used for this purpose. 

 In the use of the shield, the ditch is dug to the quicksand. 

 The shield is then placed so as to include that portion of 

 the ditch in which the next sections of tile are to be laid. 

 The -workman can usually press it down into the quick- 



