DRIVEN AND JET WELLS 101 



regions where the water is found as much as 50 feet below the sur- 

 face, and they are sometimes successfully extended to depths of 250 

 or 300 feet, or even to 400 or 500 feet or more where the materials 

 are easily penetrated and other conditions are favorable. As pre- 

 viously stated it is always advisable to carry the well from 10 to 

 15 feet or more below the point where water is first encountered. 



Cleaning and Care of Driven Wells. If the water reached by 

 a driven well contains much iron a crust will commonly accumu- 

 late in the screen and rapidly reduce or even cut off the inflow. 

 Sometimes the portion of the incrustation on the inside of the 

 casing may be jarred off by pounding sharply on the top of pipe, 

 but if on the outside, as is more commonly the case, the screen 

 will have to be removed. 



If the case is taken in hand in time, the removal of the screen 

 presents few difficulties, but in some parts of the country the 

 accumulation of incrustants, especially those of lime and silica 

 about the screen is very rapid; and often, almost before its pres- 

 ence is realized, it has reached so great a thickness that the re- 

 moval of the screen through the casing is found to be impossible. 

 The whole casing then has to be pulled up, a new screen substi- 

 tuted, and the pipe replaced, an operation costing about as much 

 as a new well. 



It has been found that wells 4 inches or more in diameter 

 seldom require a screen in sandy deposits (except when the pump- 

 ing is severe), and their substitution for the 2-inch wells, which 

 must nearly always be screened, will afford relief from incrusta- 

 tion troubles in most farm wells. Wells with screens lying en- 

 tirely in gravel give little trouble from incrustations compared 

 with those stopping in strata in which clay is mixed with the sand 

 or gravel. Where the amount of clay is comparatively small, 

 prolonged pumping at the outset will remove much of it from the 

 vicinity of the screen, and subsequent trouble is to a considerable 

 extent avoided. Artificial pockets of gravel, etc., are sometimes 

 formed about wells by introducing such material through the 

 casing before the setting of the screen (Figs. 42 and 44). 



