TYPES OF WELLS 71 



Summary of advantages and disadvantages of different types of well curbs and casings (Continued). 



Type of curbing. 



Advantages. 



Disadvantages. 



Wood (continued) 



Glazed and cement tile 

 with uncemented joints. 



Glazed and cement tile 

 with cemented joints. 



Iron casings . 



Allows all water to enter, utilizing all 



seeps. 



Does not give taste to water. 

 Does not require skilled labor. 



Safe from pollution (except that enter- 

 ing at bottom) as long as joints are 

 tight. 



Does not require expensive labor. 



Adapted both to rock and to unconsol- 

 idated materials. 



Safe from pollution except that enter- 

 ing at bottom. 



Pollution enters readily. 

 Animals gnaw through. 

 Wood rots, giving taste to water and 



favoring development of bacteria. 

 Expensive in some localities. 



Polluting matter enters readily and wejl 

 is never safe if near' source of contami- 

 nation. 



Soil may wash in through joints. 



Requires some outlay for material. 



Can be used only in soft materials con- 

 taining considerable water. 



The cost in large deep wells is consider- 

 able. 

 Practically limited to wells under 14 



inches in diameter. 



Is subject to deterioration by corrosion 

 and incrustation in some places. 

 Utilizes but one water stratum (except 

 where perforated). 



Selection of Type of Well. - - The type of well is the first and 

 perhaps the most important point to be decided. Of the many 

 kinds in use, including the dug, bored and driven types and 

 wells sunk by the jet process or drilled by rotary or percussion 

 rigs, each possesses, on the one hand, one or more points especially 

 qualifying it for use under one or more of the many varying 

 conditions encountered in drilling, and, on the other hand, some dis- 

 advantage which may disqualify it for use under certain other con- 

 ditions. The chief factors which govern the selection of type are, 

 usually, the amount of water needed, the character of the materials 

 to be penetrated, the depth to which the well must be sunk, the 

 cost of sinking the well and the safety of the resulting supply. 

 These factors are considered in detail in the following paragraphs. 



Yield as a Factor in Determining Type of Well. If an ade- 

 quate supply of ground water is available, the yield of a well will 

 depend on the character of the water-bearing material, the facility 

 of entrance of water, the size or storage capacity of the well and 

 the nature of the pumps. 



The character of the water-bearing material is of the greatest 

 importance in determining the yield of a well, as it is on the 



