DUG WELLS 85 



The excavation for dug wells that are to be curbed with wood 

 are generally square and of slight depth. The joints on which 

 the boards are nailed are preferably placed on the outside next 

 the earth walls. Such curbings, however, are undesirable in al- 

 most every particular and are to be avoided wherever possible. 



In digging open wells there is always grave danger of caving, 

 although such wells are not unfrequently carried (in stiff clayey 

 materials) to depths of 40 to 60 feet or even more without the use 

 of any support whatever for the walls. In sandy and other in- 

 coherent materials, however, temporary supports are generally 

 necessary to protect the workmen if the well is carried to any 

 depth. 



Size and Depth of Wells. Ordinary clay and the denser 

 varieties of pebbly and boulder clay or till usually contain but 

 little water, and this little is often largely in the form of inter- 

 stitial water held in the body of the material and given up slowly 

 to a well by general seepage. Under such conditions the amount 

 entering the well is often more or less proportional to the area of 

 surface exposed in the wall. This area varies with the diameter 

 of the well ; thus, three times as much surface will be exposed in a 

 given height of wall in a 6-inch well as in a 2-inch well and six 

 times as much in a 3-foot as in a 6-inch well. To give a large 

 yield a large-diameter well is very desirable in materials of the 

 character mentioned. 



Large wells are also desirable in rocks in which the water occurs 

 in a similar manner, that is, in pores rather than in open pas- 

 sages. In general, however, if water is yielded at all by the rocks, 

 it is given up more readily than by clays, hence a large bore is 

 less necessary. This is fortunate, for the range of size in rock 

 wells is usually rather scant, owing to the fact that most rock 

 wells are of the drilled type. Where the water occurs in bedding 

 or joint planes the diameter is of still less importance, as the en- 

 trance of the water is localized and is relatively free. Large 

 diameters, nevertheless, increase materially the likelihood of strik- 

 ing an opening. In the oil regions the increase of the diameter of 



