The general rules to be followed vary depending upon the existence or absence of stable clay strata. These strata, when present, have little tendency to cave about the water-bearing strata from which the water is to be pumped.
Where the well penetrates sand, gravel or other such formations of a caving nature for its entire depth and the pumping level is not deeper than 20 ft., the casing should extend 10 ft. below the pumping level. For pumping levels 20 ft. to 25 ft., the casing should extend to a depth of 30 ft., and for pumping levels deeper than 25 ft., the casing should extend at least 5 ft. below the pumping level. Water from wells of this nature (shallower than 20 ft. where an impermeable stratum underlies the unconsolidated formation) are considered unpotable unless properly treated.
Where clay, hardpan, shale or other relatively stable material overlies water-bearing sand or gravel, the upper part of the well casing should be sealed in this non-caving formation by grouting or an equivalent type of seal. An upper drill hole larger than the well casing is required to provide space for a grout type seal.
If the overburden above the aquifer contains layers of sand or gravel at a depth of less than 20 ft., it is desirable to extend the larger drill hole 5 ft. into clay below any of these loose strata. The annular space should be filled with cement grout or with drill cuttings that are of a cementing nature. The permanent well casing should extend to a depth of 5 ft. below the expected pumping level.
Where the overlying material is only clay or similar material to a depth of 25 ft. or more, the upper drill hole should be at least 20 ft. deep. The permanent well casing should extend to a depth 5 ft. below the pumping level.
During the driving of the permanent casing, the upper drill hole should be kept partly filled with clay slurry. The annular space should be completely filled with heavy clay slurry or cement grout after the well screen has been installed in the water-bearing sand. This assumes that the well has been drilled by the cable tool method and that the screen has been installed by telescoping it through the casing with the casing subsequently pulled back to expose the screen in the water-bearing sand.
Drilling by the conventional rotary method involves a different procedure to insure a tight seal around the casing in the upper drill hole. The drilling mud itself can provide the seal in the annular space provided the driller does not circulate the mud out of the upper portion of the hole when setting the well screen and developing the well. If the mud is washed out during these operations, it is necessary to fill the upper drill hole with cement grout or heavy clay slurry as one of the last steps in completing the well in a manner similar to a well drilled by a cable tool method.
The use of the telescoping method of well screen installation offers some definite advantages in sanitary well construction. The well casing can be set initially to its permanent position and grouted in place, or left undisturbed long enough to allow the drilling mud to attain its maximum gel strength. Using a smaller bit, a hole can then be drilled into the water-bearing formation below the end of the casing to permit setting the well screen by telescoping it through the casing.
Often it is desirable to drill to the full depth of the well to get samples of the water-bearing formation before deciding upon the depth to which the casing is to be set. The drilling and sampling may be carried out, and the bottom portion of the hole can be backfilled with sand after the formation samples are taken. The casing may then be set to the desired depth and the grouting accomplished to seal the casing in the drill hole.
The temporary sand backfill may be washed out by using drilling mud in preference to water to permit the well screen to be installed without disturbing the seal around the well casing.
Schemes other than those specifically described can be used to accomplish the desired results. The basic principle is to leave the casing tightly sealed in the ground with particular attention to the upper 20 ft. of the casing. The telescope method of installing the well screen lends itself to this procedure. This method also permits removal and reinstallation of the well screen at any future time, if this should become necessary, without disturbing the watertightness of the casing and its surroundings.
Wells Completed in Consolidated Formations
Two types of water-bearing rocks need to be kept in mind in devising construction details for sanitary wells drilled into consolidated formations. Sandstone is a type in which water is found and moves through small pores similar to the pores in and unconsolidated sand formation. The size of the openings is usually small enough to effectively filter the water as it moves into and through the aquifer.
Limestone and basalt aquifers, on the other hand, are rocks in which the water occurs in crevices, fractures and solution channels. These openings are of a size the produce practically no filtering effect on the water moving through them. While the real determination of the potability of water must be based on bacteriological tests rather than on arbitrary distances, limestone and similar creviced or fractured formations generally should be used as a source of ground water only where they are buried under at least 40 ft. of overburden of unconsolidated material and where the overburden extends 2,000 ft. in all directions from the well site.
These overburden conditions cannot be met in some places, yet no other source of water may be available. Procedure in those situations is to disinfect the water by continuous chlorination. Chlorinators for individual water systems are available.
Where 40 ft. of overburden exists, the well casing should pass entirely through the overburden and be firmly seated in the rock. The upper 20 ft. of the casing should be grouted in an oversized drill hole. If the overburden is sand, which will cave tightly around the well casing, the grout seal is unnecessary.
Where the overburden is less the 25 ft., the well casing should extend as far as practicable into the creviced formation and should be cemented or grouted in an oversized drill hole throughout its length.
Wells completed in sandstone formations should have the casing grouted to a minimum depth of 30 ft. where the unconsolidated overburden is 25 ft. or less. If more than 25 ft. of sand rests on top of the sandstone, the casing must be seated into the firm rock material but does not necessarily need to be grout-sealed. If the overburden is clay, the upper 20 ft. of the casing must be sealed by grouting in an oversize drill hole.
Sandstone is sometimes overlain by creviced rock. In such situations, contaminated surface water can readily flow through the creviced rock to the top of the sandstone formation. To complete a well properly in the sandstone under these conditions, the creviced formation should be cased off. The casing should extend 15 ft. into the sandstone and should be sealed by grouting throughout its length.