The Proper Way to Build a Well

Location and geology will dictate the type of well that will work and provide an adequate water supply. The most important question is whether you have an aquifer beneath your property and what type of aquifer it might be. Not every location has water, but some locations have several aquifers available. At many locations throughout Prince William County and the country, there are several aquifers that vary with depth from near the ground surface to a thousand feet below the ground surface. The aquifers’ thickness varies from a few feet to hundreds of feet thick and the water quality varies from both natural and man made contaminants.

How you should build a well is determined by type of well (dug or drilled), the local geology (sand, gravel, fractured rock, bed rock, etc.) local precipitation and environmental conditions. So when I get questions on a well the first information I need is how the well is constructed and the geology. There are a number of things that should be true in construction of all wells. The well cap should fit tightly on the top of the well casing, be vented, and have a screen to prevent insects from getting in the well. A sanitary well cap is the best option for protecting your well. The well cap should be at least 12 inches above grade, or higher if in an area that is prone to flooding, to ensure that the well cap is never covered by flood water. The area between the casing and the borehole, called the annulus, should be grouted (filled with bentonite and/or concrete) that will not allow any surface water around the well to go down the well bore or along the casing carrying surface contamination into the groundwater. 

There are three basic styles of modern well construction: Drilled Bedrock Wells or Fractured Rock Wells, Sand and Gravel Wells, Large Diameter Dug and Bored Wells. The drilled bedrock or fractured rock wells are the most basic construction. They have a casing through the overburden or the surface fractured rock to prevent surface runoff from draining into the well. Below the casing these wells are open to the bedrock aquifer. For a properly constructed well in bedrock the casing extends from just above land surface to a short distance into the bedrock where the casing is “seated” into the rock to keep it in place. The borehole continues down into the bedrock aquifer for some distance below the casing. Because bedrock is solid, it acts as the well casing in the lower portion of the well and allows water to enter the well through fractures. The fractures provide water to the well, and the borehole itself stores the water until it is pumped.

from Private Well Class.Org

 A sand and gravel well is finished in a sand and gravel aquifer, which isn’t solid and will cave in if it is not supported. These types of wells need to be completely cased- a casing extends from a foot above the surface to the bottom of the well. However to allow water to enter the well, the lower portion of the casing is a well screen. The well screen is generally made of woven steel wire formed in the shape of a pipe or casing. The screen has openings based on the size of the sand and gravel grains in the aquifer. This way, it keeps the sand out of the well, but water can flow through. 

from Private Well Class. Org

Dug and bored wells are generally around three foot in diameter and are less frequently used today because they are very susceptible to contamination. They were commonly used in unconsolidated finer materials like clay, silt, loess, etc. that may or may not have very thin sand lenses that can move small amounts of water. These wells were built big to provide storage and allow shallow groundwater to slowly seep into them. Old hand dug wells still exist and are in current use, though nearly every state has construction standards for wells that wouldn’t allow a modern well to be built in this way. Today, large diameter wells are constructed by machine and are generally of one of two types; a bored well with concrete collar or a bored well with a buried slab. In the concrete collar construction the casing is generally 4 or 5 foot sections of precast concrete that are placed on top of each other and allows water to seep into the well through the joints between these sections. Because of the possibility of surface infiltration near the well, the upper 10+ feet around the well is grouted with concrete or has a bentonite seal. Bentonite is a very fine clay that expands when wet to create a nearly impermeable layer.

from Illinois Department of Public Health. 

The buried slab construction can look like a drilled well because the upper 10 feet of the well has a standard well casing that has back filled soil or concrete around it. The well casing is fitted into a prefabricated concrete slab that tops the underground portion of the concrete collar lined well.

from  Illinois Department of Public Health.

You need to understand the construction of your well, the geology, and the aquifer to understand your well and make decisions about maintaining and repairing your well.