Grouting to Seal Well Casing
Grouting of the well casing is practiced to prevent seepage of water vertically along the outside of the casing. The grout envelope around the casing also increases the life of the casing by protecting it against exterior corrosion. Any annular space that may be produced around the well casing, either incidentally or purposely, provides a definite channel for downward movement of water unless something can be done to seal this space.
In caving formations such as sand, the opening seals itself as the sand closes in tightly around the casing. Soft, saturated clay will also squeeze tightly around the pipe in most cases. In the more stable formations (such as stiff clay, shale or rock) some artificial method of sealing the opening around the casing must be employed to provide sanitary protection.
Cement grout is the most satisfactory material for filling and sealing the annular space around the well casing. Puddled clay will provide an adequate seal in some cases, but one must be sure that the clay grout will not dry out to the extent that shrinkage cracks may develop at whatever depth the sealing effect is needed.
Cement grout should consist of either Portland cement or quick-setting cement mixed with not more than 6 gallons of water per sack of cement. Bentonite clay may be added to the grout mixture at a rate of 3% to 5% in order to improve its flow properties and reduce the shrinkage as the cement sets. Where low temperatures require a speedy setting of the grout, up to 1 ½ pounds of calcium chloride per sack of cement may be added to hasten the hardening process.
The upper drill hole in grout-type construction should be greater than the nominal size of the permanent casing. Whenever possible, grouting should be performed. This is done by adding the grout from the bottom of the space to be grouted upward to the surface in one continuous operation.
A well may or may not be the same diameter from top to bottom. After starting construction with pipe of a given diameter, it is sometimes necessary to reduce the diameter of the pipe at some depth and complete the lower portion of the well in a smaller size.
The well casing which serves to accommodate the pump must be adequately sized to permit installing the pump with proper clearance. In choosing the well diameter, this is one of the controlling factors. Well diameter is not a major factor in determining or limiting the yield of the well. Thus, the intake portion of the well may be of smaller diameter than the upper portion of the well without seriously affecting the efficiency of its operation.
Unexpected subsurface conditions are often encountered while drilling. If the well has been started with a sufficiently large bore, effective steps can be taken to exclude undesirable water from getting in the well, or to continue drilling to a deeper formation should unexpected conditions be encountered. Faults in many existing wells can be traced directly to the fact that their starting diameter was too small. This is particularly true for wells completed in consolidated rock formations.
When all factors are considered, the minimum practical diameter of the well casing is 4 inches. However, in certain circumstances, 2-inch or larger pipe can be driven to the desired depth (20 ft. or more) and cleaned out in preparation for installation of a well screen in a water-bearing sand formation. Even with 2-inch pipe, the smallest well point (1 ¼ inch) can be installed by the telescoping method. The well point can be dropped inside the 2 inch pipe and then driven into the water-bearing sand below the pipe. A short extension pipe connected to the drive point may be used where some overlap inside the lower part of the casing is desired. A suitable packer is needed to provide a sand-tight seal between the well point and the inside of the well casing, whatever its diameter.
Upper Terminal of Well
The casing pipe of any well should project not less than two inches above the pump house floor or platform which has been installed above the established ground surface. Any vent or ports for air line equipment should extend from the top of the well in the form of watertight piping to a point at least 12 inches above the floor, or in any case, not less than 24 inches above any known flood level.
The pump drop pipes along with the cable and air lines, if any, must be effectively sealed to the well casing. The sealing device employed will be dependent upon the well termination and its location with reference to the pump and/or tank. Where the pump is mounted directly over the well casing, a sanitary well seal should be used.
If the pump and/or tank is remote or offset from the well, the seal should consist of a watertight expanding sanitary well seal. This type effectively seals against the inside of the casing and at the same time seals against the drop pipes, cable and/or air line.
If the pump and/or tank is remote from the well and it is necessary to install the horizontal pump pipes below the frost line, a pitless adapter may be used. This device provides positive sealing and permits easy access to the well from the ground level. All buried pump pipes shall be pressurized to prevent contamination from surface waters. All vent and/or air lines shall be protected at open or top ends by suitable fine mesh screen to prevent entrance of any foreign materials.
Construction Material - Pipe for Well Casing
Pipe used as well casing serves to provide a structural retainer for the wall of a water well, to exclude the undesirable water from certain water-bearing formations, and to conduct water vertically from the intake portion of the well to the pump. The wall thickness and strength should be sufficient to resist the stresses imposed during and after installation without structural failure.
Steel pipe has given satisfactory service in the majority of locations. Where soil or water are somewhat corrosive, longer life can be assured by using pipe of greater wall thickness.
Galvanized sheet metal pipe is not considered suitable for well casing because it is thin and its seams and joints are not watertight. If it is necessary to use this material, it should be encased in concrete 4 to 6 inches thick.
Plastic pipe may be used for well casing where special methods can be employed to install the pipe without structural damage.
Concrete pipe or clay tile with short joint used in bored wells should also be encased with a 4-inch to 6-inch concrete envelope. This encasement should be carried to a depth of 20 ft. to insure a watertight casing that will exclude seepage of surface and shallow contaminated ground waters. If site conditions are particularly unfavorable, the concrete encasement should extend to a greater depth.
Experience has proven that steel pipe of the minimum thickness is adequate to fulfill the strength requirements and to provide an economic life for the well under most conditions provided the thinner wall pipe meets the structural requirements for the method of installation of the casing. Where additional protection against corrosive soil or water is required, thicker wall steel pipe may be used.
Sections of steel pipe used for well casing may be jointed by welding or by threaded couplings. Pipe that is to be driven should be equipped with a drive shoe at the lower end.
Temporary capping of a well until the pumping equipment is installed should be such that no contamination can enter.
Every new, modified or reconditioned water well, including pumping equipment, should be disinfected before being placed in service. The interior of the well casing should be thoroughly swabbed, to remove oil, grease and joint dope. After all construction has been completed and after pumping equipment has been installed, the well and its equipment should be thoroughly cleaned of all foreign substances. Tools, timber, rope, debris, oil, grease, joint dope, and scum should be removed. The well and pumping equipment should be disinfected with a chlorine solution made up of one part household bleach (sodium hypochlorinate 6%) and two parts water. Two quarts of this solution should be poured into the well. This should be done at least once a year regardless.