A properly designed and constructed well should balance yield and drawdown consistent with the aquifer properties.In designing a water well the objectives must be clear from the outset.
a) Provide quality water as appropriate and prevent contaminants entering the aquifer from the well;
b) Allow easy access for sampling or monitoring;
c) Be long lived;
d) Be economically viable. |
Although general guidelines can be drawn up it is important to stress that local conditions will have a major impact on well design and construction. Such conditions include quarternary and bedrock geology, storage and transmissivity values, extant water quality and the aquifer balance. Information on other wells in the area should be sought out, from the Geological Survey of Ireland, from the driller or from local people.The information on anticipating geology will predetermine appropriate drilling techniques to be adopted. The table below provides an overview of the appropriate drilling techniques viz a vis anticipated geology in the overburden and / or bedrock.
Geology |
Overburden |
Bedrock |
Drilling Technique |
Sands & Gravel |
Boulder clays cobbles & clays |
Clay Marl |
Soft Rock with Cavaties |
Stable Competent |
Down Hole Hammer Drilling |
Not Recommended |
Possible |
Not Recommended |
Possible |
Recommended |
| Air Rotary Drilling with Foam & Water |
Possible |
Possible |
Not Recommended |
Possible |
Recommended |
| Rotary Mud Drilling |
Not Recommended |
Possible |
Recommended |
Recommended |
Possible |
| Odex Drilling |
Recommended |
Recommended |
Possible |
Recommended |
N/A |
| Shell & Auger Drilling |
Recommended |
Possible |
Recommended |
Not Recommended |
Not Recommended |
| Rotary Core Drilling |
Not Recommended |
Possible |
Possible |
Recommended |
Recommended |
| Geobore 'S' Drilling |
Possible |
Recommended |
Recommended |
Recommended |
N/A |
Three broad classes of wells, based on the end use, can be defined:
a) Domestic Wells
b) Industrial Wells
c) Specialist Wells. |
The remainder of this commentary on well design considers;
a) The casing: The casing houses the pumping equipment i.e. pump chamber, and serves as a conduit for water.
b) The intake: The intake portion of the well can be screened or open, depending on the geology, but intake design must take into account the local hydrogeological conditions.
c) Sealing of borehole annulus, well head completion and protection.
d) Grouting and sealing well casing.
e) Well development. |
Casing: Design and construction of the well should address:
a) Casing diameter
b) Casing material
c) Well depth |
Intake: Design of the intake should address:
a) Length
b) Diameter
c) Screen material/slot size |
CASING
CASING DIAMETER
If the proposed well is to be used for water supply (for domestic or industrial / agricultural purposes) then the size of the pump will be the critical determinant in selecting the appropriate size of casing. The casing must be large enough to accommodate the pump with enough clearance to allow easy installation and easy access for maintenance work. In certain conditions (based on geology, drilling methods, costs) the well diameter of the lower portion of a well may have to be smaller than the upper part. If such conditions apply then the casing diameter should be chosen from the bottom of the well upwards, taking into account the driving conditions, overlap requirements, and the annular space necessary for grouting or filter packing.
CASING MATERIAL
Where there is a variety of casing materials available, the factors that affect casing selection include: cost, diameter of the well, drilling method, extant water quality, strength requirements, corrosion resistance, and planning regulations. Whatever type of casing is selected it must be suitable for the specific borehole. Bearing this in mind, resistance to collapse is perhaps the most important factor.
Steel Casing: Steel casing must always be used when the casing is driven, pulled back or if the casing is installed in holes liable to collapse.
Stainless Steel Casing: Stainless steel casing (American Society for Testing and Materials Standards A-409) is used in highly corrosive environments. While stainless steel is more expensive than mild steel, it will last longer, and if the environment is corrosive, the initial excess cost may be worth bearing.
Plastic Casing: Plastic casing is typically one of three types: Acrylonitrile butadeine styrene (ABS), Polyvinyl chloride (PVC) and rubber-modified polystyrene (SR). While the strength of plastic casing is significantly less than that of steel casing, it has certain advantages: it is corrosion resistant (even in waters where steel casing fails), it is lightweight, and easy to install. Plastic casing is more flexible than steel casing so it is important that it is centred in the borehole before backfilling is completed. A sudden formation collapse (which could be brought about by the presence of voids in the backfill) can cause plastic piping to break. If it protrudes above ground the casing must be carefully protected because it can easily be damaged by vehicles or moving equipment. Long term exposure to sunlight can also reduce the impact strength of the material. Care must be taken to avoid using plastic casing in areas prone to groundwater contamination. Although the process is not fully understood plastic can be permeable in the presence of certain chemicals.
Well Depth: The depth of the well is a function of its use. Monitoring wells are often set to very specific depths in an aquifer while water supply wells are often sunk as deep as possible in an aquifer. Water supply wells should be sunk as deep as possible to maximise specific capacity, to maximise well yield and to maintain well yields in drier periods or if the well is overpumped.
INTAKE
Well Screen Length The length of well screen will be primarily based on the type of well. In a monitoring well a very short screen may be required to sample water from a very specific location; in supply wells longer screens can allow more water to enter the well.
Screen length will be a function of the aquifer thickness, the available drawdown and the geologic make-up of the aquifer. For water supply wells the screen must be open in the portions of the aquifer with the highest hydraulic conductivity values.
Talking to the driller, or examining well logs, can give an indication of the type of material encountered in the drilling process. Well logs from wells in the area, if available, can also give broad indicators of where the most permeable units are expected to be encountered. Laboratory analysis can be carried out on the aquifer material to determine conductivity values. Borehole geophysical logging can also be used to locate the most permeable zones. The aquifer type (unconfined or confined) will also impact on screen length selection.
WELL SCREEN OPENINGS Naturally developed vis a vis imported or filter packed wells have different well screen opening requirements. In either case the best method for selecting well screen openings is sieve-analysis of samples from the water bearing formations. It is important to note that the drilling method employed can impact on the apparent make-up of the samples analysed. It is important to note the methodology and to compensate for any impact that the drilling may have had on the samples taken. (For example, in rotary drilling, drilling mud can contaminate samples, leading to selection of a screen that is smaller than necessary.)
Naturally Developed Wells The typical approach in naturally developed wells is to select an opening size that will allow 60% of material to pass through and 40% to be retained. If the water is corrosive, or the material sample is less reliable a 50:50 percent option can be used. As a general rule a more conservative opening size should be selected if there is uncertainty about the nature of the aquifer material, if the formation is well sorted or if there are time constraints on development time of the well.
Filter Pack (Gravel Pack) Wells Filter packs should be made of clean, well-rounded grains of uniform size. It is preferable to use siliceous rather than calcareous particles (especially if the water is slightly acidic, or if it is anticipated that the well may have to be acidised at some point). Design of a filter pack should be based on careful analysis of the formation materials. Filter packs are commercially available. The screen openings of the well should then be selected to ensure that at least 90% of the pack is retained after development of the well. The volume of filter pack required is easily calculated and tables of volumes of filter for various borehole diameters are in major publications. The pack should, however, extend above the screen to allow for settlement after development.
SELECTION OF SCREEN MATERIAL Water quality, potential presence of iron bacteria, and screens strength requirements will all determine the type of material used in the well screen. The primary issues involving water quality are whether the water is corrosive or encrusting. If the water is corrosive, for example, the slots in a mild-steel screen can be increased, allowing sediment to enter the well. Encrusting water can lead to closure of well slots. Chemical analysis of water samples from the formation can be undertaken to determine if the water is corrosive and/ or encrusting. Tests to be carried out should include: Dissolved Hydrogen (DO), Hydrogen Sulphide (H²S), Total Dissolved Solids, Carbon Dioxide (CO²), Chlorides, Hardness, Iron (Fe), Manganese (Mn). Although non-harmful to humans, iron bacteria are considered a nuisance organism. In wells they produce accumulations of slimy material that can clog well screens in very short time periods. Iron bacteria can be combated by using a strong solution of chlorine. However, this must be carefully flushed from the well after use, and the screen must be resistant to the corrosive effects of the chlorine. The strength of the screen is also a critical factor. Screens can be subjected to vertical compression (imposed by the weight of the overlying casing), extension forces (imposed by casing attached below the screen) and horizontal forces (primarily collapse pressure from the formation during development).
SEALING OF BOREHOLE ANNULUS, WELL HEAD COMPLETION AND PROTECTION.
OBJECTIVE Boreholes should be sealed and grouted to prevent surface contamination from entering the well and to ensure that the well does not act as a conduit for contaminants to migrate into and between aquifers. The migration of contaminants either inside or outside the casing should be prevented. The following guidelines demonstrate a number of methods used to ensure that a borehole is protected against the ingress of surface contamination.
GROUTING AND SEALING WELL CASING
a) Annular Space The length of the borehole section to be grouted will vary according to the hydrogeological characteristics of the site and the construction details (casing, screen and gravel pack). The borehole should be of sufficient diameter so that well construction can proceed to the required depth and maintain sufficient annular space for a seal and casing. A minimum 2 inch annular space is required between the inside diameter (ID) casing and borehole wall (or hollow-stem auger wall). This will allow up to a 1.5 inch outside diameter (OD) tremie tube to be used for placing the filter pack, pellet seal and grout at specific intervals. An annular space less than the 2 inch minimum is less desirable. |
Grouting Equipment for Sealing Borehole |
b) Bentonite Pellet Seal (Slug) A seal should be placed on top of the filter pack. This seal should consist of 30% solids bentonite material in the form of bentonite pellets. They should be placed by positive displacement or tremie pipe. Use of the tremie pipe method minimises the risk of pellets bridging in the borehole and assures the placement of pellets at the proper intervals. Pouring of pellets is acceptable for shallow boreholes (less than 15 metres) where the annular space is large enough to prevent bridging and to allow measuring with a tape measure to ensure that the pellets have been placed at the proper intervals. In order to ensure that the pellets have been placed at the proper intervals, the pellets should be tamped, with the appropriate tamping tool, while measuring is being conducted. The tamping process minimises the potential for pellet bridging by forcing any pellets that have lodged against the borehole wall, hollow-stem auger wall or well casing down to the proper interval.
The bentonite seal should be placed above the filter pack at a minimum of 0.5 metres vertical thickness. The hydration time for the bentonite pellets should be a minimum of eight hours or the manufacturer's recommended hydration time, whichever is greater. In all cases the proper depths should be documented by measuring and not by estimating. Other forms of bentonite such as granular bentonite and bentonite chips have limited applications and are not recommended for the bentonite seal unless special conditions warrant their use. If the water-table is below the pellet seal interval, potable water (or a higher quality water) should be used to hydrate the pellets.
c) Grouting The Annular Space Water supply wells should be drilled to the required depth as shown in table 1 and then lined by installing casing and the grout. Water wells constructed in rock that is overlain by relatively thin unconsolidated deposits are required to be pressure grouted from the ground surface to the rock. The grout should be left for 24 - 48 hours to allow the grout to reach full strength. The production section should then be drilled. The NSAI Bottled Water Standard recommends the casing and grout depths as shown in Table 1.
Depth to Bedrock (metres) |
Overburden Permeable |
Overburden Impermeable |
Minimum Depth of Casing and Grout |
< 10 |
X |
. |
15 m |
10 - 15 |
X |
. |
5m into Bedrock |
> 15 |
X |
. |
3m |
< 7 |
. |
X |
12m |
7 - 9 |
. |
X |
5m into Bedrock |
> 9 |
. |
X |
3m into Bedrock |
The annular space between the casing and the borehole wall should be filled with a neat cement grout or a cement/bentonite grout. The grout should have a minimum density of 10lb/gal. The type of grout selected should be evaluated as to its intended use and integrity. Bentonite clay can be added to the cement to hold cement particles in suspension, reduce shrinkage and improve fluidity of the mixture. Approximately 1.4 to 2.3 kg of bentonite should be mixed with 25 litres of water per bag of cement. If the amount of bentonite exceeds 6%, excessive shrinkage of the cement will occur. It is best to mix the bentonite and water first, then add cement. Potential fluid-loss conditions may call for the addition of sand or other bulky material to permit the grout to bridge larger openings without excessive fluid loss. These coarse materials render the handling and placing of grout more difficult but may be necessary to reduce the cost of materials where large openings are to be filled.
The grout should be freshly mixed and injected or displaced into the annulus between the casing and the borehole form the lowest point upwards using a tremie pipe to within 0.5 metres of the ground surface. Grout can be placed by gravity through a tremie pipe but pumping is preferred because the required volume of grout can be introduced rapidly reducing the chance of leaving voids in the grout. The casing should be centred in the borehole to prevent bridging and channelling of grout and to ensure a uniform layer of grout around the casing for the entire vertical distance to be grouted. Grout should be placed in one continuous operation to ensure a satisfactory seal. The tremie pipe should have an outlet of a side discharge port on the protruding outlet / casing as applicable. The grout should be allowed to cure for a minimum of 24 hours (24-48 hours NSAI-BW) before the concrete surface pad is installed. All grouts should be prepared in accordance with the manufacturers specifications. The casing wall thickness must be of sufficient strength to withstand the pressure exerted by cement grout particularly in deep wells.
Bentonite grouts (not cements) should have a minimum density of 10lbs/gal to ensure proper setting. The density of the bentonite grouts should be measured while mixing and should not be pumped into the borehole until the minimum density of 10lbs/gal is attained. In addition, the grouting operation should not cease until the bentonite grout flowing out of the borehole has a minimum density of 10lbs/gal. A mud balance should be used to measure the specified grout density of the bentonite grout. Estimating the grout density is not acceptable. Drilling muds are not acceptable for grouting.
Cement grout should be mixed using 25 - 26.5 litres of water per 94 lb bag of Type 1 Portland cement. The addition of bentonite (5-10%) to the cement grout is generally used to delay the setting time and may not be needed in all applications.
Bentonite grout should not extend close to the ground surface where it can dry out and shrink due to low soil moisture. Cement is always used at or near the top of the borehole.
WELL DEVELOPMENT
Well development is the process of bringing the well to its maximum efficiency, for instance by clearing out sediment and increasing the permeability of the aquifer close to the well. Well development by airlift is generally completed subsequent to emplacement of filter pack and prior to grouting of well. The following methods describe well development by using drilling / air compressor equipment.
Jetting Air to Clean Borehole |
Airlifting to Develop Well |
Methods used include
¤ Jetting: circulating a high-pressure water jet in the well.
¤ Surging: Moving a tightly-fitting plunger up and down in the well.
¤ Bailing: Moving a bailer (a long steel tube with a foot valve) up and down in the well.
¤ Dual Pipe Air-Lift System: Technique to discharge dissolved, silty water under air pressure through an annular eductor pipe to airline.
|
|
Jetting: - circulating a high-pressure
water jet in the well. |
Surging: - Moving a tightly-fitting
plunger up and down the well. |
|
|
Bailing: - Moving a bailer ( a long
steel tube with a non-return valve)
up and down in the well, bailing
out silt and sediment. |
Dual-Pipe Airlift System: - Compressed
air directed down a small 2" airline in
centre of heavy walled nominal 5" pipe
creating upward pressure outside airline,
inside 5" pipe to educt water to clear
silt and sediment aquifer. |
CONCLUSION
Well design and construction involves determining the materials and dimensions to be used in the well. These will be determined by the end-use of the well (water supply for domestic or industrial needs, monitoring, injection etc.). The most important factor to remember in constructing a well is that local conditions will affect design choices and well development technique. While there are broad rules and guidelines that can be used in constructing a well, if good information is not available about the specific site under construction the finished well may not perform to its design criteria.
Acknowledgement: Sections on Intake and Casing have been modified after Shane O' Neill, O' Neill Groundwater Engineering
