Water can be obtained from ground water storage (aquifers) to the surface through the use of either open wells or tube wells. Open wells are not commonly used because of the following reasons:
- Discharge is limited to 3 to 6 litres/s.
- Mechanical pumping of such small discharge is not economical.
- Open wells can tap only the topmost or at the most the next lower water bearing stratum.
- Water from open wells can be withdrawn only at velocity equal to or smaller that the critical velocity for the soil so as to avoid the danger of well subsidence.
To overcome these shortcomings listed above and obtain larger discharges, tube wells are used. Tube wells are long pipe or a tube (See Figure 1), that is bored or drilled deep into the ground, intercepting one or more water bearing stratum. There are two types of tube wells.
A. Cavity type tube wells: this type of tube well draws water from the bottom of the well and not from the sides and the flow is spherical.
B. Screen type tube wells: This type of well draw through screens around the pipe and flow is radial. It is commonly used and it has capacity to tap water through multiple aquifers. Screen type tube wells are further sub-divided into: Strainer tube wells and slotted pipe gravel-pack tube wells.
a. Strainer tube wells makes use of strainer lengths lowered into the borehole and located opposite the water bearing formations and plain pipe lengths located opposite the non-water bearing formations. Water enters into the well through the strainers from the sides. This type is not suitable for fine sandy strata.
b. Slotted pipe gravel pack tube wells use a slotted pipe without being covered by any wire mesh. The slotted pipe lengths are located opposite the water bearing formations. After placing the assembly of plain and slotted pipes in the borehole, a mixture of gravel and gravel shrouding is poured into the borehole between the well pipe assembly and the casing pipe so as to surround the well pipes by a designed optimum thickness of gravel packs. Gravel pack wells are generally provided in fine aquifers, where the effective grain size (D10) may be less than 0.25mm and uniformity coefficient (Cu) may be 2.0 or less. They are highly preferred for deep tube wells.
Design of well screen and gravel packs
Criteria for design of well screen
A well screen constitutes the most important part of the tube well since it serves as the intake structure for the entry of water into the wells. The design of well screen is largely influenced by the characteristics of water bearing formations around the wells.
Dry sieve analysis of aquifer
Sample obtained during drilling of the bore hole is carried out to plot the grain size distribution curve for the aquifer. The result will help us to obtain the following characteristics of the aquifer that are required to determine the design specifications of the well screen and the gravel pack.
- Effective size (D10): the sieve size through which 10% of particles shall pass and 90% retained
- Mean particle size (D50) of the aquifer and
- The uniformity coefficient (Cu): Cu is the ratio of D60 to D10 (D60/D10). It was proposed by Hazen (1893) to be a quantitative expression of the degree of the assortment of the soil, as an indicator of porosity. Generally, a material is classified as uniform if its Cu is equal to or less than 2.
How to design the size of slot openings of a screen: to fix an optimum size for the slot openings of a screen is an important component of the screen design and it depends on the size of the aquifer material. Oversized slots will pump finer material and it would be difficult to obtain clear water. Undersized slots will provide more resistance to the flow of ground water into the well, resulting in more head loss and corrosion. Fine slots are also blocked by small sand and silt particles leading to clogging. The optimum value is determined by matching the size of the opening with the grain size distribution of the material surrounding the screen. In practice, the slot size varies from 0.2 mm to 5mm.
- Slot size for non-gravel pack wells: here, the optimum slot opening size is chosen as the one which retains 40% of the sand (that is equal to D60 size of the aquifer) if the groundwater is non-corrosive and equal to D50 size of the aquifer if the ground water is corrosive. Though this may vary when more than one aquifer is tapped.
- Slot size for gravel pack wells: this is determined on the basis of the grain size distribution curve of the gravel material used for the gravel pack. On the curve, a point is located indicating D10 which shows the optimum slot size for the well screen. The actual size of the slot may be fixed within ±8% of the D10 size of the gravel pack.
Design of gravel pack: for gravel pack wells, the gravel pack design should precede the well screen slot sizes design. The gravel pack is designed based on the Pack Aquifer Ratio (PA Ratio).
Mathematically, PA ratio = D50 of the gravel/D50 of the aquifer material.
The recommendations for the PA ratio vary according to investigators and these are possibly unique to different locations. For Indian conditions for instance, the Central Board of Irrigation and Power (1967) gave the following criteria for PA ratio:
- For uniform aquifer having Cu ≤ 2.0, PA ratio should be between 9 and 12.5
- For graded aquifer having Cu ˃ 2.0, PA ratio should be between 12 and 15.5
The Cu of the gravel material should preferably be less than or equal to 2.5 (Cu ≤ 2.5). the reason for this being that higher value will cause segregation of the gravel shrouding during pouring which will lead to poor efficiency of operation of the well. As obtaining Cu ≤ 2.5 is difficult, proper field control should be done during pouring. The thickness of the gravel pack should be 7.5 cm to 20 cm.
Example
The results of sieve analysis test carried out on a 400gm sample of underground aquifer, proposed to be tapped for installation of a tube well at a location, Nawfia in Anambra state, Nigeria are given in the tables below:
| Nawfia Soil | |
| Sieve Sizes (mm) | Mass Retained (g) |
| 4.75 | 17.88 |
| 2 | 3.91 |
| 1.18 | 5.03 |
| 0.425 | 112.15 |
| 0.3 | 76.52 |
| 0.15 | 67.53 |
| 0.075 | 12.5 |
| ˂ 0.075 | 104.48 |
| 400 |
| Sieve Sizes (mm) | Mass Retained (g) | % Retained | Cumulative % retained | % Passing |
| 4.75 | 17.88 | 4.47 | 4.47 | 95.93 |
| 2 | 3.91 | 0.98 | 5.45 | 94.55 |
| 1.18 | 5.03 | 1.26 | 6.71 | 93.29 |
| 0.425 | 112.15 | 28.04 | 34.75 | 65.25 |
| 0.3 | 76.52 | 19.13 | 53.88 | 46.12 |
| 0.15 | 67.53 | 16.88 | 70.76 | 29.24 |
| 0.075 | 12.5 | 3.13 | 73.89 | 26.11 |
| ˂ 0.075 | 104.48 | 26.12 | 100 | 0 |
| 400 | 100 |
From the graph, D60 = 0.39; D50 = 0.32; D10 = 0.023
Cu = D60/D10 = 0.39/0.023 = 16.96
Since Cu ≥ 2.0, we should use PA ratio for designing the gravel pack lying between 12 and 15.5
At PA ratio = 12, D50 of gravel pack/D50 of aquifer material = 12
D50 of gravel pack/0.32 = 12; D50 of gravel pack = 0.32 x 12 = 3.84 mm
At PA ratio = 15.5, D50 of gravel pack/D50 of aquifer material = 15.5
D50 of gravel pack/0.32 = 15.5; D50 of gravel pack = 0.32 x 15.5 = 4.96 mm
It implies that the D50 of the gravel pack should lie between the limiting values of 3.84 mm and 4.96 mm
To get the limiting curves of the grain size distribution of the gravel pack, mark a straight line from the vertical axis of the grain size distribution curve of the aquifer materials corresponding to the D50 of the material. Mark out the D50 values determined above (3.84 mm and 4.96 mm) and draw a curve parallel to the central portion of the grain size distribution curve of the aquifer material. Extend the curves to axis zero on the lower horizontal axis and up to 100% on the upper horizontal axis. Read off the sieve sizes corresponding to these optimum points. The gravel pack size should lie between these optimum points and should be screened to lie within that range. The screen size for the slots of the tube well should have sizes slightly higher than the size of material corresponding to the D10 of the gravel pack curve.
How to design the length and size of the screen: The length of the screen virtually depends on the thickness of the aquifer to be tapped and the area of the screen openings. The area of the screen openings per m length of the screen usually varies between 15 to 20% of the screen area which is equal to πd where d is the diameter of the screen pipe. If the area of the openings is kept more than about 15 to 20%, the structural strength of the well screen reduces which reduces the life of the screen.
The minimum length of the screen, h = Q/Ao.ve
Where, Q = design discharge of the tubewells
Ao = area of the screen openings per ‘m’ length of the screen = πd x percent of the openings (p)
ve = optimum entrance velocity for the given k-value of the aquifer for non-gravel wells (See table below). For gravel pack wells, the mean k value for the aquifer and for the gravel pack is considered, to assume the value of ve.
| Coefficient of permeability (k) of the aquifer (cm/s) | Optimum entrance velocity (ve) in cm/s |
| 0.02 | 1.5 |
| 0.05 | 2.0 |
| 0.09 | 3.0 |
| 0.14 | 4.0 |
| 0.18 | 4.5 |
| 0.24 | 5.0 |
| 0.28 | 5.5 |
According to the construction and boring methods, tubewells are classified as:
- Driven tubewells
- Jetted tubewells, and
- Drilled tubewells
Reference
Groundwater Hydrology and Construction of wells and tube wells
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