1. ME31B: CHAPTER EIGHT DESIGN OF EXTERNAL FACILITIES TWO: GROUNDWATER, BOREHOLES AND WELLS

2. 8.1 AQUIFER DESIGNATION a) Aquifer: A geologic formation or stratum containing water in its voids or pores that may be extracted economically and used as a source of water supply. An aquifer may be confined or unconfined (see diagram).

3. AQUIFER

4. AQUIFER CONTD. b) Confined Aquifer: One in which groundwater is confined under pressure greater than atmospheric by overlying impermeable strata. It is also known as artesian or pressure aquifer. c) Unconfined Aquifer: One in which a water table serves as the upper surface of the zone of saturation. It is also known as a free, phreatic or non-artesian aquifer.

5. HYDRO-GEOLOGIC TERMS CONTD d) Flowing Artesian Well: Exists when the piezometric surface lies above the ground surface.

6. HYDRO-GEOLOGIC TERMS CONTD g) Spring: Abrupt intersection of the ground by the groundwater table. Unlike swamps, the point of intersection is usually followed by a steep slope causing the water to flow by gravity. h) Ephemeral Stream: Flows seasonally in response to groundwater fluctuations. i) Intermittent Stream: Flows only after a rain and is fed totally by surface water.

7. 8.2 POROUS MEDIA DESIGNATION i) Porosity: Groundwater occurs in voids, or pores of geologic formations. Porosity term is defined as: P = W/V where W is the volume of water required to saturate all voids or volume of all pores and V is the total volume of the rock (soil).

8. ii) Specific Retention (Sr) When groundwater is pumped or drained, some water is retained by molecular and surface tension forces. Specific retention is therefore expressed as: Sr = Wr/V (usually expressed in %). Wr is the volume of water retained and V is the total volume of rock (soil).

9. iii) Specific Yield (Sy): The water removed by the force of gravity is the specific yield or effective porosity. It is defined as: Sy = Wy/V (usually expressed in %). Wy is the volume of water drained while V is the total volume of rock(soil). Since total volume, W = Wy + Wr Porosity, : = Sr + Sy.

10. Storage Coefficient (Storativity, S):

11. v) Transmissivity (T): It is a measure of the ease of water movement in the soil. It is the hydraulic conductivity (K) multiplied by aquifer thickness (b). ie. T = K x b m 2 /d = m/d x m

12. GROUNDWATER HYDRAULICS There are two considerations: a) Steady or unsteady flow conditions (b) Confined and unconfined aquifers These flows are different and considered separately. Two basic equations are used in groundwater hydraulics: i) Laminar flow in porous media - Use Darcy's law. ii) Conservation of mass - Continuity(Water cannot be created or destroyed)

13. Darcy’s Law h1 dl h2 G/W Table Q = - K A dh/dl

14. Darcy’s Law Contd. dh/dl is hydraulic gradient in the direction of decreasing head. The negative means that flow is in the direction of the decreasing head. Darcy's law is applicable only to laminar flow i.e non-turbulent flow.

15. Reynold's Number where V is mean velocity, d e is effective diameter (90% is larger than). is kinematic viscosity = 1.14 mm 2 /s at 15 ° C When Re is less than 2000, flow is laminar and when it is more than 2000, flow is turbulent. All groundwater flows are laminar except: a) Fissured rocks: limestones (b) As water approaches wall of well.

16. Steady Radial Flow to a Well

17. Steady Flow to a Well Contd. s 1 and s 2 are drawdowns. This is the difference in level between the static water level and the pumping water level at a specified point. Assumptions: (i) There is radial symmetry in wells. (ii) If any annulus is considered, the total quantity of water is same with other annuli i.e. for continuity, the flow across any annulus is constant. As water approaches a well, the flow rate increases.

18. Assumptions of Derivation Contd. iii) The groundwater flow is horizontal (iv) The flow is steady, the drawdown is not increased further.

19. Derivation of Steady State Equation

20. Derivation of Steady State Equation Contd. Note: The replacement of the head, h with drawdown, s is because the drawdown rather than the head is normally measured during pumping tests. Given s 1 and s 2 (drawdowns) at r 1 and r 2, and knowing pumping rate, Q, Transmissivity, T can be derived

22. 8.4 WELL DRILLING METHODS AND CONSTRUCTION 8.4.1Cable Tool or Percussion Drilling: The method involves lifting and dropping a cable tool with a chisel at the end. It has a stroke of about 1 m with about 80 strokes per minute. Small water is added during the drilling to act as slurry. After some time, the tool is removed and replaced with a baler which removes water and slurry. It can be used in all rock types except that in hard rock, it will be slower. It is difficult to work in boulders.

23. Cable Tool or Percussion Drilling Contd. Advantages (i) It is cheap, simple equipment (ii)Low water needs during drilling (iii)It is easy to tell when aquifer is reached (iv)There is accurate location of the geological sampling Disadvantages (i) Need of support in unconsolidated rock (ii) The method is slow in hard rock. Speed varies from 0.5 to several metres per day. Typical capability is 600 m and 0.5 m diameter.

24. Cable-Tool Drilling Device

25. 8.4.2 Rotary – Direct Circulation This was developed to hasten speed and avoid lining in unconsolidated rocks. A rotating column is down the wells. A liquid goes through the top of mask town the column and helps remove soil drilled by the column through the sides. The lubrication is by bentonite mud. The following processes are involved: - cooling of bit (lubrication) - transport of cuttings - support of holes

26. Rotary-Direct Circulation Contd. The mud is plain water added bentonite and other additives to bring the viscosity higher. The speed is 30 to 300 r.p.m. The weight of drill increases with depth. Advantages: The method is rapid and there is no need for support in unconsolidated materials. Disadvantages: (i)High cost of equipment – 3 to 4 times that of Cable tool (i) Mud seals aquifer (iii) Loss of circulation in fracture aquifer (ii) Water need is high (about 10,000 gallons for 100 m 12 inch diameter hole.

27. Rotary-Direct Circulation Disadvantages Contd. (iii) It is difficult to recognize aquifer during drilling. This is because there is mud under pressure on circulation in the well so it is difficult to recognize when water table is reached. (iv) It is difficult to get accurate logs of the boreholes. Due to the mud sealing, another technique was devised.

28. Rotary- Direct Circulation

29. 8.4.3 Reverse Circulation Drilling Water goes in through the gap between the drill and soil wall while the slurry goes out through the center of drill. The diameter should ten be more than 400 mm minimum. The diameter can go on to 1.8 m. Clean water is used instead of mud. Advantages: Rapid at large diameters especially in unconsolidated materials Disadvantages: (i) Even larger volume of water is needed to make up for losses. This is because a mud cake is not built at the walls to reduce the permeability.

30. 8.4.3 Reverse Circulation Drilling Contd. The typical figure is 54m 3 /hr; depth of well is 120 – 300 m and the rate of operation is 12 m/hr depth typical (ii) The cost is very high.

31. 8.4.4 Compressed Air Method Compressed air is used in either direct or reverse circulation with some modification to the rig to replace mud

32. Jetting

33. 8.5 BOREHOLE DESIGN 8.5.1Aims (i) Structural stability (ii) Steady yield (iii) Local Construction (if possible) (iv) Appropriate capacity (v) Pollution control (vi) High volume/diameter/suitable diameter for pump (vii) Filter to have no detrimental effect on quality

34. BOREHOLE DESIGN CONTD. For structural stability, in consolidated rocks, this means that the top few metres will be lined with may be steel to prevent pollution and promote structural stability, and lower down will be an open hole e.g. for chalk material. In unconsolidated materials, the top section is also lined with blank casing while the lower section is lined with perforated lining that is the well screen. The well screen should have a sufficient open area to pass water without undue head losses.

35. BOREHOLE DESIGN CONTD. Narrow open holes will lead to turbulent flow, higher head losses (drawdown) and increased corrosion and encrustation rates.

36. Diameter of Bore Hole Depends on stability, discharge, storage, aquifer, and pump. The two main ones are discharge and pump. Storage is important if there is low transmissivity. (i) Discharge: If the diameter is doubled, the yield is increased by 10%, but a much greater increase in cost (See Table 8.1). The effect of diameter on yield is then small. (ii) Pump Size: The yield determines the pump size and the pump size determines the diameter (See Table 8.1). Pump size has the greatest influence on diameter.

37. Diameter of Boreholes Contd. (iii) If a gravel pack is needed, add 6 to 12 inches to the diameter. (iv) Type of drilling: There may be larger diameter on the upper surface and then reduce it with depth.

38. Well Screen Length: Recognition of aquifer requires the driller's log or geophysical logs. Aquifers can be recognised as confined or unconfined. (i) Confined Aquifers: Have a low storage coefficient, so suffer more drawdown. Ideally screen whole thickness to reduce further head losses and limit drawdown. In practice, if the aquifer material is homogenous, then 70 to 80% of the thickness is screened. If the aquifer is non- homogenous, screen the portions which have high K.

39. Well Screens Contd. There is need for maximum discharge for minimum drawdown as cost increases with drawdown. To keep confined aquifer, keep pump water level above top of aquifer.

40. Well Screen For Confined Aquifers

41. Well Screens For Unconfined Aquifer

42. 8.5.2 Gravel Pack: Decide whether or not a gravel pack is needed. The essence of a gravel pack is to provide a filter to hold back fine particles from coming into the well screen. Natural development is adopted if there is a good gradation from coarse to fine particles. Gravel packs are needed if the particles are fine and uniform (non-graded). In fine, uniform situation, fine slots are needed, which limits water flow.

43. Deciding Whether Gravel Pack is Needed

44. Deciding Whether Gravel Pack is Needed Contd. The graph relates to grain sizes and uniformity which will help decide whether or not to go for a gravel pack. (i) Effective Size, De: Particle size for which 10% is finer. It gives a measure of grain size of the material. (ii) Uniformity Coefficient: Defined by the size over which 60% is finer divided by size which 10% is finer. I.e. U.C. = D 60 /D 10 A large UC implies a non-uniform sample and a small one implies a uniform sample.

45. Deciding Whether Gravel Pack is Needed Contd. (a) Use a gravel pack if aquifer is both fine and uniform i.e. D 10 < 0.25 mm and UC < 3 (b) Gravel backfill can be used for semi consolidated aquifer which may slump onto the screen.

46. Gravel Pack Design

47. Entrance Velocity It is good to avoid turbulent flow around wells. Turbulence leads to greater drawdown and higher corrosion or encrustation rates. Limit entrance velocity to < 0.03 m/s. Entrance velocity = Q/A = Flow rate into well/Open area of well screen If e.v, is too high:  Increase the length of well screen  Increase diameter of well screen  Use gravel pack and larger slot size if it has not already been used  Use screen type with greater percentage open area or Reduce flow

48. 8.5.2 Pump Placement (i) The pump should be placed below the maximum anticipated drawdown. The anticipated drawdown will allow for:  Pumping drawdown  Loss of efficiency  Interference (ii) Place the Pump usually opposite cased section of hole (not screened). Putting the pump in the screen area will lead to direct discharge to it.

49. 8.5.7 Well Testing Purpose (a) to determine the hydraulic characteristics of the aquifer; (b) to determine the operating characteristics of the well. Two common well testing methods are: The Specific Capacity Test and The Constant-rate test.

50. Specific Capacity Test For this test the well is pumped at different levels of discharge for times equal to the expected time of well operation. It is common to start the test at 20 percent of design discharge and increment the discharge by 20 percent until 120 percent of design dis­charge is attained. The drawdown corresponding to each level of discharge is measured and used to construct what is termed the well characteristic curve as indicated in Fig. 8.6.

51. Specific Capacity Test Contd. The time of pumping is set to correspond to the expected operating time so the results of the specific capacity test may be used to estimate the expected drawdown for a given level of discharge.

52. Example of a Well Characteristic Curve

53. Specific Capacity Test Contd. The results of this test yield the specific capacity of the aquifer-well system which is defined as the ratio of the amount of well discharge to drawdown below the static piezometric surface. This parameter can be important in determining the required pump operating characteristics, the potential of interference of cones of depression of wells in the same vicinity, and in performing economic analysis of well operations.

54. Constant Rate Test The constant rate test is performed by measuring the variation in the level of draw­ down with time for a given level of discharge. If a single constant rate test is to be run, it should be at the design discharge of the system. If multiple constant rate tests are run, discharge can be varied between 60 and 125 percent of design discharge. The tests should be run for a minimum of 100 minutes and may be run for 24 hours or longer.

55. Constant Rate Test Contd. It is good practice to run the constant rate test for at least the expected operating time of the system. The principle of the constant-rate test is that drawdown increases at a constant rate of discharge if time of pumping is sufficiently long. This can be demonstrated by referring to the data shown in Fig. 8.6. The hydraulic properties of the aquifer are most conveniently determined by analysis of data over one log-cycle time of pumping. The properties are determined by rearranging the equations used to describe the hydraulics of wells.

58. Well Testing Concluded An example plot of data from this type of test is indicated in Fig. 8.7. Since solution for the storativity using equation above requires the transmissivity, must be obtained first and the result substituted into the foregoing equation. Application of the equations for transmissivity and storativity of an aquifer is indicated in the following example problem.

60. Typical Values of Transmissivity m 2 /d Suitability of Aquifer < 15Domestic boreholes e.t.c. > 150Industrial, Urban, Irrigation requirements > 15,000Exceptional