1. Overview of DRAINAGE & DEWATERING Techniques Prof. Samirsinh P Parmar Asst. Prof. Department of Civil Engineering Dharmasinh Desai University, Nadiad-387001, Gujarat, INDIA Mail: samirddu@gmail.com CL-611 ADVANCE CONSTRUCTION TECHNOLOGY/ CONSTRUCTION-II Lecture - 18

2. Definition The removal of excess water form the saturated soil mass is termed as drainage & dewatering. 2

3. in the dryThe purpose of construction dewatering is to control the surface and subsurface hydrologic environment in such a way as to permit the structure to be constructed “in the dry.” Dewatering means “the separation of water from the soil,” or perhaps “taking the water out of the particular construction problem completely.” This leads to concepts like pre-drainage of soil, control of ground water, and even the improvement of physical properties of soil. Construction Dewatering 3

4. Soil Type & Its Permeability SoilPermeability Coefficient, k (cm/sec) Relative Permeability Coarse gravelExceeds 10 -1 High Sand, clean10 -1 to 10 -3 Medium Sand, dirty10 -3 to 10 -5 Low Silt10 -5 to 10 -7 Very low ClayLess than 10 -7 Impervious 4

5. Purposes of Drainage & Dewatering Carryout construction activity below water table. To increase stability of soil. To decrease seepage & pore water pressure. Reclamation of water logged areas. Release of hydrostatic pressure behind the retaining structures. 5

6. Methods for lowering water table Ditches & sumps Well point system Shallow well system Deep well system Vacuum method Electro-osmosis method 6

7. Ditches & Sumps PERIMETER TRENCH & SUMP PUMP Initial Water Table Depressed Water Table Sump Pump 7

8. Simplest method K >10 -3Useful in Shallow excavations in Coarse grained soils whose K >10 -3 cm/s softening of lower partIf seepage is significant, it may cause softening of lower part of slope pipingPossibility of piping in the sump bottom Ditches & Sumps 8

9. Well point system WELL POINTS Natural W.T. To Pump 9

10. Dewatering system based on gravity flow 5 to 8 cm dia.A well point is a perforated pipe @ ½ to 1m long & 5 to 8 cm dia. Covered by cylindrical wire-gauge screen Well point system 10

11. Self-jetting WellPoint. 11

12. PLAN OF A TYPICAL WELL POINT SYSTEM 12

13. Suction lift limitation of WellPoint systems WellPoint system 13

14. A typical well point system dewatering 14

15. Multi stage Well point system FINAL STAGE WELL POINT 2 ND STAGE WELL POINT 1 st STAGE WELL POINT LOWERED WATER TABLE NATURAL WATER TABLE 15

16. Used for dewatering excavations which are more then 6m below the W.T. In multistage well point system, round the clock pumping schedule is necessary Interruption in pumping can create catastrophic consequences One auxiliary pump should be provided for each two pumps Multi stage Well point system 16

17. Figure : Use of well points where submergence is small 17

18. Shallow well system 30 cmA hole of 30 cm in dia. Is bored in ground 10mMax. depth of tube is 10m Strainer tube of 15 cm dia. Is lowered in the bore hole having casing tube A gravel filter is formed around the strainer tube by gradually removing the casing tube, & simultaneously pouring filter material, such as gravel etc. 18

19. A suction pipe is lowered into the filter well. The suction pipe from a number of such wells may be connected to one common header leading to the pumping unit. Shallow well system 19

20. Deep well Drainage System 16 m below the water tableAdopted, when depth of excavation is more then @16 m below the water table The system is useful where artesian water is present 15 to 60 cm diaA 15 to 60 cm dia. Hole is bored, and casing with a long screen ( 5 to 25 cm) is provided Submersible pump30mSubmersible pump with capacity to push water up to 30m or more is installed near the bottom of the well 20

21. Each well has its own pump. A row of deep wells are arranged at the toe side of the side slope of deep excavation. Also very helpful where high artesian pressure exists. Deep well Drainage System 21

22. Deep well Drainage System W.T Lowered W.T. Deep Well Pump Well Point 22

23. Figure: Deep-well system for de-watering an excavation in sand. 23

24. Deep wells with auxiliary vacuum system for dewatering in stratified material. 24

25. Vacuum method Useful for fine grained soils (fine, non cohesive soils, Silty sands etc.) particle size D 10 is smaller then @ 0.05mm & its co-efficient of permeability between 10 -3 and 10 -5 cm/s. Seal Vacuum Pump Sand Filter 25

26. It is necessary to apply a suction head in excess of the capillary head to the dewatering system. 25 cmA hole of 25 cm dia. Is created around the well point and the rise pipe by jetting water under sufficient pressure. Vacuum pumps are used to create a vacuum in the sand filling. Vacuum method 26

27. Vacuum method When the vacuum is drawn on the well point, the ground surface is subjected to unbalanced atmospheric pressure. 27

28. Figure : Vacuum WellPoint system 28

29. Electro-Osmosis Method Row of Cathodes Row of Anodes Reversed Flow -ve +ve To pump Natural W.T. Natural Direction of Flow ARRANGEMENT FOR ELECTRO_OSMOSIS PROCESS 29

30. Electro-Osmosis Method: Principle +ve -ve ANODE CATHODE SOIL PARTICLE Double Layer Natural Water Moving Force Resisting Force Shear Plane Direction Of Flow 30

31. Suitable for fine grained cohesive soils Developed by Casagrande (1952) ve = k e – i eThe velocity of flow towards the cathode can be expressed as ve = k e – i e Where, k ek e = Electro-osmotic co-eff. Of Permeability i ei e = Electric gradient, or the electric potential divided by the distance between electrodes Electro-Osmosis Method 31

32. porosityelectrolytic viscousk e depends on porosity of soils & electrolytic & viscous properties of fluid, but independent of the size of pore. k e 0.5 x 10 -4 cm/secelectric gradient of 1 volt/cm.For practical purposes, k e may be assumed as 0.5 x 10 -4 cm/sec for most soils for an electric gradient of 1 volt/cm. 40180 volts 4 to 5metersThe potentials generally used are from 40 to 180 volts with electrode spacing 4 to 5 meters. Main purpose is to increase consolidation & shear strength. Electro-Osmosis Method 32

33. Electro-osmotic WellPoint system for stabilizing an execution slope. 33

34. Ground Freezing The principle of ground freezing is to change the water in the soil into a solid wall of ice. This wall of ice is completely impermeable. Ground freezing is used for groundwater cutoff, for earth support, for temporary underpinning, for stabilization of earth for tunnel excavation, to arrest landslides and to stabilize abandoned mineshafts. 34

35. Formation of a freeze well To freeze the ground, a row of Freeze pipes are placed vertically in the soil and heat energy is removed through these pipes 35

36. Ground Freezing 32 °F0 °COnce the earth temperature reaches 32 °F (0 °C), water in the soil pores turns to ice. +20 °F –20 °FA temperature of +20 °F may be sufficient in sands, whereas temperatures a low as –20 °F may be required in soft clays. 36

37. Formation of frozen earth barrier in different soils 37

38. Freeze pipes STAGES OF GROUND FREEZING 38

39. Portable twin 60-ton brine refrigeration unit For Ground Freezing 39

40. Typical LN 2 system for ground freezing 40

41. Circular excavation support by a freeze wall 41

42. Typical freeze-pipe. 42

43. An excavation supported by gravity retaining wall of frozen earth. 43

44. Blanket Drains 44

45. Recharge of groundwater to prevent settlement of a building as a result of dewatering operations 45

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47. Grout curtain or cutoff trench around an excavation. 47

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