Outline of Presentation. Introduction Applications Design Examples

Transcription

1 Dewatering

2 Outline of Presentation Introduction Applications Design Examples

3 Introduction

4 Purposes for Dewatering For construction excavations or permanent structures that are below the water table and are not waterproof or are waterproof but are not designed to resist the hydrostatic pressure Permanent dewatering systems are far less commonly used than temporary or construction dewatering systems

5 Common Dewatering Methods Sumps, trenches, and pumps Well points Deep wells with submersible pumps

6 Sumps, Trenches, and Pumps Handle minor amount of water inflow The height of groundwater above the excavation bottom is relatively small (5ft or less) The surrounding soil is relatively impermeable (such as clayey soil)

7 Wet Excavations Sump pumps are frequently used to remove surface water and a small infiltration of groundwater Sumps and connecting interceptor ditches should be located well outside the footing area and below the bottom of footing so the groundwater is not allowed to disturb the foundation bearing surface In granular soils, it is important that fine particles no be carried away by pumping. The sump(s) may be lined with a filter material to prevent or minimize loss of fines

8 Dewatering Open Excavation by Ditch and Sump Army TM

9 Well Point Method Multiple closely spaced wells connected by pipes to a strong pump Multiple lines or stages of well points are required for excavations more than 5m below the groundwater table

10 Single Stage Well Point System Caltrans

11 Single Stage Well Point System

12 Typical Well Point System Johnson (1975)

13 Deep Wells with Submersible Pumps Pumps are placed at the bottom of the wells and the water is discharged through a pipe connected to the pump and run up through the well hole to a suitable discharge point They are more powerful than well points, require a wider spacing and fewer well holes Used alone or in combination of well points

14 Applicability of Dewatering Systems Army TM

15 Applications

16 Permanent Groundwater Control System Army TM

17 Deep Wells with Auxiliary Vacuum System Army TM

18 Buoyancy Effects on Underground Structure Xanthakos et al. (1994)

19 Recharge Groundwater to Prevent Settlement Army TM

20 Sand Drains for Dewatering A Slope Army TM

21 Grout Curtain or Cutoff Trench around An Excavation Army TM

22 Design

23 Design Input Parameters Most important input parameters for selecting and designing a dewatering system: - the height of the groundwater above the base of the excavation - the permeability of the ground surrounding the excavation

24 Depth of Required Groundwater Lowering The water level should be lowered to about 2 to 5 ft below the base of the excavation 2 to 5ft

25 Methods for Permeability Empirical formulas Laboratory permeability tests Borehole packer tests Field pump tests Accuracy Cost

26 Darcy s Law Average velocity of flow v ki h k L Actual velocity of flow v a v n Rate (quantity) of flow q kia k h L A

27 Typical Permeability of Soils Soil or rock formation Gravel 1-5 Clean sand Clean sand and gravel mixtures Medium to coarse sand Very fine to fine sand Silty sand Homogeneous clays Shale Sandstone Limestone Fractured rocks Range of k (cm/s)

28 Coefficient of Horizontal permeability of Soil (k h ) x10-4 cm/sec Note: k h based on field pumping tests Effective Grain Size (D 10 ) of Soil, mm Permeability vs. Effective Grain Size Army TM

29 Constant Head Test h L A Soil Q k QL hat

30 Falling Head Test a At t=t 1 dh h 1 At t=t 2 h 2 A Soil L Valve k al At ln h h 1 2

31 Rigid wall test Laboratory Test Methods AASHTO T215; ASTM D 2434 Typically for sandy & granular soils (k > 10-3 cm/s) Not recommended for low permeability soils (k < 10-6 cm/s) Flexible wall test ASTM D 5084 Typically for soils (k < 10-3 cm/sec)

32 Flexible vs. Rigid Wall In rigid walled permeameters Simpler apparatus Leakage along side-wall possible, especially if sample shrinks May use double ring equipment to discount side-wall leakage In flexible walled permeameters (triaxial cells) No side leakage Effective stress (hence k) varies

33 Rigid Wall Permeameter

34 Shelby Tube Permeameter Device designed to use a 6-in section of a standard 3-in diameter Shelby tube Ideal for testing loose sands and other materials (Durham Geo Slope Indicator)

35 Compaction Permeameter uses standard 4 in and 6 in compaction molds for falling or constant head permeability tests (Durham Geo Slope Indicator)

36 Rigid Wall Permeameter cm Top cap Flow Porous stone Edge Flow (discarded) Flow Compaction mould Compacted soil Porous stone Bottom cap cm Double ring permeameter introduced to measure k without including sidewall leakage which would lead to high estimates of k Flow

37 Double Ring Permeameter A standard 4 in compaction mold A stainless steel sleeve in the base divides the sample into two equal portions, allowing measurement of the permeant flow from the center and perimeter of the sample concurrently Flow is monitored with two 5 ml pipettes (Durham Geo Slope Indicator)

38 Flexible Wall Permeameter No loading Cell piston pressure Top plate Perspex walls Top cap Soil Flexible membrane O - ring Bottom cap Different at top and bottom of specimen Flow lines with valves Bottom plate

39 Flexible Wall Permeameter

40 Permeability Testing Usually test soils with very low permeability coefficient (<10-9 m/s??) v ki k dh dl To make testing practical, increase i But high i may cause cracking in soil unrepresentative flow regime (Darcy not true anymore) internal erosion edge leakage in test apparatus

41 Recommended Maximum Hydraulic Gradient k (cm/sec) i max 1x10-3 1x x10-4 1x x10-5 1x x10-6 1x < 1x

42 Field Pumping Test r r 2 r 1 Observation wells q dr Phreatic level before pumping dh Phreatic level after pumping Test well h h 2 h 1 Impermeable layer

43 Observation Wells NAVFAC (1982)

44 Permeability from Field Pumping Test Permeability k r qln 1 r h h

45 Dupuit-Thiem Approximation for Single Well r r 0 Influence range, R Q Original GWL Phreatic level before pumping y s h s Phreatic level after pumping H h w h 0 Impermeable layer Q 2 k H h ln R / r 0 2 w 1.366k H log 2 h R / r 2 w Qln r / r0 y h w k

46 Height of Free Discharge Surface C H h H 0 hs Ollos proposed a value of C = 0.5

47 Influence Range Sichardt (1928) R C' H h w k C = 3000 for wells or 1500 to 2000 for single line well points H, h w in meters and k in m/s

48 Forchheimer Equation for Multiwells Forchheimer (1930) Q 2 k H y lnr (1/ n)ln x 2 1 x 2...x n Point P Well 2 Well 1 x 1 x 2 x 3 Well 3 Circular arrangement of wells Q 2 2 k H y lnr lna a

49 1.0 h s 0.8 h 0 h Q H 0.6 h s /H 0.4 y R Height of Free Discharge Surface R/H Army TM

50 Estimation of Flow Rate Darcy s Law R H-h 0 H r 0 h 0 Q 1.571k H h H h R r 0 R r Cedergren (1967)

51 Estimation of Flow Rate Well Formulas R H-h 0 H r 0 h 0 Q 1.366k H log 2 h R / r Cedergren (1967)

52 Estimation of Flow Rate Flow Nets R H-h 0 r h 0 H 0 Flow net Q 3.14k H h R r 0 0 n n f d n f = number of flow channels n d = number of head drops Cedergren (1967)

53 Capacities of Common Deep Well Pumps Min. i.d. of well pump can enter (in.) 4 5 5/ Preferred min. i.d. of well (in.) Approximate max. capacity (gal/min) ,200 1,800 2,400 3,000 Mansur and Kaufman (1962)

54 Rate of Flow into A Pumped Well or Well Point Approximate formula Q 44 kr w h 0 k = permeability, ft/min r w = effective radius of the well, ft h 0 = depth of immersion of well, ft Bush (1971)

55 Typical Well Point Spacing in Granular Soils NAVFAC (1982)

56 Typical Well Point Spacing in Stratified Soils NAVFAC (1982)

57 Spacing of Deep Wells The spacing of deep wells required equals the perimeter of the excavation divided by the number of wells required

58 Well Point Pump Carson (1961)

59 Head vs. Discharge for Pump Carson (1961)

60 Head vs. Discharge for Pump Carson (1961)

61 Bottom Stability of Excavation z > w (h +z) Caltrans

62 Settlement of Adjacent Structures H 1 e 0 C c log ' vo ' vo h w h = reduction of groundwater level

63 Examples

64 240m lengthx150m widthx21m depth Plan View & Cross-Section Xanthakos et al (1994)

65 Design Requirement Lower the groundwater table to 1.5m below the bottom of the excavation

66 Equivalent Radius and Influence Range Equivalent radius of excavation r 0 800ft 500ft 357ft 112.5m Height of water level in well h 0 = = 85 ft Influence range R C' H h k w 340m 1130ft m m

67 Using Darcy s law Q 1.571k Rate of Flow in Wells H h H h R r 0 R r ft / min 2592gal / min Single well formula Q 1.37k log 313ft 3 2 H h R r / min 2350gal / min log 1130 /

68 Flow Rate into Wells using Flow Net R=1500 Q 3.14k 238ft H h R r (140 85) ( ) 3 0 n n / min 1782gal / min 0 f d 3 30

69 Pump Test A pump test indicates that the field permeability k = 9.2 x 10-4 cm/sec and the radius of influence R = 2200ft. The new solutions based on the pump Test results are Method Darcy s law Well formula Flow net Q (gal/min)

70 Layout of Deep Wells Xanthakos et al (1994)

71 Multiple Wells H y k Q lnr lna ft / min ln2200 ln gal / min 290/8 = 36.3 gal/min per well Deep well size: 4 dia. for 36.6 gal/min Header pipe: 4 dia. for 5 x 36.6 gal/min = 181 gal/min Discharge pump: 4 dia. Pump for 290 gal/min