Ground Improvement 8- Dewatering Techniques Asal Bidarmaghz Room CE 502 E-mail: a.bidarmaghz@unsw.edu.au Outline ⚫ ⚫ ⚫ ⚫ ⚫ Reasons of dewatering Dewatering methods Fundamental soil-water relationships Hydraulic of slots and wells Determination of: ➢ ➢ ⚫ Ground permeability Influence range Design of dewatering systems Dewatering Techniques 2 Dewatering? ⚫ Reducing water level in soil ➢ ➢ ⚫ Modifying ground by changing water regime ➢ ➢ ⚫ Increasing shear strength of cohesive soils Increase effective stress in granular soils, thereby increasing their strength Inducing consolidation in soil Diverting seepage from work area Permanent or temporary ➢ Temporary dewatering is more common for construction Dewatering Techniques 3 Dewatering? ⚫ Reducing water level in soil ➢ ➢ ⚫ Modifying ground by changing water regime ➢ ➢ ⚫ Increasing shear strength of cohesive soils Increase effective stress in granular soils, thereby increasing their strength Inducing consolidation in soil Diverting seepage from work area Permanent or temporary ➢ Temporary dewatering is more common for construction Dewatering Techniques 4 Dewatering? ⚫ Reducing water level in soil ➢ ➢ ⚫ Modifying ground by changing water regime ➢ ➢ ⚫ Increasing shear strength of cohesive soils Increase effective stress in granular soils, thereby increasing their strength Inducing consolidation in soil Diverting seepage from work area Permanent or temporary ➢ Temporary dewatering is more common for construction Dewatering Techniques 5 Dewatering? ⚫ Reducing water level in soil ➢ ➢ ⚫ Modifying ground by changing water regime ➢ ➢ ⚫ Increasing shear strength of cohesive soils Increase effective stress in granular soils, thereby increasing their strength Inducing consolidation in soil Diverting seepage from work area Permanent or temporary ➢ Temporary dewatering is more common for construction Dewatering Techniques 6 Reasons For Dewatering ⚫ To provide a dry working area, such as in excavations for building foundations, dams, and tunnels ⚫ To stabilise constructed or natural slopes ⚫ To reduce lateral pressures on foundations or retaining structures ⚫ To reduce the compressibility of granular soils ⚫ To increase the bearing capacity of foundations ⚫ To prevent liquefaction due to an upward gradient (quick conditions) ⚫ To reduce the liquefaction potential during earthquakes Dewatering Techniques 7 Reasons For Dewatering ⚫ To provide a dry working area, such as in excavations for building foundations, dams, and tunnels ⚫ To stabilise constructed or natural slopes ⚫ To reduce lateral pressures on foundations or retaining structures ⚫ To reduce the compressibility of granular soils ⚫ To increase the bearing capacity of foundations ⚫ To prevent liquefaction due to an upward gradient (quick conditions) ⚫ To reduce the liquefaction potential during earthquakes Dewatering Techniques 8 Reasons For Dewatering ⚫ To provide a dry working area, such as in excavations for building foundations, dams, and tunnels ⚫ To stabilise constructed or natural slopes ⚫ To reduce lateral pressures on foundations or retaining structures ⚫ To reduce the compressibility of granular soils ⚫ To increase the bearing capacity of foundations ⚫ To prevent liquefaction due to an upward gradient (quick conditions) ⚫ To reduce the liquefaction potential during earthquakes Dewatering Techniques 9 Reasons For Dewatering ⚫ To provide a dry working area, such as in excavations for building foundations, dams, and tunnels ⚫ To stabilise constructed or natural slopes ⚫ To reduce lateral pressures on foundations or retaining structures ⚫ To reduce the compressibility of granular soils ⚫ To increase the bearing capacity of foundations ⚫ To prevent liquefaction due to an upward gradient (quick conditions) ⚫ To reduce the liquefaction potential during earthquakes Dewatering Techniques 10 Reasons For Dewatering ⚫ To provide a dry working area, such as in excavations for building foundations, dams, and tunnels ⚫ To stabilise constructed or natural slopes ⚫ To reduce lateral pressures on foundations or retaining structures ⚫ To reduce the compressibility of granular soils ⚫ To increase the bearing capacity of foundations ⚫ To prevent liquefaction due to an upward gradient (quick conditions) ⚫ To reduce the liquefaction potential during earthquakes Dewatering Techniques 11 Reasons For Dewatering ⚫ To provide a dry working area, such as in excavations for building foundations, dams, and tunnels ⚫ To stabilise constructed or natural slopes ⚫ To reduce lateral pressures on foundations or retaining structures ⚫ To reduce the compressibility of granular soils ⚫ To increase the bearing capacity of foundations ⚫ To prevent liquefaction due to an upward gradient (quick conditions) ⚫ To reduce the liquefaction potential during earthquakes Dewatering Techniques 12 Reasons For Dewatering ⚫ To provide a dry working area, such as in excavations for building foundations, dams, and tunnels ⚫ To stabilise constructed or natural slopes ⚫ To reduce lateral pressures on foundations or retaining structures ⚫ To reduce the compressibility of granular soils ⚫ To increase the bearing capacity of foundations ⚫ To prevent liquefaction due to an upward gradient (quick conditions) ⚫ To reduce the liquefaction potential during earthquakes Dewatering Techniques 13 Reasons For Dewatering ⚫ To provide a dry working area, such as in excavations for building foundations, dams, and tunnels ⚫ To stabilise constructed or natural slopes ⚫ To reduce lateral pressures on foundations or retaining structures ⚫ To reduce the compressibility of granular soils ⚫ To increase the bearing capacity of foundations ⚫ To prevent liquefaction due to an upward gradient (quick conditions) ⚫ To reduce the liquefaction potential during earthquakes Dewatering Techniques 14 Reasons For Dewatering ⚫ To provide a dry working area, such as in excavations for building foundations, dams, and tunnels ⚫ To stabilise constructed or natural slopes ⚫ To reduce lateral pressures on foundations or retaining structures ⚫ To reduce the compressibility of granular soils ⚫ To increase the bearing capacity of foundations ⚫ To prevent liquefaction due to an upward gradient (quick conditions) ⚫ To reduce the liquefaction potential during earthquakes Dewatering Techniques 15 Is the soil suitable? Dewatering Techniques 16 Is the soil suitable? Limit of Gravity drainage Dewatering Techniques 17 Is the soil suitable? Limit of Gravity drainage Dewatering Techniques 18 Is the soil suitable? Limit of Gravity drainage Dewatering Techniques 19 Dewatering Methods ⚫ ⚫ ⚫ Open sumps and ditches Gravity flow wells Vacuum dewatering wells Dewatering Techniques 20 Open Sumps & Ditches Autumn 2008 Dewatering Techniques 21 Open Sumps & Ditches ⚫ Can be used to handle small amount of water inflow ⚫ Works well in relatively shallow excavation ➢ Water head < 2m ⚫ Works well in coarse grain soils & in fissured rocks ⚫ May be used when sinking of wells is not possible ➢ ⚫ Due to floaters or other obstructions Can be used for sheet pile walls, but checks needed for: ➢ Quick condition due to seepage in front of the wall ➢ Bottom heave and instability ➢ Slumping of the soil Autumn 2008 Dewatering Techniques 22 Open Sumps & Ditches ⚫ Can be used to handle small amount of water inflow ⚫ Works well in relatively shallow excavation ➢ Water head < 2m ⚫ Works well in coarse grain soils & in fissured rocks ⚫ May be used when sinking of wells is not possible ➢ ⚫ Due to floaters or other obstructions Can be used for sheet pile walls, but checks needed for: ➢ Quick condition due to seepage in front of the wall ➢ Bottom heave and instability ➢ Slumping of the soil Autumn 2008 Dewatering Techniques 23 Open Sumps & Ditches ⚫ Pumps are often used to remove water from sumps and ditches ⚫ Sumps and ditches should be located outside the footing area, below the footing level ⚫ It is important to prevent piping; ➢ Prevent fine particles to be carried by water ➢ May need filter to minimize loss of fines Dewatering Techniques 24 Open Sumps & Ditches ⚫ Pumps are often used to remove water from sumps and ditches ⚫ Sumps and ditches should be located outside the footing area, below the footing level ⚫ It is important to prevent piping; ➢ Prevent fine particles to be carried by water ➢ May need filter to minimize loss of fines Dewatering Techniques 25 Open Sumps & Ditches ⚫ Pumps are often used to remove water from sumps and ditches ⚫ Sumps and ditches should be located outside the footing area, below the footing level ⚫ It is important to prevent piping; ➢ Prevent fine particles to be carried by water ➢ May need filter to minimize loss of fines Dewatering Techniques 26 Open Sumps & Ditches ⚫ Pumps are often used to remove water from sumps and ditches ⚫ Sumps and ditches should be located outside the footing area, below the footing level ⚫ It is important to prevent piping; ➢ Prevent fine particles to be carried by water ➢ May need filter to minimize loss of fines Dewatering Techniques 27 Open Sumps & Ditches ⚫ Pumps are often used to remove water from sumps and ditches ⚫ Sumps and ditches should be located outside the footing area, below the footing level ⚫ It is important to prevent piping; ➢ Prevent fine particles to be carried by water ➢ May need filter to minimize loss of fines Dewatering Techniques 28 Open Sumps & Ditches ⚫ Pumps are often used to remove water from sumps and ditches ⚫ Sumps and ditches should be located outside the footing area, below the footing level ⚫ It is important to prevent piping; ➢ Prevent fine particles to be carried by water ➢ May need filter to minimize loss of fines Dewatering Techniques 29 Gravity Flow Wells ⚫ Often used to lower the water table in sandy soils ⚫ Water flows into the well under gravity if the water level in the well lowered by pumping ➢ ⚫ The water level will drop to a new equilibrium position Water is pumped through a riser pipe ➢ Usual diameters are 150-200mm ⚫ Submersible pumps has no depth limit ⚫ The water level should be lowered to a level at least 0.5m below the base of an excavation ➢ More for sand > 0.7m Dewatering Techniques 30 Gravity Flow Wells ⚫ Often used to lower the water table in sandy soils ⚫ Water flows into the well under gravity if the water level in the well lowered by pumping ➢ ⚫ The water level will drop to a new equilibrium position Water is pumped through a riser pipe ➢ Usual diameters are 150-200mm ⚫ Submersible pumps has no depth limit ⚫ The water level should be lowered to a level at least 0.5m below the base of an excavation ➢ More for sand > 0.7m Dewatering Techniques 31 Gravity Flow Wells ⚫ Often used to lower the water table in sandy soils ⚫ Water flows into the well under gravity if the water level in the well lowered by pumping ➢ ⚫ The water level will drop to a new equilibrium position Water is pumped through a riser pipe ➢ Usual diameters are 150-200mm ⚫ Submersible pumps has no depth limit ⚫ The water level should be lowered to a level at least 0.5m below the base of an excavation ➢ More for sand > 0.7m Dewatering Techniques Hausmann 32 Gravity Flow Wells ⚫ Often used to lower the water table in sandy soils ⚫ Water flows into the well under gravity if the water level in the well lowered by pumping ➢ ⚫ The water level will drop to a new equilibrium position Water is pumped through a riser pipe ➢ Usual diameters are 150-200mm ⚫ Submersible pumps has no depth limit ⚫ The water level should be lowered to a level at least 0.5m below the base of an excavation ➢ More for sand > 0.7m Dewatering Techniques Hausmann 33 Gravity Flow Wells ⚫ Often used to lower the water table in sandy soils ⚫ Water flows into the well under gravity if the water level in the well lowered by pumping ➢ ⚫ The water level will drop to a new equilibrium position Water is pumped through a riser pipe ➢ Usual diameters are 150-200mm ⚫ Submersible pumps has no depth limit ⚫ The water level should be lowered to a level at least 0.5m below the base of an excavation ➢ More for sand > 0.7m Dewatering Techniques Hausmann 34 Gravity Flow Wells ⚫ Often used to lower the water table in sandy soils ⚫ Water flows into the well under gravity if the water level in the well lowered by pumping ➢ ⚫ The water level will drop to a new equilibrium position Water is pumped through a riser pipe ➢ Usual diameters are 150-200mm ⚫ Submersible pumps has no depth limit ⚫ The water level should be lowered to a level at least 0.5m below the base of an excavation ➢ More for sand > 0.7m Dewatering Techniques Hausmann 35 Gravity Flow Wells ⚫ Often used to lower the water table in sandy soils ⚫ Water flows into the well under gravity if the water level in the well lowered by pumping ➢ ⚫ The water level will drop to a new equilibrium position Water is pumped through a riser pipe ➢ Usual diameters are 150-200mm ⚫ Submersible pumps has no depth limit ⚫ The water level should be lowered to a level at least 0.5m below the base of an excavation ➢ More for sand > 0.7m Dewatering Techniques Hausmann 36 Gravity Flow Wells ⚫ Often used to lower the water table in sandy soils ⚫ Water flows into the well under gravity if the water level in the well lowered by pumping ➢ ⚫ The water level will drop to a new equilibrium position Water is pumped through a riser pipe ➢ Usual diameters are 150-200mm ⚫ Submersible pumps has no depth limit ⚫ The water level should be lowered to a level at least 0.5m below the base of an excavation ➢ More for sand > 0.7m Dewatering Techniques Hausmann 37 Gravity Flow Wells ⚫ Often used to lower the water table in sandy soils ⚫ Water flows into the well under gravity if the water level in the well lowered by pumping ➢ ⚫ The water level will drop to a new equilibrium position Water is pumped through a riser pipe ➢ Usual diameters are 150-200mm ⚫ Submersible pumps has no depth limit ⚫ The water level should be lowered to a level at least 0.5m below the base of an excavation ➢ More for sand > 0.7m Dewatering Techniques 38 Gravity Flow Wells ⚫ Often used to lower the water table in sandy soils ⚫ Water flows into the well under gravity if the water level in the well lowered by pumping ➢ ⚫ The water level will drop to a new equilibrium position Water is pumped through a riser pipe ➢ Usual diameters are 150-200mm ⚫ Submersible pumps has no depth limit ⚫ The water level should be lowered to a level at least 0.5m below the base of an excavation ➢ More for sand > 0.7m Dewatering Techniques 39 Multi-Well Systems ⚫ ⚫ Multiple closely spaced wells connected by pipes to a powerful pump Multiple lines or stages of well points are required for excavations more than 5 to 7m below the groundwater table Dewatering Techniques 40 Multi-Well Systems ⚫ ⚫ Multiple closely spaced wells connected by pipes to a powerful pump Multiple lines or stages of well points are required for excavations more than 5 to 7m below the groundwater table Johnson (1975) Dewatering Techniques 41 Multi-Well Systems ⚫ ⚫ Multiple closely spaced wells connected by pipes to a powerful pump Multiple lines or stages of well points are required for excavations more than 5 to 7m below the groundwater table Johnson (1975) geoquipwatersolutions.com Dewatering Techniques 42 Multi-Well Systems ⚫ ⚫ Multiple closely spaced wells connected by pipes to a powerful pump Multiple lines or stages of well points are required for excavations more than 5 to 7m below the groundwater table Johnson (1975) geoquipwatersolutions.com Dewatering Techniques 43 Multi-Well Systems ⚫ ⚫ Multiple closely spaced wells connected by pipes to a powerful pump Multiple lines or stages of well points are required for excavations more than 5 to 7m below the groundwater table Johnson (1975) geoquipwatersolutions.com Dewatering Techniques 44 Multi-Well Systems ⚫ ⚫ Multiple closely spaced wells connected by pipes to a powerful pump Multiple lines or stages of well points are required for excavations more than 5 to 7m below the groundwater table Johnson (1975) geoquipwatersolutions.com Dewatering Techniques 45 Multi-Well Systems ⚫ ⚫ Multiple closely spaced wells connected by pipes to a powerful pump Multiple lines or stages of well points are required for excavations more than 5 to 7m below the groundwater table Johnson (1975) geoquipwatersolutions.com Dewatering Techniques 46 Multi-Well Systems Dewatering Techniques 47 Multi-Well Systems Dewatering Techniques 48 Multi-Well Systems Dewatering Techniques 49 Multi-Well Systems Dewatering Techniques 50 Vacuum Dewatering Wells ⚫ Use in fine soils where water does not flow freely under gravity ➢ ⚫ ⚫ ⚫ ⚫ Capillary tension is high The well is sealed off close to filter section or around the riser pipe Vacuum increases the difference in pressure heads Water inflow is low Well spacing must be close Dewatering Techniques 51 Vacuum Dewatering Wells ⚫ Use in fine soils where water does not flow freely under gravity ➢ ⚫ ⚫ ⚫ ⚫ Capillary tension is high The well is sealed off close to filter section or around the riser pipe Vacuum increases the difference in pressure heads Water inflow is low Well spacing must be close Dewatering Techniques 52 Dewatering Techniques 53 Dewatering Techniques 54 Dewatering Techniques 55 http://www.hoax-slayer.com/dubai-construction-flood.shtml Dewatering Techniques 56 http://www.hoax-slayer.com/dubai-construction-flood.shtml Dewatering Techniques 57 http://www.hoax-slayer.com/dubai-construction-flood.shtml Dewatering Techniques 58 http://www.hoax-slayer.com/dubai-construction-flood.shtml Dewatering Techniques 59 http://www.hoax-slayer.com/dubai-construction-flood.shtml Dewatering Techniques 60 http://www.hoax-slayer.com/dubai-construction-flood.shtml Dewatering Techniques 61 http://www.hoax-slayer.com/dubai-construction-flood.shtml Dewatering Techniques 62 http://www.hoax-slayer.com/dubai-construction-flood.shtml Dewatering Techniques 63 Dewatering Techniques 64 Dewatering Techniques 65 Dewatering Techniques 66 Dewatering Techniques 67 Dewatering Techniques 68 Dewatering Techniques 69 Dewatering Techniques 70 Dewatering Techniques 71 Dewatering Techniques 72 Soil-Water Relationships Dewatering Techniques 73 Stresses in Dry Soil z Autumn 2008 Dewatering Techniques 74 Stresses in Dry Soil t =0 z sh sv Autumn 2008 Dewatering Techniques 75 Stresses in Dry Soil t =0 W = gd z A W z sh sv Autumn 2008 Dewatering Techniques 76 Stresses in Dry Soil t =0 W = gd z A W W = sv A z sh sv Autumn 2008 Dewatering Techniques 77 Stresses in Dry Soil t =0 W = gd z A W W = sv A z sv = gd z sh sv Autumn 2008 Dewatering Techniques 78 Stresses in Dry Soil q t =0 W = gd z A W W = sv A z sv = gd z +q sh sv Autumn 2008 Dewatering Techniques 79 Stresses in Dry Soil q t =0 W = gd z A W W = sv A z sv = gd z +q sh sv Autumn 2008 sh = Ko sv Ko = Coefficient of lateral earth pressure at rest Dewatering Techniques 80 Stresses in Saturated Soil ⚫ Effective stress concept: Saturated soil Autumn 2008 Dewatering Techniques 81 Stresses in Saturated Soil ⚫ Effective stress concept: Soil loaded by an applied weight W W W Soil loaded by water weighing W Saturated soil Autumn 2008 Dewatering Techniques 82 Stresses in Saturated Soil ⚫ Effective stress concept: Soil loaded by an applied weight W W W Compression Autumn 2008 Soil loaded by water weighing W No deformation Dewatering Techniques 83 Stresses in Saturated Soil ⚫ Effective stress concept: Soil loaded by an applied weight W W W Compression Soil loaded by water weighing W No deformation Deformation is a function of the stresses applied to the soil. Autumn 2008 Dewatering Techniques 84 Stresses in Saturated Soil ⚫ Effective stress concept: Vw F=Vw gw Autumn 2008 Dewatering Techniques 85 Stresses in Saturated Soil ⚫ Effective stress concept: Vw Vs F=Vw gw Autumn 2008 Dewatering Techniques 86 Stresses in Saturated Soil ⚫ Effective stress concept: Vw Vs F=Vw gw +Vs gs-Vs gw Autumn 2008 Dewatering Techniques 87 Stresses in Saturated Soil ⚫ Effective stress concept: Vw Vs F=Vw gw +Vs gs-Vs gw =Vw gw+Vs (gs- gw) Autumn 2008 Dewatering Techniques 88 Stresses in Saturated Soil ⚫ Effective stress concept: Vw F` Vs F=Vw gw +Vs gs-Vs gw =Vw gw+Vs (gs- gw) F= Fw + F’ Inter-granular force Autumn 2008 Dewatering Techniques 89 Stresses in Saturated Soil ⚫ Effective stress concept: Vw F` Autumn 2008 Dewatering Techniques 90 Stresses in Saturated Soil ⚫ Effective stress concept: Vw F = Vw gw Autumn 2008 Dewatering Techniques 91 Stresses in Saturated Soil ⚫ Effective stress concept: Vw Vsoil F = Vw gw +Vsoil gsat-Vsoil gw Autumn 2008 Dewatering Techniques 92 Stresses in Saturated Soil ⚫ Effective stress concept: Vw Vsoil F = Vw gw +Vsoil gsat-Vsoil gw =Vw gw+Vsoil (gsat - gw) Autumn 2008 Dewatering Techniques 93 Stresses in Saturated Soil ⚫ Effective stress concept: Vw Vsoil F = Vw gw +Vsoil gsat-Vsoil gw =Vw gw+Vsoil (gsat - gw) F = Fw + F’ Autumn 2008 Dewatering Techniques 94 Stresses in Saturated Soil ⚫ Effective stress concept: Vw Vsoil F = Vw gw +Vsoil gsat-Vsoil gw =Vw gw+Vsoil (gsat - gw) F = Fw + F’ Autumn 2008 F’ = F - Fw Dewatering Techniques 95 Stresses in Saturated Soil ⚫ Effective stress concept: sv = sv - u Effective stress Autumn 2008 Total stress Pore water pressure =gw zw Dewatering Techniques 96 Stresses in Saturated Soil ⚫ Effective stress concept: sv = sv - u Effective stress Total stress Pore water pressure =gw zw sh = Ko sv Coefficient of earth pressure at rest Autumn 2008 Dewatering Techniques 97 Stresses in Saturated Soil ⚫ Effective stress concept: sv = sv - u Effective stress Total stress Pore water pressure =gw zw sh = Ko sv Coefficient of earth pressure at rest sh = sh + u Autumn 2008 Dewatering Techniques 98 Stresses in Saturated Soil ⚫ Effective stress concept: Autumn 2008 Dewatering Techniques 99 Stresses in Saturated Soil ⚫ Effective stress concept: s s Autumn 2008 Dewatering Techniques 100 Stresses in Saturated Soil ⚫ Effective stress concept: s u s u u Autumn 2008 s Dewatering Techniques u s s 101 Stresses in Saturated Soil ⚫ Capillary tension: Water rises above the water table due to tension between soil particles and water. WT Dewatering Techniques 102 Stresses in Saturated Soil ⚫ Capillary tension: Water rises above the water table due to tension between soil particles and water. uWT u+ z Dewatering Techniques 103 Stresses in Saturated Soil ⚫ Capillary tension: Water rises above the water table due to tension between soil particles and water. - Negative pore pressure - Suction - Tensile stress. uWT u+ z Dewatering Techniques 104 Stresses in Saturated Soil ⚫ Capillary tension: Water rises above the water table due to tension between soil particles and water. - Negative pore pressure - Suction - Tensile stress. uWT The depth and the value of the capillary suction are difficult to determine. u+ z Dewatering Techniques 105 Stresses in Saturated Soil ⚫ Capillary tension: Water rises above the water table due to tension between soil particles and water. - Negative pore pressure - Suction - Tensile stress. uWT The depth and the value of the capillary suction are difficult to determine. - 0.05m in gravel - >1m in fine sand + u - > 3m in silt z Dewatering Techniques 106 Drainable Water Dewatering Techniques 107 Drainable Water ⚫ ⚫ Water does not drain completely from the voids above the new water table Specific yield: ➢ ➢ ⚫ Specific retention: ➢ ➢ ⚫ The ratio of water drains from soil to the soil total volume Varies from 0.2-0.3 The amount of water retained after the specific yield has released Specific retention = n – Specific yield Some water will be released from soil below water table due to increase in effective stress and consolidation Dewatering Techniques 108 Drainable Water ⚫ ⚫ Water does not drain completely from the voids above the new water table Specific yield: ➢ ➢ ⚫ Specific retention: ➢ ➢ ⚫ The ratio of water drains from soil to the soil total volume Varies from 0.2-0.3 The amount of water retained after the specific yield has released Specific retention = n – Specific yield Some water will be released from soil below water table due to increase in effective stress and consolidation Dewatering Techniques 109 Drainable Water ⚫ ⚫ Water does not drain completely from the voids above the new water table Specific yield: ➢ ➢ ⚫ Specific retention: ➢ ➢ ⚫ The ratio of water drains from soil to the soil total volume Varies from 0.2-0.3 The amount of water retained after the specific yield has released Specific retention = n – Specific yield Some water will be released from soil below water table due to increase in effective stress and consolidation Dewatering Techniques 110 Drainable Water ⚫ ⚫ Water does not drain completely from the voids above the new water table Specific yield: ➢ ➢ ⚫ Specific retention: ➢ ➢ ⚫ The ratio of water drains from soil to the soil total volume Varies from 0.2-0.3 The amount of water retained after the specific yield has released Specific retention = n – Specific yield Some water will be released from soil below water table due to increase in effective stress and consolidation Dewatering Techniques 111 Water Flow Factors affecting the rate of flow: Dewatering Techniques 112 Water Flow Factors affecting the rate of flow: Dewatering Techniques 113 Water Flow Factors affecting the rate of flow: No flow Dewatering Techniques 114 Water Flow Factors affecting the rate of flow: Dewatering Techniques 115 Water Flow Factors affecting the rate of flow: Water level difference causes flow. DH Dewatering Techniques 116 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) DH Dewatering Techniques 117 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) DH Dewatering Techniques 118 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) Q = f (DH) DH DL Dewatering Techniques 119 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) Q = f (DH) Q = f (1/ DL) DH DL A=Cross section area Dewatering Techniques 120 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) Q = f (DH) Q = f (1/ DL) Q = f (A) DH DL A=Cross section area Dewatering Techniques 121 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) Q = f (DH) Q = f (1/ DL) Q = f (A) DH DL A=Cross section area Dewatering Techniques 122 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) Q = f (DH) Q = f (1/ DL) Q = f (A) DH DL A=Cross section area Dewatering Techniques 123 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) Q = f (DH) Q = f (1/ DL) Q = f (A) DH DL A=Cross section area Dewatering Techniques 124 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) Q = f (DH) Q = f (1/ DL) Q = f (A) DH DL A=Cross section area Dewatering Techniques 125 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) Q = f (DH) Q = f (1/ DL) Q = f (A) Q = f (k) k=Coefficient of permeability DH DL A=Cross section area Dewatering Techniques 126 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) Q = f (DH) Q = f (1/ DL) Q = f (A) Q = f (k) DH=Head difference DL=Length of the soil sample A=Cross section area k=Coefficient of permeability DH DL Dewatering Techniques 127 Water Flow Factors affecting the rate of flow: Rate of flow = Q (m3/s) Q = f (DH) Q = f (1/ DL) Q = f (A) Q = f (k) DH=Head difference DL=Length of the soil sample A=Cross section area k=Coefficient of permeability Darcy' s law DH Q = k.A DL DH DL Dewatering Techniques 128 Hydraulic Gradient A DL B Dewatering Techniques 129 Hydraulic Gradient HA A DL B HB Dewatering Techniques 130 Hydraulic Gradient Hydraulic gradient (i) is the rate of head loss between two points in a soil. HA A DL B HB Dewatering Techniques 131 Hydraulic Gradient Hydraulic gradient (i) is the rate of head loss between two points in a soil. H A − HB i= DL AB HA A DL B HB Dewatering Techniques 132 Hydraulic Gradient Hydraulic gradient (i) is the rate of head loss between two points in a soil. DH A B H A − HB i= = DL AB DL AB HA A DL B HB Dewatering Techniques 133 Darcy’s law DH AB Q = k . A. DL AB HA A DL B HB Dewatering Techniques 134 Darcy’s law DH AB = k . A. i Q = k . A. DL AB HA A DL B HB Dewatering Techniques 135 Darcy’s law DH AB = k . A. i Q = k . A. DL AB Rate of flow Coefficient of permeability Cross section area Hydraulic gradient Distance Head between A difference &B between A & B HA A DL B HB Dewatering Techniques 136 Dewatering Techniques 137 Aquifer Type ⚫ Aquifer: ➢ ⚫ Confined or artesian aquifer ➢ ⚫ Fully saturated soil confined by impervious layers Unconfined aquifer ➢ ⚫ A permeable soil which stores significant amount of water No upper impervious boundary exists Several water bearing layers may exist ➢ Perched aquifer: ❖ ⚫ Unconfined aquifer separated from ground water by unsaturated soil In analysis of flow problem, aquifers are idealised ➢ ➢ Have horizontal boundaries Homogeneous and isotropic porous material Dewatering Techniques 138 Aquifer Type ⚫ Aquifer: ➢ ⚫ Confined or artesian aquifer ➢ ⚫ Fully saturated soil confined by impervious layers Unconfined aquifer ➢ ⚫ A permeable soil which stores significant amount of water No upper impervious boundary exists Several water bearing layers may exist ➢ Perched aquifer: ❖ ⚫ Unconfined aquifer separated from ground water by unsaturated soil In analysis of flow problem, aquifers are idealised ➢ ➢ Have horizontal boundaries Homogeneous and isotropic porous material Dewatering Techniques 139 Aquifer Type ⚫ Aquifer: ➢ ⚫ Confined or artesian aquifer ➢ ⚫ Fully saturated soil confined by impervious layers Unconfined aquifer ➢ ⚫ A permeable soil which stores significant amount of water No upper impervious boundary exists Several water bearing layers may exist ➢ Perched aquifer: ❖ ⚫ Unconfined aquifer separated from ground water by unsaturated soil In analysis of flow problem, aquifers are idealised ➢ ➢ Have horizontal boundaries Homogeneous and isotropic porous material Dewatering Techniques 140 Aquifer Type ⚫ Aquifer: ➢ ⚫ Confined or artesian aquifer ➢ ⚫ Fully saturated soil confined by impervious layers Unconfined aquifer ➢ ⚫ A permeable soil which stores significant amount of water No upper impervious boundary exists Several water bearing layers may exist ➢ Perched aquifer: ❖ ⚫ Unconfined aquifer separated from ground water by unsaturated soil In analysis of flow problem, aquifers are idealised ➢ ➢ Have horizontal boundaries Homogeneous and isotropic porous material Dewatering Techniques 141 Aquifer Type ⚫ Aquifer: ➢ ⚫ Confined or artesian aquifer ➢ ⚫ Fully saturated soil confined by impervious layers Unconfined aquifer ➢ ⚫ A permeable soil which stores significant amount of water No upper impervious boundary exists Several water bearing layers may exist ➢ Perched aquifer: ❖ ⚫ Unconfined aquifer separated from ground water by unsaturated soil In analysis of flow problem, aquifers are idealised ➢ ➢ Have horizontal boundaries Homogeneous and isotropic porous material Dewatering Techniques 142 Hydraulic of Slots & Wells ⚫ Assumptions: ➢ Darcy’s law is valid ❖ ➢ Q=kiA Dupuit-Theim approximation ❖ The hydraulic gradient at any point is equal to the slope of drawdown curve i = dy/dx Dewatering Techniques 143 Hydraulic of Slots & Wells ⚫ Assumptions: ➢ Darcy’s law is valid ❖ ➢ Q=kiA Dupuit-Theim approximation ❖ The hydraulic gradient at any point is equal to the slope of drawdown curve i = dy/dx Dewatering Techniques 144 Hydraulic of Perfect Slots Dewatering Techniques 145 Hydraulic of Perfect Slots Dewatering Techniques 146 Hydraulic of Perfect Slots q=k i A Dewatering Techniques 147 Hydraulic of Perfect Slots q=k i A dy q=k y dx Dewatering Techniques 148 Hydraulic of Perfect Slots q=k i A dy q=k y dx h L q h y dy = k 0 dx w Dewatering Techniques 149 Hydraulic of Perfect Slots q=k i A dy q=k y dx h L q h y dy = k 0 dx w h 2 − h 2w qL = 2 k Dewatering Techniques 150 Hydraulic of Perfect Slots q=k i A dy q=k y dx h L q h y dy = k 0 dx w h 2 − h 2w qL = 2 k ( h q= 2 ) − h 2w k 2L Dewatering Techniques 151 Hydraulic of Perfect Slots q=k i A dy q=k y dx h L q h y dy = k 0 dx w h 2 − h 2w qL = 2 k ( h q= 2 ) − h 2w k 2L The total flow will be 2q Dewatering Techniques 152 Hydraulic of Perfect Slots q=k i A dy q=k y dx h L q h y dy = k 0 dx w h 2 − h 2w qL = 2 k ( h q= 2 −h 2L 2 w )k The total flow will be 2q Equation of drawdown: L−x 2 h −y = h − h 2w L 2 Dewatering Techniques 2 ( ) 153 Hydraulic of Perfect Wells Dewatering Techniques 154 Hydraulic of Perfect Wells Q=k i A Dewatering Techniques 155 Hydraulic of Perfect Wells Q=k i A dy Q = k ( 2 x ) y dx Dewatering Techniques 156 Hydraulic of Perfect Wells Q=k i A dy Q = k ( 2 x ) y dx L h dx r Q x = 2k h ydy w Dewatering Techniques 157 Hydraulic of Perfect Wells Q=k i A dy Q = k ( 2 x ) y dx L h dx r Q x = 2k h ydy w ( k h 2 − h 2w Q= ln( L / r ) ) Dewatering Techniques 158 Hydraulic of Perfect Wells Q=k i A dy Q = k ( 2 x ) y dx L h dx r Q x = 2k h ydy w ( k h 2 − h 2w Q= ln( L / r ) ) Equation of drawdown: Q ln( x / r ) y −h = k 2 2 w Dewatering Techniques 159 Free Discharge Height ⚫ ⚫ hs = h w – ho For rough calculation: ➢ ⚫ hw = ho h s= ? ➢ ➢ For slots use chart For wells: Dewatering Techniques 160 Free Discharge Height ⚫ ⚫ hs = h w – ho For rough calculation: ➢ ⚫ hw = ho h s= ? ➢ ➢ For slots use chart For wells: Dewatering Techniques 161 Free Discharge Height ⚫ ⚫ hs = h w – ho For rough calculation: ➢ ⚫ hw = ho h s= ? ➢ ➢ For slots use chart For wells: Dewatering Techniques 162 Free Discharge Height ⚫ ⚫ hs = h w – ho For rough calculation: ➢ ⚫ hw = ho h s= ? ➢ ➢ For slots use chart For wells: Dewatering Techniques 163 Free Discharge Height ⚫ ⚫ hs = h w – ho For rough calculation: ➢ ⚫ hw = ho h s= ? ➢ ➢ For slots use chart For wells: Dewatering Techniques 164 Free Discharge Height ⚫ ⚫ hs = h w – ho For rough calculation: ➢ ⚫ hw = ho h s= ? ➢ ➢ For slots use chart For wells: 0.5( h − h o ) 2 hs = h Dewatering Techniques 165 Influence Range ⚫ L=? ➢ ➢ ⚫ Important for slots Less important for wells If no recharge ➢ L varies with time at decreasing rate: Dewatering Techniques 166 Influence Range ⚫ L=? ➢ ➢ ⚫ Important for slots Less important for wells If no recharge ➢ L varies with time at decreasing rate: hkt L = 1 .5 n Dewatering Techniques 167 Influence Range ⚫ L=? ➢ ➢ ⚫ Important for slots Less important for wells If no recharge ➢ L varies with time at decreasing rate: hkt L = 1 .5 n ➢ Or with sufficient accuracy: L = C(h − h w ) k = C s k ➢ C=3000 for wells and 1500-2000 for slots Dewatering Techniques 168 Influence Range ⚫ L=? ➢ ➢ ⚫ Important for slots Less important for wells If no recharge ➢ L varies with time at decreasing rate: hkt L = 1 .5 n ➢ Or with sufficient accuracy: L = C(h − h w ) k = C s k ➢ C=3000 for wells and 1500-2000 for slots Dewatering Techniques 169 Hydraulic of Multi-Wells ⚫ For n wells, the water level at point P will be equal to y if the total discharge is Q: Dewatering Techniques 170 Hydraulic of Multi-Wells ⚫ For n wells, the water level at point P will be equal to y if the total discharge is Q: Q1 = ( k h 2 − y 2 ) ln( L / x 1 ) Dewatering Techniques 171 Hydraulic of Multi-Wells ⚫ For n wells, the water level at point P will be equal to y if the total discharge is Q: Q1 = ( k h 2 − y 2 ln( L / x 1 ) ) = k (h − h o2 ln( L / r ) 2 ) Dewatering Techniques 172 Hydraulic of Multi-Wells ⚫ For n wells, the water level at point P will be equal to y if the total discharge is Q: Q1 = Q= ( k h 2 − y 2 ln( L / x 1 ) ) = k (h 2 ( ) − h o2 ln( L / r ) k h 2 − y 2 ) ln L − (1 / n ) ln( x 1x 2 x 3 .......x n ) Dewatering Techniques 173 Hydraulic of Multi-Wells ⚫ For circular arrangement of n wells, the water level at the centre of the circle will be equal to y if the total discharge is Q: Dewatering Techniques 174 Hydraulic of Multi-Wells ⚫ For circular arrangement of n wells, the water level at the centre of the circle will be equal to y if the total discharge is Q: k h 2 − y 2 Q= ln L − (1 / n ) ln( x 1x 2 x 3 .......x n ) ( ) Dewatering Techniques 175 Hydraulic of Multi-Wells ⚫ For circular arrangement of n wells, the water level at the centre of the circle will be equal to y if the total discharge is Q: k h 2 − y 2 Q= ln L − (1 / n ) ln( x 1x 2 x 3 .......x n ) ( Q= ( k h − y 2 2 ) ) ln L − ln a Dewatering Techniques 176 Hydraulic of Multi-Wells ⚫ For circular arrangement of n wells, the water level at the centre of the circle will be equal to y if the total discharge is Q: k h 2 − y 2 Q= ln L − (1 / n ) ln( x 1x 2 x 3 .......x n ) ( Q= ⚫ ( k h − y 2 2 ) ) ln L − ln a The water level ho of an individual well: ( ) k h 2 − h o2 Q= ln L − (1 / n ) ln( r x 2 x 3 .......x n ) Dewatering Techniques 177 Ground Permeability Dewatering Techniques 178 Ground Permeability ⚫ ⚫ The most important factor in determination of discharge and water level Can be (in the order of increasing cost and accuracy): ➢ ➢ ➢ ➢ Approximated from empirical relationships Laboratory tests Borehole tests Field pumping tests Dewatering Techniques 179 Ground Permeability ⚫ ⚫ The most important factor in determination of discharge and water level Can be (in the order of increasing cost and accuracy): ➢ ➢ ➢ ➢ ⚫ Approximated from empirical relationships Laboratory tests Borehole tests Field pumping tests Typical range: ➢ 10-10 Soils exhibit a wide range of permeabilities. 10-9 Clays 10-8 10-7 10-6 10-5 10-4 Silts 10-3 Sands Coarse Fines Dewatering Techniques 10-2 10-1 10-0 Gravels (m/s) 180 Ground Permeability Can be related to particle size. For 1<Cu< 3 Coefficient of permeability ➢ D5 (mm) Dewatering Techniques 181 Ground Permeability Can be related to particle size. For Cu> 3 For 1<Cu< 3 permeability Coefficientofofpermeability Coefficient ➢ D D55 (mm) (mm) Dewatering Techniques 182 Ground Permeability ➢ ➢ Can be related to particle size. Empirical relationships: ❖ For uniform sands: Terzaghi and Peck (1967) ❖ For compacted sands: Sherard et al. (1984) ➢ ➢ Small amount of silt and clay will change coefficient of permeability Care must be exercised whenever using empirical relationships – examine basis for relationship and limits of observations used Dewatering Techniques 183 Ground Permeability ➢ ➢ Can be related to particle size. Empirical relationships: ❖ For uniform sands: Terzaghi and Peck (1967) ❖ k=C 2 D10 For compacted sands: Sherard et al. (1984) ➢ ➢ Small amount of silt and clay will change coefficient of permeability Care must be exercised whenever using empirical relationships – examine basis for relationship and limits of observations used Dewatering Techniques 184 Ground Permeability ➢ ➢ Can be related to particle size. Empirical relationships: ❖ For uniform sands: Terzaghi and Peck (1967) ❖ k=C 2 D10 For compacted sands: Sherard et al. (1984) ➢ ➢ Small amount of silt and clay will change coefficient of permeability Care must be exercised whenever using empirical relationships – examine basis for relationship and limits of observations used Dewatering Techniques 185 Ground Permeability ➢ ➢ Can be related to particle size. Empirical relationships: ❖ For uniform sands: k=C Terzaghi and Peck (1967) ❖ For compacted sands: Sherard et al. (1984) ➢ ➢ 2 D10 2 k = 0 .35 D 15 Small amount of silt and clay will change coefficient of permeability Care must be exercised whenever using empirical relationships – examine basis for relationship and limits of observations used Dewatering Techniques 186 Ground Permeability ➢ ➢ Can be related to particle size. Empirical relationships: ❖ For uniform sands: k=C Terzaghi and Peck (1967) ❖ For compacted sands: Sherard et al. (1984) ➢ ➢ 2 D10 2 k = 0 .35 D 15 Small amount of silt and clay will change coefficient of permeability Care must be exercised whenever using empirical relationships – examine basis for relationship and limits of observations used Dewatering Techniques 187 Ground Permeability ⚫ Pumping tests with observation wells: Q ln( x / r ) y −h = k 2 Dewatering Techniques 2 w 188 Ground Permeability ⚫ Pumping tests with observation wells, unconfined aquifer: Q ln( x 2 / x 1 ) y −y = k 2 2 2 1 y1 y2 x1 x2 Dewatering Techniques 189 Ground Permeability ⚫ Pumping tests with observation wells, unconfined aquifer: Q ln( x 2 / x 1 ) k= ( y 22 − y12 ) Q ln( x 2 / x 1 ) y −y = k 2 2 2 1 y1 y2 x1 x2 Dewatering Techniques 190 Ground Permeability ⚫ Pumping tests with observation wells, unconfined aquifer: Q ln( x 2 / x 1 ) k= ( y 22 − y12 ) ⚫ Q ln( x 2 / x 1 ) y −y = k 2 2 2 1 Confined aquifer: Q ln( x 2 / x 1 ) k= 2 m ( h 2 − h 1 ) y1 y2 x1 x2 Dewatering Techniques 191 Design of Dewatering System Dewatering Techniques 192 Design of Dewatering System ⚫ ⚫ Most important input parameters: ➢ Water level after dewatering ➢ Coefficient of permeability Outcome of design: ➢ Arrangement of wells ➢ Number of wells ➢ Capacity of pumps Dewatering Techniques 193 Design of Dewatering System ⚫ ⚫ Most important input parameters: ➢ Water level after dewatering ➢ Coefficient of permeability Outcome of design: ➢ Arrangement of wells ➢ Number of wells ➢ Capacity of pumps 0.5-1.5m Dewatering Techniques 194 Design of Dewatering System ⚫ ⚫ Most important input parameters: ➢ Water level after dewatering ➢ Coefficient of permeability Outcome of design: ➢ Arrangement of wells ➢ Number of wells ➢ Capacity of pumps Dewatering Techniques 195 Well Discharge Capacity ⚫ Discharge capacity: Q i = A k ie Dewatering Techniques 196 Well Discharge Capacity ⚫ Discharge capacity: Q i = A k i e= 2 r h o k i e Dewatering Techniques 197 Well Discharge Capacity ⚫ Discharge capacity: Q i = A k i e= 2 r h o k i e ⚫ Limitation: ➢ To prevent turbulence, large head loss, filter instability, etc (i e )max 1 = 15 k Dewatering Techniques 198 Well Discharge Capacity ⚫ Discharge capacity: Q i = A k i e= 2 r h o k i e ⚫ Limitation: ➢ To prevent turbulence, large head loss, filter instability, etc (i e )max (Q i )max 1 = 15 k Therefore: k = 2 r h o 15 Dewatering Techniques 199 Well Discharge Capacity ⚫ Discharge capacity: Q i = A k i e= 2 r h o k i e ⚫ Limitation: ➢ To prevent turbulence, large head loss, filter instability, etc (i e )max (Q i )max 1 = 15 k k = 2 r h o 15 Therefore: Or: Dewatering Techniques (h o )min 15Q i = 2 r k 200 Well Discharge Capacity ⚫ Discharge capacity: Q i = A k i e= 2 r h o k i e ⚫ Limitation: ➢ To prevent turbulence, large head loss, filter instability, etc (i e )max 1 = 15 k Therefore: k (h o )min Or: (Q i )max = 2 r h o 15 ⚫ Discharge capacity according to well formula: ( k h 2 − h o2 Qi = ln( L / r ) 15Q i = 2 r k ) Dewatering Techniques 201 Well Discharge Capacity ⚫ Discharge capacity: Q i = A k i e= 2 r h o k i e ⚫ Limitation: ➢ To prevent turbulence, large head loss, filter instability, etc (i e )max 1 = 15 k Therefore: k (h o )min Or: (Q i )max = 2 r h o 15 ⚫ Discharge capacity according to well formula: ( k h 2 − h o2 Qi = ln( L / r ) 15Q i = 2 r k ) Dewatering Techniques 202 Well Diameter, Spacing, Depth ⚫ ⚫ ⚫ Maximum head is limited to pump capacity ➢ <8 meter for normal centrifuge pumps ➢ No limits for submerged pumps, but < 30-40m Diameter and spacing ➢ >3 – 4m spacing for 150mm diameter wells ➢ >5 - 6m spacing for 300-350mm diameter wells Spacing for well-points are closer Dewatering Techniques 203 Well Diameter, Spacing, Depth ⚫ ⚫ ⚫ Maximum head is limited to pump capacity ➢ <8 meter for normal centrifuge pumps ➢ No limits for submerged pumps, but < 30-40m Diameter and spacing ➢ >3 – 4m spacing for 150mm diameter wells ➢ >5 - 6m spacing for 300-350mm diameter wells Spacing for well-points are closer Dewatering Techniques 204 Well Diameter, Spacing, Depth ⚫ ⚫ ⚫ Maximum head is limited to pump capacity ➢ <8 meter for normal centrifuge pumps ➢ No limits for submerged pumps, but < 30-40m Diameter and spacing ➢ >3 – 4m spacing for 150mm diameter wells ➢ >5 - 6m spacing for 300-350mm diameter wells Spacing for well-points are closer Dewatering Techniques 205 Dewatering of Excavations ⚫ Standard design steps: 1: Replace the actual excavation with a equivalent circular area of radius “a” and estimate the quantity of water: Dewatering Techniques 206 Dewatering of Excavations ⚫ Standard design steps: 1: Replace the actual excavation with a equivalent circular area of radius “a” and estimate the quantity of water: Q tot = ( k h 2 − y 2 ) ln L − ln a y is the depth of water required at the centre of the excavation. ho must be assumed here. Dewatering Techniques 207 Dewatering of Excavations ⚫ Standard design steps: 1: Replace the actual excavation with a equivalent circular area of radius “a” and estimate the quantity of water: Q tot = ( k h 2 − y 2 ) ln L − ln a y is the depth of water required at the centre of the excavation. ho must be assumed here. 2: Estimate the number of wells needed using the (Qi)max: (Q i )max k = 2 r h o 15 Dewatering Techniques 208 Dewatering of Excavations ⚫ Standard design steps: 1: Replace the actual excavation with a equivalent circular area of radius “a” and estimate the quantity of water: Q tot = ( k h 2 − y 2 ) ln L − ln a y is the depth of water required at the centre of the excavation. ho must be assumed here. 2: Estimate the number of wells needed using the (Qi)max: (Q i )max k = 2 r h o 15 Therefore: Dewatering Techniques Q tot n= Q max 209 Dewatering of Excavations (cond) 3: Check the assumption made for ho: ( ) k h 2 − h o2 Q= ln L − (1 / n ) ln( r x 2 x 3 .......x n ) Dewatering Techniques 210 Dewatering of Excavations (cond) 3: Check the assumption made for ho: ( ) k h 2 − h o2 Q= ln L − (1 / n ) ln( r x 2 x 3 .......x n ) Calculate a new improved ho and calculate L and Q ….. Repeat steps 1 to 3 until the assumed value of ho is sufficiently close to that calculated Dewatering Techniques 211 Dewatering of Excavations (cond) 3: Check the assumption made for ho: ( ) k h 2 − h o2 Q= ln L − (1 / n ) ln( r x 2 x 3 .......x n ) Calculate a new improved ho and calculate L and Q ….. Repeat steps 1 to 3 until the assumed value of ho is sufficiently close to that calculated 4: Return to the original excavation: Distribute the wells around the perimeter of the excavation ❖ Calculate the water level at critical depth ❖ Verify the design or increase the number of wells ❖ Dewatering Techniques 212 Dewatering of Excavations (cond) 3: Check the assumption made for ho: ( ) k h 2 − h o2 Q= ln L − (1 / n ) ln( r x 2 x 3 .......x n ) Calculate a new improved ho and calculate L and Q ….. Repeat steps 1 to 3 until the assumed value of ho is sufficiently close to that calculated 4: Return to the original excavation: Distribute the wells around the perimeter of the excavation ❖ Calculate the water level at critical depth ❖ Verify the design or increase the number of wells ❖ This method overestimate ho! Dewatering Techniques 213 Dewatering of Excavations (cond) ⚫ Modified design method: ➢ Estimate ho from: Dewatering Techniques 214 Dewatering of Excavations (cond) ⚫ Modified design method: ➢ Estimate ho from: f Q i ln( b / r ) ho = y − k 2 (h o )min Dewatering Techniques 15Q i = 2 r k 215 Dewatering of Excavations (cond) ⚫ Modified design method: ➢ Estimate ho from: f Q i ln( b / r ) ho = y − k 2 (h o )min 15Q i = 2 r k for large well spacing for small well spacing (b<5r) Dewatering Techniques 216 Dewatering of Excavations (cond) ⚫ Modified design method: ➢ Estimate ho from: f Q i ln( b / r ) ho = y − k 2 (h o )min 15Q i = 2 r k for large well spacing for small well spacing (b<5r) Dewatering Techniques 217 Settlement of Adjacent Structures ⚫ Increase in effective stress results in settlement of the ground ➢ ➢ ⚫ It may affect the adjacent building Normal methods of settlement calculation is applicable If settlement is excessive: ➢ ➢ Cut off wall may be used Water may be recharged into the layer close to the building Dewatering Techniques 218 Settlement of Adjacent Structures ⚫ Increase in effective stress results in settlement of the ground ➢ ➢ ⚫ It may affect the adjacent building Normal methods of settlement calculation is applicable If settlement is excessive: ➢ ➢ Cut off wall may be used Water may be recharged into the layer close to the building Dewatering Techniques 219 Settlement of Adjacent Structures ⚫ Increase in effective stress results in settlement of the ground ➢ ➢ ⚫ It may affect the adjacent building Normal methods of settlement calculation is applicable If settlement is excessive: ➢ ➢ Cut off wall may be used Water may be recharged into the layer close to the building Dewatering Techniques 220 Settlement of Adjacent Structures ⚫ Increase in effective stress results in settlement of the ground ➢ ➢ ⚫ It may affect the adjacent building Normal methods of settlement calculation is applicable If settlement is excessive: ➢ ➢ Cut off wall may be used Water may be recharged into the layer close to the building Dewatering Techniques 221 Settlement of Adjacent Structures ⚫ Increase in effective stress results in settlement of the ground ➢ ➢ ⚫ It may affect the adjacent building Normal methods of settlement calculation is applicable If settlement is excessive: ➢ ➢ Cut off wall may be used Water may be recharged into the layer close to the building Dewatering Techniques 222 Settlement of Adjacent Structures ⚫ Increase in effective stress results in settlement of the ground ➢ ➢ ⚫ It may affect the adjacent building Normal methods of settlement calculation is applicable If settlement is excessive: ➢ ➢ Cut off wall may be used Water may be recharged into the layer close to the building Dewatering Techniques 223 Settlement of Adjacent Structures ⚫ Increase in effective stress results in settlement of the ground ➢ ➢ ⚫ It may affect the adjacent building Normal methods of settlement calculation is applicable If settlement is excessive: ➢ ➢ Cut off wall may be used Water may be recharged into the layer close to the building Dewatering Techniques 224 Autumn 2008 Dewatering Techniques 225
