Commonly used pumped groundwater-control methods in soils
| Method | Applications | Depth | Installation | Spacing between wells or sumps | Notes |
|---|---|---|---|---|---|
| Open pumping | |||||
| Sump pumping: groundwater is allowed to seep into the excavation, where it is collected in pits or low points (sumps) within the excavation or structure, from where it is pumped away | Shallow excavations in coarse-grained soils | Typically constrained by sump construction and excavation stability | Excavation: sumps may be open or supported using perforated concrete rings or steel liners | Subject to size of excavation, may be extended using trenches or horizontal collection drains feeding to the sump | May not give sufficient drawdown to prevent seepage from emerging on the cut face of a slope or the walls of structures, possibly leading to loss of fines and instability |
| Cannot give a completely ‘dry’ excavation, and will not control groundwater in any confined aquifers below an excavation | |||||
| Takes up space in excavation and may obstruct construction works | |||||
| May generate silt- or sediment-laden discharge water, causing environmental problems if pumped water is not adequately treated prior to discharge | |||||
| Pre-drainage pumping methods | |||||
| Wellpoints (single stage): lines or rings of closely spaced small-diameter (≈50 mm) shallow wells (known as wellpoints) are installed around an excavation and pumped by a suction system | Shallow excavations in silty sands, sands and sandy gravels | Limited to 5–6 m below pump level and may be further limited in silt/silty soils | Typically rotary drilling or jetting in favourable conditions | 1–3 m | Installation by jetting may be slow and difficult in dense gravels and if cobbles and boulders are present |
| Close spacing of wellpoints means that access to excavation may be restricted | |||||
| Site strip may be required to allow wellpoints, headermain and pumps to be installed as low as possible to overcome limitations on suction lift | |||||
| Wellpoints (two stage): initial stage of wellpoints allows excavation for a second stage of wellpoints at low level (deeper excavation using three or more stages can be an option); further stages can be used for deeper excavations | Excavations in sandy gravels sands and possibly silty sands | Excavation depth between stages limited to approximately 4.5 m | Typically rotary drilling or jetting in favourable conditions | 1–3 m | Notes for single-stage wellpoints apply |
| Excavation needs to be paused to allow installation of second and any subsequent stages of wellpoints | |||||
| Space required for excavation side slopes | |||||
| Pumping on upper stages can be reduced or sometimes eliminated when lower stages are operational | |||||
| Deepwells: bored wells are drilled and installed with slotted liner (≈100–300 mm dia.) and filter media surround; wells are pumped by slimline electrically driven borehole pumps | Deeper excavations in sandy gravels and sands | Only limited by performance and capacity of pump | Typically rotary or cable percussion drilling | 10–30 m or more in favourable conditions | Depth and drawdown may be controlled by soil stratification |
| Number of wells must be optimised to control costs; requires good ground investigation data, experienced design team and experienced installers | |||||
| Well yields will be high in coarse high-permeability soils requiring larger-diameter wells to accommodate pumps of sufficient capacity | |||||
| In strata of relatively low permeability, well yields may be low and large numbers of wells may be required | |||||
| Vacuum deepwells: deepwell with annular bentonite/grout seal and well head cap with head space connected to a surface vacuum pump | Deeper excavations in fine sand and silts, where drainage may be slow | Only limited by performance and capacity of pump | Typically rotary or cable percussion drilling | 5–20 m | Depth and drawdown may be controlled by soil stratification |
| Number of wells must be optimised to control costs; requires good ground investigation data, experienced design team and experienced installers | |||||
| Sufficient flow required to cool pumps> | |||||
| Note NPSH (net positive suction head) and risk of pump cavitation damage | |||||
| Ejector wells: small-diameter (≈50–100 mm) bored wells installed with slotted liner and filter media pumped by water-driven nozzle and venturi system; if the top of the well is sealed a vacuum can be generated in the well | Deeper excavations in silty sand, silts or laminated or fissured clays | Generally limited to 20–50 m depending on equipment | Typically rotary or cable percussion drilling | 3–10 m | Low energy efficiency, may not be significant concern if flow rates are low |
| Number of wells must be optimised to control costs; requires good ground investigation data, experienced design team and experienced installers | |||||
| Can be prone to clogging and other operational problems caused by groundwater chemistry, principally iron-related biofouling | |||||
| Artificial recharge: pumped groundwater is re-injected back into the ground, under careful control, by way of an array of wells or trenches | A wide range of conditions in soils of moderate to high hydraulic conductivity | Only limited by soil stratigraphy and hydrogeological conditions | Varies | Varies | Used to mitigate environmental or other adverse impacts of groundwater lowering; can also be used as means to dispose of dewatering water, where other discharge routes are not available |
| Must be designed to match the hydrogeological conditions on site, and specialist advice may be required | |||||
| Often complex to operate and maintain; recharge wells often suffer from clogging due to water chemistry effects; there may be a requirement for water treatment prior to recharge and/or periodic back-flushing and cleaning of recharge wells | |||||
| Method | Applications | Depth | Installation | Spacing between wells or sumps | Notes |
|---|---|---|---|---|---|
| Sump pumping: groundwater is allowed to seep into the excavation, where it is collected in pits or low points (sumps) within the excavation or structure, from where it is pumped away | Shallow excavations in coarse-grained soils | Typically constrained by sump construction and excavation stability | Excavation: sumps may be open or supported using perforated concrete rings or steel liners | Subject to size of excavation, may be extended using trenches or horizontal collection drains feeding to the sump | May not give sufficient drawdown to prevent seepage from emerging on the cut face of a slope or the walls of structures, possibly leading to loss of fines and instability |
| Cannot give a completely ‘dry’ excavation, and will not control groundwater in any confined aquifers below an excavation | |||||
| Takes up space in excavation and may obstruct construction works | |||||
| May generate silt- or sediment-laden discharge water, causing environmental problems if pumped water is not adequately treated prior to discharge | |||||
| Wellpoints (single stage): lines or rings of closely spaced small-diameter (≈50 mm) shallow wells (known as wellpoints) are installed around an excavation and pumped by a suction system | Shallow excavations in silty sands, sands and sandy gravels | Limited to 5–6 m below pump level and may be further limited in silt/silty soils | Typically rotary drilling or jetting in favourable conditions | 1–3 m | Installation by jetting may be slow and difficult in dense gravels and if cobbles and boulders are present |
| Close spacing of wellpoints means that access to excavation may be restricted | |||||
| Site strip may be required to allow wellpoints, headermain and pumps to be installed as low as possible to overcome limitations on suction lift | |||||
| Wellpoints (two stage): initial stage of wellpoints allows excavation for a second stage of wellpoints at low level (deeper excavation using three or more stages can be an option); further stages can be used for deeper excavations | Excavations in sandy gravels sands and possibly silty sands | Excavation depth between stages limited to approximately 4.5 m | Typically rotary drilling or jetting in favourable conditions | 1–3 m | Notes for single-stage wellpoints apply |
| Excavation needs to be paused to allow installation of second and any subsequent stages of wellpoints | |||||
| Space required for excavation side slopes | |||||
| Pumping on upper stages can be reduced or sometimes eliminated when lower stages are operational | |||||
| Deepwells: bored wells are drilled and installed with slotted liner (≈100–300 mm dia.) and filter media surround; wells are pumped by slimline electrically driven borehole pumps | Deeper excavations in sandy gravels and sands | Only limited by performance and capacity of pump | Typically rotary or cable percussion drilling | 10–30 m or more in favourable conditions | Depth and drawdown may be controlled by soil stratification |
| Number of wells must be optimised to control costs; requires good ground investigation data, experienced design team and experienced installers | |||||
| Well yields will be high in coarse high-permeability soils requiring larger-diameter wells to accommodate pumps of sufficient capacity | |||||
| In strata of relatively low permeability, well yields may be low and large numbers of wells may be required | |||||
| Vacuum deepwells: deepwell with annular bentonite/grout seal and well head cap with head space connected to a surface vacuum pump | Deeper excavations in fine sand and silts, where drainage may be slow | Only limited by performance and capacity of pump | Typically rotary or cable percussion drilling | 5–20 m | Depth and drawdown may be controlled by soil stratification |
| Number of wells must be optimised to control costs; requires good ground investigation data, experienced design team and experienced installers | |||||
| Sufficient flow required to cool pumps> | |||||
| Note | |||||
| Ejector wells: small-diameter (≈50–100 mm) bored wells installed with slotted liner and filter media pumped by water-driven nozzle and venturi system; if the top of the well is sealed a vacuum can be generated in the well | Deeper excavations in silty sand, silts or laminated or fissured clays | Generally limited to 20–50 m depending on equipment | Typically rotary or cable percussion drilling | 3–10 m | Low energy efficiency, may not be significant concern if flow rates are low |
| Number of wells must be optimised to control costs; requires good ground investigation data, experienced design team and experienced installers | |||||
| Can be prone to clogging and other operational problems caused by groundwater chemistry, principally iron-related biofouling | |||||
| Artificial recharge: pumped groundwater is re-injected back into the ground, under careful control, by way of an array of wells or trenches | A wide range of conditions in soils of moderate to high hydraulic conductivity | Only limited by soil stratigraphy and hydrogeological conditions | Varies | Varies | Used to mitigate environmental or other adverse impacts of groundwater lowering; can also be used as means to dispose of dewatering water, where other discharge routes are not available |
| Must be designed to match the hydrogeological conditions on site, and specialist advice may be required | |||||
| Often complex to operate and maintain; recharge wells often suffer from clogging due to water chemistry effects; there may be a requirement for water treatment prior to recharge and/or periodic back-flushing and cleaning of recharge wells | |||||
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