Dewatering – its Meaning, Importance and Consequences if Uncontrolled

Dewatering – its Meaning, Importance and Consequences if Uncontrolled

Definition of Terms.

Dewatering is the artificial means of removing excess groundwater from the soil for favourable condition of any construction. It aims at lowering the groundwater level. This leads to concepts like pre-drainage of soil, control of groundwater, and even the improvement of physical properties of soil (Kamran, 2007).

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 Dewatering has existed as a specialty for a long time (Kamran, 2007). Consequently, a number of well – established techniques have been developed to lower the groundwater table during excavation (kamran, 2007). The geology, groundwater conditions, and type of excavation all influence the selection of dewatering technology (Kamran, 2007).

Dewatering can become a costly issue if over looked during project planning. In most contracts, dewatering is the responsibility of the contractor. The contractor selects the dewatering method and is responsible for its design and operation.

Planning Dewatering Operation

In planning a dewatering operation, a comprehensive assessment of the potential environmental impacts of dewatering should be conducted or commissioned as part of the project feasibility stage, to identify issues and develop management strategies to address such issues.

      The assessment must include but not be limited to:

             Define the commencement date, duration, anticipated quality and frequency of dewater discharge;

             Determine via scientific modeling the radius of influence and profile of any water table drawdown cone (including threat to vegetation or existing structures from land settling);

             Determine the quality of water to be discharged; including probable contaminant concentrations based on natural groundwater contaminants and the local land use history;

             Determine whether the soil contains iron pyrites or other characteristics likely to result in acid sulfate release during excavation or dewatering. These conditions may be caused when peat swamps are exposed to air after the water table is lowered. Dewatering should not take place unless effective measures are taken to prevent acid water (low PH) causing the release of arsenic or toxic metals to environment;

             Assess the need and viability of dewater treatment e.g. settling, biological stabilization, pH adjustment, chemical flocculation or filtration;

             Include baseline assessment of the recovering environment (before dewatering); including seasonal changes of water flow, water table levels  and water quantities;

             Verify that the discharge water quality will consistently comply with federal, state and local government statutory requirements, where dewatering could effect local resource values. Alternative criteria may be proposed to regulatory bodies for assessment based on-site detailed scientific studies;

             Notify council of the results of consultation with any local residents or businesses likely to be effected during dewatering.  A contact person must be available during dewatering operations to manage any issue; and 

             Seek approval from other agencies with jurisdiction for the removal of marine plants.

         Also the discharge of treated water to the environment should not cause any of the following effects:

             Detrimental impact upon environmental values of receiving waters or significant threat to those values.

             Harm to native vegetation, or erosion of structures or services;

             Soil erosion or local flooding;

             Sediment build-up in drain, water ways or wetlands;

             Nuisance to the local community e.g. noise, odour, impacts on plants or property, or hazards, and

             Loss or reduced flows in public or private water sources.

 Dewatering activities near coastal or estuarine environments must ensure that there is no potential to draw salt water into a less saline aquifer or discharge saline waters to any receiving environment.

DEWATERING METHODS

Deep wells

Deep wells can be used to dewater pervious sand or rock formations or to relieve artesian pressure beneath an excavation (Terzaghi and Peck, 1948). They are particularly suited for dewatering large excavations for dams, tunnels, locks, powerhouses, and shafts. Excavations and shafts as deep as 30feet can be dewatered by pumping from deep wells with submersible pumps. The principle advantages of deep well are they can be installed around the periphery of an excavation and thus leave the construction area unencumbered by dewatering equipment, and the excavation can be pre-drained for its full depth (Terzaghi and Peck, 1948).

Deep wells for dewatering are similar in type and construction to commercial water wells (Terzaghi and Peck, 1948). They commonly have a screen with a diameter of 6 to 24 inches with lengths up to 300feet and are generally installed with a filter around the screen to prevent the infiltration of foundation materials into the well and to improve the yield of the well.

 Deep wells may be used in conjunction with a vacuum system to dewater small, deep excavations for tunnels, shafts or caissons sunk in relatively fine grained or stratified pervious soils or rock below the groundwater table. The addition of a vacuum to the well screen and filter will increase the hydraulic gradient to the well and will create a vacuum within the surrounding soil that will prevent or minimized seepage from perched water into the excavation (Roberts and Preene, 1994).  Installations of this type require adequate vacuum capacity to ensure efficient operations of the system.

 This method work best in soils consisting of sand and gravel mixtures.

  Well Points

Well points are small-diameter (less than 6 inches) tubes with slots near the bottom that are inserted into the ground from which water is drawn by a vacuum generated by a dewatering pump.

 A well point system consists of a number of well points spaced along a trench or around an excavation site, all connected to a common header, which is attached to one or better point pumps. Well point systems are most suitable in soils consisting of sand or sand and gravel mixtures that are shallow aquifers where the water level needs to be lowered not more than 15 or 20feet. Due to the vacuum limitation of the pump, excavations that are deeper will require multiple stages of well point systems. A multiple stage well point system consists of the installation of well points at two or more levels. It is important the lowest well point stage be located at an elevation within reasonable suction lift or the desired final water level (Powers, 1992). Single pump or multiple pumps may be used, depending on availability of equipment and job conditions. If multiple pumps are used, they should be spaced along the header, or they can be grouped into one single pump station. In a single pump station only one discharge one line is used, but in this case large header pipes are used to bring the water to the central pump station without excessive friction.

The construction steps in the well point system.

             The well points are jetted into the ground;

               The annular void is filled with filter media;

               The well point is connected to a header pipe by means of a riser;

               The header pipe is connected to suction pumps for pumping.

When designing a well point system , it is necessary to give first consideration to the physical conditions of the site to be dewatered.

                Things to consider include:

             The physical layout

             Adjacent areas

             Soil conditions

             Permeability

             The amount of water to be pumped

             Dept to imperviousness

             Stratification.

The normal range of well point spacing is from 3 to 12 feet.

Well point systems generally cost more than either sumps or deep wells, and require near-continual maintenance.

 Sumps

Sump dewatering system involves excavation of a temporary pit and installation of sumps within the excavation, from which water entering the excavation can be pumped. This method of dewatering generally should not be considered where the groundwater head must lowered more than a few feet, as seepage into the excavation may impair the stability of excavation slopes or have a detrimental effect on the integrity of the foundation soils.

The number and location of sump pits should be determined and placed on erosion and sedimentation control plans (Mitchell and Soga, 2005). The contractor can relocate in the field to optimize use but discharge location changes should not be made without proper review and approval. A design is not required for the sump but construction should conform to the general description as follows:

A perforated vertical standpipe is wrapped with 1/2 inch hardware cloth and geotextile. The standpipe is then placed in an excavated pit which is then backfilled with filter material consisting of washed stone. The pump is then placed in the standpipe and water is pumped from the centre to a suitable discharge area such as a sediment trap, sediment basin or a stabilized area.

Disadvantages of a sump dewatering system are slowness in drainage of the slopes; potentially wet conditions during excavation and backfilling, which may impede construction and adversely affect the sub grade soil; space required in the bottom of the excavation for drains, ditches, sumps and pumps and the frequent lack of workmen who are skilled in the proper construction or operation of sumps (Leonards, 1962).

Ground Freezing

The principle of ground freezing is to change the water in the soil into a soil 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. The principles of ground freezing are analogous to pumping groundwater from wells. To freeze the ground, a row of freeze pipes are placed vertically in the soil and heat energy is removed through these pipes. Isotherms (an isotherm is a line connecting locations with equal temperature) move out from the freeze pipes with time similar to groundwater contours around a well. Once the earth temperature reaches 32oF (or 00C), water in the soil pores turns to ice. Then further cooling proceeds. The groundwater in the pores readily freezes in granular soils, such as sands. If the temperature is lowered further, the strength increases marginally. In cohesive soils, such as clays, the groundwater is molecularly bonded at least in part to the soil particles. If soft clay is cooled down to freezing temperature, some portions of its pore water will begin to freeze and it causes the clay to stiffen. With further reduction in temperature, more pore water freezes and consequently more strength gain is achieved. When designing for frozen earth structures in cohesive soils, it may be necessary to specify substantially lower temperatures to achieve the required strength, than in cohesionless soils (Kamran, 2007). A temperature of + 200F may be sufficient in sands, whereas temperatures as low as -200F may be required in soft clays.

 The most common freezing method is by circulating brine. Chilled brine is pumped down a drop tube to the bottom of the freeze pipe and flows up the pipe, drawing heat from the soil.

The liquid nitrogen (LN2) process has been applied successfully to grounded freezing. The cost per unit of heat extracted is much higher than with circulated brine.

The freezing method is remarkably versatile, and with ingenuity it can be adapted to a great number of project conditions (Kamran, 2007).

IMPORTANCE OF DEWATERING

Ground Excavation

Dewatering plays a vital role in ground excavation. Some subsurface excavations (Figure 5) are jeopardized due to the level of the water table. In order to carry out such works, dewatering is carried out to permit excavation and construction within a relatively dry environment as the water table is lowered.

Stable Foundation

As the soil is dewatered, it controls the hydrostatic pressure and seepage thereby increasing the stability of excavation slopes or foundation soil so as to make them suitable for supporting structures.

Irrigation

 After excess water is extracted from the construction site, property owners may use it for their landscaping needs, based on acreage and plant type. The important thing to keep in mind when thinking about using this process for water discharge is that the volume required for irrigation needs to exceed the amount of water that is collected during dewatering. If the irrigation needs are not large enough, the property owners may need to attain a permit to discharge the leftover water to the sewer system. Using excess water for irrigation (Figure 7) is a moneysaving and environmentally friendly option.

CONSEQUENCES OF UNCONTROLLED DEWATERING

Ground Subsidence

Subsidence is the vertical deformation of a rock formation without loading but probably due to internal processes or re-arrangement of grains, pores, in and out flow of materials. It is also the lowering of the land–surface elevation from changes that take place underground.

As groundwater pumping increases during the process of dewatering, ground subsidence also will increase. In many aquifers, groundwater is pumped from pore spaces between grains of sand and gravel (ILRI, 2000). If an aquifer has beds of clay or silt within or next to it, the lowered water pressure is a loss of support for the clay and silt beds (Roberts, Roscoe, Powrie, and Butcher, 2007). Because these beds are compressible, they compact, and the effects are seen as ground subsidence. This ground subsidence causes many problems including

             Changes in elevation and slope of streams, canals and drains.

             Damage to bridges, roads and railways,

             Damage to private and public buildings,

             Failure of well casings from forces generated by compaction of fine–grained materials in aquifer system.

Flooding

The excess water flowing into discharge areas during dewatering can cause minor flooding. If the water is not used for any activity such as irrigation. Also the discharge of turbid water (water full of sediment) into storm drains or bodies of water can cause clogging of existing drainage facilities, which causes flooding during storm events .

Structural Collapse

Uncontrolled dewatering increases the load on foundation soil below the original groundwater table (Leonard, 1962). As most soils consolidate upon application of additional load, structures located within the radius of influence of a dewatering system may collapse.

Groundwater Depletion

Groundwater depletion refers to the withdrawal of water at greater rates than replenishment or exhaustion of water well. It is also defined as long–term water level decline caused by sustained groundwater pumping. Uncontrolled dewatering can cause compaction and porosity loss in rock and soil, and this can permanently ruin a good aquifer (Plummer and Carlson, 2008).

Some of the negative effects of groundwater depletion include increased pumping costs, deterioration of water quality, reduction of water in streams and lakes.

SUMMARY AND CONCLUSIONS

Summary      

Dewatering is very necessary in construction site where the groundwater level is high. Its aim is to lower the groundwater level and create a favourable condition for any construction. Dewatering can be a costly issue if overlooked during project planning. In most contracts, dewatering is the responsibility of the contractor. The contractor selects the dewatering method and is responsible for its design and operation.

Uncontrolled dewatering can be detrimental to the environment as it can cause ground subsidence, flooding, structural collapse and groundwater depletion.

 Conclusions

Previous researches have shown that with dewatering, the level of the water table can never be a hindrance to any sub-surface excavation. And at such, contractors should always make proper planning for dewatering operations.

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Comments

  1. Ibrahim Bello says:

    It is a good decument to me as building proffessional.

  2. ali bradoo says:

    good decument >>

  3. This is a very good compilation. I have really benefited from dis as a geologist

  4. Reference list please

  5. GOOD EASSY

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