BEST DEWATERING METHOD FOR MASS EXCAVATION

Water oozing in earth excavation from sides and bottom creates a big and tedious problem for the site-in-charge. Trenches will have to keep water free during excavation and foundation work. Various methods are adopted for dewatering the trenches. These are:
  1. By constructing the drains.
  2. Providing sumps and pumping units.
  3. Cement grouting.
  4. Freezing process.
  5. Chemical consolidation of the soil.
  6. Electro-osmosis process.
  7. Well-point system.
  8. Deep well system.

1. By constructing the drains: In this method drains are constructed on both the sides of the foundation trench. The groundwater which otherwise would have reached the trench is intercepted by these drains and trenches of foundation can be maintained free from oozing water. These drains are given suitable bed slope and connected to nearby low area. If no low area is available in the vicinity the water may be collected at the central place and bailed out from time to time.

Best dewatering method for mass excavation


ADVANTAGES: This method is the cheapest of all dewatering methods and useful only in shallow foundations in waterlogged areas.

2. Providing sumps and pumping units: This method is based on more or less same principle as the first one. In this system sumps or wells are constructed along the sides of the trenches. Side drains are constructed along the bottom of the trenches by providing suitable slope, so that water getting access to these drains is led to the supms. The water thus being collected is continuously pumped out by pumps. The sumps may be 1m x 1m x 1m in size and side drains 20 cm diameter semi-circular. Sizes of sumps and drains depend upon the area and intensity of seepage from subsoil. Center to center spacing of sumps along the centerline of the trenches may vary from 40m to 60m.
best dewatering method for mass excavation

ADVANTAGES: This is most suitable for deep excavation and large quantity of sub-soil water to be handled.

3. Cement grouting: This method is adopted for stopping the leakages from the rocks mostly. It can be used for stopping leakage from soft soils also.
Stopping leakage from rock: Whenever soft patches are noticed in the rocks, holes are drilled. There is hard and first rule in regard to spacing depth and sizes of the holes. Their direction, depth and inclination are varied depending upon the kind of the rock. Cement grout is prepared by adding sufficient water to the cement and drilled holes are filled by grout under pressure. The process of grouting is continued till no grout is coming up through the hole. To avoid rupturing of the rocks, the grouting pressure is varied suitably. Once a hole is started grouting, it should not be left unfilled.
best dewatering method for mass excavation


Stopping of leakage from soft soils: In case of soft soil, before actual grouting, the formation of channels of least resistances are carried out. For this, two pipes are driven at small distance, and water is forced through one of the pipes until water appears in the second pipe. This forms channels of least resistance between two pipes. Similarly, a number of such channels are formed in the soil. Cement grout is applied through the same pipe through which water was forced. Cement grouting is continued till grout appears in the other pipe. After placing valve in the pipe, the process of forcing the grout is continued. The grout is thus forced in the soil around the trenches and thus a waterproof layer is formed and water leakage into the trenches prevented.

4. Freezing process: In this method, the area around the excavation is frozen and the soil is converted into a wall of frozen earth. For that, 10cm to 15cm diameter steel pipes closed from bottom are driven into the ground at an interval of 1m to 1.5m surrounding the area to be excavated. 2.5cm to 5cm diameter pipes are inserted into the large pipes and connected to a common pipe which is connected to the refrigeration plant. The small inserted pipes may be open at the bottom end or may have perforated ends.

best dewatering method for mass excavation

Cold refrigerant liquid is circulated through the circuit at -23degrees to -30degrees centigrade. The cold liquid coming through small pipes goes up through large pipes. This makes larger diameter pipe to cool and the soil surrounding this pipe freezes. Maintaining proper control in the process a wall of considerably thick frozen earth can be developed and excavation of enclosed area can be done without difficulty.
For excavation of larger area, the layout of pipes is to in circular fashion so that advantages of arch action is taken. For smaller area, layout of pipes is kept in rectangular or square fashion.
DISADVANTAGES:
  • This method is suitable only for works of short duration.
  • It is effective only in case soil to be frozen is water bearing sand, gravel or silt.
  • The stability of frozen soil is not reliable for long time.
  • This process is very expensive.
5. Chemical consolidation of soil: In this method soil around the area to be excavated is solidified using solutions of chemical compounds like silicate of soda and calcium chloride. In this method pipes like well-points are riven into the ground surrounding the area to be excavated. While pipes are being driven one of these chemicals is forced through the pipes. Similarly, after reaching the desired depth when pipes are withdrawn second chemical is forced through the pipe. Both the chemicals react chemically and solidify the soil.
DISADVANTAGES:
  • The process of chemical solidification is very expensive and hence is used only special conditions.
6. Electro-osmosis process: It is an established fact that when direct current is passed through the two electrodes driven in the saturated soil, water contained in the soil is repelled by positive electrode(anode) and is attracted by negative electrode (cathode). When cathode electrodes are put into the well-points and anode electrodes at mid distance between two well-points and direct current passed through them, the flow of groundwater towards cathodes i.e., well-point is increased, from where it is pumped out and drainage of the area is achieved.
best dewatering method for mass excavation
best dewatering method for mass excavation

DISADVANTAGES:
  • This process is only suitable for very fine soils having very poor permeability.
  • It involves use of very expensive equipment hence high cost.
7. Well-point system: This is a method of keeping an excavated area free of groundwater by intercepting the groundwater flow with the help of well-points driven deep into the ground. The well-points are driven surrounding the area to be excavated, at nearly 1m to 1.5m intervals. The essential parts of a well-point system are well-points, riser pipe, swinger arm, and header pipe.

best dewatering method for mass excavation

  • Well-point: It is a pipe of 4cm to 5cm diameter and 1m to 1.8m long. The pipe is fitted with a valve at its bottom. The valve acts in such a way that it opens when water is forced into the pipe and closes when suction is applied to the pipe. Wellpoint pipe is provided with holes and these holes are covered with a fine mesh screen.
  • Riser pipe: It is a vertical pipe sunk into the ground by jetting method and is connected to the well-point at its bottom end. Diameter of this pipe is also same as that of well-point pipe.
  • Swinger arm: It is a pipe connecting upper end of the riser pipe with the header pipe above ground level.
  • Header pipe: It is a pipe laid surrounding the area to be excavated. All the well-points are connected to header pipe through swinger arms which in turn is connected to a pumping unit. The diameter of the header pipe may vary from 15cm to 25cm depending upon the intensity of groundwater flow and also the number of well-points it has to handle.
best dewatering method for mass excavation


Process of well-point system: First of all, positions of well-points to be driven are marked around the area to be excavated. Spacing of well-points depends upon the nature of soil and intensity of groundwater flow. Generally, minimum spacing between well-points may be 1m or so. The well-point is jetted down to the required depth by forcing water through it at the rate of 20 - 25 liters per second. The water jet emanating from the well-point pipe dislodges the soil and enables well-point pipe and riser pipe to be sunk to the desire depth. When well-point has reached its required depth, water jet is allowed to run for some time so that the water while rising up may create an annular space around the well-point. The flow of water jet is stopped and annular space around the well-point is filled with filter media of sand and gravel. This filter media prevents the entry of sand particles into the well-point and also clogging of screen wrapped around the well-point pipe. Similarly, all the well-points are driven with riser pipe fitted to them. After that, all the well-points are connected to the common header pipe through individual Tee-piece and swinger arm. The header pipe is finally connected to the suction pump.

When suction pump is run, it creates suction or vacuum inside the well-point pipes and water starts coming out from ground and groundwater table is lowered. When suction pump is operated, valve provided at bottom of well-points get closed and water cannot enter well-point from bottom end. Water enters into the well-point through holes provided in the well-points.

In a single stage, water can be lifted by suction is about 5m hence water table can be lowered to 5m only from the surface of the ground and excavation can be done for only 5m depth. If excavation has to go deeper than 5m, two stage or multi-stage well-point system may be needed.

If the area to be excavated is clayey, having its permeability less than 0.0001 cm per second, the filter media is filled around the well-point and riser pipe only up to the water table and the rest annular space is filled with tamped clay to act as a clay seal. Clay seal prevents the entry of air into the well-point through filter media. During pumping by powerful pump, considerable vacuum is created in well-point as well as the surrounding filter. The pressure on the ground is that of atmosphere and that in well-point and surrounding filter only a fraction of it. This difference in pressure causes consolidation of the soil and water from the consolidated soil oozes into the well-point and pumped out, thus water table is lowered.
ADVANTAGES: 
The well-point system of dewatering is the most economical method for dewatering up to a depth of 5m in a single stage.

8. Deep well system: In deep well system, deep bore well is done in suitable locations 4 to 6 nos as per requirements. Depth of boring is more than 40m. Diameter of boring pipe is 20cm or more and provided with a powerful submersible pump (10 H.P). A single deep well dewatering system can pump more than 3000 gallons of water per minute. The excavation work needs removal of large volume of water at a fast speed, deep well system is useful. The performance of deep well system depends upon the permeability of the soil, it can work wide range of soil condition. This system specially adopted for high permeable soil such as sandy areas, where groundwater recharges at a faster rate.

best dewatering method for mass excavation

DISADVANTAGES:
The deep well dewatering system is expensive and needs expert to implement the system. This is only suitable for excavation more than 30m deep where huge quantity of water is to be removed at a faster rate.

CONCLUSION

Different construction sites have different site conditions and hence different dewatering systems are needed as per the site situation. But economical and cost-effective method is always preferable. In most of the sites, beyond 1.8m depth during earth excavation water oozes out from the trenches. It becomes severe beyond 3.0m depth. Therefore, 1.8m to 3.0m deep excavation, providing sumps and and pumping units (5 H.P) is the better option in terms of economical aspect. Whereas 3.0m to 8.0m depth and for larger area of excavation the well-point system is the most economical method compared to the other methods as discussed above.


BEST DEWATERING METHOD FOR MASS EXCAVATION





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Green Environment

Hi, I’m Gobinda Burman (Green Environment). More than 27 years in the field of Building Construction industry, I’ve completed many high-rise building projects successfully. I have got the opportunity to learn and solve the critical practical problems related to building construction. I love to share my knowledge with those people who wants to build their own home and the budding civil engineers willing to build their career in this field..

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1 comments:

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