Drainage and Dewatering

In the saturated soil mass, the process of withdrawl of superfluous water from it whether in natural or any artificial form is referred as drainage. Before any kinds of engineering works for example excavation for basement of foundation for any structure or for foundations of dams, mining projects etc., excavations are widely popular below the ground water level. To perform such kinds of excavations, the ground water has to be lowered to the lowest point of excavation in order to obtain workable dry place for the construction work. In this process, drainage is also an essential part of it in order to decrease pore water pressure and minimize the seepage effect which certainly betters the strength and effective strength of it as well as increases safety and formulates stable excavation. So it can be extracted that generally in waterlogged areas, drainage helps for lowering the water table and enables to do subsurface excavation work.

Various difficulties are generated due to the high ground water. For solving those, many methods are adopted. Some of them are listed as follows:

Well point system
Sump and Ditches
Shallow well system
Electro-osmosis method
Vacuum method
Deep well system

Well point system:

It is applicable generally for the excavation carried out in the shallow depths and is mainly executed for unstable and weak ground conditions and also soils with moderate permeability. In this method, a long perforated pipes along with well point systems are used for lowering the groundwater level and enhancing the construction work. The pipe used is generally about 1m long with 5cm in diameter. Usually for even spaced small-diameter wells, this well point system is operated. Water is pumped out with the help of high- efficiency water pump as all the well points with numerous riser pipes are connected to a single main pipe known as a header pipe. Vacuum will be created in that main pipe as draws the water from the depth to the ground surface. Generally in the form of rings or in lines, the well points are installed around the excavation point and are spaced according to the permeability of the soil usually 1 m or more.

It works fine for coarse and fine clean sands. It can be swiftly positioned as well as removed. Furthermore it is not effective for less permeable fine soil with effective particle size D10 finer than 0.05mm.

Sump and Ditches

They are the most elementary and simplest form of dewatering in the construction work. It is post preferred for the coarse grained soils in the shallow type of excavation whose bearing capacity exceeds 10^(-3) cm/s. Drainage ditches are created around the location where dewatering is to be done. The ditches are perforated, far down the work area and at an appropriate location different pits are designed which are known by the names sump from where the collected water is pumped out. By using the “principle of gravity water flow”, the water in the slumps are collected from slopes. The main disadvantage of it is probability of formation of piping or boiling over them. Subsequently, there is also uninterrupted loss of soil fines due to the pumping which may result in the erosion in the underground and assist for the further failure of the lateral slopes. Due to this reason, it is recommended to use linings known by inverted filters for the protection of slumps and ditches. They have multiple layers with the coarser materials in the bottom to upwards of the sump pit.

Shallow well system

It is a slight modification of deep well system and has some similarities with well point system on the basis of pumping actions. In this system, a diameter of 30cm or more dug for a depth up to 10m extending below the pump axis. By steadily eliminating the casing tube and concurrently adding filter materials like gravel, gravel filter is created between discoid space between casing and strainer tube. On the filter well being formed, suction pipe is laid down. From the number of such types of the filter wells, they are attached to a common header. With the help of one or more well point pumps, the header pipe is suctioned.

Shallow well system is more functional in the high permeability soil. As the diameters of the well are larger compared to that of well points. Hence, they have broader centers which even reduces the number of installation of wells, which significantly reduces the drilling cost.

Electro-osmosis method

The methods described above are unable to dewater in case of fine grained cohesive soil. By using this electro-osmosis method, those particular type of soils can be drained as well. This technique was mainly established by L. Casagrande during 1952. Two electrodes are impelled into saturated soil with the flow of current between one another in this system. Then there is very slow movement of soil water from anode (positive) to cathode (negative). Usually pierce metal tube are taken as a cathode through which pumping action is done for the seepage water. Whereas, sheet piling for excavation or steel rod are taken as an anode. Generally, the designation of electrodes is made in such a way that the original path for the flow of water is altered away from excavation which consequently rises the stability of slope. It also increases shear strength of that particular soil.

Generally during the practical application of this method, 40-180 volts of electrical potential are used with the gap between electrodes as 4.5 cm and 15 to 25 amperes of direct current are operated. Though this method is highly effective for draining of fine grained soils, but due to the use of electricity, the cost increases and even experts or specialist supervision is also required. It results in enhancing the shear strength as well as the consolidation of the soil.

Vacuum method

This method is applicable in case of the fine-grained soils with permeability around 10^(-3) to 10^(-5) cm/s with the size minimum than 0.05mm and D10 as the effective particle size. For this method, a trench of nearly around 25 cm is formed nearby the well point, and with the help of enough pressure by spurting water through the riser pipe. The medium to abrasive sand is speedily showelled when the jet water is yet flowing into the hole and load it about from the top until 0.75 to 1 m. With the help of clay by tamping it, the upper portion is completely sealed. Also, the vacuum is generated nearby the riser pipe and well point in the sand filling with the help of vacuum pumps. An unbalanced atmospheric pressure is generated in the ground surface when vacuum is formed in well point. In spite of less amount of water is extracted out, but the consolidation of the sub soil occurs due to the inequitable pressure in the ground surface. However many weeks may be needed for the consolidation to occur and stabilization of the soil.

Deep well system

From the name itself, this system is suitable for the deep excavation. If the depth of excavation exceeds 16m below the surface of ground, this method is applicable. Even for the presence of artesian water at a significant depth, this technique is very beneficial. Generally a hole of diameter ranging from 15- 60m deep is bored for the required amount of depth along with the strainers for casing in the earlier zones. At the base of the well, immersing type of pump efficient enough to lift water up to 30m or more is fixed. For the separate well, they have a different pumps as well. In the outer side of the location with excavation being carried, wells are situated. However, in the toe side of the location for deep excavation, there is frequent installation of the row of well point.

The major drawback for this method is crash of pumps or power supply during the execution may outcome into the quick formation of seepage as well as the hydrostatic pressure during the excavation which results into risky situation. Hence adequate amount of substitute power supply as well as pumping units must be made easily available.

Protective filters

For the secured way out of the seepage water as well as to avoid the transportation of the earth particles, protective filters are mandatory. In the case of earth dams and weighting filters, such filters are essential to be kept at the base of sumps and trenches of the drainage. The protective filter can eliminate the risk of collapse of downstream slope or rockfill as the phreatic lines lies inside the body of the earthen dam.

Drainage filter usually comprises of coalition of various layers of pervious matters and while construction they are planned in such a manner that there is very speedy drainage of seeping water whereas the motion of the soil particles is restricted. For the filter, the following layers are progressively coarser than the earlier ones. So it can also be identified by the name of inverted or base filter. The initial concept for the design of protective filters was innovated by Terzaghi. After that, number of laboratory researches were being conducted by U.S Corps of Engineers. The result from the experiment conducted has turned out to be very helpful and number of rules for designs were developed.
Usually the layers for horizontal filters are comparatively thinner compared to that of vertical or inclined type of filters. So for the sand, the minimum thickness of the horizontal filter is 150 mm and that for the gravel is 300mm. Nevertheless in the case of inclined or the vertical filters or the transition filters between pervious and the impervious core in earth dam, the nominal width for each respective layer is considered as 1m to 1.5m. Similarly, during the compaction of different filter layers, as the non- cohesive zones they should be compacted to the same density. Furthermore during the placing of the filter materials, attention should be paid in order to avoid the segregation as well as the transmission of the fines into core material.

Conclusion

From any saturated soil mass, the expulsion of the surplus water is the dewatering which is very essential for any underground excavation work. Various techniques and systems are defined for the dewatering and drainage process. Upon the basis of size, number and spacing the design of the dewatering system depends upon. For the safe passage of the seeping water and forbid the passage of the soil particles, protective filter layers and designed.