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Meaning of Seepage:
The movement of water through soil is referred to as seepage.
The total head at any point of a saturated soil mass, through which water is flowing consists of-
(i) Velocity head;
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(ii) The piezometric head; and
(iii) The datum head. Since the velocity through soils is very small, the velocity head is generally neglected.
Flow will occur between the two points, only when there exists some difference in the potential of those two points.
Seepage of water results in the following types of problems, viz.-
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(i) Loss of stored water through an earth dam or foundation,
(ii) Instability of slopes and foundations of hydraulic structures due to force exerted by the percolating water,
(iii) Settlement of structures founded on or above compressible layers due to expulsion of water from the voids caused by load application.
The difference between the potential heads of two points h, represent the head difference and is called available potential. This available potential lost (h) inflow between two points (at L distance apart), is in fact consumed by the friction offered by the soil to the movement of water through it.
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This frictional drag pressure would be equal to h. γw. This is counterbalanced by the flow force, which is called the seepage pressure. This seepage pressure can be defined as the pressure exerted by water on the soil through which it percolates.
The total seepage force transmitted to the soil mass can be easily obtained by multiplying the seepage pressure by the total cross-sectional area of the soil mass.
This seepage pressure always acts in the direction of flow.
The flow is generally laminar. The path taken by a water particle is represented by a flow line although an infinite number of lines can be drawn.
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At certain points on different flow lines, the total head will be the same.
The lines connecting points of equal total hand can be drawn.
These lines are known as equipotential lines. As flow always takes place along the steepest hydraulic gradient, the equipotential lines cross flow lines at right angles. The flow lines and the equipotential lines together form a flow net. The flow net gives a pictorial representation of the path taken by water particles and the pressure variation along that path.
Quick Sand Condition:
It is clear that by increasing the total head difference h, it is possible to reach a condition when the effective stress in the soil becomes equal to zero, that is –
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Hγ’ – iH γw = 0
This condition occurs when the hydraulic gradient –
i = icr = γ’/γw
icr is called the critical hydraulic. gradient
When upward flow takes place at the critical head gradient, a soil such as sand loses all its shearing strength and it cannot support any load. This soil is said to have become ‘quick’ or ‘alive’ and boiling will occur.
The popular name for this phenomenon is quick sand. Quick and is not a type of sand but only a hydraulic condition. In this case the seepage pressure iH. γw has become equal to the effective pressure Hγ so that the effective stress throughout the soil is reduced to zero.
In practice, boiling condition may occur when excavations are made below water table and water is pumped out from the excavation pit to keep the area free from water.
It can be prevented by lowering the water table at the site before excavation or alternatively, by increasing the length of upward flow. Boiling condition is also common when a pervious sand stratum underlying a clay soil is in an artesian pressure condition.
To Sum Up:
1. Quick sand is not a kind of sand; it is a hydraulic condition.
2. A sand soil becomes ‘quick’ when flow is upward under a hydraulic gradient which reduces the effective stress to zero.
3. In a typical sand soil, the critical hydraulic gradient is about 1.
4. Quick sand condition occurs mainly in fine sands.
5. High artesian pressure in a coarse sand is one of the most important reasons for the development of quick sand condition.
Seepage Force:
The seepage pressure are sometimes converted into forces by multiplying them by the total cross-sectional area of the soil samples. The seepage force is applied by the flow water to the soil skeleton through frictional drag. In an isotropic soil, the seepage force always acts in the direction of flow.
Seepage force is usually expressed as force per unit volume of soil,
To analyse the force acting on a soil element, we can either use seepage forces in combination with submerged weight or boundary water forces with total weights.
Piping:
The undermining of the subsoil starts from the tail end of the work. It begins at the surface due to the residual forces of seepage water at this end being in excess of the restraining forces of the subsoil which tend to hold the latter in position.
Once the surface is disturbed, the dislocation of subsoil particles works further down and, if progressive, leads to the formation of cavities below the floor in which the latter may collapse.
This undermining is supposed to result from what is known as piping, that is, the erosion of the subsoil by the high velocities of flow of water through it when such velocities exceed a certain limit.
But this concept of undermining is incomplete. Water has a certain residual force at each point along its flow through the subsoil which acts in the direction of flow and is proportional to the pressure gradient at that point.
At the tail end, this force is directed upward and will tend to lift up the soil particle if it is more than the submerged weight of the latter. Once the soil particles are disturbed, the resistance against upward pressure of water will be further reduced, tending to progressive disruption of the subsoil. The flow gathers into a series of pipes in the subsoil and dislocation of particles is accelerated.
This concept of undermining by ‘foundation’ was put forward by Prof. Terzaghi (1925).
Protective Filters:
Downstream drainage filters and drains in earth dams, and “weighting filters” at the bottom of drainage sumps and trenches arrest the percolating water at the exit and allow its safe exit without inducing piping. With a proper drainage filter under the d/s slope or a rock fill toe.
A protective filter consists of a combination of layers of pervious materials and is designed in such a manner as to provide quick drainage, yet prevent the movement of soil particles due to flowing water.
In such a filter, each subsequent layer becomes increasingly coarser than the previous one, and hence, it is also called reverse filter or inverted filter. The soil to be protected is known as the base materials.
The four main requirements to be satisfied by filter material are as follows:
1. The filter material should be sufficiently fine and so graded that the voids of the filter are small enough to prevent base material particles from penetrating and clogging the filter.
2. The filter material should be sufficiently coarse and pervious compared to the base material so that the incoming water is rapidly removed without any appreciable build-up of seepage forces within the filter.
3. The filter material should be coarse enough not to be carried away through the drainage pipe opening. The drainage pipe should be provided with sufficiently small openings or perforations, or additional coarser layer should be used if necessary.
4. The filter layer should be sufficiently thick to provide good distribution of all particles size throughout the filter and to be able to carry the seepage discharge. The filter thickness should ensure an adequate safety against piping and proper insulation for frost susceptible base material, as the case may be.
To achieve the above stipulations, Terzaghi laid down certain criteria for protective filters. These have been subsequently extended by the Corps of Engineers, USA.
The filter specifications are given below:
D15, D50 and D85 refer to the particle size from the grain size distribution curves.
No significant invasion of particles from the protected soil to the filter takes place, will be taken care by the first specification. Sufficient head is lost in flow through the filters without a build-up of seepage pressure will be taken care by the first part of the second criterion.
If these criteria cannot be met by one filter layer or the layer thickness is insufficient, several layers of filters, each coarser than one below it and each layer satisfying the specified filter criteria with respect to the lower layer, are to be used. Such a multi-layered filter is called a graded filter or an inverted filter.
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