Failure of Hydraulic Structure (With Control) | Seepage Analysis | Soil Engineering

In this article we will discuss about the failure of hydraulic structure by piping along with its control measures.

Failure of Hydraulic Structures by Piping:

Hydraulic structures, such as weirs and dams, built on pervious foundations may fail by a phenomenon that is known as piping. There are two types of such piping failure.

1. Backward Erosion Piping:

When the hydraulic gradient of the flowing water near the downstream floor of a hydraulic structure, known as exit gradient, is high (greater than the critical hydraulic gradient), the water rises above the ground surface with a high velocity. The associated high seepage pressure removes some of the fine soil particles near ground level, forming a hole.

When the soil near the exit is removed, the flow net for the foundation soil gets modified, with more concen­tration of flow lines and equipotential lines in the remaining soil mass, resulting in a further increase in the exit gradient. This causes further removal of the soil particles, which, in turn, increases the exit gradient.

A cylindrical void extends from the downstream side to the upstream side in the foundation soil below the hydraulic structure, as shown in Fig. 10.19. As soon as this void approaches the reservoir, a large amount of water rushes through it and the hydraulic structure fails due to high settlement and loss of bearing capacity.

Backward erosion piping may also occur in the body of an earth structure such as an earth dam. This takes place when the phreatic line cuts the downstream face of the dam and the seepage pressure is high. It is indicated by a progressive sloughing of the downstream slope. Such a failure can occur even when the exit gradient is low.

If the downstream slope has a slope angle equal to the angle of internal friction of a cohesionless soil, the critical gradient at which failure occurs is approximately equal to zero. Thus, it has to be ensured that the phreatic lines never touch the downstream side of the earth dam to avoid failure.

Backward erosion piping may also occur along any weak bedding plane in the foundation or along the periph­ery of a conduit embedded in the earth dam when the seepage pressure is high.

2. Heave Piping:

Failure by heave piping occurs in the form of a rise or heave of a large mass of soil due to seepage pressure acting in the upward direction. This may occur on the downstream side of a hydraulic structure, where the flow takes place in the upward direction and the seepage pressure also acts in the upward direction.

When the seepage pressure due to the upward flow of water at any level is greater than the submerged weight of the soil above that level, the entire soil mass in that zone heaves up and is blown out by the percolating water. This type of failure is known as heave piping failure.

Heave piping may occur on the downstream of a sheet pile or cutoff wall of a hydraulic structure. As per Terzaghi, heave piping may occur within a distance of D/2 on the downstream of the sheet pile, where D is the embedment depth of the sheet pile. It occurs in the zone marked abcd in Fig. 10.20 when the upward seepage pres­sure is greater than the submerged weight of the soil in this zone.

In Fig. 10.20, the equipotential line of 0.4h potential head passes through point a, and that of 0.3h potential passes through b. Hence, the average hydraulic pressure on the base ab of the prism abed is –

Upward seepage force per meter length of the wall is –

Downward force due to the submerged weight of the soil is –

Heave piping would occur when F_s > W. The factor of safety against heave piping can be obtained as –

where γ’ is the submerged density of soil on the downstream side of the hydraulic structure, D is the depth of embedment of the sheet pile, and h_ab is the average hydraulic head for a distance of D/2 from the downstream end of the sheet pile.

Seepage Control in Hydraulic Structures – Prevention of Piping Failures:

The occurrence of piping within or below a hydraulic structure may cause its failure or affect its service.

The follow­ing measures may be adopted to prevent piping failures:

1. Increasing Path of Percolation:

The exit gradient can be reduced by increasing the path of seepage by:

i. Increasing the base width of the hydraulic structure.

ii. Providing upstream/downstream cutoff walls below the hydraulic structure.

iii. Providing an impervious blanket on the upstream slope. This reduces the chance of backward erosion piping through the base.

2. Reducing Seepage:

By reducing seepage through the body of the earth dam, the chances of piping failure across the body of the dam can be reduced. This can be done by providing an impervious clay core in the body of the dam.

3. Providing Drainage Filter:

A drainage filter can be provided to change the direction of flow away from the downstream face, besides prevent­ing the movement of soil particles along with the flowing water.

A drainage filter may be horizontal, as shown in Fig. 10.21(a), or rock toe, as shown in Fig. 10.21(c). A chimney drain, as in Fig. 10.21(b), is effective for stratified soil deposits in which the horizontal permeability is greater than the vertical permeability. This prevents backward erosion piping through the body of the dam.

4. Providing Loaded Filter:

A loaded filter, when provided, increases the downward force without increasing the upward seepage force. It con­sists of graded sand and gravel. It is placed at the exit point, where the water emerges from the foundation. For the sheet pile wall, this filter is placed over the potentially unstable prism abed in Fig. 10.20 such that the factor of safety against heave piping is improved, as shown in –

F = W + W_f/F_s …(10.47)

where W is the downward force due to the submerged weight of the soil, W_f is the downward force due to the weight of the filter, and F_s is the upward seepage force per meter length of the wall.