Piping failures

Water that percolates through earth dams and their foundations can carry soil particles that are free to migrate. The seepage forces tend to cause the erodible soil or soft rock to move towards the downstream face of the dam. That is if the seepage forces are large enough and the pore spaces in the material are large enough. Along the unprotected discharge face AB, the soil will heave if the gradients are large enough.

Every seepage discharge surface, both internal and external, which could be susceptible to piping or heave must be covered with filters that permit water to pass but will hold the soil particles firmly in place.

Filter criteria

Since the core is stabilised with rock or gravel zones, it is necessary to prevent the fine core material being sucked into the upstream shell material during rapid drawdown of the reservoir, or forced into the downstream shell by seepage water under reservoir head. Transition or filter zones must therefore be provided on each side of the core.

The upstream filter, if non-cohesive and of proper grading, can serve a valuable service by providing material for induced self-healing should a transverse crack appear in the core. Selection of the best material for this purpose is well justified. Although its prime function is to retain the core material against movement into the rockfill, the downstream transition material should be selected and placed so as to inhibit the propagation of a core crack into the compacted rockfill. It is good practice to widen the transition zones towards each abutment, i.e. where tension and oblique cracking may occur.

To prevent migration of fines from the core:

D15/D85 < 4-5
(filter)/(zone being filtered)

D50/D50 < 25
(filter)/(zone being filtered)

For sufficient permeability:

D15/D15 > 4-5
(filter)/(zone being filtered)

To prevent segregation of the filter:

D60/D10 < 20

Problems associated with natural formations

The foundations and abutments of dams are usually stable under the influence of the natural groundwater flow. However, reservoir filling greatly changes the groundwater regime and may lead to piping and internal erosion. The potential for internal erosion and piping may occur at joints in rock, beds of gravel and in cavities left by rotting roots, animals burrows or other buried organic matter.

Recommendations for preventing piping in natural formations

Field exploration and geological mapping for dam projects should identify the important soil and rock formations that could cause failure by internal piping or heave. The geotechnical properties of these materials should be thoroughly investigated. If the materials are proven to be unsuitable then remedial action should be taken to improve their geotechnical properties.

All new dams and reservoirs should be carefully observed and monitored once in service to detect the development of unsafe conditions. If seepage quantities increase or if there is an unexplained change in seepage conditions then protective measures should be put into action.

Such actions should include lowering the reservoir and placing weighted filters over areas where seepage discharges occur.