Internal erosion is the formation of voids within a soil caused by the removal of material by
seepage
Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids (usually air and wat ...
. It is the second most common cause of failure in
levee
A levee (), dike (American English), dyke (English in the Commonwealth of Nations, Commonwealth English), embankment, floodbank, or stop bank is a structure that is usually soil, earthen and that often runs parallel (geometry), parallel to ...
s and one of the leading causes of failures in
earth dam
An embankment dam is a large artificial dam. It is typically created by the placement and compaction of a complex semi-plastic mound of various compositions of soil or rock. It has a semi-pervious waterproof natural covering for its surface an ...
s, responsible for about half of embankment dam failures.
Internal erosion occurs when the hydraulic forces exerted by water seeping through the pores and cracks of the material in the
dam
A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use ...
and/or
foundation
Foundation may refer to:
* Foundation (nonprofit), a type of charitable organization
** Foundation (United States law), a type of charitable organization in the U.S.
** Private foundation, a charitable organization that, while serving a good cause ...
are sufficient to detach particles and transport them out of the dam structure.
Internal
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
is especially dangerous because there may be no external evidence, or only subtle evidence, that it is taking place. Usually a
sand boil
Sand boils or sand volcanoes occur when water under pressure wells up through a bed of sand. The water looks like it is boiling up from the bed of sand, hence the name.
Sand volcano
A sand volcano or sand blow is a cone of sand formed by the ...
can be found, but the boil might be hidden under water. A dam may breach within a few hours after evidence of internal erosion becomes obvious.
Piping is a related phenomenon and is defined as the progressive development of internal erosion by seepage, appearing downstream as a hole discharging water. Piping is induced by regressive erosion of particles from downstream and along the upstream line towards an outside environment until a continuous pipe is formed.
Internal erosion and the piping process
According to the
International Commission on Large Dams
The International Commission on Large Dams, or ICOLD, (french: Commission Internationale des Grands Barrages ''or CIGB'') is an international non-governmental organization dedicated to the sharing of professional information and knowledge of the de ...
(ICOLD), there are four general failure modes for internal erosion of
embankment dam
An embankment dam is a large artificial dam. It is typically created by the placement and compaction of a complex semi-plastic mound of various compositions of soil or rock. It has a semi-pervious waterproof natural covering for its surface and ...
s and their foundations:
* Through the
embankment
Embankment may refer to:
Geology and geography
* A levee, an artificial bank raised above the immediately surrounding land to redirect or prevent flooding by a river, lake or sea
* Embankment (earthworks), a raised bank to carry a road, railwa ...
* Through the foundation
* Embankment-into-foundation
* Associated with through-penetrating structures
The process of internal erosion occurs across four phases: initiation of erosion, progression to form a pipe, surface instability, and, lastly, initiation of a
breach
Breach, Breached, or The Breach may refer to:
Places
* Breach, Kent, United Kingdom
* Breach, West Sussex, United Kingdom
* ''The Breach'', Great South Bay in the State of New York
People
* Breach (DJ), an Electronic/House music act
* Miroslava ...
. Internal erosion is also classified in four types, dependent on failure path, how the erosion initiates and progresses, and its location:
* Concentrated leak: seeping water erodes and enlarges a crack until a breach occurs. The crack may not progress to the exit (albeit failure is still possible), but eventually the continued erosion forms a pipe or a sinkhole.
* Backward erosion: initiated at the exit point of the seepage path, this type of erosion occurs when the hydraulic gradient is sufficiently high to cause particle detachment and transport; a pipe forms backwards from the exit point until breach.
* Suffusion: occurs in soils with a wide range of particle sizes. Finer soil particles are eroded through the voids between coarser particles. Soils susceptible to suffusion are termed internally unstable. Suffusion can only occur if the volume occupied by the finer particles is lower than the available void space between the coarse particles.
* Soil contact erosion: a phenomenon called sheet flow occurs at interfaces between coarse and fine soils. Water seeps along the interface between the two soils, eroding the particles from the finer layer into the coarser layer.
Concentrated leak
''Concentrated leaks'' occur when cracks form in the soil. The cracks must be below reservoir level, and water pressure needs to be present to maintain the open pipe. It is possible for water flow to cause the sides of the pipe to swell, closing it and thus limiting erosion.
Additionally, if the soil lacks sufficient cohesion to maintain a crack, the crack will collapse and concentrated leak erosion will not progress to a breach.
Cracks that allow concentrated leaks can arise due to many factors, including:
* Cross-valley arching resulting in vertical stresses on the sides of the dam
* Core arching on the shoulders of the embankment
* Differential settlement (above 0.2% differential, a crack forming is almost certain)
* Small scale irregularities during core treatment (e.g. due to poor compaction)
* Cracks and gaps adjacent to
spillway
A spillway is a structure used to provide the controlled release of water downstream from a dam or levee, typically into the riverbed of the dammed river itself. In the United Kingdom, they may be known as overflow channels. Spillways ensure tha ...
s or
abutment walls or around conduits
* Various environmental factors like
desiccation
Desiccation () is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container.
...
, settlement during
earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
s, freezing, animal
burrow
An Eastern chipmunk at the entrance of its burrow
A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of locomotion. Burrows provide a form of sh ...
s, vegetation/roots.
Longitudinal cracks arise from the spreading of the embankment, while transverse openings, which are much more common, are due to vertical settlement of the dam. The hydraulic shear stress τc required for the initiation of concentrated leak erosion can be estimated using laboratory testing, such as the hole erosion test (HET).
Backward erosion
![Hanging valley - geograph](https://upload.wikimedia.org/wikipedia/commons/d/d0/Hanging_valley_-_geograph.org.uk_-_715576.jpg)
''Backward erosion'' often occurs in non-plastic soils such as fine
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
s. It can occur in sandy foundations, within the dam or levee, or in cofferdams under high flood pressures during construction, causing unraveling at the downstream face. It also occurs in landslide and flood-prone regions where slopes have been disturbed.
Backward erosion is most often exhibited by the presence of
sand boil
Sand boils or sand volcanoes occur when water under pressure wells up through a bed of sand. The water looks like it is boiling up from the bed of sand, hence the name.
Sand volcano
A sand volcano or sand blow is a cone of sand formed by the ...
s at the downstream side of dams. Experiments from Sellmeijer and co-workers have shown that backwards erosion initiates in a slot through the strata that overlays the eroding soil (e.g. through excavations or drainage ditches)
and then progress in many, smaller pipes (less than 2mm in height) rather than a single one. The stability of the pipes is dependent on the head, and once this is larger than a critical value (0.3-0.5 of flow path length), the channel extends upstream. Beyond this, at any head greater than the critical value, erosion progresses until eventually, the pipes break through to the upstream reservoir, at which point a breach occurs. In order for backward erosion to occur, the dam or levee body must form and maintain a ‘roof’ for the pipe.
Suffusion
''Suffusion'' occurs when water flows through widely-graded or
gap-graded, cohesionless soils.
The finer particles are transported by seepage, and the coarse particles carry most of the effective stress. Suffusion can only occur provided the fine soil particles are small enough to pass between the coarse particles and do not fill the voids in the coarser soil. Water flow velocity must also be sufficient to transport those fine particles.
Suffusion leads to increased permeability in the embankment core, greater seepage velocities and possibly hydraulic fractures. It can also lead to settlement if it occurs in the dam foundation. Soils subject to suffusion also tend to be affected by
segregation Segregation may refer to:
Separation of people
* Geographical segregation, rates of two or more populations which are not homogenous throughout a defined space
* School segregation
* Housing segregation
* Racial segregation, separation of humans ...
. The Kenney-Lau approach is a renowned method for the analysis of suffusion, which uses the particle size distribution to assess the internal stability of a soil, which directly affects the likelihood of suffusion occurring.
Soil contact erosion
''Soil contact erosion'' occurs when sheet flow (water flow parallel to an interface) erodes fine soil in contact with coarse soil.
Contact erosion is largely dependent on the flow velocity, which must be sufficient to detach and transport the finer particles, as well as the finer soil particles being able to pass through the pores in the coarse layer. When contact erosion is initiated, a cavity is formed, leading to a reduction of stress. The roof of the cavity then collapses; the collapsed material is transported away resulting in a larger cavity. The process continues until the formation of a sinkhole. It is possible for a cavity to not collapse; this will lead to backward erosion occurring.
Soil contact erosion can occur between any granular layer and a finer soil such as in silt-gravel, and often results in a loss of stability, increases in pore pressure and clogging of the permeable layer.
Experimental results show that close to the geometrical limit, the point at which the fine particles can ''just pass'' between the coarse particles(the filter criterion), erosion initiation and failure are much more likely.
Prevention using filters
It is possible to interrupt the process of internal erosion with the use of
filter
Filter, filtering or filters may refer to:
Science and technology
Computing
* Filter (higher-order function), in functional programming
* Filter (software), a computer program to process a data stream
* Filter (video), a software component tha ...
s. Filters trap eroded particles while still allowing seepage, and are normally coarser and more permeable than the filtered soil. The type of filter required and its location is dependent on which zones of the dam are most susceptible to internal erosion. Per regulation, filters need to satisfy five conditions:
* Retention: the filter must limit or interrupt the transport of eroded soil particles.
* Self-filtration: also defined as stability, the filter must be internally stable.
* No cohesion: the filter must not have the ability to maintain cracks or the ability to
cement
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mix ...
.
* Drainage: the filter must be sufficiently permeable to allow water pressure to dissipate.
* Strength: the filter must be capable of transferring stresses within the dam without being crushed.
References
{{DEFAULTSORT:Internal Erosion
Dams
Erosion