ASCE Newsletter Article-Earth Dams

8/2/2019 ASCE Newsletter Article-Earth Dams

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NEW APPROACH TO ASSESS PIPING POTENTIAL IN EARTH DAMS AND LEVEES

By Kevin S. Richards and Krishna R. Reddy

ASCE Nationals 2009 Infrastructure Report Card gave grades of D and D- for the current status on

dams and levees in the U.S., respectively. It has been reported that there are more than 85,000

dams in the nation which are over 50 years old with 4,000 deficient dams, including 1,819 high

hazard potential dams. The 2009 ASCE Illinois Infrastructure Report Card gave a grade of C

for the current status of dams in Illinois. 445 Dams (about 32%) in Illinois are more than 50

years old; 329 of theseare unpermitted. These dams likely require significant repair and

rehabilitation to ensure their continued safe operation. Many of the nation's estimated 100,000

miles of levees are also over 50 years old and were originally built to protect crops from flooding;

however, their reliability under the recent changing conditions is unknown. Any failure of dams or

levees can pose a significant risk to public health and safety. There is an urgent need to assess the

reliability of dams and levees and repair and rehabilitate them to ensure adequate performance and

public safety. Unfortunately, the safety of dams and levees is often ignored until a disastor strikes

leading to loss of life as well as damage to property and ecology.

Approximately half of all dam failures in the world are attributed to piping phenomena. Amongother possible modes piping phenomena include heave, internal erosion and backwards erosion.

While the most common piping failure mode is internal erosion, most often associated with

conduits or other structural penetrations through dams, up to one third of all piping failures might

be attributed to backwards erosion piping (Figure 1). A number of dam failures are also due to

piping into foundations or abutments with untreated geologic deficiencies (Figure 2).

In 1922, Terzaghi performed experiments to study heave-type piping and developed the following

equation for prediction of heave:

b/w=icrit (1)

Where b=buoyant unit weight of soil, w=unit weight of water, and icrit=critical hydraulic gradient

at which the soil mass becomes unstable. Terzaghis equation is adopted by practitioners for all

piping failure modes, such as backwards erosion piping or piping along internal fractures (internal

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erosion), although the theoretical basis for this adaptation of Terzaghis equations has not been

confirmed with laboratory experiments. Heave is a special case of piping where the seepage force

is acting against the force of gravity. The factor of safety against piping based on this geometry

may not provide a conservative value for cases where a component of the seepage force may be

working with gravity.

Early researchers (e.g., Bligh in 1918 and Lane in 1934) recognized an important difference

between intergranular seepage and seepage along soil-structure boundaries. Modern practitioners

define these two different piping mechanisms as either backwards erosion piping (for the

intergranular-flow case), and internal erosion for the case of flow along pre-existing openings

(either soil-structure openings or cracks through an embankment). Recently, a laboratory

investigation has been performed at the University of Illinois at Chicago to investigate factors and

mechanisms affecting piping in soils. This study included developing a new test apparatus, called

the True-Triaxial Piping Test Apparatus (TTPTA) and conducting over one hundred piping tests

using a wide range of materials. The TTPTA consists of a cubical cell that can accommodate

cohesive or non-cohesive soil samples, allow application of confining stresses to the soil in three

mutually perpendicular directions, and allow introduction of seepage water through inlet and outlet

at controlled pore pressures and hydraulic gradients. Based on the laboratory experiments, a

relationship between the maximum principal stress and the velocity required to initiate piping in

granular soils is developed.

It appears that an energy component in piping exists as demonstrated by the influence of the rate of

increase of inflow on the critical velocity, and that there are at least two important characteristic

piping behaviors that depend on the plasticity of fines. In granular non-plastic soils, piping occurs

at higher critical velocities and lower hydraulic gradients for a given pore pressure. In contrast,

piping in plastic clayey soils occurs at higher critical gradients but lower velocities for a given pore

pressure. We also found there may be a coupling between the void ratio and the hydraulic gradient

in non-plastic soils, and it is therefore recommended that seepage velocity (rather than the

hydraulic gradient alone) be used as a better index property of piping in noncohesive soils.

Experiments also revealed a relationship between the critical velocity and the pore pressure in the

cases where soils were tested at different pore pressures below, and above the buoyant condition.

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Clearly, soils in a buoyant state pipe at lower critical seepage velocities (vcrit); however, soils below

the buoyant threshold may still be prone to piping, albeit at higher seepage velocities. There is an

apparent trend between vcrit and the pore pressure when soils are in a non-buoyant state; however, it

is not much larger than the standard deviation of the data (=0.1 cm/sec). No clear relationship

was found between pore pressure and critical velocity in non-buoyant noncohesive soils. The

conclusion is that pore pressure does not play a key role in lowering the critical velocity for piping

in granular materials until the soil is in the buoyant state, at which point the critical velocity drops

substantially. Hence, evaluating piping risk in soils prone to heave should be done separately than

soils below the heave threshold. Overall, it is concluded that Terzaghis equation for calculating a

factor of safety based on buoyancy alone is not sufficient.

In addition to the buoyancy state, the seepage angle plays a significant role in piping. When

seepage is vertically upward, the full affect of gravity is working to stabilize soil particles and the

required seepage velocity to dislodge particles is much greater than if the seepage is horizontal or

in a downward direction. Seepage angle must be considered when evaluating piping potential.

Finally, the other important finding from these investigations was that the plasticity and amount of

fines greatly influences piping behavior. A small amount of plastic fines added to uniform sand

greatly reduces the susceptibility of a soil to backwards erosion, whereas the addition of even a

small amount of non-plastic fines can lower the critical seepage velocity that induces backwards

erosion. Hence, soil tests are recommended to assess site-specific backwards erosion potential due

to the amount and type of fines that may be present.

With these improvements to our understanding, we explored a couple new methods for better

predicting the risk of backwards erosion based on site characteristics. The total hydraulic energy

available to initiate piping can be given by:

Etot = Ei +Ev + E (2)

Where Etot = total hydraulic energy available to initiate piping, Ei = energy contribution due to

hydraulic gradient, Ev = energy due to increased seepage velocity at the exit, and E= potential

elevation energy loss, or gain due to seepage angle. The dimensionless friction head at pipe

initiation can be written as:

crit = {[(p1/i)-(p2/o)]+[((v1/n)2/2g)-((v2/n)

2/2g)]+}/L (3)

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Where,crit= Dimensionless Friction Head at the initiation of piping,p1,2= Component of

Differential Pressure Head (corrected for horizontal flow), i,o= Unit weight of water at the inlet

and outlet, respectively, v1,2= Darcy velocity of seepage at inlet and outlet, respectively, n=

Porosity, g= gravity, = Elevation head change due to seepage angle (length of seepage path times

sin, =0 for horizontal flow); and L = length of flow path (0.155 m in test cell). In the

experiments using TTPTA, differential pressure gages measure the E i, Ev and Ecomponents. The

Dimensionless Friction Head was computed for several granular soil tests that exhibited simple

backwards erosion piping. It was found that the velocity head component contributes very little to

the overall energy of piping in freely draining granular materials if the total energy of the system is

considered. As illustrated by the dimensionless friction head equation shown above, the equation

does not consider energy losses due to friction as water percolates through the embankment. These

frictional losses do not necessarily contribute to piping but are largely responsible for the energy

loss causing hydraulic gradients. This is evidenced by the drop in head in all dams as the water

approaches the seepage face, particularly in cases where piping is not an outcome. Hence, the Ei

and Emay contribute only a small part of the total energy used for piping. One characteristic

observed with piping cases is the apparent high velocity concentrated seepage exiting the pipe. It

is suggested here, that this high velocity discharge is providing the bulk of the energy responsible

for the erosion. For this reason, we recommend that the kinetic energy be used in-lieu of the

dimensionless friction head for evaluation of backwards erosion piping, as this component of

energy may be more responsible for the backwards erosion phenomena. More studies of case

histories could yield the relative contributions of each of the three energy terms to piping, but

based on the laboratory experiments to-date, it was determined that backwards erosion piping is

largely a surficial phenomena caused by high velocity seepage at the discharge point influenced by

intergranular flow.

Using the kinetic energy of seepage alone as an index property for backwards erosion piping would

require the following expression. This equation is best used for soils without plastic fines:

Ekcrit = (mv2) (nv2) (4)

Where:Ekcrit= critical kinetic energy of seepage at initiation of backwards erosion, m = mass of

percolating water per cubic centimeter of soil (for distilled water at 4 C, m = n * 1.0 g/cm3

* 1.0

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cm3 ; n is porosity), and v = Darcy velocity of intergranular seepage (v=ki, where kis the hydraulic

conductivity and i is the hydraulic gradient).

As previously discussed, experimental results showed a marked difference in piping behavior

when as little as 10% plastic fines are present in granular soils. The results also show that for soils

with plastic fines, shear stress is a better index property to use for the prediction of internal erosion

due to the relatively large pore pressures and low seepage velocity that characterize internal

erosion. So, for a soil with plastic fines the shear stress may be expressed by:

crit = whpic (5)

Where crit = critical hydraulic shear stress (N/m2), w = unit weight of water (N/m3), hp = pressure

head when piping is initiated (m), and ic = critical hydraulic gradient.

The methods described here may be easily used to assess the factor of safety against either

backwards erosion or internal erosion at existing dams. The factors of safety can be computed by:

Cohesionless Soils (backwards erosion)- F.S. piping = Ekcrit / Ekavailable (6)

Cohesive Soils (internal erosion)- F.S. piping = crit/ available (7)

These piping factors of safety can be computed for a variety of hydraulic loading conditions that

may be encountered using an apparatus similar to the TTPTA used in our work. It is important that

Ekcrit or the crit be evaluated using site specific soils tested under in-situ stress states. The plasticity

and amount of fines significantly affect piping behavior; hence, it is important to test the soils in

the laboratory to determine the magnitude of these effects. If field stress states, buoyancy state, or

soil density differs from laboratory test conditions, the differences in void ratio, stress state, etc.

should be accounted in the evaluations.

In summary, improved knowledge of pipe initiation and propagation mechanisms allows for better

risk evaluation of piping potential in existing structures through the use of seepage models that can

predict hydraulic conditions within the soils. The methodology presented provides a new method

for evaluation of backwards erosion piping. Having knowledge of the critical kinetic energy for

noncohesive soils and critical shear stress for cohesive soils allows for better prediction of the

performance of embankment dams or levees. Proper application of these techniques should help to

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better predict the piping risk at dams and to better allow limited resources to be directed towards

projects with the highest risk.

Kevin S. Richards, Ph.D., P.E., P.G., is Senior Civil Engineer in the Office of Energy Projects,

Division of Dam Safety and Inspections, Federal Energy Regulatory Commission, Chicago, and he

has over 30 years of experience in geotechnical and geological engineering, including the

assessment and design of earth dams.

Krishna R. Reddy, Ph.D., P.E. is Professor of Civil & Environmental Engineering at the

University of Illinois at Chicago and has over 20 years of teaching, research and consulting

experience in the design of foundations, earth structures, landfills, and site remediation systems.

They can be reached for additional information at: [email protected]@uic.edu

Figure 1 Backwards erosion piping through an embankment.

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mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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a. Quail Creek Dam failure 1989, Utah b. Teton Dam failure 1976, Idaho

c. Swift No. 2 power canal dike failure 2002, Washington

Figure 2 Some dam failures resulting from piping through geologic deficiencies in the

foundation.

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ASCE Newsletter Article-Earth Dams