The hole erosion test (HET) is a method used in

geotechnical engineering Geotechnical engineering, also known as geotechnics, is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics to solve its engineering problems. I ...

to quantify the resistance of a

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

, and is specifically relevant to the topic of

internal erosion Internal erosion is the formation of voids within a soil caused by the removal of material by seepage. It is the second most common cause of failure in levees and one of the leading causes of failures in earth dams, responsible for about half of e ...

embankment dams 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 ...

. The test can be performed in a

laboratory A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratories are found in a variety of settings such as schools ...

on a remolded soil sample, and provides estimates of both the critical shear stress for erosion of the soil sample as well as a numerical measure of soil erodibility. In the

design A design is the concept or proposal for an object, process, or system. The word ''design'' refers to something that is or has been intentionally created by a thinking agent, and is sometimes used to refer to the inherent nature of something ...

and

engineering Engineering is the practice of using natural science, mathematics, and the engineering design process to Problem solving#Engineering, solve problems within technology, increase efficiency and productivity, and improve Systems engineering, s ...

of embankment dams, the critical shear stress provided by this test indicates the maximum

shear stress Shear stress (often denoted by , Greek alphabet, Greek: tau) is the component of stress (physics), stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross secti ...

that a

fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...

(such as water) can apply to a soil before a concentrated leak forms and erosion begins. The numerical measure of soil erodibility can be used to predict how quickly this erosion will progress, and it can be found as an input in various

computer simulations Computer simulation is the running of a mathematical model on a computer, the model being designed to represent the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be determin ...

for

dam failure A dam failure or dam burst is a catastrophic type of structural failure characterized by the sudden, rapid, and uncontrolled release of impounded water or the likelihood of such an uncontrolled release. Between the years 2000 and 2009 more than ...

Procedure

The standard hole erosion test consists of first compacting the soil sample in a standard

mold A mold () or mould () is one of the structures that certain fungus, fungi can form. The dust-like, colored appearance of molds is due to the formation of Spore#Fungi, spores containing Secondary metabolite#Fungal secondary metabolites, fungal ...

. Then, a small hole (typically 6mm) is drilled lengthwise through the soil. Next, the downstream

hydraulic head Hydraulic head or piezometric head is a measurement related to liquid pressure (normalized by specific weight) and the liquid elevation above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22, eq.3.2a. It is usually meas ...

is set to a standard value, and the initial upstream hydraulic head is chosen using

trial-and-error Trial and error is a fundamental method of problem-solving characterized by repeated, varied attempts which are continued until success, or until the practicer stops trying. According to W.H. Thorpe, the term was devised by C. Lloyd Morgan ( ...

. As the liquid (typically water) flows through the hole, the soil should erode and the hole will expand. The flow rate should be measured throughout the procedure. Directly after the test, the diameter of the hole should be measured. The hydraulic

along the surface of the hole at time ''t'' can be calculated as:

\tau = \rho g \frac \frac

where ''ρ'' is the

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

of the liquid, ''g'' is the

gravitational acceleration In physics, gravitational acceleration is the acceleration of an object in free fall within a vacuum (and thus without experiencing drag (physics), drag). This is the steady gain in speed caused exclusively by gravitational attraction. All bodi ...

, ''Δh'' is the difference in hydraulic head across the sample, ''L'' is the length of the sample, and ''Φ_t'' is the diameter of the hole at time ''t.'' While the diameter of the hole is not directly measured throughout the test, it can be estimated using the measured flow rate as well as an estimated friction factor. From the change in diameter of the hole over time, the rate of erosion can thus be plotted against applied hydraulic shear stress and fit to the following equation:

E_r = k_d(\tau - \tau_c)

where ''E_r'' is the rate of erosion over time, ''k_d'' is the soil erodibility, and ''τ_c'' is the critical shear stress for erosion.

Modified hole erosion test (HET-P)

One criticism of the standard hole erosion test is that the use of the hydraulic head rather than the

total head Bernoulli's principle is a key concept in fluid dynamics that relates pressure, speed and height. For example, for a fluid flowing horizontally Bernoulli's principle states that an increase in the speed occurs simultaneously with a decrease i ...

implies that the change in velocity head is negligible, which may not be a valid assumption given the sometimes high velocities downstream of the hole. The difference in hydraulic head used to calculate the shear stress also does not factor in the energy dissipated due to flow recirculation and expansion losses downstream of the test specimen. Furthermore, estimating the diameter of the hole throughout the test using an assumed friction factor has been reported as problematic.{{Cite journal , last1=Wahl , first1=T L , last2=Regazzoni , first2=P , last3=Erdogan , first3=Z , date=2008 , title=Determining Erosion Indices of Cohesive Soils with the Hole Erosion Test and Jet Erosion Test , journal=Hydraulic Laboratory No. DSO-08-05, US Dept. of the Interior, Bureau of Reclamation, Technical Service Center , location=Denver, Colorado The modified hole erosion test (HET-P) seeks to rectify these issues with the addition of a pitot-static tube. This allows for the direct measurement of total hydraulic head, thus accounting for the total energy loss between the upstream and downstream ends of the soil sample. While the diameter of the hole is still not measured directly throughout the test, the pitot-static tube provides an independent estimate of the mean flow velocity, which can then be used to calculate the diameter of the hole more directly using the

continuity equation A continuity equation or transport equation is an equation that describes the transport of some quantity. It is particularly simple and powerful when applied to a conserved quantity, but it can be generalized to apply to any extensive quantity ...

. The modified hole erosion test results in significantly smaller values for the critical shear stress - this is makes the results of the test more consistent with other tests, such as the Rotating Cylinder Test or the

Jet Erosion Test The jet erosion test (JET), or jet index test, is a method used in geotechnical engineering to quantify the resistance of a soil to erosion. The test can be applied in-situ after preparing a field site, or it can be applied in a laboratory on eithe ...

References

Soil erosion Tests in geotechnical laboratories