Piping failure mechanism under buildings, highways, dams, and earthen structures is a clear sign of Dispersive (or Saline-Sodic) soils. Refer to (Singh 1992) that the "dispersive" clayey soils are the soils that erode in which individual clay particles go into suspension. Unlike the case of normal erosion, the dispersive soils are highly erodable even under low-velocity flow.
Now, the question arisen is how to detect the dispersive soil?
Different methods do exist to detect the dispersive soils such as Pinhole test, Crumb test, and Hydrometer dispersion test refer to (Sherard et al. 1972, 1976a, and 1976b). The first thing we have to know is that the dispersive soils have a very high content of Sodium cations! Also, a pH of 1:2.5 dispersive_soil/water is usually > 7.8.
In the hydrometer dispersion test; the dispersive soil is detected by a parameter called "Dispersion Percent%." This parameter is defined as the ratio of the percentage of particles of size < 0.005 mm without a dispersing agent to the percentage of particles of size < 0.005 mm with a dispersing agent. The soil is considered dispersive if the "Dispersion Percent%" is > 50%.
Another method to detect the dispersive soils is the one described by (von Harmse 1975) as shown in the following equations:
Exchangeable Sodium Percentage in the soil (ESP) % = (Na/CEC) * 100
Exchangeable Magnesium Percentage in the soil (EMgP) % = (Mg/CEC) * 100
Where;
Na = Total exchangeable (Na+) bases, in Meq/100g of air-dried soil.
Mg = Total exchangeable (Mg++) bases, in Meq/100g of air-dried soil.
CEC = Cation Exchange Capacity of soil, in Meq/100g of air-dried soil.
If (ESP > 8) and (ESP + EMgP > 15) then the soil is considered as dispersive soils! If the (ESP>15); then the soil is dispersive and have a serious piping potential. Hopefully, you find this post helpful and might support you in detecting the dispersive soils.
Thank you and best regards!
Hamzah M. B. Al-Hashemi, A.M.ASCE
Geotechnical (Civil) Design Engineer, MSc
Dear Hamzah,
You have given an interesting discussion about how to identify dispersive nature of clay.. in the sake of interest towards research on this topic, do you have any idea on to calculate the percent dispersion based on free swell test results..is it possible to find out the degree of dispersion of soil based on free swell test results..as degree of swell and dispersion is some what interrelated so can we arrive any correlation based on free swell test results about dispersive nature of soil..ok thanks for the interesting discussion..
Ineterting discussion.
. It is not common to perform these tests in normal geotechncial practice. Normal geotechnical engineer may not identify the dispersive soil in the field. This may result in design and construction issues. Is there anyway identify the dispersive soil in the field?
Eastern Montana has a significant number of formations containing dispersive soils. For field identification, there are two soil characteristics to look for. Deep erosional rills in road cuts or on natural bluffs and cliffs suggest dispersive soils. Also, soil caverns and collapse features in drainages are also a good indication of dispersive soils. Surface features alone are not positive proof of dispersive soils to my knowledge, but they can be a guide indicating testing is warranted. If dispersive soils are suspected, material should be collected for testing.
