Dispersive/Saline-Sodic Soils

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 Strong Force-MGC W.L.L., #Bahrain

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