For civil, geotechnical or geological engineers, dispersive soils represent one of the problems in soil mechanics and geotechnics, which need to be solved to make construction works safe, profitable and durable over time.
What are dispersive soils?
Dispersive soils are those that are made up of a certain type of clay (dispersive clays) that when exposed to water tend to form fissures or channels that allow internal soil erosion and generate problems in civil engineering works.
What are dispersive clays?
The dispersive clays are those in which their mineralogical constitution is such that the attractive forces between the clay particles are less than the repulsive force, in addition, they are enriched and surrounded by sodium ions (Na +).
How does erosion occur in dispersive soils?
As mentioned above, dispersive clays are surrounded by sodium ions (Na+), which are more likely to be washed away.
When water comes into action, sodium ions are dragged, therefore, the space between the clay particles increases, that is, microspaces are formed in the volume of clays, and also the attractive force decreases, so the material is prone to be eroded through the spaces formed.
These microspaces are formed as small channels that, over time and continuous water erosion, generate large channels or siphons, which decrease the stability of the soil and achieve the loss of the initial engineering properties.
What problems do dispersive clays cause?
The main problem caused by dispersive clays before the action of water is the internal erosion of the soil through the spaces formed, which in turn decreases the stability of the material where the civil works are located.
Internal erosion in dispersive soils can cause changes in the volume of the material, therefore, generate breakage or deformation in the construction works that sit on those soils.
In addition, settlements may occur in the structures.
Stabilization of dispersive soils
Basically, the stabilization of dispersive soils is aimed at resolving the dispersivity of the clays that make up dispersive soils.
As mentioned above, soil dispersivity is related to the high percentage of sodium ions (Na+) adhered to clays.
Therefore, the correct way to stabilize or reduce the instability of dispersive soils will be to replace unstable sodium ions (Na+) with other stable ions such as (Ca2+), (Al3+), (Mg2+), through chemical exchange.
Another way to stabilize the dispersive soil will be with additives that function as binders or that compact the soil.
The most commonly used methods are summarized below.
Stabilization with hydrated aluminum sulfate
The addition of hydrated aluminum sulfate (Al2(SO4)3∙18H2O) in dispersive soils induces the substitution of aluminum cations for sodium.
For this, approximately 1.5% hydrated aluminum sulfate must be added with respect to the total volume of the dispersive clays.
In addition to eliminating soil dispersivity, this method reduces the liquid limit of the soil by up to 50% and thereby reduces the plastic index.
Stabilization with portland cement
Portland cement works as a binder when mixed with the dispersive clays of the soil, for this reason it is necessary that the mixture is made before or during the realization of the work, that is to say that this method does not work for engineering works already executed.
The method is effective when values above 95% compaction are reached.
Lime stabilization
With quicklime
Quicklime or also calcium oxide (CaO) is used to reduce soil moisture (amount of water in the soil) so that water is prevented from causing internal erosion in dispersive soils, when it drags sodium ions.
With hydrated lime
Hydrated lime, or calcium hydroxide, is practically insoluble, its purpose is to allow the migration of calcium ions from hydrated lime particles to clay particles, displacing less stable ones.
This process generates agglomeration and as a result the soil becomes more granular and friable.
This method can be used to stabilize soils of dams already built, that is, there is no need to remove the soil, but it must be taken into account that it is only used in small to medium works with caution.
The process is simple: first, calcium hydroxide is introduced into the volume of reservoir water from the micro-dams, which is then dragged through the embankment. In this way the water crosses the soil mass, where cation exchange occurs and therefore the stabilization of the dispersive clays in the soil is generated.