Soil mechanical properties as influenced by exchangeable cations

Abstract

The tensile strengths of dried, undisturbed aggregates and of dried, artificial aggregates and the shear strengths of freshly-moulded, moist soils were investigated as functions of their contents of exchangeable cations.Tensile strength increased with increasing exchangeable sodium and decreased with increasing exchangeable calcium. Shear strength of moist soil, as assessed through the Atterberg limits, increased with increasing exchangeable calcium, potassium and sodium. The influence of exchangeable magnesium was variable in that it tended to increase the tensile strength of the soils when dry, whereas it tended to decrease the shear strength of one of the sets of soils in the moist state.It is concluded that the exchangeable cations which give rise to greater repulsion between, and dispersion of, clay particles in water also give rise to greater strength in the dried soil. It is proposed that this is because greater particle repulsion in the soil water during dryingenables particle rearrangements to take place more readily and this results in denser packing arrangements and increased strength. I N T R O D U C T I O NSoil strength is controlled by a number of factors including the properties of the particle surfaces which are in contact or near contact. The properties of the particle surfaces are strongly dependent on the types of ions adsorbed onto them. Many of these ions are described as exchangeable because they can be replaced by other ions. Richards (1953) showed that, for briquets of soil which had been prepared by moulding when moist and then air-drying, samples which were sodic (with a predominance of exchangeable Na ions) had greater strength, as measured by the modulus of rupture test, than samples which were calcic (with a predominance of exchangeable Ca ions). Similarly, Warkentin & Yong (1962) showed that the shear strength of Na montmorillonite was greater than that of Ca montmorillonite.Reeve et al. (1954) showed that modulus of rupture increased linearly with increasing exchangeable Na, but that there was no significant effect with K. An increase in modulus of rupture with increasing exchangeable Na was also shown by Gerard (1965).The effects of a wider range of cations on the tensile strength of oriented montmorillonite films were investigated by Dowdy & Larson (1971). In general, strength decreased in the order:Fe > K 2 Na > A1 > Ca. It was thought that the mechanism of bonding with Fe was different from that with the other ions.All of the above results are for relatively dry soils or clays (drier than around -10 MPa total water potential). At the other epd of the scale, at saturation, or in the presence of free water, clay particles may repel one another and disperse. When dispersion occurs, the strength of the material

Desert Soils

“…For example, Wang, Zhou and Zheng (2006) measured the tensile strength of a crust from Shapotou in China, and reported rupture strengths of 30-42.8 kPa. These values are comparable to those reported for natural aggregates from soils of southern Australia (Dexter and Chan, 1991), suggesting that the additional strength arising from the microorganisms in the crusted sand can be similar to that arising from the cohesion generated in soils by silts and clays. The elastic behaviour that some BSCs exhibit allows surface deformation to reduce the impact stress caused by saltating grains, but nevertheless sustained impacts progressively cause a loss of structural integrity (McKenna Neuman, Maxwell and Boulton, 1996;McKenna Neuman and Maxwell, 2002).…”

The changes in the microstructure of ion-exchanged clays

“…clay fraction affect to their engineering parameters [1]. Also, connections between the type of exchangeable cation and soil mechanical properties [2,3] and changes in microstructural parameters [4,5] are confirmed. Despite this, looking for the connections between the microstructure features with the soil properties is very essential.…”

Soil friability: A review of the concept, assessment and effects of soil properties and management

“…They found that tensile strength increased more strongly with clay content for smectite dominated than for illite and especially kaolinite dominated soils. In other studies, Barzegar et al (1994) and Dexter and Chan (1991) showed that tensile strength also depends on the composition of exchangeable cations. The strength of remolded dry soils saturated with different exchangeable cations decreased in the order Na N Na-Mg N Na-Ca N Na-Ca N Mg N CaMg N Ca (Barzegar et al, 1994).…”