The Effects of Salt Solutions on the Properties of Bentonite-Sand Mixtures

Abstract

The swelling behaviour and hydraulic conductivity of Na-bentonite powder and bentonite-sand mixtures (10 and 20% of bentonite by dry weight) have been measured with distilled water and various salt solutions (0.01, 0.1 and 1 mol/l concentrations), It was found that in dilute solutions, the bentonite in mixtures subjected to small confining stresses swells sufficiently to separate the sand particles and reach a clay void ratio similar to that achieved by bentonite alone. At high stresses, or in strong solutions, the bentonite in a mixture has insufficient swelling capacity to force the sand particles apart and swelling is limited by the sand pore volume. The hydraulic conductivity of a mixture depends on the bentonite void ratio, and the porosity and tortuosity of the sand matrix. A design model is proposed to predict the engineering properties of a mixture over a range of confining stresses from the properties of its constituents and the permeant.

The factors controlling the engineering properties of bentonite-enhanced sand

“…Above about 200kPa the void ratio is less sensitive to changes in stress, but still appears to decrease linearly with the logarithm of vertical stress. Studds et al (1998) also report similar tests where the same air-dried bentonite is allowed to swell with various chloride salt solutions. In such tests there is always concern that the bentonite can act as a semi-permeable membrane once it is partially hydrated (drawing the water in more readily than the salt).…”

“…Studds et al (1998) report the behaviour of air-dried bentonite powder (SPV200 Wyoming bentonite supplied by Volclay) swelling one-dimensionally against a surcharge (see figure 1a). At vertical effective stresses below about 200kPa the bentonite void ratio is very sensitive to the applied stress, decreasing approximately linearly with the logarithm of vertical stress.…”

“…Subsequent consolidation results in realignment of the bentonite unit particles, but not the formation of particle clusters. Studds et al (1998) measured the "hydraulic" conductivity to various chloride salt solutions of bentonite specimens that were initially prepared in an air-dry state, then allowed to swell in the selected solution against a surcharge stress ( Figure 3). For each strength of solution, there is a linear trend between the logarithm of hydraulic conductivity and the logarithm of void ratio (different chloride salts at the same concentration produced broadly similar trends; see Studds, 1997, for more details).…”

The strength of unsaturated bentonite-enhanced sand

“…Table 3 presents the results of the drained triaxial tests on Sherburn sand. As the relative density [I D (e max À e)= (e max À e min )] increased from about 0´1 to 0´55, ö (the maximum angle of shearing resistance) increased from just under Studds et al (1998). { Determined by methods described in Head (1980).…”

“…SPV 200 bentonite originates from Lovell, Wyoming, USA, and is a well-ordered sodium montmorillonite with minor quartz and cristobolite impurities (Studds et al, 1998). Sherburn sand is predominantly a quartz sand (suppliers speci®-cation).…”