Effect of freeze–thaw cycling on the mechanical properties of lime-stabilized expansive clays

Outline

• Abstract
• Keywords
• 1. Introduction and Scientific Background
• 2. Materials
• 2.1. Material Properties
• 2.2. Sample Preparation
• 3. Experimental Procedures
• 3.1. Freezing and Thawing Test Procedure
• 3.2. Unconfined Compression and Direct Shear Tests
• 3.3. Swelling Test
• 3.4. Mercury Intrusion Porosimetry Test
• 4. Results and Discussion
• 4.1. the Effect of Lime Content on the Swelling Pressure
• 4.2. the Effect of F–t Cycles on the Unconfined Compressive Strength
• 4.3. the Effect of F–t Cycles on the Volume Change
• 4.4. the Effect of F–t Cycles on the Shear Strength
• 4.5. the Effect of F–t Cycles on the Soil Fabric: Pore Size Distribution
• 5. Conclusions
• Acknowledgments
• References

Conclusions

In this research, a parametric study was carried out to investigate the effect of the curing time, the percent of lime added and F–T action on frost resistance and swelling pressure. The conclusions, which can be drawn from the laboratory tests, are:

1. Lime addition has a strong influence on the plastic and swelling properties of these two soils. The swelling pressure was eliminated in the case of bentonite with the addition of 5% quicklime, and of 3% quicklime in the case of kaolinite.
2. Lime addition improves the UCS of clayey soils, whether subjected to F–T cycles or not, as compared to the untreated soils. However, for the lime-stabilized bentonite cured for 28 days, the UCS decreased by 40% after 10 F–T cycles but was still higher than the UCS of the untreated material. In the case of lime-stabilized kaolinite, no significant losses in strength were observed, only slight decreases in samples cured for short periods—yet, compared to bentonite, the kaolinite does not gain as much strength. These behaviors can be explained by the kinetics of the lime treatment reactions which depends on the different mineralogical and crystallographic nature of the two clayey soils tested. The fast cation exchange reaction immediately consumes added lime in the smectite soil and not the kaolinite. The effectiveness of lime treatment in the long-term is determined by the reactivity of the pozzolanic reaction and the different amounts of lime that are not consumed by the different types of clay mineral in the short-term. Whatever the mineralogical composition of the clay soil, treatment with lime in the presence of water induces the formation of Ca-hydrates composed of different proportions of Ca, Si, and Al, resulting from the destruction of clay minerals (Al-Mukhtar et al., 2014). The improvement measured in the mechanical properties of the two tested soils depends on these reactions.
3. The bentonite soil presents higher volume changes than kaolinite soil, even after lime-stabilization. In the case of untreated and 28 days cured materials, the volume increases with the number of F–T cycles. Changes in the pore size distribution towards higher pore diameters play an important role in these volume changes when samples are subjected to F–T cycles. When the stabilized clay is cured for a longer period of time (300 days in this case), the volume changes are smaller.
4. F–T cycles affect the shear strength parameters of the soil samples. After 5 F–T cycles, a loss of approximately 37% and 75% in cohesion occurred in the stabilized kaolinite and bentonite respectively, for a curing time of 300 days. Even if the strength properties of clays are improved through stabilization, when subjected to F–T action these properties are strongly affected. The effects of F–T cycles on the direct shear strength are mainly reflected in the cohesion, while the frictional component of the shear strength did not vary significantly.

The laboratory experiments carried out assume that a curing of 28 days at 20 °C and UCS strength is considered as the basis for the assessment before F–T tests. The experimental conditions applied can be considered as the worst conditions faced in-situ for these soils: partial achievement of the reaction between lime and clay after 28 days and accelerated F–T cycles. Therefore, the transfer of the test results to construction works in the field involves the following recommendations concerning the lime treatment protocol before the freezing period. It is recommended to treat the soils with lime outside the freezing period and to apply watering in the field after lime stabilization in order to prevent strength losses and to enhance lime–clay interactions. Finally, lime treatment improves the behavior of clayey soils against freezing– thawing, but in construction projects it is important to consider a longer curing time for kaolinite soils than for bentonite soils before the arrival of these environmental conditions (F–T) on the treated soils.