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In nature, expansive soil is generally in a state of swelling during rainfall, and its soil–water characteristic curve (SWCC) exhibits a three-dimensional space shape under the action of an increasing void ratio. In this study, expansive soil from Hefei, China, was used. SWCC and hygroscopic tests were performed. A three-dimensional hygroscopic swelling mathematical model of the dry density–void ratio–saturation degree was constructed based on the volume change behavior of expansive soils resulting from the hygroscopic test. In addition, depending on the variation in suction with the saturation degree of expansive soil under different values of dry densities, an SWCC mathematical model considering the swelling effect was constructed. The three-dimensional surface model of the hygroscopic swelling of the expansive soil was steeper in the lower saturation degree area. As the saturation degree increased, the three-dimensional surface gradually became flat. The SWCC model of the expansive soil under the action of swelling in a three-dimensional space is an S-shaped surface. The model exhibited dynamic changes in the suction and saturation degrees as the dry density of the soil decreased continuously. The model comprehensively revealed the hydraulic and swelling properties of expansive soil. [ABSTRACT FROM AUTHOR]

Freeze–thaw cycles can deteriorate the expansive soil performance. Phase change materials (PCMs) absorb and store heavy latent heat from the environment, which can be utilized to mitigate freeze–thaw deteriorations in expansive soil. In this study, an environmentally friendly paraffin-based PCM was selected, which included two different forms: liquid (pPCM) and microcapsules (mPCM). The volume change, mechanical properties, thermal properties, and microstructure of the improved soil were studied. These results showed that PCMs can release considerable thermal energy during phase transitions, which improves the thermal stability of the soil. The pPCM enhanced the plasticity of soil and prevented brittle failure; mPCM reduced the damage to the soil microstructure caused by freeze–thaw cycles. Macroscopically, it weakened the swell–shrink behavior and limited the degradation of the resilient modulus, unconfined compressive strength, and failure strain of expansive soil. The optimal PCM content was determined based on the mechanical and thermal properties of the improved soil. This study provides a new idea for the engineering treatment of expansive soil in cold regions. [ABSTRACT FROM AUTHOR]

The present work aims to evaluate expansive soil stabilization using different additives. Twenty soil mixtures consisting of 10%, 20%, 25%, and 30% zeolitic tuff (ZT) with 2%, 4%, and 6% lime in various combinations were used to stabilize the soil for pavement subbase application. A comprehensive test program, including Atterberg's limits, compaction, unconfined compression strength (UCS), California Bearing Ratio (CBR), resilient modulus, and permanent deformation, was performed on natural and stabilized soil specimens. Scanning electron microscopy (SEM) supplied with energy-dispersive X-ray spectroscopy (SEM/EDX) was used to confirm the study findings. Adding ZT increases the availability of the silica and alumina needed for the pozzolanic reaction; test results revealed that adding ZT increases the maximum dry unit weight, CBR, UCS, resilient modulus, decreases plasticity, and permanent deformation. As such, ZT can improve the physical and mechanical properties of lime-stabilized soil as a pavement's subgrade and subbase layers. [ABSTRACT FROM AUTHOR]

In this study, the suction induced by stress release was investigated by measuring the suction of instantaneously unloaded MX80 bentonite specimens after wetting in an oedometer either under different vertical stresses or under a constant-volume condition. Swell-consolidation tests were also performed, and the results were compared with those of specimens wetted under different vertical stresses or a constant-volume condition. The microstructure of the unloaded specimens was investigated using mercury intrusion porosimetry (MIP). The water content and void ratio as well as the degree of saturation of specimens were also determined. Results showed that in the case of limited water exchange between macropores and micropores during stress release, the change in suction was equal to that of stress (−ds/dσv=1). By contrast, with water exchange between macropores and micropores, soil swelling occurred, and the variation of suction was higher than that of stress (−ds/dσv>1). A method was proposed allowing such water exchange between macropores and micropores to be determined (−ds/dσv−modified=1 , where σv−modified is the modified stress considering water exchange), evidencing the macropore drying due to stress release. Comparison of results from MX80 bentonite and other clayey soils showed that when the nonclay (grains such as quartz, calcite, and so on) fraction (NCF) was lower than 35% by mass, NCF had an insignificant effect on −ds/dσv−modified (equal to 1). By contrast, in the case of higher NCF (>35%), −ds/dσv−modified became lower than 1, and the higher the NCF, the lower the −ds/dσv−modified value. These findings provide useful information for better understanding the mechanism involved at the microstructure level in the stress/suction coupling during stress release in clayey soils. [ABSTRACT FROM AUTHOR]

This paper describes key research on expansive soils and the methods employed to characterize them; fallacies in the current characterization of expansive soils are also explained. Novel swell characterization models that account for hydro, chemical, and mechanical behaviors of soils are introduced and used to demonstrate in case studies to improve expansive soil stabilization practices. The first two case studies present the results of expansive soils stabilized by incorporating clay mineralogy and soluble soil sulfate measurements. An innovative design method for successful stabilization of expansive soil is introduced in the first case study, which incorporated both basic clay mineralogy and unsaturated soil behaviors as well as performance-based durability studies. Sulfate soil stabilization works on medium-to-high sulfate soils, including rigorous laboratory and field validation studies, are presented in the second case study. The third case study, which involves a steep earthen embankment built with expansive clayey soils and experiencing recurring surficial slope failures and maintenance issues, is also discussed. Forensic studies explaining the causes of slope failures and their mitigation methods are also included. All case studies reveal the need for detailed data about soil chemistry, including clay mineralogy and sulfate studies, to improve the current field stabilization and infrastructure design on expansive soils. The last section summarizes recent innovations for better health monitoring and management of civil infrastructure built on expansive soils using unmanned aerial vehicle platforms and visualization tools, which will be valuable for validating the application of new materials, designs, and construction processes. [ABSTRACT FROM AUTHOR]

A laboratory index defined as gravimetric water content at ambient relative humidity of 75% (w75) is proposed for qualitatively classifying the swelling potential of clays and clayey soils (e.g., low, moderate, high). The methodology is calibrated by comparison with existing plasticity-based and suction-based classification methodologies for a series of natural clays from Missouri and Colorado and clay mixtures prepared to represent a wide range of swelling potential. Procedures are described for obtaining w75 by placing samples in the headspace of an environmental chamber maintained under controlled humidity using a saturated NaCl solution. The proposed methodology has potential advantages over existing expansive soil classification methods because a large number of samples may be tested concurrently, no specialized testing equipment is required, and testing procedures may be readily automated. Measurements may be obtained in as little as 3–5 days with coefficients of variation ranging from 5 to 25%. [ABSTRACT FROM AUTHOR]