Abstract

Expansive soil is highly susceptible to water-softening and desiccation cracking under alternating wet–dry conditions, often resulting in severe geotechnical and geological hazards. To mitigate these undesirable engineering properties, a composite improvement approach utilizing biochar and sisal fiber was employed. The shear strength and cracking characteristics of the improved expansive soil were systematically investigated through direct shear tests and desiccation cracking tests on specimens prepared with varying biochar contents, sisal fiber contents, and fiber lengths. Scanning electron microscopy (SEM) was further conducted to elucidate the underlying microstructural mechanisms. The results indicated that the individual addition of biochar or sisal fiber enhanced the shear strength of the expansive soil. Increasing the biochar content from 4% to 10% yielded an 8–19% strength gain, whereas raising the sisal fiber content from 1.5‰ to 6‰ led to a more substantial 36–110% improvement. Conversely, extending the fiber length from 10 mm to 30 mm diminished the shear strength by 11–21%. Higher biochar contents progressively increased the internal friction angle (from 14.84° to 18.52°) but were accompanied by a decline in cohesion (from 9.0 kPa to 4.0 kPa). In contrast, increasing the sisal fiber content markedly enhanced cohesion (from 4.7 kPa to 50.3 kPa) while marginally reducing the internal friction angle (from 15.2° to 12.8°). In terms of crack suppression, a 10% biochar content achieved an 86.8% reduction in crack ratio, while 6‰ sisal fiber yielded a 72.4% reduction. Range analysis revealed that crack length and crack ratio were most sensitive to biochar content, whereas crack width was predominantly governed by fiber content. Notably, surface cracking was completely eliminated in the composite specimen prepared with 10% biochar, 4.5‰ sisal fiber, and a fiber length of 20 mm. Microstructural analysis revealed that biochar particles featured rough surfaces and abundant internal pores, while the sisal fibers formed a randomly interwoven network within the soil matrix. The synergistic interplay between the rigid biochar skeleton and the flexible fiber network contributed to the substantial enhancement in both mechanical strength and crack resistance.

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