In this paper, an extensive series of direct shear box tests (99 tests) were conducted to explore and compare the effects of raw and treated natural fibers, specifically Doum fibers on the mechanical behavior of three categories of sandy soils with distinct mean particle sizes (D50 = 0.63, 1, and 2 mm). Specimens from every soil category, containing 0 to 0.8% raw Doum fibers and 0 to 1% treated Doum fibers in incremental step of 0.2%, were reconstituted at an initial relative density of (Dr = 87 +/- 3%) and subjected to three different initial normal stresses (100, 200, and 400 kPa). The obtained results indicate that incorporating raw or treated Doum fibers improve the mechanical and rheological properties (internal friction angle, ductility, and maximum dilatancy angle) of the tested mixtures up to specific thresholds Doum fiber content (FD = 0.6% and FTD = 0.8% for raw and treated Doum fibers respectively). Beyond these limiting values, the mechanical and rheological properties decreased with further increases in Doum fiber content. Additionally, specimens reinforced with treated Doum fibers exhibit higher shear strength than that of the raw Doum fibers for all tested parameters. Based on the experimental results, it has been found to suggest a reliable correlation between Particle Size Distribution (PSD) characteristics and mechanical properties for all reconstituted specimens. The recorded soil trend is especially pronounced for the mean grain size (D50) ranging between 1 and 2 mm, where a notable increase in shear resistance is noticed. The analysis of the obtained outcome suggests the introduction of new enhancement factors (EF tau peak and EF phi degrees) as useful parameters for predicting the mechanical behavior of sand-fibers mixtures. Furthermore, new relationships have been developed to forecast changes in mechanical properties (peak shear strength, internal friction angle, and maximum dilatancy angle) of the tested mixtures under the impact of the selected parameters (FD/TD, D50, and sigma n).

Traditional soil stabilization methods, including cement and chemical grouting, are energy-intensive and environmentally harmful. Microbial-induced carbonate precipitation (MICP) technology offers a sustainable alternative by utilizing microorganisms to precipitate calcium carbonate, binding soil particles to improve mechanical properties. However, the application of MICP technology in soil stabilization still faces certain challenges. First, the mineralization efficiency of microorganisms needs to be improved to optimize the uniformity and stability of carbonate precipitation, thereby enhancing the effectiveness of soil stabilization. Second, MICP-treated soil generally exhibits high fracture brittleness, which may limit its practical engineering applications. Therefore, improving microbial mineralization efficiency and enhancing the ductility and overall integrity of stabilized soil remain key issues that need to be addressed for the broader application of MICP technology. This study addresses these challenges by optimizing microbial culture conditions and incorporating polyethylene fiber reinforcement. The experiments utilized sandy soil and polyethylene fibers, with Bacillus pasteurii as the microbial strain. The overall experimental process included microbial cultivation, specimen solidification, and performance testing. Optimization experiments for microbial culture conditions indicated that the optimal urea concentration was 0.5 mol/L and the optimal pH was 9, significantly enhancing microbial growth and urease activity, thereby improving calcium carbonate production efficiency. Specimens with different fiber contents (0% to 1%) were prepared using a stepwise intermittent grouting technique to form cylindrical samples. Performance test results indicated that at a fiber content of 0.6%, the unconfined compressive strength (UCS) increased by 80%, while at a fiber content of 0.4%, the permeability coefficient reached its minimum value (5.83 x 10-5 cm/s). Furthermore, microscopic analyses, including X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), revealed the synergistic effect between calcite precipitation and fiber reinforcement. The combined use of MICP and fiber reinforcement presents an eco-friendly and efficient strategy for soil stabilization, with significant potential for geotechnical engineering applications.

The issue of water and wind erosion of soil remains critically important. Polymeric materials offer a promising solution to this problem. In this study, we prepared and applied an interpolyelectrolyte complex (IPEC) composed of the biopolymers chitosan and sodium carboxymethyl cellulose (Na-CMC) for the structuring of forest sandy soils and the enhancement of the pre-sowing treatment of Scots pine (Pinus sylvestris L.) seeds. A nonstoichiometric IPEC [Chitosan]:[Na-CMC] = [3:7] was synthesized, and its composition was determined using gravimetry, turbidimetry, and rheoviscosimetry methods. Soil surface treatment with IPEC involved the sequential application of a chitosan polycation (0.006% w/w) and Na-CMC polyanion (0.02% w/w) relative to the air-dry soil weight. The prepared IPEC increased soil moisture by 77%, extended water retention time by sixfold, doubled the content of agronomically valuable soil fractions > 0.25 mm, enhanced soil resistance to water erosion by 64% and wind erosion by 81%, and improved the mechanical strength of the soil-polymer crust by 17.5 times. Additionally, IPEC application resulted in slight increases in the content of humus, mobile potassium, mobile phosphorus, ammonium nitrogen, and mineral salts in the soil while maintaining soil solution pH stability and significantly increasing nitrate nitrogen levels. The novel application technologies of biopolymers and IPEC led to a 16-25% improvement in Scots pine seed germination and seedling growth metrics.