Mechanical Behavior and Formation Mechanisms of Hyphae-Based Lightweight Geomaterials

Abstract

As the main by-product of wheat, wheat bran is mostly used as animal feed or incinerated, which adversely affects the environment. Mycelium-based biocomposites, which are mixtures of agricultural by-products (e.g., wheat bran) and fungi, minimize wheat bran wastage and have attracted attention as building materials owing to their high strength, low density, and environmental friendliness. Nonetheless, the practical application of mycelium-based materials in geotechnical engineering remains rudimentary. Building on mycelium-based biocomposites, this study develops lightweight sand-mycelium soil (LSMS), consisting of soil, substrate materials (wheat bran), and hyphae (P. ostreatus), as an eco-friendly lightweight backfill material in geotechnical engineering. The impact of substrate material content, effective confining pressures, and hyphae on the mechanical properties of P. ostreatus-based LSMS was studied. In addition, a detailed investigation of the formation mechanisms of P. ostreatus-based LSMS was conducted. The results indicated that increased substrate material content reduces the strength but increases the ductility of P. ostreatus-based LSMS. The presence of hyphae alters the structure of P. ostreatus-based LSMS, reducing volumetric contraction and improving strength, although this depends on confining pressure. The formation of P. ostreatus-based LSMS involves biophysical mechanisms (hyphal network bundling and filling of void spaces) and biochemical mechanisms (self-bonding, secretion bonding, and aragonite deposition). Biochemical mechanisms may provide a more prolonged effect on the stability of P. ostreatus-based LSMS compared to biophysical mechanisms. P. ostreatus LSMS shows great potential as an eco-friendly alternative to conventional lightweight backfill materials in geotechnical engineering. Expanded polystyrene/tire shred–soil mixtures have been widely used as lightweight backfill materials in geotechnical engineering. However, these mixtures may pose adverse implications for ecosystems. Mycelium-based biocomposites are composed of fungi and agricultural by-products. They exhibit environmentally friendly properties, high compressive strength (0.17–1.1 MPa), and low density (0.16–0.28 g/cm3), providing a new approach for the preparation of sustainable lightweight backfill materials. LSMS, as proposed in this paper, is a lightweight backfill material consisting of soil, wheat bran, and fungi, integrating conventional lightweight backfill materials with mycelium-based biocomposites. LSMS exhibited cohesion values ranging from 12.1 to 41.5 kPa, internal friction angles of 15.8° to 34.0°, and compressive strengths of 136 to 558 kPa, which are comparable to those of EPS–soil mixtures (cohesion: 1.26–22.4 kPa, internal friction angle: 11°–34.3°, compressive strength: 31.3–530 kPa). LSMS is potentially suitable for use as a shallow backfill material in geotechnical engineering.