Clayey soils are challenging in geotechnical engineering because of their high compressibility, moisture sensitivity, and low shear strength. This study investigates lightweight and sustainable pumice, locally termed Ponza, for stabilizing clayey soil collected from Al-Amiriya, Baghdad, Iraq. Pumice was incorporated at replacement ratios of 10%, 20%, 30%, 40%, and 50% by dry mass. For each mixture, the dry constituents were blended, mixed with water at the corresponding optimum moisture content, and compacted before testing; CBR specimens were then soaked for 72 h. The experimental program included modified Proctor compaction, soaked California Bearing Ratio tests, and large-scale direct shear tests under normal stresses of 25, 50, and 75 kPa for unreinforced and Geostrip-reinforced specimens. A 200-mm-wide shear box was selected to accommodate the 4.75–9.5 mm pumice particles and minimize particle-size and boundary effects. Increasing pumice content reduced the maximum dry density and produced an overall increase in optimum moisture content because of the lightweight and porous nature of pumice. CBR generally improved at higher pumice contents owing to greater particle interlocking and the development of a frictional load-bearing skeleton. The shear response gradually changed from cohesion to friction-controlled behavior. For unreinforced samples, the friction angle increased from 16.3° for natural clay to 31.0° for 50% pumice, and cohesion decreased from 19.69 kPa to 10.50 kPa. Average interface interaction coefficients of 0.94–0.99 are shown to be efficient at stress transfer and compatible with the clay-pumice mixtures and Geostrip reinforcement.
The demand for sustainable soil stabilizers has driven interest in soil–biochar mixtures for geotechnical applications such as landfill liners and subgrade systems. However, the role of biochar particle size in governing compressibility, shear strength, and heavy metal immobilization remains unclear. This study examines the influence of fine (FB) and coarse biochar (CB) derived from bamboo on the compressibility, shear strength, and Pb2+ immobilization in clayey soil at dosages of 1–10%. One-dimensional consolidation tests reveal that FB at 10% increased the void ratio by 31.7%, whereas CB reduced it by 22.7% relative to untreated soil. Both biochar types decreased compression and swell indices at higher dosages, with FB-treated soils exhibiting greater stiffness under elevated stress. Void index analysis indicates the formation of a stable and stress-resistant soil–biochar matrix. Microstructural observations show denser particle packing in FB mixtures, while CB mixtures exhibit particle fragmentation at higher dosages. Toxicity characteristic leaching procedure (TCLP) tests demonstrate that 5% FB reduces Pb2+ concentrations below regulatory limits within 14 days, whereas CB requires higher dosages or longer curing periods. These findings highlight the critical role of biochar particle size in optimizing soil performance and contaminant immobilization for barrier applications.
This study examines the small-strain shear modulus ( G max ) and water retention behavior of unsaturated sand–silt mixtures with a focus on the effects of fines content (FC), mean net stress ( p n ), and hydraulic hysteresis. A comprehensive experimental program was conducted, incorporating saturated and unsaturated bender element tests, soil water retention curve measurements, and scanning electron microscope imaging. The tests were performed on compacted specimens of Firuzkooh No. 161 silica sand mixed with varying silt contents (0-100%) as fines under p n of 50, 100, and 200 kPa along both drying and wetting paths. Two unsaturated triaxial systems equipped with bender elements and a hanging water column controlled matric suction ( ψ ) via axis translation and water head control techniques. The results revealed significant changes in soil structure and hydromechanical behavior with increasing FC. In contrast to other soil mixtures, clean sand exhibited distinct hydraulic and mechanical behaviors, with G max demonstrating a nonmonotonic variation with ψ . As FC increased, a pronounced reduction in G max emerged at FC of 20%, at which point the soil structure began to transform from a fines-in-sand to a sand-in-fines structure. Notably, the impact of hydraulic hysteresis became increasingly prominent with FC: for mixtures with 10%–100% FC, G max during wetting was higher than during drying at the same ψ levels, whereas an opposite trend was observed for clean sand. Additionally, for sand specimens, the suction stress concept effectively described the variation of G max with ψ , while for sand–silt mixtures (10%–100% FC), an extended framework incorporating the equivalent void ratio into a double hardening mechanism concept was adopted. The consistency between experimental observations and theoretical interpretations suggested that these concepts could provide a robust basis for predicting G max behavior in unsaturated sand–silt mixtures.
The replacement of conventional aggregates in concrete with alternative and waste-based materials has become an important research area for sustainable construction. This study experimentally investigated the effects of natural perlite aggregate replacement and fiber reinforcement on the mechanical and microstructural properties of concrete. Natural perlite aggregate was used to replace conventional crushed stone aggregate at replacement levels of 0%, 30%, 70%, and 100%, while hemp shives, recycled plastic waste fibers, and polypropylene (PP) fibers were incorporated at volume fractions of 0.0%, 0.5%, 1.0%, and 1.5%. After 28 days of curing, compressive strength, modulus of elasticity, flexural strength, splitting tensile strength, ultrasonic pulse velocity (UPV), compressive stress–strain behavior, and scanning electron microscopy (SEM) analyses were performed. The results showed that 30% perlite replacement increased the compressive strength and modulus of elasticity by approximately 6.67% and 4.08%, respectively, compared with the reference mixture. In contrast, replacement levels of 70% and 100% reduced the mechanical performance due to the porous structure of perlite and the weaker interfacial transition zone. Among the fiber-reinforced mixtures, the optimum fiber content was found to be 1.0%, while PP fibers provided the best overall performance in terms of crack-bridging ability and energy absorption. Overall, the combination of 30% natural perlite aggregate replacement and 1.0% PP fiber was identified as an effective approach for producing low-carbon concrete with balanced mechanical properties.
Cement-stabilized soil is widely used for ground improvement but suffers from brittleness and low tensile strength. This study investigates combined modification of cement-stabilized soil with waste brick powder (WBP, 5%) and glass fiber (GF, 0–2.0%) to enhance mechanical performance and microstructural integrity. Unconfined compressive strength (UCS), splitting tensile strength (STS), and qualitative scanning electron microscopy were evaluated at 3, 14, and 28 days. Results reveal stress-state-dependent reinforcement: UCS peaked at 2.0% GF with gains of 18.1%, 7.3%, and 9.1%, while STS maximized at 1.5% GF (43% increase), declining 14.3% at 2.0% due to fiber agglomeration. Post-peak ductility improved markedly, with the residual strength ratio increasing from 12.4% to 43.2% and the ductility index from 1.18 to 1.85. Microstructural analysis suggests that uniform fiber dispersion enables crack bridging and interfacial load transfer, whereas agglomeration creates localized weak zones. Preliminary assessment indicates that WBP substitution may avoid approximately 85 kg CO2eq per cubic meter by valorizing construction waste. These preliminary laboratory findings suggest potential stress-state-dependent dosage trends for sustainable cement-stabilized soil composites, pending further validation.
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