This study investigates the potential of using scoria, a lightweight volcanic aggregate, as a sustainable replacement for natural sand in the production of structural lightweight concrete, addressing the challenges posed by sand scarcity in Ethiopia. The concrete mixtures were proportioned to achieve a minimum compressive strength of 25 MPa, with natural sand partially or fully replaced by scoria at volumetric substitution levels of 35%, 50%, 70%, and 100%. Key concrete properties were assessed at curing ages of 7, 28, and 56 days. The experimental program included slump, compressive strength, split tensile strength, flexural strength, water penetration, and ultrasonic pulse velocity (UPV) tests. The 35% replacement mix achieved the highest compressive strength at 43.6 MPa, surpassing the control sample (39.6 MPa), while the 50% replacement yielded optimal flexural (4.88 MPa) and tensile strengths (3.36 MPa). Workability decreased with increasing scoria content, and densities dropped from 2405 kg/m³ (control) to 2164 kg/m³ (100% scoria). Scoria’s microstructural properties, observed via SEM and XRD, confirmed its glassy, porous structure, contributing to enhanced water resistance, with penetration depths decreasing to 20 mm at 35% scoria replacement. These findings suggest that scoria, at optimal levels, can improve concrete’s mechanical and durability properties, offering a viable, eco-friendly sand alternative.

Growing emphasis on sustainable urban development has intensified the use of underground urban space for transport and protection. During war or terrorism, underground tunnels can function as protective structures, so accurately assessing their response to surface blasts is essential. This study presents a three-dimensional numerical investigation of a circular metro tunnel embedded in sandy clay and subjected to surface detonation. The soil is modelled with Mohr–Coulomb plasticity, while the tunnel lining and reinforcement adopt concrete damaged plasticity and Johnson–Cook models. Blast pressures are applied via the CONWEP formulation (TM 5-855-1), and the simulation captures coupled interactions between soil, lining, reinforcement and blast wave. The framework is validated against analytical peak overpressure predictions, empirical crater dimensions and experiments on blast-loaded reinforced concrete slabs, showing acceptable deviations. Crater-size formulas are recalibrated for sandy clay using simulations with 25–1000 kg TNT, improving agreement within this range. An energy-based framework relates TNT weight to kinetic, plastic dissipation and strain energy in soil, providing meaningful insight into blast energy transmission, absorption and dissipation. The effect of charge magnitude on soil stress, liner deformation and tensile damage is assessed for overburden depths of 15, 12 and 9 m; shallow tunnels suffer greater deformation and damage under increasing blast intensity. Performance criteria identify safe and unsafe burial depths for different charges. Regression-based energy and deformation relationships enable quantitative comparisons of tunnel responses within the defined scope and assumptions.

The demand for sustainable development in cities has enhanced the utilization of underground urban space (UUS) by constructing underground structures. During war, this UUS may be utilized as protective structures (tunnels, bunkers, and strategic storage). Hence, their behavior in extreme events such as blasts, fires, earthquakes, etc., must be evaluated properly. This paper presents a numerical study on an underground metro tunnel constructed in sandy clay, subjected to a surface blast. The Mohr-Coulomb plasticity model was used to simulate soil behavior, while the concrete damaged plasticity (CDP) model captured the nonlinear response of concrete. Tunnel liner reinforcement was modeled using the Johnson-Cook plasticity model with a hardening law and rate dependence. Surface blast analysis was conducted using the CONWEP (Conventional Weapons Effects) tool, based on the US Department of Army technical manual (TM5-855-1) and Trinitrotoluene (TNT) explosive parameters. The simulations were carried out for 0.1 seconds. The validation of the model and process was performed with peak pressure calculations from the TM5-885-1 manual and empirical relationships established in published literature. However, a reassessment of these formulas has been done by regression analysis of simulation data. Further, relationships were established between energies (kinetic, plastic dissipation, and strain) generated in the soil and different TNT weights by regression analysis. Additionally, the stresses in the soil, deformation, and tensile damage in the tunnel liner due to different TNT weights have been examined for overburden depths of 15m, 12m, and 9m.