Underground green corridors are getting popular worldwide intending to integrate greenery into built environments. Green corridors provide greenery benefits and aesthetic pleasing view. Proper irrigation is necessary to maintain live plants in green walls. Automated irrigation with adequate amounts and proper drainage is vital to avoid overflows and drowning of plants. Due to improper irrigation plants in green walls can die quickly, creating a potential fire risk in underground green corridors. This paper aims at investigating the ignition, fire propagation and smoke propagation in underground green corridors. Numerical simulations with computational fluid dynamics (CFD) and physical scale modeling experiments were performed. The simulation has been completed based on heat release rate (HRR, kW/m²) data obtained from the cone calorimeter experiments for three plant species, namely; Hedera helix, Peperomia obtusifolia and Aglaonema commutatum. CFD simulations were conducted for an underground passage with a green wall. Results showed that a fire does not occur in underground corridors with fresh and live plants. However, fire risk increases gradually with drying out of plants. For dry plants, the surface temperature may reach above 850 °C. The fire spreads rapidly in about 20 s. An experimental study was carried out on a reduced-scale underground green corridor to investigate the fire and smoke propagation. It is observed that in the vertical direction fire spreads up to the ceiling rapidly. However, fire spreads slowly in the horizontal direction along the green wall. The experiments show that when the green wall is ignited, it may tremendously affect the evacuation due to smoke spread along the corridor and fire propagation in the vertical direction. It is recommended to decrease the fire risk in underground corridors with properly maintained green walls, especially in evacuation paths. This work is an initial study towards the analysis of fire and smoke propagation in underground green corridors.

Green roofs (GRs) and green walls (GWs) are good strategies for urban greenery. This study explores the cooling load benefits of GRs and GWs simultaneously for comparison. EnergyPlus simulation programme is used for estimating annual cooling load reduction for different buildings and scenarios in Hong Kong. In simulating GR, a built-in thermal model is used. For GWs, a self-developed thermal model is used, which has been developed and validated in our previous study. The simulation covers a single-storey building and two multi-storey buildings, each with four different coverage areas for GR and GWs. The GWs are assumed to be on building facade facing the west, east, north, and south. Results reveal that both GRs and GWs are capable of protecting building envelop from reaching higher temperatures and of reducing cooling load. In a single-storey building with an equal area of GR and GW, GR is more effective in energy saving. However, in a multi-storey building GR can provide energy benefits only to the topmost floor. An equal area of GW can provide benefits to multiple floors, which may result in higher benefit. Furthermore, the available area for GWs is larger. When considering the effect of orientation of GW, the west-facing GW contributes to the highest annual energy saving. It should be noted that the effect of orientation may differ with location and climatic conditions, and also with the shading effect of surrounding buildings. Therefore, installation of GRs or GWs should be considered case by case, taking into consideration the scale and surroundings of the building, the climatic condition, and area of greenery coverage.