Carbon monoxide poisoning associated with blasting operations close to underground enclosed spaces. Part 1. CO production and migration mechanisms

Abstract

Explosives used for blasting operations in civil engineering works can generate large volumes of carbon monoxide (CO). The production of 10–24 L of CO per kilogram of explosives blasted is theoretically possible. CO can migrate a considerable distance in the fractured rock of the blasted areas and then infiltrate closed spaces (sewage systems, manholes, basements of houses). In the Province of Quebec, in the last 10 years, seven people were poisoned by CO in their houses to the extent that they had to be treated in a hyperbaric chamber. Underground conduits broken by blasting, filling around underground conduits in road or house trenches, or fractured rock created by blasts between houses or between a house and a road are the different CO pathways identified in the Quebec incidents. Field tests done by our group show that (i) the structural geology of the rock formation (schistosity, family of joints and fractures) controls the direction and extent of gas migration in fractures generated by blasts; (ii) the confinement of the rock can affect the quantity of gas migrating in the fractured rock; (iii) significant concentrations of CO may persist in the fractured rock 7 days after a blast; (iv) advection is the initial mechanism of CO migration immediately after a blast, and the distance of migration varied from 8 m in the fractured rock to 20 m in the fills of a road trench; and (v) further CO migration by diffusion up to 15 m in the induced fractures and 30 m in fills may occur in the 3 days following a blast.Key words: carbon monoxide, blasting, poisoning, enclosed spaces, gas migration, house.

Carbon monoxide poisoning associated with blasting operations close to underground enclosed spaces. Part 2. Special working procedures to minimize CO migration

“…A second family of joints is also present but not well developed. The rock formation was tight and relatively impervious before the blasts (Martel et al 2004). The broken rock of the main trench is highly permeable, however, and constitutes a preferential migration path for gases produced during blasting operations.…”

“…Some increase in permeability was also related to an increase in induced rock fractures along the main trench and around houses 1 and 2 due to past blasting operations. Based on CO concentrations and pressures measured in the monitoring system in the surrounding rock (see Martel et al 2004), significant increases in permeability are possible within 5 m of the perimeter of the houses and the main and private trenches.…”

“…The instrumentation was aligned perpendicular to the four sides of the houses. A detailed description of the monitoring system consisting of many multilevel observation wells for gas sampling and pressure wells is given in Martel et al (2004). More wells for gas sampling were also installed in the main trench and neighbouring houses at various depths to follow the movement of CO.…”

“…In the previous three tests (see Martel et al 2004), the CO migration in the fractured rock was studied at two sites after construction blasts for a medium-sized house. At the Beauport experimental site, the blast of an equivalent isolated house was done in a limestone formation confined by 0.46 m of permeable sand.…”

“…The test at Rock Forest house 2 was performed previously and the results are presented in the companion paper (Martel et al 2004). The blast design shown in Fig.…”