Causes of confined space hypoxia during underground construction in the Lambeth Group beneath London

Abstract

Hypoxic ground gas is rarely reported in London despite its frequent occurrence in the Upnor Formation and the serious health and safety implications for underground working. The death of two engineers by asphyxiation and recent interception of pressurised hypoxic gas by two ground investigation boreholes highlight the urgent need for research into this hazard which until now has been extremely limited.

Glauconite is legally and widely assumed to cause the oxygen loss in confined spaces but this research proves it incapable of doing so. Visual inspection and laboratory analysis, using optical and infrared spectroscopy, scanning electron microscopy and X-ray diffraction techniques confirm fresh glauconite within highly oxidised soil samples throughout the lower Lambeth Group and Thanet Sand Formation. The low concentration of pyrite and organic materials eliminates these as alternative causes.

A poorly known but common bright blue-green mineral, green rust, is instead shown to be a very potent and plausible reducing agent. However, laboratory analysis under anoxic conditions has yet to prove its existence in the Upnor Formation, largely because of its extremely rapid oxidation, although the results are consistent with its presence in fresh samples. Significant preservation techniques were implemented to prevent oxidation of soil samples prior to laboratory analysis and improvements identified for future research. The presence of green rust during underground construction within the Upnor Formation will have serious implications for the health and safety of personnel.

Measurements of soil suction, using the filter paper technique, show that the air entry value of the Lower Mottled Beds is such that they will act as an impermeable barrier to upward migration of hypoxic gas, trapping it within the underlying Upnor Formation. Field tests indicate that the gas is formed during periods of dewatering and may be influenced by changes in barometric pressure.