ABSTRACT
INTRODUCTION Hazardous gas incidents in civil works excavations involve so many variables that patterns may seem indiscernible, making meaningful extrapolation appear impossible. On the contrary, gas occurrences in the near-surface underground follow consistent geological patterns, and the consequences of gas in an excava tion relate to the effectiveness of atmosphere control measures. This chapter iden tifies common geologic environments that may contain all the factors necessary to pose a gas risk to construction. It describes environments in which bacterial, thermogenic, and geothermal gases are generated, migrate, are retained, and can enter excavations.
