On-Site Rescue Inc.

http://www.cos-mag.com/personal-process-safety/34267-are-excavations-the-new-confined-space/

This is a really good article on risks of excavating - here is the key paragraph...

The particular tasks being carried out in an excavation, the processes and products used can also create harmful conditions, says Jason Hindson, senior safety consultant at Ronin Safety and Rescue in Coquitlam, B.C.

A worker may be required to remove coal tar with asbestos from a utility pipe, for example. Welding in a confined space produces oxygen deficiency, which can cause dizziness, confusion, asphyxiation and death. When a sewer main is opened in an excavation, the space should be considered a confined space. Hazardous energy sources must be locked out.

click link to read this well written article by Linda Johnson in "Occupational Safety"

Are excavations the new confined space? | Canadian Occupational Safety Considering every excavation as a potential confined space can expose hidden hazards