Document ID: chunk:federal_register_of_legislation:F2016L00416:reg:4:p12
Version: federal_register_of_legislation:F2016L00416
Segment Type: reg
Provision Reference: reg 4 (pt 12/21)
Character Range: 61906–64707

virtually no time to escape, especially
if the collapse is extensive. A buried worker is likely to die from suffocation before help arrives
(for example either the head is buried, or the chest is so restricted by the weight of ground that the worker can no longer breathe).
Figure 8 shows a typical example of ground failure where material collapses onto a worker pinning them against the wall of a trench. Trench collapses of this nature can cause fatal injuries.

    Figure 8 Trench collapse and associated ground forces

When planning the work and selecting appropriate excavation methods and control measures,
it is important to consider:
    * the type and strength of the material to be excavated (e.g. whether the ground is natural
      and self-supporting or has been previously backfilled)
    * the moisture content of the soil
    * if the ground is level or sloping
    * if groundwater is present
    * if there are any discontinuities or faults in the strata
    * if there are any other nearby water courses, drains or run-off that might affect the stability
      of the excavation
    * the work area and any access or operational limitations
    * the planned height of the excavated face
    * if vehicular traffic and/or powered mobile plant will operate near the excavation
    * if there will be other construction activity nearby that may cause vibration
    * any other loads adjacent to the planned excavation (e.g. buildings, tanks, retaining walls, trees)
    * if the need for persons to enter the excavation can be eliminated, and
    * any underground essential services.
The ground conditions will have a significant impact on the selection of an excavation method and the control measures implemented.

Ground conditions
In their natural condition, soils have varying degrees of cohesive strength and frictional resistance. Examples of materials with virtually no cohesive strength are dry sand, saturated sand and gravels with minimum clay content. Ground encountered in excavations can generally be categorised as one of three types:
    * hard, compact soil
    * soil liable to crack or crumble, and
    * loose or running material.
Of these materials, hard compact soil is the type that can cause the most trouble because
the face 'looks good' and this often leads to risks being taken. Loose or running material is
often the safest, because the need for safety precautions is obvious from the start.
Soil liable to crack or crumble is doubtful and should be given careful consideration before
the treatment to be given is determined. Useful information can often be obtained from local authorities.
Non-cohesive faces can be very hazardous. With the right amount of moisture they can look safe and solid. A little loss of water by evaporation from the face or