OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 32 Excavating and Tunnelling

Section 442 Classification of soil type

The employer is responsible for classifying the soil being excavated into one of the three types described in this section and summarized in Table 32.1.

Table 32.1 Classification of soil types

Soil Characteristics

Soil Type

Hard and Compact
Soil

Likely to Crack and Crumble Soil

Soft, Sandy
or Loose
Soil

Consistency Hard, very dense in compactive condition Stiff, compact in compactive condition Firm to very soft, loose to very loose in compactive condition
Ability to penetrate Only with difficulty by a small, sharp object With moderate difficulty with a small, sharp object With ease
Appearance Dry Damp after it is excavated, has low to medium natural moisture content Appears solid but flows or becomes unstable when disturbed. Can be dry, running easily into a well-defined conical pile, or wet.
Ability to excavate with hand tools Extremely difficult Moderately difficult With ease
Water seepage Shows no signs of water seepage Shows signs of localized water seepage  
Other Does not include previously excavated soil Shows signs of surface cracking Is granular soil below the water table, unless the soil has been dewatered.

Exerts substantial hydraulic pressure when a support system is used.

The soil type helps to determine how stable the walls of an excavation will be. When the walls of an excavation are composed of layers – seams of gravel or debris may lie behind seemingly solid walls – the weakest layer is most likely to slump or slide. The total cross-section of soil must therefore be classified as the weakest soil type and the support system designed accordingly. Assume the worst and base precautions on the most unstable soil type that is likely to be present.

For example, a stronger layer overlain by a weaker layer could result in the uppermost, weaker layer slumping into the excavation, exposing workers to risk of injury. Similarly, if a stronger layer lies above a weaker layer, the slumping of the weaker layer could cause a large block of the upper layer to become unstable. Either case presents an unacceptable risk of injury or death to workers.

Because of the nature of classifying soil types, a competent person should be assigned to carry this out. A geotechnical engineer (a professional engineer) experienced in soil classification can assist with this.

Trench wall failure

Worker deaths resulting from trench wall collapse are all too common, and completely preventable. The material removed from the ground to form a hole, trench or cavity is extremely heavy. It may weigh more than 1476 kilograms/cubic metre (100 pounds/square foot), the equivalent weight of a car in a space less than the size of the average office desk. Wet soil, rocky soil or rock is usually heavier.

Undisturbed soil is kept in place by the horizontal and vertical forces of adjacent soil. Once soil is removed to create a trench, it is no longer available to provide support for the soil left behind in the trench wall. Without support, soil from the trench wall eventually moves downward and inward into the excavation. This creates a serious life-threatening hazard for workers in the trench.

Figure 32.1 shows the three areas of failure in a trench wall. The first failure occurs in Zone 1 at the base of the trench wall. This movement creates an undercut area, allowing soil in Zone 2 to collapse. The failure of Zones 1 and 2 leaves the remaining trench wall, Zone 3, unsupported. Zone 3 will break away from the wall under its own weight and fall into the trench. How long it takes for Zones 2 and 3 to collapse is unpredictable. Many rescue attempts are unsuccessful because rescuers attempt to save victims before the second and third failures take place. The would-be rescuers are often trapped along with the first victim(s).

Figure 32.1 Mechanics of trench wall failure involving previously disturbed soil

Figure 32.2 shows where soil that has already been excavated and backfilled is most likely to collapse. Previously disturbed soil takes a long time to return to its previous condition.

Figure 32.2 Areas of a trench in previously disturbed soil most likely to collapse

A trench wall collapse might involve 2.5 to 4 cubic metres of soil, weighing from 3700 to 7400 kilograms (8100 to 16,300 pounds). The human body cannot support such heavy loads without being injured.

A worker buried to a depth of less than one metre of soil experiences enough pressure on the chest to prevent the lungs from expanding and drawing in a breath. Suffocation occurs within approximately three minutes. Even if the worker is quickly rescued, the heavy weight of the soil is likely to cause serious injuries, particularly if the worker’s body comes to rest in an awkward position.

Factors that may cause wall collapse

Figure 32.3 shows examples of factors that may cause the wall of an excavation to collapse. Moisture in soil reduces its strength. Once an excavation is opened, the walls are exposed to the elements. Moisture content and soil stability can change rapidly.

Figure 32.3 Factors that may cause cave-in of an excavation or trench

Any large, heavy movement near an excavation causes vibration of the surrounding soils. This movement can result in soil failure. Moving machinery, nearby traffic, pile driving and blasting all cause vibration in surrounding soils.

Vibration-related soil failures can occur in all types of soil. However, certain types of soils are more susceptible to vibration failures than others. For example, sandy soils tolerate less vibration than clay soils. Since soil conditions may be a mixture of more than one soil type, it is better to play it safe when protecting an excavation from wall collapse.

Adjacent buildings and structures can reduce soil stability by placing extra pressure on the walls of an excavation. An excavation can cause nearby building walls to collapse because the soil that otherwise provided support to the walls has been removed.

Spoil piles and supplies placed near the excavation, and mobile equipment operating nearby, can put extra pressure on the walls of the excavation. These sources of pressure or loading should be kept as far away from the excavation as reasonably practicable.

For more information
A Guide to the OSHA Excavations Standard
North Carolina Department of Labor, 2002