OHS Code Explanation Guide

Published Date: July 01, 2009
Bookmark this page

Part 32 Excavating and Tunnelling

Section 451 Cutting back walls

If the walls of an excavation are cut back, the design specifications of this section must be followed. Since the specifications are based on a subjective interpretation of soil type, a professional engineer should be consulted whenever there is a question of doubt related to the soil type and potential risk to workers. The regulatory responsibility for compliance rests with the employer, who must make the related decision.

Hard and compact soil

As shown in Figure 32.7, the walls must be sloped to within 1.5 metres of the bottom of the excavation at an angle of not less than 30° measured from the vertical.

Figure 32.7 Cut back of excavation walls in “hard and compact soil”

Likely to crack or crumble soil

As shown in Figure 32.8, the walls must be sloped to within 1.5 metres of the bottom of the excavation at an angle of not less than 45° measured from the vertical.

Figure 32.8 Cut back of excavation walls in “likely to crack or crumble soil”

Soft, sandy or loose soil

As shown in Figure 32.9, the walls must be sloped from the bottom of the excavation at an angle of not less than 45° measured from the vertical.

Figure 32.9 Cut back of excavation walls in “soft, sandy or loose soil”

Benching as a safe alternative to cutting back

Based on the results of a report prepared for Workplace Health and Safety by a geotechnical engineer (March 2009), benching is an acceptable alternative to the practice of cutting back the walls of an excavation as required by section 451. If benching is used by an employer, the following practices need to be followed:

  1. benching can be a safe alternative to the straight cutting back of excavation walls in hard and compact, and likely to crack or crumble soils. Benching is not acceptable for soft, sandy or loose soil;

  2. the rise and run for hard and compact soil should be at least 1 Vertical: ¾ Horizontal (or flatter) with a maximum rise of 1.2 metres. The maximum unsupported vertical cut at the base is 1.2 metres rather than the 1.5 metres allowed by section 451 if the walls are sloped. The run of the first bench must be twice that of the succeeding benches. This wide first bench provides a more stable slope base. See Figure 32.10;

  3. the rise and run for likely to crack or crumble soil should be at least 1 Vertical: 1 Horizontal (or flatter) with a maximum rise of 1.0 m. The maximum unsupported vertical cut at the base is 1.0 metre rather than the 1.5 metres allowed by section 451 if the walls are sloped. The run of the first bench must be twice that of the succeeding benches. This wide first bench provides a more stable slope base. See Figure 32.11;

  4. the maximum depth of a benched excavation is limited to 6 metres. Increasing the depth of the excavation or increasing the height of the slope and benches increases the risk of slope failure, compromising the safety of workers within the excavation. Benched excavations deeper than 6 metres need to be certified by a professional engineer;

  5. heavy equipment and spoil piles of soil should not be allowed within 1.2 metres of the edge of the uppermost bench. Vibration caused by construction equipment may cause instability of the bench;

  6. although benching may result in an overall more stable slope, it may have some adverse effects on the sidewalls. Benching exposes more surface area which allows more evaporation and drying of the soil. This can lead to cracking and fissures in some soils. Also, water can pool on the horizontal bench surfaces and then infiltrate the slopes and benches. The employer should provide surface drainage such as drainage ditches on benches, minimize the infiltration of water and try to minimize any rise in the ground water table. Horizontal bench surfaces should not be sloped away from the wall to drain water collecting on the bench. Doing so will cause water to cascade over each bench and run down the slope, causing erosion of the soil; and

  7. benches need to be formed during the excavation process and not by cutting the slope from the bottom. Cutting a slope at its base can momentarily destabilize the slope until the soil at the top is removed. Therefore top down construction of the benches is required.

Figure 32.10 Benching profile for hard and compact soil

Figure 32.11 Benching profile for likely to crack or crumble soil

The effect of benching on overall slope stability

To see the effect of benching on the overall stability of a slope, it is necessary to understand the effects of applying a force on the surface of the slope. A force applied on the surface of a slope has a stabilizing effect if it is applied above the neutral point. The neutral point is defined as a point on the surface of a slope where a force has neither a stabilizing nor destabilizing effect (see point A in Figure 32.12). A downward force applied above a neutral point will have a destabilizing effect.

Figure 32.12 Neutral point on a slope

On the other hand, if a downward force is applied below the neutral point, it will have a stabilizing effect. The reverse is true if upward forces are applied. It should be noted that the neutral point is not a fixed point on the slope. In slope stability analysis, it is the critical slip surface that is the most important slip surface in considering stability.

A slip surface is a potential failure surface on a slope which separates the slide soil mass from the unmoving ground. The critical slip surface is the one that has the lowest factor of safety i.e. the most likely to fail. The critical slip surface not only depends on the geometry of the slope and properties of the soil, it also depends on the forces applied on the boundary of the sliding mass. If the location of the applied force is moved along the surface, the critical slip surface changes, which also changes the location of the neutral point.

Benching can be viewed as adding and removing soil masses from the surface of a slope. As shown in Figure 32.13, adding a bench at the bottom of the slope has a stabilizing effect since a force is applied below the neutral point due the weight of the bench. Creating a bench by removing soil at the top of the slope also has a stabilizing effect since it is the equivalent of applying an upward force above the neutral point.

Therefore benching, if carried out properly, should enhance the overall stability of the slope.

Figure 32.13 Loading which enhances the stability of a slope

Although benching has an overall stabilizing effect on a slope if it is carried out properly, it does have adverse effects on a slope. As stated earlier, benching creates a larger surface area than a plane cut, exposing more soil to evaporation which promotes drying. This may lead to cracking and fissures in cohesive soil.

Benching creates a flat surface on top of each bench which can potentially promote infiltration of water and pooling of water in localized areas. Increased water content in the soil and a rise in the ground water table have destabilizing effects on a slope and bench. Therefore, infiltration of water needs to be minimized.