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CROSS Safety Report

Concerns with propping design to large excavations

Report ID: 298 Published: 1 January 2019 Region: CROSS-UK

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Overview

Concerns are raised about the design of propping to large excavations and particularly about the design of the connections for a raking prop to a horizontal waler.

Key Learning Outcomes

For temporary works designers:

  • Careful consideration needs to be given to load paths and forces generated when adjoining members do not meet at the same point. Good detailing with allowances for tolerances is often required.

  • If industry guidance is not sufficient, it is important to raise awareness and discuss concerns with your team and others

  • Raising awareness is the first step in the process of bringing about improvements to industry guidance and practices

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A reporter is concerned about the design of propping to large excavations and particularly about the design of the connections for a raking prop to a horizontal waler.

They have found standard guidance confusing and that opinions from other engineers differ. They therefore suggest that authoritative and detailed guidance is necessary for temporary works design.

The only useful guidance the reporter found is the Construction Industry Research and Information Association (CIRIA) C517, Temporary propping of deep excavations - guidance on design (1999), where Figure 10.5 shows 'Typical end detail for a raking prop (after Goldberg et al, 1976)'. The raking prop is shown as meeting the centreline of the waler at the outer flange (Figure 1). The inner flange is shown fixed to a vertical soldier beam, which is assumed could equally be a sheet-piled wall or a contiguous piled wall.

Image
Figure 1: raking prop to outer flange of waler beam; without kicker

Issues with identifying the correct load path

An inclined 'kicker' or 'spur brace' is provided (Figure 2), presumably to resist the upwards component of the prop force which has to be resisted where the prop joins the outer flange. The idea is presumably that the horizontal force component from the prop force passes into the waler web and then along the waler to resist the horizontal forces from the soil.

Image
Figure 2: raking prop to outer flange of waler beam; with kicker

Unfortunately, when the forces at the top of the prop are resolved, if the kicker is to resist the vertical force, its axial force must be similar to the prop force, unless it is extremely steep; in fact, it is usually approximately in line with the prop.

This means (resolving forces) that the prop force passes up through the kicker to where the kicker meets the wall - and the waler doesn't spread the horizontal prop force along the wall.

There may be an alternative way of looking at this, where the soil forces travel along the waler, back into the wall at the back of the waler local to the prop, up through the wall to where the kicker meets the wall, hence into the kicker and down to the prop. However, proving such a load path would be difficult; the local wall stressing, and the connections would need very close examination.

Solutions identified

The reporter has shown this to several temporary works designers. Some of them already know about the problem but some just can't see what the problem is. Solutions identified so far (and used) are to either have a near-vertical kicker (or tie-down underneath), then checking that this force can be resisted vertically, or to join the prop at the inner flange, not at the outer flange, which requires some overhead welding on site.

Expert Panel Comments

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If forces from adjoining members do not meet at a point, then a moment will be induced and in this case the forces must be resolved to eliminate torsion on the waling.

If the junction between prop and waler beam is an assumed pin, then the system can be a structure with the eccentric vertical component of the prop creating bending in the piling (just below the waler fixing point).

Alternatively, if the prop to waler junction is assumed to be fixed, the bending will be shared between the waler and the piling. This could only work if the piling/raker have adequate bending capacity.

The reporter is correct in that, while the inclusion of a kicker provides a load path for the vertical force component of the raking prop, the horizontal force component of the raking prop will also pass through the kicker if it is aligned with the raking prop. In this case, the waler beam will not spread the prop force along the wall.

This issue can be resolved through good detailing, which could include aligning the raking prop with the inner flange of the waler. However, tolerances should always be allowed for, particularly for sheet pile walls where there can be packing between the sheet pile wall and the waler beam, so there can still be a moment. To overcome any torsional effects, stiffeners can be effective.

The Temporary Works forum (TWf) are aware of the issue and are considering producing a short guidance document.

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