QUALITY CONTROL AND HES ENGINEERING: General Sloping or Shoring Requirements (Safety Inspection)

GENERAL SLOPING OR SHORING REQUIREMENTS

SAFETY INSPECTION

Oleh : Bayu Nurwinanto

The trench excavation works must be closely supervised to ensure compliance with the statutory requirements, permit conditions and contract specifications, including all safety precautions. Many previous failures resulting in casualties or social disruption were likely to have occurred because of non-compliance due to inadequate site supervision. The permit holders, their consultants if employed and contractors all have an important role to play during the construction of the trench excavation works. That is to ensure the safety of the personnel involved in the trench excavation works and the general public who may be affected by the works.

Safety Inspection

Apart from the daily on-site supervision by the contractors and the consultants if employed, the project proponents, client Departments or utility undertakings concerned should arrange inspections of trench excavation works sites as and when necessary in order to check that all statutory requirements, permit conditions and contract specifications related to safety measures are in place. Any non-conformities identified during site safety inspection should be documented and brought to the attention of the relevant parties for immediate rectification.

Safety Checklist

This is a basic checklist for site supervisory staff who have a key role in ensuring compliance with statutory requirements, permit conditions and contract specifications. Other items should be added or items can be marked as not applicable as appropriate to suit particular trench works. For example, if the shoring design has already allowed for the loading from construction vehicles and stockpiled materials placed within 1.5 m from the edge of the trench, Item (i) of the checklist may be marked as not applicable. In cases where non-compliance is observed, the supervisor should investigate whether the variations are permissible. If variations are confirmed to be not permissible, all works should be suspended until appropriate steps have been taken to rectify the situation. The site supervisory staff should fill in and sign the checklist, and should list those steps, if any, that have been taken to rectify the situation in the checklist for record purposes.

Construction vehicles, excavated spoil, materials, etc. are being kept at least 1.5 m away from the edge of the excavation.
Spoil heaps are being properly placed and covered, and will be kept sheltered in wet weather, or will be removed for maintaining access for pedestrians and traffic.
Adequate safe access to and egress from any trench is being provided and properly maintained.
The open trench is properly lit and fenced off in accordance with Code of Practice for the Lighting, Signing and Guarding of Road Works (HyD, current version).
The location of any buried services has been identified and clearly marked.
Crossing services are properly supported
Trenches are being supported according to the design and contract specifications.
The workers are working at safe distances from each other .
The assumptions used in the design are still valid.
There is no movement or deterioration of the ground that may put adjacent services, roads, structures or slopes at risk.
The area is unaffected by vibration induced by the operation of heavy machinery.
The ground water level is as used in the design (i.e. not higher).
The work is being done in accordance with the specifications/drawings. If not, is the variation permissible?
Unsupported trench faces are safe, with no sign of peeling away, progressive collapse, etc.
The method of withdrawing support during backfilling is safe.
Backfill material is being properly compacted.

Drainage Provisions (particularly important for trench excavations above man-made slopes, retaining walls or sloping natural ground in the wet season).

Flooding is not observed in excavations.
There are proper sumps.
There are adequate working and stand-by pumps of sufficient capacity on site.
Upstands along either side of the trench are provided.
Gaps/voids between support and the vertical trench sides are filled.
The open trench is being securely covered when work is not in progress.
A full-time watchman is making frequent spot-checks during rainstorms.

General Sloping or Shoring Requirements

Minimum sloping or shoring requirements and maximum spacing of timbers are given in Tables 1 and 2 and illustrated in Figures 1 to 16. Variations from these standards or alternate designs must be in accordance with written instructions from a professional engineer.

When or where shoring is required, no worker shall be in an excavation until support systems are installed. Excavation support systems must be installed as soon as possible after excavation to minimize soil movement. The support system must be installed firmly in contact with the excavation walls. Any voids or spaces between the shoring system and the excavation walls must be backfilled or blocked, as shown in Figures 10 and 11. Shoring walers and struts should be installed starting near the top of the excavation, and progressing downward. Removal of these members should progress upward from the bottom of the trench.

Shoring uprights must extend at least 300 mm (1 ft) above the top of trench walls (except where road plates are being used). Uprights must go as close to the bottom of the trench as permitted by the material being installed in the trench, but in no case more than 600 mm (2 ft) from the bottom. Uprights must not be inclined outward more than 15˚ from vertical when viewed along the trench. (See Figure 10).

Sawn lumber used for shoring and timbering must be a minimum of No. 2 or better from the following groups :

Douglas fir-larch.
Hemlock-fir.
Spruce-pine-fir.
Coast Sitka spruce.

Hydraulic or pneumatic jacks must have a means to ensure they will not collapse. A combination of sloping and shoring may be used, as shown in Figure 15, Excavation slopes or supporting systems must be inspected daily or more frequently if required, and must be effectively maintained.

The sides of the excavation must be trimmed or scaled to remove any loose material, rocks, or other objects that could endanger workers. A level area extending 600 mm (2 ft) back from the edges of the trench must be maintained free of materials and equipment.

In trenches of similar excavations over 1.2 m (4 ft) deep, a ladder or other safe means of entry and exit shall be provided in the immediate area where workers are employed. The ladder must extend from the bottom of the excavation to at least 1 m (3 ft) above the ground level. Walkways for access to a bulk excavation must be at least 500 mm (20 in) wide, and have handrails and guardrails where required by the Occupational Health and Safety Regulation. Water must not be allowed to accumulate and remain in excavations. Erosion of excavation faces or bearing surfaces must be controlled.

End shoring may be omitted where the soil conditions at the end of the trench do not require shoring or where the maximum allowable spacing of uprights equals or exceeds the width of the trench. End shoring is required where the spacing of uprights must be close and tight or less than the trench width. Where end shoring is required, the walers for the end shoring should be installed to bear against the walers that extend along each side of the trench, or alternately in a manner that will provide equivalent structural restraint. End shoring must be designed by a professional engineer where the trench width exceeds 1.8 m (6 ft).

Where the base width of an excavation exceeds 3.7 m (12 ft), shoring may require engineering design or be impracticable. In an unsloped excavation that has not been certified by a professional engineer, workers should remain clear from the base of the slope a distance not less than the depth of the excavation. To ensure that workers do not violate this minimum distance, barricades, warning ribbons, and/or signage should be installed.

Size and Spacing of Members

Table 1: Trench Support Structures (Metric)

Note :

The dimensions shown are minimum and must be increased if necessary to meet job conditions.The dimensions of members in millimeters are actual dimensions for surfaced dry materials. The dimensions in inches are the nominal values for surfaced dry materials. Members must be at least No. 2 or better, Douglas Fir-Larch, Hemlock-Fir, Spruce- Pine-Fir, or Coast Sitka Spruce.

Trenches less than 1.2 m (4 ft) deep must be shored when the potential for hazardous ground movement is likely, as in ground subject to hydrostatic pressure or vibration. Walers may be omitted in trenches not exceeding 2.4 m (8 ft) in depth provided the soil is sufficiently hard and solid to safely permit waler deletion, and the trench is not in proximity to previously excavated ground.

At least two struts must be installed in each vertical plane where struts are required. For trenches 4.6 to 6.1 m (15 to 20 ft) deep, shoring for each soil type should be similar in general appearance to that show in Figure 7.

Size and Spacing of Members (continued)

Table 2: Trench Support Structures (Imperial)

Notes :

on Table 1 apply to Table 2

Hard and Solid Soils (Type A)

Figure 1: Depth 1.2 m to 3 m (4 to 10 ft)

Figure 2: Depth 3 m to 4.6 m (10 to 15 ft)

Note:

Hard and Solid Soils (Type A) (continued)

Figure 3: With Walers Omitted

Depth 1.2 m to 2.4 m (4 to 8 ft)

Figure 4: Depth 1.2 m to 3 m (4 to 10 ft)

Soils Likely to Crack or Crumble (Type B) (continued)

Figure 5: Depth 3 m to 4.6 m (10 to 15 ft)

Note:

Soft, Sandy, Filled, or Loose Soils (Type C)

Figure 6: Depth 1.2 m to 3 m (4 to 10 ft)

Note:

The third level of bracing is optional, depending on the depth of excavation.

Soft, Sandy, Filled, or Loose Soils (Type C) (continued)

Figure 7: Depth 3 m to 4.6 m (10 to 15 ft)

Figure 8

End Shoring — Typical Detail

Note:

For trenches 4.6 m to 6.1 m (15 to 20 ft) deep, refer to Table 1 or 2 for size and spacing of members. The general appearance is shown in Figure 6 or 7, except that additional walers and struts will be required. Uprights must extend to bottom of trench. Uprights for end shoring are not shown for clarity. Refer to Table 1 or 2 for appropriate size and spacing of uprights and walers. Soil type at end of trench may be different than at sides of trench. Walers and uprights installed for end shoring must be appropriate for local soil type.

Soft, Sand, Filled, or Loose Soils With Plywood Sheathing (Type C)

Figure 9: Depth 2.7 m (9 ft) Maximum

Figure 10

Position and Slope of Uprights

Notes:

Shoring must be sized for the full depth of the trench. There should be no indication while the trench is open of a possible loss of soil from behind or below the bottom of the shoring. Generally, the stability of Type C soils (soft, sandy, filled, or loose) will not be adequate to permit significant excavation below the shoring.

Figure 11

Typical Blocking in Lieu of Backfilling

Figure 12

Required Joint Arrangement for Walers

Figure 13

Sequence for the Installation and Removal of Shoring

Figure 14

Required Slope of Unshored Excavation Walls

Figure 15

Combined Sloping and Shoring

Unshored trench and excavation walls must be sloped flatter than the angle of repose, but in no case steeper than 3 horizontal to 4 vertical unless otherwise specified in writing by a professional engineer. The angle of repose is the natural stable slope loose excavated material forms when dumped on a level surface.

Figure 16

Where side slope of original ground is steeper than 3H:1V,
soil pressures are greater and engineering is required

Benching

All benched excavations 6.1 m (20 ft) or less in depth shall have a maximum rise between benches of 1.2 m (4 ft). For unrestricted worker access at any level, the width of the bench immediately above any particular rise shall not be less than 1.5 times the height of that rise. For example, the minimum bench width for a 1.2 m (4-ft) rise would be 1.8 m (6 ft).