6.2 Protecting Excavation Sides: Battering, Benching, Shoring and Trench Support Systems

Two ways to keep the sides up

1. Cut the sides back (batter or bench) to a slope at which the ground will stand under all foreseeable conditions of work and weather.
2. Support the sides with timbering, sheeting, sheet piles, trench boxes or hydraulic frames designed for the ground, water and surcharge.

CDM 2024 regulation 24(1) requires whichever measures are practicable to prevent collapse, burial and falling material, "including providing supports or reinforcement". DOSH's 2015 Tapak Selamat excavation bulletin (paragraph 11) is blunt: no employee shall be permitted to enter any excavated area unless supports such as sheet piling, shoring or other necessary safeguards are provided. Designing supports or slopes needs reliable soil and rock parameters from site investigation.

Battering (sloping)

An unsupported trench must be cut back to a safe slope, so that the material in the sides can stand under all expected work and weather conditions. DOSH's bulletin gives typical safe angles from the HSE (UK) guidance it cites:

Use the table with care:

• Natural soils are usually mixtures.
• A small rise in water content can greatly reduce strength.
• Battered excavations need regular monitoring.
• Rock faces may need regular scaling to remove loose material.

The bulletin also notes that in a shallow trench in uniform material known to be stable, which will be backfilled soon, vertical faces may be safe up to 1.5 m. This is a judgement for a competent person, not a free pass: water, vibration, fill or surcharge cancel it.

Worked example: a 3 m deep trench in dry sand battered at 30° needs each side to extend horizontally about 3 / tan 30° = 3 / 0.577 = 5.2 m from the toe. In a narrow lane that space rarely exists, which is why support systems are used.

Benching (stepping)

Benching cuts the sides into a series of steps. DOSH's bulletin says:

• step dimensions follow the safe slope angles above
• the vertical rise of each step should not exceed 1.2 m (HSE)
• each step should be at least 1.2 m wide (DOSH 2000)
• where a trench exceeds 3 m in depth and is not otherwise supported, it should be benched or stepped (DOSH 2000)

Benching is unsuitable for granular soils that run, and for wet ground where the steps themselves slump.

Supporting the sides

Timbering and sheeting

• Sheeting means steel sheets or timber boards against the trench walls. Close sheeting (boards or sheets touching) is used in loose or wet ground. Open sheeting (spaced boards) is suitable only in firm clay.
• Walings (wales) are horizontal members holding the sheeting. Struts span between walings across the trench.
• DOSH's bulletin, quoting the former FMA rules, says planks used as sheet piling should be at least 50 mm thick, wale spacing must keep the planks within their safe bending stress, and each end of each waling must be separately braced.
• Earth-supported shores or braces must bear against footings large and stable enough, or be toed in deeply enough, not to shift.
• Use timber packing so steel struts do not bear directly on steel.
• Where banks are undercut, provide shoring to support the overhang. Do not remove temporary sheet piling for a retaining wall until the wall has developed its full strength.

Proprietary systems

DOSH's bulletin (paragraph 25) says proprietary ground support systems must always be used in accordance with the manufacturer's instructions.

Where a trench needs two lengths of sheet piling, one above the other, the lower piling is set inside the bottom walings of the upper piling and is driven and braced as the excavation proceeds (bulletin paragraph 69).

Installing and removing support safely

• Never enter an unsupported trench to install supports. Install from the surface, or progressively: dig, then immediately support, before anyone goes below.
• Install struts from the top down. Remove them from the bottom up, as backfilling proceeds, so the trench is never left without support. DOSH's bulletin says temporary supports should not be withdrawn until enough backfill is in place to prevent collapse, and backfill must be placed and compacted in layers.
• Re-inspect after any change to the support, after rain and at every shift start (Section 6.4).

Spoil and surcharge

CDM regulation 24(1)(e) requires steps so that no part of an excavation, or the ground next to it, is overloaded by plant or material. DOSH's bulletin (paragraph 55) recommends:

• placing excavated material and other loads at least 610 mm from the edge, or a distance of the trench depth. Choose the distance by considering the excavation's depth and slope, the soil and the weight of the load.
• piling or retaining spoil so it cannot fall back in, make the banks slip, or heave the excavation bed
• placing spoil on the lower side of sloping ground where possible, without holding back runoff water, and shaping it to channel rainwater away from the excavation

For plant:

• Keep heavy plant, cranes, concrete trucks and stockpiles out of the zone of influence unless the support design allows for their loads (bulletin paragraphs 53 and 54).
• Use stop blocks or bunds where vehicles tip near edges (CDM regulation 30(5)).
• Never let workers stay in a trench while plant is depositing material into it (bulletin paragraph 37).

Scenario: a 2.5 m sewer trench in silty sand

The method statement shows a vertical-sided trench with no support, the excavator dumping spoil on both edges, and pipe-layers working in the trench. The SSS stops the work because:

1. Silty sand with groundwater cannot stand vertically at 2.5 m, and the 1.5 m "shallow trench" allowance does not apply.
2. Spoil on the edge overloads the sides (regulation 24(1)(e)).
3. Nobody may enter without supports (bulletin paragraph 11).

The revised method:

• a trench box or hydraulic frame system within its rated depth, installed from the surface
• spoil loaded straight into trucks, or placed beyond the zone of influence on one side
• dewatering through a silt trap
• ladder access inside the protected zone
• shift-start inspections by a designated person, recorded under regulation 26