6.1 Excavation Hazards, Soil Behaviour and Pre-Excavation Investigation

Why excavations kill

Trench collapses happen suddenly and without warning. DOSH's Tapak Selamat bulletin on excavation safety (Issue 3/2015) points out that one cubic metre of soil can weigh as much as one tonne. It also records that of the 33 construction-site fatalities DOSH studied in 2015, five were caused by excavation and groundwork. A worker buried to the chest cannot breathe even if their head is clear, and rescuers who climb into an unsupported trench are often caught by a second collapse.

Other excavation hazards include:

• people, vehicles or materials falling in
• striking underground electricity cables or gas mains
• contact with overhead lines by excavator booms
• flooding, drowning and entrapment in silt
• toxic or oxygen-deficient air in deep pits
• undermining neighbouring buildings, roads or services
• contaminated ground and infections such as leptospirosis
• noise and hand-arm vibration from breakers

The law: CDM 2024 regulations 24 to 27

Definition (regulation 3): an excavation includes any earthwork, trench, well, shaft, tunnel or underground working.

Regulation 24(1). The construction work contractor must take all practicable steps, including providing supports or reinforcement, to ensure that:

• (a) no excavation or part of it collapses
• (b) no material forming its walls or roof, or next to it, is dislodged or falls
• (c) no person is buried or trapped by falling or dislodged material
• (d) no person, plant or material falls into the excavation
• (e) no part of the excavation or nearby ground is overloaded by plant or material

Regulations 24(2) to 26 add inspection and reporting rules (Section 6.4). Regulation 25 applies the same approach to cofferdams and caissons. Regulation 27(4) prohibits work that could create risk from underground services unless suitable measures have been taken. Regulation 30(5) requires measures to stop vehicles falling into excavations. Most breaches carry fines up to RM500,000 or one year or both.

Excavations involving a risk of burial appear in the Fourth Schedule ("work involving particular risks"), so the construction phase plan must include specific measures for them.

How soils behave

You do not need to be a geotechnical engineer, but you must recognise ground that will not stand.

Factors that make any soil less stable:

• Water: rain, groundwater and burst pipes reduce soil strength and add pressure. Malaysian monsoon rainfall can change a stable cut overnight.
• Surcharge: spoil heaps, stacked materials, vehicles and plant near the edge (regulation 24(1)(e)).
• Vibration: piling, compaction, traffic and breakers.
• Time: excavations left open longer deteriorate; DOSH's bulletin lists "the length of time that the excavation will be open" as a planning factor.
• Previous disturbance: backfilled trenches and old foundations.

DOSH's bulletin notes that a small increase in water content can significantly reduce some soils' strength, and that all but the shallowest excavations need some form of support unless they are cut back to a safe slope (Section 6.2).

Pre-excavation investigation

DOSH's excavation bulletin (paragraph 5) says the excavation supervisor's assessment must follow a site investigation by the main contractor during planning. Points to consider:

1. Underground services: gas, water, sewerage, telecoms, electricity, chemical, fuel or refrigerant lines
2. Previous use of the site
3. Existing buildings and new structures
4. Soil investigation results and ground contamination
5. Water courses, including underground, and the water table
6. Storage and disposal of spoil
7. Transport, haul routes, working space and storage
8. The most suitable method of temporary support
9. The number of people involved
10. Local weather
11. How long the excavation will be open
12. Emergency arrangements

The bulletin also says that clients contribute by funding adequate site investigation, and further investigation if conditions differ from those predicted. Under CDM 2024 this is part of the client's pre-construction information (regulation 5) and the designers' duty to eliminate or reduce risks (regulation 11).

Services

• Verify the position of all underground installations before work begins. Records alone are not enough (bulletin paragraph 7).
• Where pipes or cables feeding occupied buildings must be worked on, isolate them first, communicate with the occupants, and provide alternative escape routes if exits are blocked (paragraph 8).
• Obtain records from the utility owners: TNB for electricity, the state water operator, the gas distributor and the telecoms operators. Then scan with a cable locator and hand-dig trial holes to confirm positions before machine digging (Section 6.4).

Neighbouring structures

• DOSH's bulletin (paragraph 27) advises that foundations less than twice the excavation depth from the face are more likely to be affected by ground movement. Underpinning may be needed.
• Where the stability of adjoining structures is in question, support them by underpinning, sheet piling, shoring or bracing designed by a Professional Engineer (paragraph 26). Monitor movement during the works (Section 6.3).
• Meet the occupants of surrounding buildings before work starts and keep in contact throughout (paragraph 9).

Health hazards in the ground

DOSH's bulletin lists leptospirosis (from rat urine in water and soil), gastro-enteritis, hepatitis A and tetanus, as well as contaminated ground containing asbestos, coal-tar residues or heavy metals from previous use. Controls include:

• welfare facilities with washing
• covering cuts
• gloves and boots
• hygiene briefings
• assessing contaminated ground before digging (Section 10.3)

Scenario: a drain upgrade beside shophouses

A 2.8 m deep drain trench is planned 3 m from the front wall of a row of old shophouses on shallow footings, in sandy fill, two weeks into the monsoon season. The SSS's pre-start review finds:

1. The footings are within twice the excavation depth (5.6 m), so the shophouses may move. The temporary works designer must check the support system and the need for underpinning, and set up monitoring points.
2. Sandy fill below the water table needs close sheeting or a trench support system, with dewatering planned (Section 6.2).
3. TNB and water records show cables and a main along the shop fronts, so permit-to-dig, cable locator scans and trial holes are needed before machine digging.
4. Spoil cannot be stored beside the trench in the narrow lane. Plan truck removal and set stop blocks for vehicles.
5. The construction phase plan needs specific measures for this Fourth Schedule work, and the method statement needs an inspection and rescue plan.