Excavation - Hazards and Precautions

Excavation - Hazards and Precautions

The following questions are asked by the interviewer during a safety officer interview.

backhoe

What is excavation?

Simply a cavity or depression formed by the earth removal is called excavation. Excavations are common where underground utilities, water lines, sewage pipes or cables are being laid.

What are the hazards and precautions of excavation?

Unsafe excavations have caused many accidents and even death at work.

1. Cave-in

A cave-in of excavation sidewalls is serious and life-threatening hazard. This happens when the sidewalls are not properly supported and collapse due to pressure from the surrounding soil. A cave-in can severely injure or kill workers because of the weight of soil.

A cubic meter of soil weighs approximately 1.5 ton, enough to serious injuries or fatalities.

2. Damage underground facility

Provide cable detector and metal detector to identify the underground facilities and countersign of an electrical engineer to assure that there is no electric, phone, instrument and cathodic protection cable.

Must countersign of a mechanical engineer to confirm that there is no underground pipeline or utilities.

3. Presence of toxic gas, high combustible gas concentration:

Gas testing required if excavation more than 1.2 meters in depth (OSHA). The worker can be exposed to Radon gas which is considered a carcinogen, H2S (toxic) and methane (flammable) gas.

4. Electrocution

an underground utility such as a power cable might be got cut and cause electrocution and fire, must be assessed before excavation and take the countersignature of an electrical engineer.

5. Slip, trip and fall

The edge of excavation must be a hard barricade, post signage and keep the area free from any obstruction, maintain good housekeeping to avoid slip, trip and fall.

6. Hit/struck by excavating machine

Provide hard barricade, post signage, assign flagman, worker stand away from the machine, proper supervision,

7. Asphyxiation due to lack of oxygen

Oxygen deficiency means oxygen level is less than 19.5% volume can cause dizziness, unconsciousness and asphyxiation, should be ventilated. The presence of CO2 inside the trench also replace the oxygen ( water react with limestone and produce CO2)

8. Dropped object

Keep spoil soil minimum 60 cm from the edge of the excavation (maintain repose 45 degrees)

9. Manual handling hazards 

Strenuous activity with the wrong posture can cause work-related upper limb disorder (URLD) and MSD (musculoskeletal disorder) such as a sprain, hernia etc.

10. Dust inhalation

Regularly dust inhalation over a period can cause lungs disease, silica-containing material such as concrete, sandstone which is known as respirable crystalline silica can cause silicosis (asthma).

Precautions/requirements of excavation,

• provide documentation such as Hazard identification, work permit, JSA, risk assessment, the rescue plan

• All excavations deeper than 1.2 meters (4 feet) are confined spaces. You must have a Confined Space Entry Permit and a proper excavation plan and design must be developed before any person can go inside. (29 CFR Part 1926)

• As you know, gas testing is the only way to find out if the atmosphere inside an excavation is safe.

• Motor vehicles, cranes and heavy equipment shall be kept away from the edge of the excavation a distance of 2 m (6.5 ft) or the depth of the excavation into 1.5 times.

• Excavation 4ft (1.2 meter) deeper requires a Hot Work Permit (ARAMCO) and Confined Space Entry Permit.

• The appropriate method of excavation should be determined, benching, shoring or sloping, the methods of the combination depends on the soil, the nature of operation and space available.

• Spoils piles should be placed at least 2ft (ARAMCO) away from the edge of the excavation.

• Hard barricade should be placed at least 1meter (less than 1.2 meter deep) and 2 meters (more than 1.2 meters deep) – SABIC) away from the edge of the excavation.

• The scaffold should not be erected near the excavation; it should be at least 1.5 times the depth of the excavation.

• Cranes shall be operated away from the trench more than the depth of the trench.

• If the excavation is left unattended overnight, it shall be barricaded, with flashing lights to warn the passer.

• Emergency rescue equipment shall be immediately available at the site for excavation considered as confined space such as SCBAs, safety harnesses, lifelines, basket stretchers, and mechanical lifts etc.

• Everyone working in an excavation must be properly trained for an emergency.

• Excavating machines are not allowed to operate closer than 3 meters (10 feet) from the nearest underground hazard.

• Excavating machines are not allowed to operate closer than 3 meters (None hydrocarbon pipeline) and 5 meters ( carrying hydrocarbon pipeline)

• Provide a ladder if travel distance is more than 30 meters, if excavation more than 1.2 meters deep then provide a ladder with every 7.62 meters travel distance

• Regular inspection by a competent person

Shoring

Shoring means supporting the sidewalls of excavation with the system. Shoring is the third-best choice to prevent sidewall collapse. Sloping and benching is safer protective methods. 

Sloping

Sloping means laying back the sides of excavation at an angle. Sloping is the best protective system. The "safe" angle of the slope varies with different types of soil and loads.

Classification of soil - OSHA

Type 'A' soil - max slope 3/4:1 (53 degree)

A - Cohesive soil that has an unconfined compressive strength of 1.5 ton per square foot (TSF) or greater - clay, clay loam

Type 'B' soil - max slope 1:1 (45 degree)

B - Cohesive soil that has an unconfined compressive strength of greater than 0.5 TSF but less than 1.5 TSF - silt, silt loam

Type 'C' soil - max slope 1.5:1 (34 degree)

C - Cohesive soil that has an unconfined compressive strength of 0.5 TSF or less - sandy soil

Benching

Benching is the second-best choice after sloping. This excavation is in the process of being benched so that a sidewall collapse will not harm the workers. Multiple benches allowed cohesive soil only

Excavation Plan/Certificate

Excavation of more than 10 cm need an excavation plan, If excavation more than 3 meters attached shoring plan with calculation. (SHEM 08.11)