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Excavation and Trenching. Introduction.
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Introduction An excavation is any man-made cut, cavity or depression in the earth’s surface, formed by earth removal. This includes a trench, which is a narrow type of excavation made below the ground that is deeper than it is wide. Trenches can be up to 15 feet wide. Trenches and excavations are common at many construction sites and are considered one of the most dangerous construction operations. It is estimated that 100 plus deaths per year occur due to cave-ins and other excavation hazards. OSHA requires that workers in trenches and excavation areas be protected. Most fatalities and injuries could be avoided with proper knowledge, utilization of OSHA safety standards and an effective health and safety program.
Hazards Probably the most feared hazard is a cave-in, which makes very little noise and gives very little warning prior to the collapse. Some causes of cave-ins include: • Vibrations from equipment or traffic in the area. • Weight of equipment near the trench edge. • Weight of excavated material (spoil) too close to the trench edge. • Poor soil condition. • Rain/water which weakens the trench walls. If more than one condition is present, then the risk of a cave-in is even greater.
Hazards Other hazards include: • Asphyxiation due to lack of oxygen in a confined space. • Inhalation of toxic fumes released during digging. • Drowning, electrocution, explosions and fire as a result of coming into contact with water, sewer, electrical, natural gas or other types of utility lines. • Being hit by vehicles when working near traffic.
Competent Person A competent person must be on-site at all times to oversee all aspects of excavating and trenching. In many cases this person will also serve as the construction-site supervisor. A competent person is someone that has a greater level of training and experience than the average worker. This person is knowledgeable about trenching and excavation standards, is able to identify hazards and unsafe conditions and is authorized to stop work when necessary to protect employees and equipment. This person must also have training in soil analysis and in the use of proper protective systems.
Soil Analysis An analysis of the soil must be made to determine the type of protection system that should be utilized. The competent person should perform the two OSHA mandated test and classify the soil. Soil has four possible classifications. • Solid Rock - Most Stable • Type A - Very Good • Type B - Fair • Type C - Least stable, Very Poor
Protective Systems Sloping - means excavating to form sides that are inclined away from the excavation so as to prevent cave-ins. Soil condition determines the degree of slope required. Benching - means excavating the sides of an excavation to form one or a series of, horizontal levels or steps, usually with vertical or near-vertical surfaces between levels. Shoring - is the process of placing metal or wood panels inside a trench, against the soil walls, and securing them in place with bars and vises. Proper shoring prevents cave-ins. Shielding - is a structure that is able to withstand the forces imposed on it by a cave-in. Shields used in trenches are usually referred to as trench boxes or trench shields.
Safety Precautions Before work begins at an excavation site, all underground utilities should be located and marked. A call to Nationwide One Call at 811 is necessary to inform the various utilities of the pending work and to have them perform this safety measure. If an underground utility is damaged, notify the utility operator promptly. If the damage results in the release of hazardous gases or liquids, both the utility operator and appropriate emergency response officials should be notified immediately. If working near traffic, safety measures should be taken to protect workers. The use of high visibility clothing is necessary. A designated, trained flag person along with signs, signals and barricades should also be used.
Safety Precautions The competent person must perform site inspections daily at the start of each shift. Additionally, inspections are required: • Following a rainstorm, snowstorm or windstorm; • When equipment is used near the edge of an excavation; • When there is a change in the size, location or placement of the spoil pile; and • When there is any indication of change or movement in adjacent structures.
Access and Egress A safe means of access and egress is required in all trenches deeper than four feet. A ladder is the most commonly used method. The ladder should extend three feet above the top of the excavation and be located within 25 feet of workers. Structural and earthen ramps can be used but must be designed and evaluated by a competent person. If water is present or possible in the excavation, appropriate protections must be provided. Pumps and drainage systems should be utilized.
Excavated Materials (Spoil) Excavated material (spoil) can be very dangerous if not placed a safe distance away from the trench or excavated area. • Weight of the spoil can cause a cave-in. • Spoil and equipment can fall back onto workers located in the trench and should be set back at least two feet from the excavation/trench. • When the worksite does not permit a two foot setback, spoils should be temporarily moved to another site. • Use a retaining device, such as a trench box, which extends above the top of the trench to prevent spoils and equipment from falling back into the excavation/trench.
Confined Spaces Some excavations and trenches are considered confined spaces. Employees must be protected from exposure to harmful levels of atmospheric contaminants, oxygen deficiency, hazardous fumes and toxic gases. Adequate ventilation must be ensured or respiratory equipment provided. If the work site is considered a confined space, additional confined space training is necessary to protect all employees.
Mobile Equipment Mobile equipment should have a warning device or system. Steps should also be taken to prevent vehicles from accidentally falling into the trench. • Barricades must be installed where necessary. • Hand or mechanical signals must be used as required. • Stop logs must be installed if there is a danger of vehicles falling into the trench. • Soil should be graded away from the excavation; this will assist in vehicle control and channeling of run-off water.
Surface Crossing Surface crossing of trenches should be discouraged. If trenches must be crossed, appropriate walkways or bridges with guardrails must be provided. In the event of an accident, follow safety procedures. • Alert the competent person, appropriate emergency response personnel, necessary utility companies, and others according to your company policy. • Never attempt to rescue a fellow worker during a cave-in. A second cave-in is very likely. Leave rescue attempt to trained rescue personnel.
Conclusion This program has discussed many aspects of trenching and excavating. Remember to follow all safety rules. If a situation seems unsafe, alert the competent person and stop work until it is safe to continue. Never take shortcuts or try to skip necessary safety procedures. Finish excavation work as quickly as possible in a safe manner. Remember, you are the one working in the trench or excavation. It is your life that may be at risk.