Trenching & Shoring Safety Course Outline

Transcription

1 Trenching & Shoring Safety Course Outline The following outline summarizes the major points of information presented in the program. The outline can be used to review the program before conducting a classroom session, as well as in preparing to lead a class discussion about the program. Introduction to Trenching & Shoring Safety Excavations & Trenching Pre-Planning, Training, & Equipment Confined Spaces & Hazardous Atmospheres Competent Person Soil Classifications Type A, B, & C Soils Conclusion Excavations & Trenching o OSHA defines an excavation as any man-made cut, cavity, trench, or depression in the earth s surface as formed by earth removal. This can include anything from excavations for home foundations to a new highway. o A trench refers to a narrow excavation made below the surface of the ground in which the depth is greater than the width-and the width does not exceed 15 feet. Trenching is common in construction and utility work, where underground piping or cables are being installed or repaired. o If an excavation is more than 5 feet in depth, there must be a protective system in place while workers are in the excavation. Pre-Planning, Training, & Equipment o Pre-job or project planning is an extremely important aspect of a safe trenching and excavation project. This is where many incidents can be avoided well before any soil is excavated. The competent person must take an active role in the process.

2 o Ladders, ramps and stairways must be provided in all excavations 4 feet or more in depth. Secured ladders must be placed every 25 feet of lateral travel. o In addition, ladders must extend at least 2-3 feet above the top of the excavation. o Employees must never be allowed to climb in and out of the trench by means of the shoring system. o Employees should be trained in the following areas:! The nature of Trenching and Shoring hazards in the work area;! The correct procedures for erecting, maintaining, disassembling, and inspecting Trenching and Shoring protection systems;! The use and operation of Trenching and Shoring equipment! The role of each employee in the Trenching and Shoring safety monitoring system when the system is in use;! The limitations of safety equipment during the performance of work;! The correct procedures for equipment and materials handling and storage;! The details in the specific plan. Confined Spaces & Hazardous Atmospheres o Before any work is attempted in confined spaces, all employees must be thoroughly trained in all aspects of the standard and safe work practices of confined space entry. No employee may be allowed to work or enter without training and understanding of Confined Space hazards. o A Confined Space means a space that:! Is large enough and so configured that an employee can bodily enter and perform assigned work; and! Has limited or restricted means for entry or exit (for example: tanks, vessels, silos, storage bins, hoppers, vaults and pits are spaces that may have limited means of entry); and! Is not designed for continuous employee occupancy. o Hazardous Atmospheres! The OSHA standard states that when working in trenches that are dug in locations where hazardous atmospheres are likely

3 to be present, atmospheric testing, ventilation, and respiratory protection, must be provided.! Areas such as landfills, hazardous waste sites, chemical plants, refineries, and areas where underground storage tanks are present are all locations which may produce hazardous atmospheres.! Planners who perform pre-construction site surveys should look for potential atmospheric hazards as well as the physical conditions of the area to be excavated. Competent Person o Every excavation or trenching site must have a competent person who inspects the site daily and has the authority to shut down work efforts if needed. o The Competent Person must inspect the trench or excavation for any evidence of any situation which could result in:! Cave-ins! Indications of a trench protective system failure! Hazardous Atmospheres! Other hazardous conditions o Where there is evidence of a potential cave-in or other hazardous condition, the Competent Person must remove the affected employees until the necessary precautions have been taken to assure safety. Soil Classifications o A number of stresses and deformations can occur in an open cut or trench. For example, increases or decreases in moisture content can adversely affect the stability of a trench or excavation. The following information explains some of the more frequently identified causes of trench failure. o TENSION CRACKS! Tension cracks usually form at a horizontal distance of 0.5 to 0.75 times the depth of the trench, measured from the top of the vertical face of the trench. See the accompanying drawing for additional details. o SLIDING! Sliding or sloughing may occur as a result of tension cracks, as illustrated below.

4 o TOPPLING! In addition to sliding, tension cracks can cause toppling. Toppling occurs when the trench's vertical face shears along the tension crack line and topples into the excavation. o SUBSIDENCE AND BULGING! An unsupported excavation can create an unbalanced stress in the soil, which, in turn, causes subsidence at the surface and bulging of the vertical face of the trench. If uncorrected, this condition can cause face failure and entrapment of workers in the trench. o HEAVING OR SQUEEZING! Bottom heaving or squeezing is caused by the downward pressure created by the weight of adjoining soil. This pressure causes a bulge in the bottom of the cut, as illustrated in the drawing above. Heaving and squeezing can occur even when shoring or shielding has been properly installed. o BOILING! Boiling is evidenced by an upward water flow into the bottom of the cut. A high water table is one of the causes of boiling. Boiling produces a "quick" condition in the bottom of the cut, and can occur even when shoring or trench boxes are used. Type A, B, & C Soils o Stable Rock! Stable Rock is natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed. It is usually identified by a rock name such as granite or sandstone. o Type A Soil! Type A Soils are cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (tsf) (144 kpa) or greater. Examples of Type A cohesive soils are often: clay, silty clay, sandy clay, clay loam and, in some cases, silty clay loam and sandy clay loam. o Type B Soil! Type B Soils are cohesive soils with an unconfined compressive strength greater than 0.5 tsf (48 kpa) but less

5 than 1.5 tsf (144 kpa). Examples of other Type B soils are: angular gravel; silt; silt or loam. o Type C Soil! Type C Soils are cohesive soils with an unconfined compressive strength of 0.5 tsf (48 kpa) or less. Other Type C soils include granular soils such as gravel, sand and loamy sand, submerged soil, soil from which water is freely seeping, and submerged rock that is not stable. Conclusion o Insure that there is a safety plan for all trenching and shoring operations o The competent person must conduct a checklist inspection of the site every day before work commences o All workers should be trained on trenching operations and be informed of the safety plan. o The competent person must have the authority to shut down operations if needed.

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