Presentation on theme: "Trenching & Shoring Safety Susan Miller Safety Training Coordinator Murray State University."— Presentation transcript:
Trenching & Shoring Safety Susan Miller Safety Training Coordinator Murray State University
Soil is Heavy A cube of soil measuring 1 ft. on a side weighs at least 100 lb (more in many cases). A cubic yard of soil (3 ft. on a side) contains 27 of these, or 2,700 lbs. total. This weighs about as much as a mid-sized automobile.
Dangers On average, 50 workers are killed at excavation sites yearly. Cave-ins account for 76% of fatalities. Being struck by or crushed by equipment, dropped loads, equipment rollovers, electrocutions, and other events account for the remainder.
Excavation or Trench? What’s the difference? Excavation – any man cut, cavity, trench, etc., formed by earth removal. Excavations can include a building basement, roadbed or trench. Trench – narrow underground excavation that is deeper than it is wide and no wider than 15 feet. All trenches are excavations, but not all excavations are trenches. Trenches are usually more dangerous.
Overview of OSHA’s rule 29 CFR –Assign a competent person –Keep spoil piles and heavy equipment away from the edge of trench –Use adequate protective systems –Train on hazard recognition and avoiding unsafe conditions
Notifying the Authorities Before starting work, OSHA requires: Determine the approximate location of underground utilities. Contact the utility company to inform them of proposed work and have them mark location of lines.
Competent Person One who can identify existing and predictable hazards in the surroundings, or unsanitary, hazardous, or dangerous working conditions, and who has the authority to stop work until they are fixed. Does not have to be at the excavation jobsite at all times.
How Deep is the Excavation? Four-foot rule- OSHA requires that workers have a means to get in and out of a trench if it is four or more feet deep. You must not have to travel more than 25 feet to reach the means of egress.
How Deep is the Excavation? Five-foot rule- OSHA requires protection from cave-ins by protective systems unless the excavation is: –Entirely in stable rock, or –Less than five feet and the competent person inspects the excavation and determines there is no indication of a potential cave-in.
Testing & Classifying the Soil If your jobsite trench is not in stable rock or is five-feet or more deep, it must be protected from a cave-in. Testing and classifying soil is a critical component to protecting employees from a cave-in. The competent person is responsible for testing and classifying the soil. If there is ever a doubt about the soil type, it must be treated as type C.
Stable Rock Natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed. Usually identified as granite or sandstone. Probably not going to be on MSU campus.
Type A Soil TYPE A SOILS are cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (tsf) 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. No soil is Type A if it is fissured, is subject to vibration of any type, has previously been disturbed, is part of a sloped, layered system where the layers dip into the excavation on a slope of 4 horizontal to 1 vertical (4H:1V) or greater, or has seeping water.
Type B Soil TYPE B SOILS are cohesive soils with an unconfined compressive strength greater than 0.5 tsf but less than 1.5 tsf. Examples of Type B soils are: angular gravel; silt; silt loam; previously disturbed soils unless otherwise classified as Type C; soils that meet the unconfined compressive strength or cementation requirements of Type A soils but are fissured or subject to vibration; dry unstable rock; and layered systems sloping into the trench at a slope less than 4H:1V (only if the material would be classified as a Type B soil).
Type C Soil TYPE C SOILS are cohesive soils with an unconfined compressive strength of 0.5 tsf or less. 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. Also included in this classification is material in a sloped, layered system where the layers dip into the excavation or have a slope of four horizontal to one vertical (4H:1V) or greater.
Why bother classifying? The most important reason for classifying soil is that the results can be used to determine what type of protective system can be used. If it is decided that the soil will not be classified, then: –Excavations must have a slope of 1 ½ horizontal to 1 vertical (34 degrees). 1 1/2 1
What’s Next? Once the soil type at an excavation is determined, the next task is to select what protective system will be used. –Sloping or benching of the sides –Supporting the sides with shoring –Placing a trench shield between the sides MSU is free to choose the most practical design approach depending on the jobsite circumstances.
Sloping & Benching If the soil has been classified, there are more options for sloping and benching. Soil TypeHeight/Depth Ratio Slope Angle (in degrees) Stable RockVerticalVertical 90 o Type A¾:153 o Type B1:145 o Type C1½:134 o
Type A Soil Slope – ¾:1
Type A Bench Simple Bench Multiple Bench
Type B Soil Slope – 1:1
Type B Bench Simple Bench Multiple Bench
Type C Soil Slope – 1½:1
Type C Soil – Sloping & Shielding
Shoring & Shielding Shoring and shielding is used when the location or depth of the cut makes sloping back to the maximum allowable slope impractical. Shoring provides a support system for trench faces to prevent movement of soil, underground utilities, roadways and foundations.
Shoring Systems Shoring systems consist of posts, wales, struts and sheeting. Two basic types of shoring: –Timber –Aluminum hydraulic
Hydraulic Shoring Hydraulic shoring is a prefabricated strut and/or wale system made of aluminum or steel. Hydraulic shoring offers a critical advantage over timber shoring because you do not have to enter the trench to install or remove it. Most systems are light enough to be installed by one worker.
Example of Hydraulic Shoring
Shielding Trench boxes are different from shoring because, instead of shoring the trench face, they are intended primarily to protect workers from cave-ins and similar incidents. The excavated area between the outside of the trench box and the face of the trench should be as small as possible. Box should extend 18 inches above surrounding area if sloping toward excavation.
Example of Trench Box
Personal Protective Equipment Hard hats are required for trench work as there are always overhead hazards whether working alongside the trench or in the trench. Ventilation equipment may be required if there is a possibility of an atmospheric hazard.
Installing & Removing Protective Structures Connect support system members securely. Avoid overloading system members. Install other structural members to carry loads imposed on the support system when you need to remove an individual member. Remove from the bottom up. Backfill the excavation as soon as possible.
Getting In & Out of the Trench Structural ramps Ladders Earthen ramps
Jobsite Hazards Vehicle exposure – wear warning/reflective vest Spoil pile – 2 feet away minimum Falling loads – do not work under equipment Water – leave the trench during rainstorms, divert surface water away from trench Crossing over – never a good idea
Hazardous Atmospheres Oxygen levels 23.5% Combustible gas >20% of LEL High concentrations of hazardous substances When testing for contaminants or oxygen levels- –Testing must be conducted before employees enter trench and regularly to ensure safe atmosphere. –Frequency of testing should be increased if equipment is operating in the trench. –Testing frequency should also be increased if welding, cutting, or burning is done in the trench.
Emergency Rescues Respirators may be required, you must be trained and in the MSU respirator program. Lifelines must be provided and attended to at all times if you enter bell-bottom pier holes, deep confined spaces or other similar hazards. If you enter a confined space, you must be trained to recognize and deal with the hazards. Normally, trenches are not considered confined spaces because they are well ventilated.
Inspections Daily and before the start of work As work conditions change After every rainstorm When fissures, cracks, undercutting, water seepage, bulging at bottom, etc. occur When there is a change in the size, location or placement of the spoil pile When there is any indication of change or movement in adjacent structures
What’s wrong with this trench?
No means of egress Spoil pile too close to edge Shoring not complete Missing backfill No edge stabilization No hard hats No air monitoring
Summary Cave-ins account for most fatalities Four-foot rule – must have means of egress within 25 feet of all workers in trench Five-foot rule – protection from cave-ins must be provided by using shoring, sloping or trench box Soil classifications – Solid rock, A, B, C Hazards