Excavation Safety LCS 6465 (07-11)

Transcription

1 Excavation Safety LCS 6465 (07-11)

2 Excavations: Hazard Recognition in Trenching and Shoring A. Weight of Soil The weight of soil varies with type and moisture content. One cubic foot of soil can weigh from 110 pounds to 140 pounds or more, and one cubic yard (27 cubic feet) of soil can weigh more then 3000 pounds, about as much as a small pickup truck. Natural Forces A number of stresses and deformations can occur in an open cut or trench. The most common results are: Tension cracks usually form at a horizontal distance of 0.5 to 0.75 times the depth of the trench. Sliding or sloughing may occur as a result of tension cracks. In addition to sliding, tension cracks can cause toppling. Subsidence at the surface and bulging of the vertical face of the trench. Bottom heaving or squeezing is caused by the downward pressure created by the weight of adjoining soil. Boiling is evidenced by an upward water flow into the bottom of the cut. The forces of nature are quite impressive and unless disturbed are held in a state of equilibrium. When equilibrium is constant, no movement will occur. This can be shown by looking outside at the ground, the ground is not moving because these forces are in equilibrium. It is when we excavate, and remove soil, that a state of instability occurs. Now that this state has been disturbed, the soil needs to go back to its original state and begins to move about by these forces. Example: A stack of concrete blocks will stand vertically provided that they have been set on a level surface. No movement will occur unless a horizontal force is applied to one side. This is the same force that is applied to a soil column that has no equal force pushing back. With only one side being applied pressure, it will eventually fail, resulting in a cave-in. B. Soil Types OSHA categorizes soil and rock deposits into four types. 1. Stable Rock 2. Type A Soils Type A soils are cohesive soils with an unconfined compressive strength of

3 1.5 tons per square foot (tsf) or greater. This soil typically has a high clay content. 3. Type B Soils Type B soils are cohesive soils with an unconfined compressive strength greater than 0.5 tsf but less than 1.5 tsf. This soil will usually contain clay. 4. Type C Soils Type C soils are cohesive soils with an unconfined compressive strength of 0.5 tsf or less. This soil can contain clay but will be soft. Typical Type C soil would be very granular like a sandy soil. Proper Method for Evaluating Soil To properly determine a soil type, the competent person must perform. At least one (1) visual analysis and, At least one (1) manual analysis of soil at the site of the excavation. It is the responsibility of the Competent Person to perform both tests, then use the combined information to determine the soil type. If both visual and manual methods are not used in combination, the soil cannot be properly evaluated and could result in a cave-in. It is also important to remember that as conditions change, the competent person must re-evaluate the soil. Changing conditions include rain or if the operation is moving and new soil is being excavated such as would be found during pipelaying operations. Visual Analysis of Soil A visual test is a qualitative evaluation of conditions around the site. In a visual test, the entire excavation site is observed, including the soil adjacent to the site and the soil being excavated. If the soil remains in clumps, it is cohesive; if it appears to be coarse-grained sand or gravel, it is considered granular. Other visual considerations include: Existing water Fissuring of the soil Heavy equipment or traffic that can create vibration Layered soil systems Manual Test Equipment and Methods Many kinds of equipment and methods are used to determine the strength of soil prevailing in an area. Pocket Penetrometer Penetrometers are direct-reading, spring-operated instruments used to determine the unconfined compressive strength of saturated cohesive soils. Once pushed into the soil, an indicator sleeve displays the reading.

4 Thumb Penetration Test The thumb penetration procedure involves an attempt to press the thumb firmly into the soil in question. Type A soil can only be barely penetrated by the thumbnail Type B soil can only be penetrated up to or just past the thumbnail Type C soil can be easily penetrated up past the thumbnail The thumb test is subjective and is therefore the least accurate of the three methods. Soil can not be classified by this test alone. Once a visual and manual test have been performed, the competent person must look at the definitions for each soil type and based on the findings, choose the soil type by referring to Appendix A found in the OSHA excavation standard. C. Protective Systems No matter what type of protective system is used at the jobsite, always stay inside of the protection at all times. Shoring Shoring is the provision of a support system for trench faces used to prevent movement of soil, underground utilities, roadways, and foundations. Shoring or shielding is used when the location or depth of the cut makes sloping back to the maximum allowable slope impractical. Shoring Types Timber Uses either Douglas Fir or Mixed Oak Aluminum Hydraulic Uses hydraulic pressure to stop the soil from moving. All shoring should be installed from the top down and removed from the bottom up. Hydraulic shoring should be checked at least once per shift for leaking hoses and/or cylinders, broken connections, cracked nipples, bent bases, and any other damaged or defective parts. Trench Shields, Boxes, Shoes Trench boxes are different from shoring because, instead of shoring up or otherwise supporting the trench face, they are intended primarily to protect workers from cave-ins and similar incidents. Sloping This is a method of cutting back the vertical dirt wall of the excavation in order to prevent the soil from sliding or falling into the excavation due to forces applied to it. OSHA has included diagrams in Appendix B of the excavation standard that can be used to determine proper slope based on type of soil being excavated. Type A soil Slope at 3/4 H : 1V Type B soil Slope at 1H : 1V Type C soil Slope at 1 1/2 H : 1V

5 Benching There are two basic types of benching, simple and multiple. The type of soil determines the horizontal to vertical ratio of the benched side. Spoil Temporary spoil must be placed no closer than 2 ft from the surface edge of the excavation, measured from the nearest base of the spoil to the cut. D. Excavation Hazards Utilities Always use the one-call system to mark underground utilities Hand dig when inside of marked zones Be aware of overhead powerlines that may be contacted by high reaching equipment Surface Crossing of Trenches Any ramps used to cross trenches must have standard guardrails installed Access and Egress Trenches 4 feet or more in depth should be provided with a fixed means of egress. All ladders, stairs or ramps must be with 25 feet of workers in the excavation. Exposure to Vehicles Workers exposed to vehicles must wear warning vests Always stay inside work zone areas Never walk out from in front of vehicles without looking for oncoming traffic Exposure to Falling Loads No workers are allowed under overhead loads from any equipment Be aware of any loads being lifted Hazards of Water Never work in an excavation that has standing water or where water is seeping in from the side walls. Methods for controlling standing water and water accumulation must be provided.

6 Atmospheric Hazards Carbon monoxide from vehicles can collect in a trench. Contamination from leaking sewer or gas lines can also cause dangerous concentrations of gas in a trench. Manholes may not contain enough oxygen to sustain life or could contain carbon monoxide, hydrogen sulfide or explosive methane gas. E. Competent Person The designated competent person should have and be able to demonstrate knowledge of excavation hazards, soil analysis and the requirements of the OSHA excavation standard and have authority to stop work. Inspections Inspections shall be made by a competent person and should be documented. Inspections must be made daily, after rainstorms and after other occurrences which might negatively affect the excavation or protective system. Look for: Protective system failure Evidence of water Evidence of fissuring Hazardous atmospheres Emergency Procedures In the event of a cave-in, equipment rollover or other emergency it is important that proper emergency procedures be followed. Know the phone number and method of communication for getting professional help Shut down all equipment to minimize vibration Keep away from the edge of the excavation to reduce surcharge loading Never jump into the excavation to attempt a rescue, many would be rescuers become victims of secondary cave-ins.