Chapter 11 Flashcards
In the U.S., an average of 60 to 100 workers die each year in trench cave ins.
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Before any trench rescue or entry operations are initiated, the area around the trench opening must be evaluated for stability.
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A shoring team is assigned the function of measuring and placing shoring members in the trench environment.
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Trench lip - top edge of a trench.
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Cut team - a team assigned the function of building shoring and cribbing elements and other items as needed.
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Shoring teams create a safe working environment by strategically placing components in a trench environment.
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Cut stations use powered and hand tools to build lumber components needed to support a trench rescue.
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Trenching tools include short handled shovels with a blade that can be swiveled 90 degrees to form a hoe.
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Collapsible canvas buckets are used to remove dirt from trenches during rescue operations.
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Vacuum trucks provide a means of quickly removing loose soil from the bottom of the trench.
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Generally, protective equipment is disassembled in the reverse order of placement.
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A trench should be considered an attractive nuisance and may make a tempting location for children to explore, or it might draw curious onlookers.
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According to NFPA 1006, the skill set of a Level 1 rescuer is limited to conducting trench rescue operations to a non-intersecting trench that is no deeper than 8 feet.
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Section 650 of 29 CFR 1926 defines the term trench at the US federal level as a narrow excavation made below the surface of the ground.
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In general, the depth is greater than the width, but the width of a trench is not greater than 15 feet.
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Trenches may be excavated in many different configurations including, straight wall, T, L, and X.
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A registered professional engineer must design shoring plans for trenches deeper than 20 feet.
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Soil is divided rock composed of natural mineral materials present in the earth’s crust.
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Sedimentary rock is composed of layers that include natural fissures between the layers.
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Trench lip protection should include indications of unstable areas.
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Cohesive soils get their strength through the friction and chemical bonding that occurs between particles.
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Cemented soils can lose stability through fissuring, and water can dissolve the minerals that bond the soil.
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Composed primarily of clay, cohesive soils vary based on the type of clay, the moisture content, and the amount of granular soil in the mixture.
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Granular soils are characteristically grainy and, depending on the size of particles, may be identified as gravel, sand, or silt.
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The stability of granular soils is dependent upon the angularity of the grains, the amount of natural cements present, and the moisture content.
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Loams are mixtures of cohesive soils and granular soils.
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The stability of loams depends on the percentage of each of the constituents and the moisture content.
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Cohesive soils may not be vary stable, depending on the moisture content of the soil and other environmental conditions.
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To simplify the field classification of soils, regulatory agencies in the US classify soils into four general types - stable rock, A, B, C.
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Depending upon its composition and moisture content, a cubic yard of soil can easily weigh over a ton.
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Testing soil samples from the spoil pile is an effective method of determining the condition of the soil in the trench walls.
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Cohesiveness is a factor of grain size.
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Cohesive soils remain in clumps, granular materials break up easily.
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Tension cracks and evidence of material spalling off the vertical trench walls could be indication of dangerously unstable soil.
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Evidence of layering in trench walls may show a slope toward the trench.
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The visual analysis of an excavation takes into account the site in general, the soil adjacent to the trench, the soil forming the trench walls, as well as soil samples taken from within the trench.
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Manual tests of soil from the trench walls are done to provide additional quantitative and qualitative evidence of the soil’s composition and to assist in the proper classification of the soil.
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First, a moist or wet ball of the soil should be tested for plasticity.
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It is cohesive if a 1/8 inch piece of thread at least 2 inches in length can be held up by one end without breaking.
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In a test of dry strength, if a sample is dry and crumbles into individual grains or a fine powder with little or no pressure, it is considered to be granular.
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Inconsistencies or spalling in a trench wall can indicate instability.
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Another manual test, a responder’s thumb can be used to estimate the unconfined compressive strength of cohesive soils.
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While rescuers may not have time to precisely determine the soil type, observing the spoil pile and the exposed trench wall can give a good visual indication of the type of soil involved.
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Deep fissures near the trench lip should be monitored for changes over time.
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In technical language, a cave in refers to the movement of soil or rock in large enough quantity to cause damage or entrapment to a person.
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All responders at the scene should remain vigilant throughout the incident for signs of an impending collapse.
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Slough in often occurs when less cohesive soils near the middle or bottom of a trench collapse.
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The vertical and lateral forces at work in an excavation include the natural force of gravity on a vertical wall that lacks the lateral support that it formerly had.
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Given an average weight of about 2,700 pounds per cubic yard, the weight of the unsupported trench walls can generate incredible forces.
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When soils form layers, especially layers that incline into the excavation, the chances of trench wall failure greatly increase.
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When soils become saturated, the cementation and friction bonding of the individual particles begin to disappear, and the particles start to float apart.
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If a trench remains open for an extended period, the soil can dehydrate.
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Surcharge loads include any additional weight in proximity to the lip of the trench that increases trench wall instability and may contribute to initial or secondary cave ins.
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The full area of the lip of the trench is considered to be a horizontal distance from the lip of the trench equal to the depth of the trench.
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Undermining of the trench walls can result from over digging or from areas of the wall sloughing off into the bottom of the trench.
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Heavy machines operating near trench lip are the source of several types of hazard, including surcharge loads.
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Regardless of all other conditions that are present, the two factors that have the most effect on the stability of the trench walls are surcharge loads and vibration.
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Subsequent collapses that bury responders coworkers in the trench attempting a rescue creates a stratification of victims in which the uppermost victims must be removed before victims at a lower level can be reached.
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An initial collapse or a subsequent collapse and entrapment may create a victim stratification.
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Laying sheets of plywood or other types of protection, such as 2 inch lumber on the ground along the lip of the trench creates stabilization.
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Plywood that is used for lip protection should have a minimum thickness of 3/4 inch.
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The industry standard in the US is for the spoil pile to be a minimum of 2 feet away from the lip of the trench.
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Personnel assigned to move the spoil pile should begin on the side of the pile away form the trench lip and work towards the lip.
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In any case, rescuers should not have to travel more than 25 feet to reach a ladder.
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Sampling the atmosphere will help determine the need for mechanical ventilation.
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When rescuers and other employees must work in trenches deeper than 4 feet, employers are required to protect workers from cave ins.
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In sloping, both sides of the trench are inclined away from the bottom of the trench.
-Usually less than 34 degrees.
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Benching, also known as stepping, is a method of preventing cave in by excavating one or a series of horizontal steps or benches, usually with vertical or near vertical surfaces between levels.
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Shielding uses specialized equipment called trench boxes to protect workers in excavations.
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Because of the extreme impact forces that could be encountered, 1 1/8 inch CDX or 3/4 inch arctic white birch plywood should be used for sheeting.
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Uprights used in trench rescue operations serve the function of shielding the inside of the trench against debris falling from the trench wall.
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After vertical supports are in place, a ladder may be placed into the trench, with at least 3 feet of the ladder above the lip of the trench.
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A responder can then descend the ladder, no farther than waist deep into the trench, to measure the distance between the uprights at the level of the top brace.
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Horizontal timbers called wales hold planks in place.
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The wales are spaced vertically at the same intervals as the cross braces.
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Wale - timber placed against sheeting planks in an excavation to keep the sheeting planks in place.
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Horizontal supports, often cross braces or struts, hold the uprights firmly against the trench walls.
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Because only the vertical planks and the horizontal shores contribute to the strength of the system, the cross braces need to be installed as level as possible.
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If joist hangers are used, they should be placed on one side of the trench only.
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Duplex nails are well suited to rescue work because they can be removed and reused.
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Timber horizontal shores are placed from the top toward the bottom, in sequence.
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Over tightening the first cross brace may cause one or both uprights to shift out of place at the bottom.
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Wedges or adjustable brackets may be used to secure horizontal shores in a trench rescue environment.
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Placement of shoring components should use utility rope to prevent items falling out of reach.
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Safe zone - the space within 2 feet in all directions of an installed support component of an existing approved shoring system.
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Once the shoring is in place, level 1 responders may enter the trench, but they must stay within the shoring safe zone.
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Given the weight of soil, this treatment may include the initiation of care for a crush injury before completely freeing a victim’s trapped limbs.
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The safe zone in a trench is the area where shoring is intact and correctly placed.
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