Trench shield

Wet methods Use wet methods to minimize contamination with dusts. Clean vessels chemically vs. sandblasting. Use water sprays for cleaning. Clean areas frequently. Use water sprays to shield trenches or pump seals. 

Dry storage in a narrow, shielded trench has much to recommend it. The initial cost and maintenance are lower than water storage and, if the shielding plug to fill the top of the trench is made integral with the radiation source, a simple and reliable system results. 

Soil testing must be done on freshly excavated samples from the work site the testing determines the soil type and type of soil protective system that should be used. The three basic protective systems are sloping and benching, shoring, and shield (trench boxes) (Figure 6-2). 

In pipelines, especially those that are highly stressed from internal pressure, uniform and adequate support of the pipe in the trench is essential. Unequal settlements may produce added bending stresses in the pipe. Lateral thrusts at branch connections may greatly increase the stresses in the branch connection itself, unless the fill is thoroughly consolidated or other provisions are made to resist the thrust. Rock shield shall not be draped over the pipe unless suitable backfill and padding are placed in the ditch to provide a continuous and adequate support of the pipe in the trench. 

Samoa, like Iceland, has a moderate plume flux, but it is located on a fast moving plate near the Tonga Trench. It shows high, but variable, He/ He ratios (11-24 Ra) during its alkalic shield-building stage (Farley et al. 1992 Poreda and Farley 1992). These variations have been attributed to mixing between a plume source and recycled components derived from the nearby subduction zone (Farley 1995). 

A permanent or portable structure designed to withstand a cave-in in excavations or trenches. These structures can be pre-manufactured or job-built in accordance with the Occupational Safety and Health Administration (OSHA) regulation 29 CFR 1926.652 (c) (3) or (c) (4). It may also be called trench boxes or trench shields. See also Trench Shield. 

An engineered metal box that is placed in an excavation for the protection of workers. It does not provide structural strength to the excavation, but provides workers a safe worksite that protects them from collapsing material of the trench. A registered professional engineer must design the trench shield or trench box system, which can be pre-manufactured or built on site as necessary. 

To prevent cave-ins, excavations can be shored using timber or other materials. Sides can be sloped to reduce the overburden (weight and pressure exerted by large amounts of soil on the sides). Also, manufactured shield or trench box systems, or those designed by qualified engineers, can be use to protect construction workers. 

Contractors may also use a trench box or shield that is either designed or approved, or is based on tabulated data prepared or approved by a RPE. Timber, almninum, or other suitable materials may also be used. OSHA standards permit the use of a trench shield as long as the protection provided is equal to or greater than the protection that would be provided by the appropriate shoring system. 

Nathan approaches Mike. Mike, I just checked the new excavation on the south end. The soil was okay before it rained yesterday. Now I think we re going to have to put in a trench shield. ... 

Shields intended for trenches that are more than 20 feet deep. 

An employee was installing a small-diameter pipe in a trench 3 feet wide, 12-15 feet deep, and 90 feet long. The trench was not shored or sloped, nor was there a box or shield to protect the employee. Further, there was evidence of a previous cave-in. The worker may not have been aware of that. The employee apparently entered the trench and a second cave-in occurred, burying him. He was found face down in the bottom of the trench. 

Cave-in means the separation of a mass of soil or rock material from the side of an excavation, or the loss of soil from under a trench shield or support system, and its sudden movement into the excavation, either by falling or sliding, in sufficient quantity so that it could entrap, bury, or otherwise injure and immobilize a person. 

Shield (Shield system) means a structure that is able to withstand the forces imposed on it by a cave-in and thereby protect employees within the structure. Shields can be permanent structures or can be designed to be portable and moved along as work progresses. Additionally, shields can be either premanufactured or job-built in accordance with 1926.652(c) (3) or (c)(4). Shields used In trenches are usually referred to as trench boxes or trench shields. ... 

Additional requirement for shield systems used in trench excavations. Excavations of earth material to a level not greater than 2 feet (.61 m) below the bottom of a shield shall be permitted, but only if the shield is designed to resist the forces calculated for the full depth of the trench, and there are no indications while the trench is open of a possible loss of soil from behind or below the bottom of the shield. 

Q. Can workers enter a trench with water accumulation if the workers are protected from cave-in by shoring, shielding, or sloping and the water level is controlled ... 

A. Paragraphs 1926.1053(b)(6) and (7) address ladder footing displacement which is not normally a problem in trenches. If a ladder needs to be secured against tipping, it may be secured to a shield or member of a protective structure provided the ladder does not alter the effectiveness of the protective system. 

Trench shields must be installed to prevent lateral movement in a collapse —... 

A. Assuming that cave-in protection were required under 1926 Subpart P (Excavations), and that a shield were used to provide that protection, 1926.652(g)(1)(H) would require the shield to be ...installed in a safe manner to restrict lateral or other hazardous movement of the shield in the event of the application of sudden lateral loads. Therefore, although our standard does not set a maximum distance between a shield box and a trench face, an employer would be required to ensure that, in the event of a collapse of the face, the shield would not move laterally. Trench work is frequently performed with limited room for maneuver, which means that workers can be injured with just a little movement by a trench shield. 

The closer a properly constructed trench shield is to the trench wall, the less chance there is that it could be dislodged by a lateral force. As a practical matter, unless the shield were braced to prevent this type of movement, in most instances an employer would have to keep the shield quite close to the face. The employer s competent person would have to make the determination of whether the shield was close enough to the wall to ensure that there would be no lateral movement. 

Allowable gap requirement between the trench shield and trench sides. — 1926.652(g)(ii) —... 

In this letter, you request that OSHA accept and publish the 3-6 inches allowable gap between the trench shield and trench sides as an industry standard. The allowable gap requirement is defined and discussed in your position paper, dated July 3,2012. We have reviewed your position paper and provide the following comments for your consideration. You have stated ... 

However, other trench shield manufacturers have specified the allowable gap to be no more than 12 inches for both sides of the trench shield i.e., if a 3 inches gap occurs on one side of the shield. 

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