«Leaders Guide Trenching and Shoring Safety - Competent Person The manual for Excavations Trenching and Shoring is produced in Adobe Acrobat. The ...»
Cohesive Soil A Cohesive Soil has a high clay content providing cohesive properties. A Cohesive soil does not crumble and can easily be excavated with vertical sides.
Cohesive soils include: sandy clay, silty clay, clay, etc....Often Type "A" unless previously disturbed or wet.
Dry Soil A dry soil is one that does not exhibit any visible signs of moisture. This is usually a Type "B" or "C" soil.
Fissured Soil A fissured soil has a tendency to crack with little resistance such as tension cracks. This is a very hazardous type of soil.
Granular Soil A granular soil has little or no clay content and no cohesive strength. Can not be molded when moist and crumbles easily when dry. Never Type "A" soil.
Plastic Soil A plastic soil can be rolled into 1/8" threads and held with out cracking. A plastic soil can be a Type "A" soil if previously undisturbed.
Saturated Soil A saturated soil is a soil in which the voids are filled with water. If submerged or has been under water then always Type "C" soil.
Protective systems are methods of protecting workers from cave-ins of material that can collapse into an excavation. As mentioned earlier if an excavation is less than 5 feet deep, OSHA does not require a protective systems (It is important to remember that a wall collapse in a trench four and 1/2 feet deep can still have serious results!) For trenches between 5 feet and 20 feet deep, are all acceptable protective measures. It is up to the planners of the construction project and the competent person to determine which systems will work best. If an excavation is greater than 20 feet deep, a must design the protective system.
All employees must be protected from the hazard of a potential cave-in by the use of trench protection systems designed in accordance with the standard.
A. The excavation is made entirely in stable rock. Be aware, cutting, blasting or breaking rock can de-stabilize it.
B. The excavation is less than 5 feet in depth and after examination by the Competent Person is deemed safe and there is no potential for a cave-in.
Any excavation 5 feet or greater will always require the use of a protective system. In certain circumstances excavations less than 5 feet may also require protection depending upon the judgment of the Competent Person and soil stability.
REMEMBER Protective systems in trenches or excavations greater than 20 feet in depth will require design by a registered civil engineer.
Shoring Systems Shoring systems are structures of timber, mechanical, or hydraulic systems that support the sides of an excavation and which are designed to prevent cave-ins.
is a type of shoring system that keeps the earth in position. It can be driven into the ground or work in conjunction with a shoring system. Driven sheeting is most frequently used for excavations open for long periods of time.
Another type of sheeting, in which plates or shoring grade plywood (sometimes called Finland form) is used in conjunction with strutted systems such as hydraulic or timber shoring. These strutted systems are also referred to as. The most frequently used strutted system involves which are lightweight, re-usable and installed and removed completely from above ground.
Design of slope or benching systems must comply with current standards.
Depending upon the soil classification, excavations will be sloped no greater than 3/4 horizontal to one vertical or 53 degrees, measured from the horizontal.
Slopes and benching systems will be selected and used by the employer and their designee (Competent Person or Registered Civil Engineer) must be in compliance with the requirements of the standard. (Sections 1541.1(b)(1) or in the alternative, Section 1541.(b)(3) or Section 1541.(b)(4)) Slopes 20 feet or greater designed by a professional engineer using the options
in 1541.1 (b) must be in written form and include the following at a minimum:
(A) The magnitude of slopes that were determined to be safe for the particular project (B) The configurations that were determined to be safe for the project (C) The identity of the Registered Professional Engineer At least one copy of the design must be maintained at the job site while the slopes are being constructed. After that time a copy must be made available to CAL/OSHA upon request.
Sloping and Benching Systems Sloping and benching are another means of protecting workers from cave-in hazards. Sloping is a method of cutting back the trench walls at such an angle that there is little chance of collapse. This is sometimes referred to as an "angle of repose", and must be suitable to the type of soil.
In the real world, there are very few applications where sloping and/or benching can be used. Why? Most often, the luxury of available space is the first consideration. Many excavations are dug in right-of-ways where the presence of other utilities and traffic become major considerations. Moreover, for every cubic yard of soil that is removed, it is very likely that nearly the same amount of material must be put back, and compacted as well
Table 5 Maximum Allowable Slopes
PROTECTIVE SYSTEMSThere are a number of ways to protect employees from the hazards of moving ground. Shoring should be your last consideration, as other means of protection should be considered first such as: Sloping and/or benching which cuts back the soil to prevent a cave-in from occurring.
Shoring Types 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 systems consist of posts, wales, struts, and sheeting. There are two basic types of shoring, timber and aluminum hydraulic.
TIMBERTimber used has been used for shoring material for centuries. Timber does have its place in trenching and excavations work but can be costly and a laborious process.
Timber used may be used as a method of trench protection in excavations less than 20 feet in depth.
Timber used may not be just any type of material purchased at the local do it yourself store. It must meet certain criteria such as actual and not nominal wood dimensions. In general, 4 inch material means four inch material, not 3 3/4 inch.
Refer to the bending strength of wood for mixed oak and Douglas fir.
Timber may also be used in conjunction with or in place of other types of shoring such as hydraulic and pneumatic systems.
Generally timber is used in excavations that are at least Type "A" or "B" soil. Be sure to refer to the tables provided by CAL/OSHA for minimum size, dimension, wood type and placement of timber to be used in a shoring system. (Refer to appendix C to section 1554.1) Though timbers can provide very good trench protection, consideration must be given to the cost of material, its working life, and the labor to construct to a shoring system.
Timbers can be used as typical shores, constructed into waler systems such as what would be used for "tight sheathing" situations in saturated soil or other Type "C" material.
Refer to the tables for the proper size and spacing of cross braces.
ALUMINUM HYDRAULIC SHORINGThe trend today is toward the use of hydraulic shoring, a prefabricated strut and/or wale system manufactured of aluminum or steel. Hydraulic shoring provides a critical safety advantage over timber shoring because workers do not have to enter the trench to install or remove hydraulic shoring. Other advantages
of most hydraulic systems are that they:
Are light enough to be installed by one worker;
Are gauge-regulated to ensure even distribution of pressure along the trench line;
Can have their trench faces "preloaded" to use the soil's natural cohesion to prevent movement; and Can be adapted easily to various trench depths and widths.
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.
Aluminum hydraulic shoring again is only to be used as a trench protection method in excavations that do not exceed 20 feet in depth and only if the type of soil allows the use.
Aluminum hydraulic shoring is intended to be used in Type A and B soil only, and in trenches that do not exceed 20 feet in depth.
Like timber, this type of shoring must also meet certain specifications for us in trenches. Note: The width of the trench will dictate the cylinder diameter to be used.
The soil type and trench depth will also determine how far apart to space the shores.
Like timbers, this type of shoring can be used individually or in a waler system for greater protection.
Remember that the manufacturers‘ tabulated data must be on site for this equipment.
2 inch cylinders must have a minimum of 2" inside diameter with a safe working load of no less than 18,000 pounds compressive load.
3 inch cylinders must have a 3 inch inside diameter with a safe working load of no less than 30,000 pounds.
A hydraulic hand pump is required to apply the proper pressures to the wall of the trench. Depending up the manufacturer, the pump is charged into the green zone or about 750 psi – which causes the shoring to be squeezed against the trench wall at about 2250 psi. The pump uses water and water soluble oil for the hydraulic fluid.
Caution must be taken when working with aluminum hydraulic shoring as there are several pinch points, therefore be sure to use only shoring with guards in these areas. Shoring can be heavy and care must taken to avoid back injuries by using good lifting techniques and/or using help. Be sure to take advantage of the shoring release tools provided with the equipment. Not only do they speed installation, but make it safer.
Be sure to inspect the shoring prior to use to ensure that they are in good condition and have no leaks.
Screw jacks must only be used with the proper type of iron pipe (schedule 40) cut to length to fit the trench opening.
The concern with screw jacks is that there is no means to determine what pressures you are exerting upon the trench sides. In addition, to install the shores it requires that an employee enter the unprotected trench to apply the screw jack to the timber. This flies in the face of trench safety recommendations The safest method to install this jack would be to place the ladder in the middle of the trench, stand on a ladder beginning at 2 feet from the top of the trench and step down the ladder to within 2 feet of the bottom of the trench; tightening the jack handle as you proceed down the ladder.
Be sure to nail the feet of the screw jack to the timber to prevent the jack from falling out when tightening the other screw jacks.
A, also known as a trench box, is another common protective system used by contractors. Trench boxes are not designed to prevent cave-ins, but rather serve to "shield" workers within the structure should a cave-in occur. This is an excellent choice when placing continuous installations, as in pipe laying.
The box is placed in the trench and dragged along with the progress of the work.
A few important points about shields:
Personnel should be out of the box and above ground when the shield is being moved. You could be caught between the moving box and fixed object(s);
Employees must remain in the confines of the box when working The top of the shield should extend at least eighteen (18) inches above the level of any materials that could cave or roll into the trench;
Some shields are designed to be stacked, one on top of another. Never stack shields that are not designed for that purpose, and do not stack shields from different manufacturers, as they may not be compatible.
The forces of a cave-in can literally push a box sideways, causing a crushing hazard. After a box is positioned for the work, the voids between the box and the trench wall should be filled with excavated material to prevent displacement caused by a cave-in.
Shielding should always be used according to manufacturer‘s tabulated data.
Trench shields are an effective means of providing trench protection in poor soil conditions.
Shields allow employees to work safely in an enclosed area. Shields can be very versatile allowing the units to be stacked for very deep trenches. Remember: Be sure a Registered Professional Engineer approves systems for trenches that exceed 20 feet in depth.
Shield systems must be designed to have the capacity to resist without failure all loads that are intended or could reasonably be expected to be transmitted to the trench protective system.
CONSTRUCTIONConstruction. OSHA's Alliance Program. This is one of OSHA's Strategic Management Plan Focus Areas.
Construction. OSHA. A Spanish version is also available. Helps workers identify and control the hazards that cause serious construction-related injuries.
Trenching and Excavation Trenching and Excavation Cave-ins are perhaps the most feared trenching hazard. But other potentially fatal hazards exist, including asphyxiation due to lack of oxygen in a confined space, inhalation of toxic fumes, drowning, etc. Electrocution or explosions can occur when workers contact underground utilities.
OSHA requires that workers in trenches and excavations be protected, and that safety and health programs address the variety of hazards they face. The following hazards
cause the most trenching and excavation injuries: