Excavation Protection: Choosing the Correct System
By: Dylan Hipple CHST, GSP
The number of deaths related to trenching and excavation has more than doubled over the last year. Excavations remain a national special emphasis for OSHA. OSHA has increased the enforcement of excavation and trenching activities throughout the US. This increased enforcement is due to the high number of collapses and number of deaths and injuries related.
Trenching and excavation activities are executed around the world every day. Work inside of excavations is considered one of the most perilous jobs in the construction industry. This type of work presents serious hazards to all workers involved. Multiple fatalities are recorded every year in collapses and cave-ins of trenches and excavations. One cubic yard of soil can weigh as much as a car. If left unprotected, an excavation can lead to an early grave for workers. It is the employer’s responsibility to ensure that workers are adequately trained and protected before entering an excavation. This article will outline the different protection systems and the correct application of each of them.
Choosing the Correct Protection System:
In excavation and trenching operations protecting is preventing. OSHA outlines numerous requirements in CFR 1926 subpart P regarding excavation and trenching. The following are a few of those requirements outlined by OSHA; Trenches/Excavations exceeding 20 feet require Professional Engineering Systems with tabulated data. OSHA’s Subpart P Appendices A-F also outline specific requirements when installing protective systems. Manufacturer’s recommendations must be followed for all protective systems utilized. All components of the protective system must be inspected daily by the Competent Person. When figuring out what the best method is for protecting works in excavations there are four common methods. Those methods are sloping, benching, and shoring or shielding.
Sloping is a viable option as a protective system. Sloping is often the most cost-effective way to protect employees from potential trenching and excavation hazards. However, sloping can only be chosen when there is adequate space around the excavation to get the required slope ratio. OSHA outlines different ratios for sloping dependent on the soil classification. Type A soil uses a width to depth ratio of ¾ to 1 or approximately 54 degrees. For example, a 12 ft deep excavation should be sloped back 9 ft. Type B soil uses a ratio of 1 to 1. For example; a 12 ft deep excavation should be sloped back 12 ft. Type C soil uses a ratio of 1 ½ to 1. An excavation that is 12 ft deep in type C soil should be sloped back 18 ft.
Sloping is beneficial over benching for a multitude of reasons. Sloping removes less material than benching does and requires the same amount of real-estate to achieve. Sloping is an easier process to accomplish on equipment and is easy to construct in the field. If depths reach greater than 20’ then engineered sloping is required.
Benching is the least common type of protection used in construction. Often sloping is chosen over benching operations for the reasons listed earlier. Benching is only feasible on type A and type B soils, you cannot bench type C soil. Benching must be constructed in accordance with OSHA’s 1926 Subpart P. There are different difficulties of a benching system. There is a simple bench and a multiple bench. It is often difficult for employees to understand the many requirements of a benching system, thus the reason it is rarely used in construction.
Shielding systems offer the most effective protection from potential collapses. There are multiple types of shoring and shielding systems available and they can be used in all soil types. Shielding systems include trench boxes, steel plates, and/or combination of protective systems. Shielding does not protect against soil failures. They are intended to protect workers, not support the excavation. Shielding systems do not support the face of excavations, rather they protect the workers inside of them. For that reason, shielding systems do not allow anyone to work outside the protection of the system being used. These systems must be used in accordance with manufacturer recommendations. Tabulated data sheets for all trench boxes must be present with the Competent Person. All components must be free of damage.
Trench boxes are separate from shoring. Unlike a shoring system, a trench boxes’ primary purpose is not to support the face of the trench. The primary purpose of a trench box is to protect the worker’s inside from cave-in like incidents. The area between the trench box and the trench wall should be limited to be as small as possible. The spacing between the sides of the excavation and the trench box are backfilled with soil or other similar material to prevent the movement of the trench box. “Shields may not be subjected to loads exceeding those which the system was designed to withstand”. In trench boxes, the braces and spreaders used must be approved by the manufacturer to use. All braces should be inspected and free of any damage. Safety pins must always be in place. Adequate access into and out of the trench box must always be provided. This means that the access/egress should be located inside of the trench box so that employees are protected 100% of the time. There are specific requirements for the installation of trench boxes. They must be backfilled not to exceed 6” of space between box and trench wall, must be no more than 2 feet above the bottom o fa trench in Type A & B soil, and must maintain solid contact with trench floor in Type C soil.
There have been multiple trench box failures do to the following scenarios: The gap between the bottom of the trench box and floor was too great and allowed the excavation to fill. Workers have been injured or killed in trench box mishaps when they have been outside the area of protection, the system was not installed properly, and working with damaged components of the trench box system.
When should shoring be chosen over shielding? There are numerous reasons to choose shoring systems. Some of those reasons to shore include;
- Utilized when protecting of adjacent buildings is needed. Shoring may be needed to support the weight and added pressure that adjacent buildings have on the soil around the excavation.
- They may also be used when protecting existing systems (sidewalks, roadways, etc.) All these man-made structures impose added pressure on the soil and should be accounted for.
- Real estate is not available to utilize conventional systems. In certain scenarios there is simply not enough space to utilize a system such as a trench box. Shoring systems can be tailored to the specific dimensions of the excavation in which they are intended.
- May reduce cost to straight cut and shore vs. sloping or shielding.
A shoring system is used to support the face of an excavation and to prevent movement of soil, underground utilities, roads, and foundations. A shoring system is typically used when sloping is an inadequate solution due to the depth of the cut or the location. A shoring system will consist of posts, wales, struts, and sheeting. Below are some of the different types of shoring systems that are available.
Different types of shoring:
- Sheet Pile - used in wetland areas where water intrusion is expected. Sheet pile systems are typically utilized in type B & C systems.
- Pneumatic Shoring - is a system operated by air. Common brands of pneumatic shoring include; Paratech, Airshore, Holmatro. A hydraulic shoring system is pressurized by air. Common pressures for pneumatic shoring systems are in the range of 200 to 350 psi. Pneumatic shoring systems can be costly.
- Hydraulic Shoring - The most common brand of hydraulic shoring is speed shoring. Hydraulic shoring is pressurized by a water/mineral oil mixture. There is a required pressure setting of 1800 lbs. Hydraulic shoring also has distance requirements; 18” from lip, 4 ft vertical/horizontal. Panels are required in this type of system. Typical panels used in hydraulic shoring systems include finland form (“Fin Form”). Depending on soil type or working room needed, whalers can be installed. Sheeting is required to be as close as possible for type C soil.
- Screw jack struts - Screw jacks are a type of system that is most commonly used with timber shoring. Screw jack systems differ from hydraulic and pneumatic systems in that the struts of a screw jack system must be adjusted manually. This creates a hazard because the worker is required to be in the trench to adjust the strut. In addition, uniform "preloading" cannot be achieved with screw jacks, and their weight creates handling difficulties. The requirements are outlined in OSHA Subpart P Appendix. The minimum strut size that can be used is 6x6. The spacing requirements for screw jacks are as follows; 2 feet from lip, 2 feet from floor, 4 feet vertical/horizontal. Lumber can be spanned to certain lengths and maintain load capacities.
- Timber shoring – A lot of lumber is required. OSHA outlines specific timber shoring requirements for each soil type in tables that are in OSHA Subpart P Appendix. Timber shoring is not convenient when installing utilities. Timber sizing requirements are also listed in the OSHA subpart.
- Lagging - Lagging is used in deep excavations and when large areas need to be protected. Typically lagging systems are installed for excavations that will be open for an extended periods. A lagging system is installed in stages. The lagging system typically stays in place and is backfilled. Lagging is a common system for large building excavations.
- Engineered systems - Engineered systems are utilized when an excavation exceeds 20 feet in depth. They are also used when additional exposures exist, such as an adjacent structure. An engineered system will also be required when special hazards exist.
Of those listed above, the most commonly used protective system today is a hydraulic shoring system. Hydraulic shoring consists of pre-fabricated struts and/or whaler systems, typically made of aluminum or steel. The most appealing aspect of hydraulic shoring vs. timber shoring is the safety advantage. With a hydraulic shoring system, workers do not have to enter the excavation to install or remove the shoring. Other advantages include:
- Lightweight and can be installed by one worker
- Have gauges to ensure there is even pressure distribution
- Can be “preloaded” to use the soil’s natural cohesion to prevent soil displacement
- Are versatile in various depths and widths of trenches and excavations
Proper installation should be done from the top down and removed the opposite way (bottom up). This method of installation and tear down will ensure that no employees are ever exposed to a potential collapse. Just like any other tools/equipment, hydraulic shoring should be checked before use and during the shift to inspect for leaking or broken hoses, bad connections, cracked nipples, bent bases, and any other damage.
Worker safety is the number one priority when deciding what protective system will be used. The different systems available all offer unique advantages and disadvantages. There is a plethora of solutions available for protecting workers in excavations. It is up to you to determine which one best suits your individual project needs.
Dylan Hipple CHST, GSP
UNITED STATES DEPARTMENT OF LABOR.” Occupational Safety and Health Administration, www.osha.gov/dts/osta/otm/otm_v/otm_v_2.html.
OSHA CFR 1926 Subpart P. Excavations