Engineer’s Corner: Trench Shield Inspection and Repair

BY Steve Barnhardt


It is well understood that the use of shoring equipment requires regular inspection and maintenance. Understanding how to inspect a shoring device, what needs to be focused on, and how to make practical decisions about what is observed, is not so clear.  Steel shoring shields have very long lifetimes, which depend on how the product was manufactured, the method of use, frequency of use, and overall maintenance. This article discusses the major aspects of the inspection and repair of steel shoring shields.

It should be noted that in this article I make suggestions and recommendations regarding decisions that the owner and his/her Competent Person has to make. These recommendations are based on my engineering knowledge and experience with shoring shields, but are by no means meant to be an exclusive, exhaustive, or definitive list of what should be inspected as that responsibility shall remain with the duly identified Competent Person. OSHA does not provide any specific assistance or instruction regarding inspections, only that they must be performed; however, the decisions have to be made from the contractor’s perspective, without an approved criterion set currently available.  The North American Excavation Shoring Association (NAXSA) is one group that’s attempting to make strides to provide customers with this information.  NAXSA is formed predominantly of the premier trench safety manufacturers and suppliers in North America with decades of experience working with shoring equipment.  The group is trying to establish general recommendations that a contractor could refer to for guidance on its website.  The information provided will be of a more general nature, and as such geared more toward education, and where there is any question about decisions to be made those questions should be addressed in consultation with a knowledgeable engineer.

Inspection Requirements

The major requirement to inspect shoring shields is written in OSHA Subpart P:

1926.651(k)(1) Daily inspections of excavations, the adjacent areas, and protective systems shall be made by a competent person for evidence of a situation that could result in possible cave-ins, indications of failure of protective systems, hazardous atmospheres, or other hazardous conditions. An inspection shall be conducted by the competent person prior to the start of work and as needed throughout the shift. Inspections shall also be made after every rainstorm or other hazard-increasing occurrence. These inspections are only required when employee exposure can be reasonably anticipated.

1926.651(k)(2) Where the competent person finds evidence of a situation that could result in a possible cave-in, indications of failure of protective systems, hazardous atmospheres, or other hazardous conditions, exposed employees shall be removed from the hazardous area until the necessary precautions have been taken to ensure their safety.

These inspections can be separated into two categories:

  1. Condition of Equipment, and
  2. Proper Installation

A condition of equipment inspection should also be made every time the shield leaves the yard, again before it is put into the ground and every time it is brought back to the yard.

What to look for and how to evaluate the condition

A word of caution, before we dive into this discussion, evaluating the condition of a piece of safety equipment, has many degrees of evaluation. The analysis below is very much
a general discussion and general application of what may typically happen.  There will always be exceptions to these rules, the reader’s focus should thus be on the process and adapting that process, along with the company’s risk management and or safety departments, into a policy that will work for the reader’s company.


Shoring shields are continually being lifted, pounded, and pulled, resulting in scrapes, dents, punctures, and bent elements. Some of this “wear and tear” is acceptable, while others may require immediate repair.  It is helpful to develop a set of criteria that can be used to evaluate the seriousness and impact of the damage. A lot of times, “cosmetic” wear and tear has no impact on the strength of the shield but the Competent Person must inspect the shield to make this determination.

A thorough inspection of the critical areas of structural elements, as well as damage to the surface that could hide these damages, of the shield, is what should be looked at during inspections. The structural elements of the shoring shield are:

    • The wall: A trench shield’s structural weighting can be envisioned as a very wide beam that is simply supported at the ends. If the beam is too heavily
      loaded it will bend in the middle. The middle third of the wall is the critical area, for instance, the middle third of a 24-ft long shield is the 8-ft section in the middle of the shield. This area should have no punctures, dents, or waffling (skin pushed in around the ribs). If these conditions are present the shield should be evaluated further by the Competent Person to assess whether it should be removed from use immediately or the structural capacity down-rated.  For the Competent Person, if this is discovered in the field the Competent Person will need to make a decision on how to address the excavation.  Some companies will operate on a zero-risk policy and require the removal of the shield.  Other companies may allow the Competent Person to make that decision by looking at the depth rating of the shield and the depth of the excavation.  For example, a company may allow the Competent Person to decide to keep the shield in the ground if it has a depth rating of 18 feet in the given soil type but is only being used at 12 feet for minimal damage.  For excavations where removing a system is a concern, it’s possible the contractor could reinforce the structural weakness to prevent a larger issue or maintain the production cycle.  The help of a registered professional engineer could also be enlisted in this instance to provide a stamped engineered plan to address the issue and limit the contractor’s and Competent Person’s liability for the adaptation.  Shield walls deflect when they are loaded and should return to the original position upon removal unless exposed to pressure beyond the shield’s yield strength, in which case there may be a permanent set of deflection.  All manufacturers and engineers calculate loading somewhat differently as there are a lot of assumptions to be made regarding spoil piles, setback, surcharge loads and adjacent structures, etc., however, generally speaking, the allowable loading is based on approximately 2/3 of the yield strength.  Once the shield reaches its yield strength and bends, even after the load is removed the shield will not return to its original state. To straighten the shield, some think it would have to be loaded again beyond yield strength in the opposite direction to bend again. This process would bend the shield back to its original shape, however, the process has structurally weakened the shield and it will no longer have the same capacity it originally did.  The contractor should enlist the counsel of a trained professional experienced in rating the shield.  If desired, a registered professional engineer can re-rate the shield based upon the deflection and repair process.  As a very rough general rule, an unloaded steel shield wall should not be deflected more than the length divided by 150. For example, a 24-ft shield is 288-in long, 288-in/150= 1.92” or approximately 2” for a 24-ft shield, and 1-in for a 10-ft shield.  If a shield wall deflects more than that it should be rejected from use until an engineer reviews the shield and provides a new capacity rating, if possible, given the amount of deflection.  During use in the field, shields become deflected due to soil loading.  When the Competent Person does the field inspection he needs to have some type of criteria to determine how much deflection is acceptable.  Table 1 gives the deflection that results from loading to the maximum allowable load.
      Deflection is the best indication of when the shield is within the allowable load range. These deflections should never be exceeded and should be watched closely when deflection is within 80% of allowable. At this point, due to allowable stress design factors, the shield is still within a 2:1 factor of safety from rapid bending failure.  Cosmetic wall punctures, scrapes or dents 6-in long or less found outside of the middle third and outside the end beam critical area will typically have a minimal impact on the shield’s strength.  If punctures show up during daily in-service inspections in these critical areas the shield should be retired from use until they are repaired. For this type of dent or ding outside the critical areas, repairs can wait until after the shield is taken out of the ground and before the next use.
  • The end beam: This can be envisioned as a cantilever beam similar to a diving
    board. Bending at the bottom end of a shield is evidence of yield strength failure. If this deflection is greater than 1-in when it is out of the ground it should be straightened and down-rated because the welds have been over-stressed. The deflection due to ground loading should never be
    allowed to exceed 20% of the wall thickness, for example, the deflection at the bottom corner for a 6-in shield should never be allowed to exceed 0.2 x 6 = 1.2-in, with certain applications requiring even lower tolerances.


    • Spreader receivers: The tension receiver hole should be round and have no stress cracks emanating from the hole toward the open end or around the pipe from one end of the hole to the other end. Oval-shaped holes are cause for rejection until repair. This member is a critical element and has to be looked at closely before and after the shield is shipped because the Competent Person will not be able to look at it after the spreader is attached. Failure is rapid and will cause the toe of the shield to rotate in. During my career, I have evaluated receiver failure that resulted from excessive loading and also from defective spreader material – two areas the Competent Person should keep in mind during system selection.  Overloading is more likely to happen with high-rated shields because the main failure mechanism changes from wall bending to overloading the end
      beam and tension spreader.  The lower receiver (compression receiver) is a less critical member because it is in compression and steel compression strength is much greater than steel tension strength.
  • Spreaders: The lower spreader acts as a column and has the potential for
    buckling. The lower spreaders should always be straight and not dented. The
    upper spreader acts as a tension member like a cable and is subject to compression failure at the pinhole resulting in an oval-shaped hole, and stress cracking leading from the hole to the end of the spreader. On upper spreaders, the holes should always be round and inspected for stress cracks.  Spreaders are always Schedule 80 pipe material.


  • Pins and keepers: These elements connect the spreaders to the spreader
    sockets. They should be the correct diameter, steel rating, PSF rating and not be bent. Substitute bolts or other objects are never acceptable. Keeper pins should always be in place.



Other things to look for during inspections are:

  • Crushed top beam: Shields are often pounded into the ground. Once the top beam in the middle third is dented from pounding it is compromised and the shield should not be used until it is repaired. If older beat-up boxes are used they should be used on the top stack because the soil loading is less and they should also be down-rated by about 20%.  If shields are planned to be pounded some sort of pounding cap should be used to prevent damage to the top beam.


  • Skeleton reveals, and punctures outside the middle third: Over time and heavy use the shield skins get deflected inward and reveal the ribs. Dings and small punctures are also common. Underground contractors usually own these shields as part of their equipment fleet. These shields are still usable and can still be put to practical use, but the Competent Person should inspect them and down-rate the shield accordingly, typically a minimum of at least 20%. A registered engineer should evaluate these shields and issue a new tabulated datasheet.  In the field, the Competent Person should see to it that these types of shields are used at the top level and in OSHA Type A and B soils where there is little danger of them becoming overloaded.  Shields used for deep-down critical work and places like the bottom shields in a bore pit should be in excellent condition with no defects. These shields are hard to remove or replace should they be rejected during the competent person’s equipment inspection.

Competent Person inspection for installation and field conditions

In addition to inspection for the condition of the shoring shield, the Competent Person should also inspect for proper installation. The soil type and anticipated loading have to be determined and compared to the allowable depth and loading on the datasheet. The shield cannot be held more than 2-ft off the bottom and the soil should be within 12-in of the shield wall. Stacked shields should have stacking pins in place. Anticipated surcharge loads and setbacks must be determined and some sort of positive means of enforcing the setbacks should be established. Proper access from inside the shields should be available. Dewatering to the bottom of the shield should be evident.

Repair, down rating, and retirement

Repairs should always be performed by certified welders and with the consultation of the engineer that is going to recertify the tabulated data. For the contractor that owns his own shields, he should repair in consultation with a registered professional engineer familiar with the recertification of tabulated data. If the repair is performed independently the owner picks up all liability for defects, shield failure, and accidents related to the shield. Rented shields should be inspected when they are received on the job, similar to car rental the user will be charged for damage to the equipment.

As a practical matter, there is nothing wrong with using old user-owned shields on a project as long as they are maintained, safe, and have tabulated data stamped by an engineer. As the shield ages, it needs to be reviewed and potentially down-rated. Shields that have been completely rebuilt and or maintained to the original condition, like those most equipment rental companies supply, do not need to be downrated. In other words, steel retains its properties and strength forever so trench shields that have been rebuilt to the original condition are not worn down or weakened because they have been used for several years. It is also important to remember that the condition of the contractor’s equipment in the field reflects his pride in workmanship, safety culture, and company as a whole.

When shields are rented the user should expect to receive those shields in perfect condition. One major reason for renting versus owning equipment is that the contractor does not have to purchase, maintain, repair, or yard, and focuses project acquisition on a specific size or model that he owns. These are things that equipment rental companies specialize in and do well. At National Trench Safety we have three major repair centers across the United States that work full time and often double shifts repairing and refurbishing our shields to ensure contractors have safe equipment designed to operate at maximum capacity. At the distribution yards, the shields are always inspected before they are sent out. We are always searching for new innovations in shoring shields to bring to the construction shoring market.


DISCLAIMER: the information contained in this article is provided for general and illustrative purposes only and is not to be considered Site Specific and or designated engineering for any project or work zone, nor is it to be used or consider to be tabulated data, technical data, advice and or counsel to be used on any job site.  Each project is different and is the responsibility of the employer’s designated Competent Person to make decisions upon what systems and methods may be used in compliance with the federal and local regulations, manufactures tabulated data, engineered drawings and other plans.

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