Excavation, Trenching and Shoring
This section has been reviewed and updated as needed: May 2014
These requirements apply to all open excavations made in the earth’s surface into which employees are expected to enter. An excavation is defined as, “Any man-made cut, cavity, trench, or depression in an earth surface, formed by earth removal.” (OSHA 1926.650)
A trench (trench excavation) is defined as, “A narrow excavation made below the surface of the ground where the depth is greater than the weidth, but the width of a trench (measured at the bottom) is less than 15 feet (4.6 m) or less (measured as the bottom of the excavation), the excavation is also considered to be a trench”. (OSHA 1926.650)
Environment, Health and Safety:
- EHS will perform periodic inspections of excavation sites conducted by UNC-CH employees to ensure that compliance with OSHA regulations is met.
- EHS will review the Excavation, Trenching, and Shoring policy annually or as changes to OSHA regulations occur.
University employees are to comply fully with the following requirements:
- At least one ‘competent person’ must be on the job site at all times to ensure that OSHA regulations are being followed to the fullest.
- A ‘competent person’ is defined by OSHA as, ‘one who is capable of identifying existing and predictable hazards in the surroundings, or working conditions which are unsanitary, hazardous, or dangerous to employees, AND who has authorization to take prompt corrective measures to eliminate them’.
- This person must have proper training, knowledge, an experience in understanding excavations/trenching/shoring issues, AND must be given the authority by their employer to shut down a job site if they deem it as unsafe.
- Those deemed as competent persons must be retrained every two (2) years in excavations/trenching/shoring so that they are up to date with current regulations.
- A Checklist must be completed before entering the excavation, and before each work day. This checklist must be kept on site at all times to verify that the items listed were covered during the checklist inspection of the job site.
- Both visual and manual soil testing shall be performed by a “competent person” to determine soil type before employees are allowed to enter a trench. One (1) visual and one (1) manual test must be conducted by the “competent person”.
- All materials in proximity to the excavation site must be stored, arranged, or secured in such a manner as to prevent the material from accidentally falling into the trench so that Fall Hazard Injuries can be prevented. Keep all materials at least two (2) feet away from the trench.
- Prior to the commencement of the excavation, the Work Unit Supervisor is to notify the UNC Utilities Division, 2-8394, to request that underground utilities or installations be located. ULOCO should be contacted if public right-of-way property is involved.
- Adequate means of egress shall be maintained at all times. No A-frame ladders are to be used if a ladder is used as the means of egress.
- Excavations located near public vehicular traffic shall be barricaded and employees shall be provided with and wear warning vests.
- Caution tape or other means of barriers shall be placed around the excavation/trenching site.
- Atmospheric testing must be performed prior to the entry of employees in excavations greater than four (4) feet in depth, or where oxygen deficiency or other hazardous atmospheres could reasonably be expected to exist. Contact Kim Haley (843-2735) from EHS Industrial Hygiene for air monitoring questions and equipment.
- If a hazardous atmosphere is verified at a trenching site, emergency rescue equipment must be available and attended (SCBA, LIfelines, etc.) as required by the Confined Space Standard (see Confined Space section of the IMAC Manual).
- Inspection of trenching operations for hazardous conditions must be performed daily or when changing conditions warrant (rain, different soil type, etc.). Upon detection of a hazardous condition employees must be removed from the excavation at once.
- Protective systems for excavations are required to be inspected and certified by a registered professional engineer of the appropriate excavation discipline or field.
- Fall protection shall be provided for trenches that are 6 feet in depth AND 30 inches wide.
- “Where employees or equipment are required or permitted to cross over excavations, walkways or bridges with standard guardrails shall be provided.” (OSHA 1926.641, (1) 1)
- “Adequate barrier physical protection shall be provided at all remotely located excavations. All wells, pits, shafts, etc., shall be barricaded or covered. Upon completion of exploration and similar operations temporary wells, pits, shafts, etc., shall be backfilled.” (OSHA 1926.651, (1) 2)
- If the protective systems have been approved by a registered professional engineer, a copy of the Tabulated Data and Shoring Material Certification must be available on the site at all times.
- Scope, Applications, & Definitions. (OSHA 1926.650)
- Hazard Recognition in Trenching and Shoring. (OSHA Technical Manual: Section V, Ch. 2)
- Specific Excavation Requirements. (OSHA 1926.651)
Angle of Repose
When the area in which an excavation is being made is large enough to permit it, the sides of the cut shall be sloped at the natural angle of repose. This is the angle that the sloped surface would develop during natural weathering processes.
Excavations shall be sloped at an angle no steeper than once and one half feet (1½ ft.) on the horizontal to each foot on the vertical (34 degrees measured from the horizontal).
**Excavations shall be sloped in this manner unless the employer uses one of the other options that can be found in OSHA 1926.652.
When it is not possible to slope to the angle of repose, shoring must be erected to prevent a cave-in.
From an accident prevention standpoint, trenching is a major area of convern in excavation work. A trench is a narrow excavation made below the surface of the ground. In general, the depth is greater than the width, but the width of a trench is not greater than 15 feet.
Cave-ins may be caused by:
- Hydrostatic pressures from freezing or thawing;
- Vibration from vehicles or equipment operating inside the trench, or nearby;
- Improper sloping of trench walls;
- Placement of materials:
- Soil should be placed at least two feet from the trench.
- Materials should be placed a safe distance away from the trench to prevent injuries that may occur from falling objects (etc. chop saw, tool boxes, drills, pipes, etc.).
- Failure to shore properly and to brace trench walls;
- A common, contributing factor to cave-ins is the tendency to omit shoring when the trench is to be left open for only a short time.
**Caution** Cave-ins can sometimes occur when shoring is being installed or removed from within a trench.
**Cave-ins can result in possible injury or death.
Lack of Space
To some extent, all employees engaged in trenching operations are exposed to the hazards of cave-ins. Trenches are usually narrow and the lack of space sometimes complicates the danger of a cave-in.
Experienced employees can sometimes detect approaching danger of cave-ins from tension cracks, moisture, and changes in soil texture. Young or less experienced employees are less likely to recognize these indications.
Trench employees are exposed to hazards involving much more than soil movement. Underground utility lines often present dangerous surprises and account for a large number of fatalities in excavations.
The OSHA standards are specific on the need to locate underground utility lines before digging begins. However, even with advanced survey techniques, the exact location of such lines is difficult to determine until digging is in progress.
In work areas where the exact location of underground electrical power lines is unknown, employees using jackhammers, bars, or other hand tools which may contact a line shall be provided with insulated protective gloves.
Utility lines for water, steam, and gas present hazardous conditions that require the worker to use personal protective equipment. These conditions also require employees to obtain as much advanced information about the location as possible. Carbon monoxide can reach hazardous concentrations in excavations. Accordingly, provisions shall be made for detection and suppression of heavier than air gasses.
The failure to use personal protective equipment has resulted in many injuries which could have been prevented.
Shoring as defined by OSHA is, “A structure such as a metal hydraulic, mechanical or timber shoring system that supports the side of an excavation and which is designed to prevent cave-ins”.
The reasons for earth support in trenching (or other excavation) are:
- Control of property damage
- Safety of personnel
In a remote location, or where buildings are some distance away, the reasons for sidewall support may include limiting the quantity of material to be moved, expediting the job, and protecting workers; but structure protection would not be involved. In such a case, extreme rigidity of the means of the sidewall support would not be required. In any case, the timber stresses should not exceed the safe working stress value of the support.
Nearby Structures and Utilities
When a structure is close enough to lie within the limiting plane of rupture, the timbering must be rigid enough to prevent all movement of the earth which supports such a structure.
Sheeting and timbering must be designated to minimize any flexing of planks and timbers. This is because masonry is very weak in tension and any slight movement which alters the distribution of stresses will cause the foundation and walls to crack. Spalling (surface failure) of interior plaster may result.
Underground utilities and nearby structures, therefore, require that displacement of the soil adjacent to the cut be kept at an absolute minimum. Sheeting and bracing must be used and must be kept “tight.” In the “active state” of trenching there are settlements and lateral movements of the solid adjacent to the sheeting, which can be as much as 0.5 percent of the depth of the cut, notwithstanding good quality material and the best workmanship.
Damage to nearby structures from soil movement as described above can be controlled, or reduced, by stressing the struts-exerting pressure upon the sheeting and forcing it against the soil walls.
Sheeting thus designed and braced has the effect of holding the “at rest” pressure of the soil. The struts must be re-stressed regularly to insure that the shoring remains tight. Pouring dry sand between sheeting and supported ground helps to keep the ground subsidence to a minimum if it is completed as soon as shoring is set.
The Sliding Trench Shield
The sliding trench shield, or box, is:
A shoring system composed of steel plates and bracing, welded or bolted together, which supports the walls of a trench from the ground level to the trench bottom and which can bemoved along as work progresses.
It is not intended to entirely displace conventional shoring, but will substantially reduce its use.
Portable trench boxes or sliding trench shields may be used for the protection of personnel in lieu of a shoring system or sloping. Where such trench boxes or shields are used, they shall be designed, constructed and maintained in a manner which will provide protection equal to or greater than the sheeting or shoring required for the trench.
A sliding shield usually is constructed of about one (1) inch gauge steel plate. The length and side panel height is optional, depending upon the need. The front end is closed, with some models having a sled-type plow attached to facilitate ease in dragging the unit. The rear end may be closed or have an open structural arch. Cross-bearing must be used. Braces can be removed to permit insertion of pipe, etc., into the unit. If the unit requires cross-braces, they should always be replaced after being raised for any reason.
A unit can be designed for various widths, heights, and lengths through the use of interchangeable bolted plates.
When the sheeting reaches down to an underlying rock formation into which the cut must be carried by drilling and blasting, the joint between the bottom of the sheeting and the rock formation should be kept tight.
- In this situation, the upper part of the cut being made through the soil should be at least a foot wider than the cut which is to be made through the rock. This is to provide a ledge to hold the sheeting and the earth behind it.
- There are two important reasons for preventing such voids:
- To prevent pressure from the opposite side of the trench or cut from thrusting the sheeting and timbering system into the void.
- To prevent slumping of the soil from under structures. This may be located on the same side of the trench as the void.
- In the event of blasting, the sheeting system, including any joints which have been sealed, should be closely inspected to make sure there has been no disturbance that would permit earth movement. This should be done even in cases where a void problem has been corrected.
Very Soft Locations
When sheeting must be installed in very wet conditions, such as bogs, or marshes, it is subjected to unequally distributed stresses and unpredictable earth movement. This can cause one wall of the trench sheeting to raise or lower. Adequate provision, such as diagonal cross-bracing, must be made to prevent the timber system from jack-knifing and collapsing.
- Under such conditions, such as marshes, bogs, mud and quicksand, the sheeting and timbering structure must be sufficiently rigid to hold the sheeting vertically true to prevent any lifting tendency.
In fine sands, improperly installed drainage can, in some cases, remove soil particles and cause a subsidence of the ground surface. It can also cause the sand to lose it capillary moisture and dry out to the point at which the sand loses the strength provided by the moisture, and cause the face of the trench to slide.
- Small tension cracks can be grouted or filled with asphalt to keep out the water. Canvas, plastic, or similar material can be used to cover large cracks or the face of the trench.
- Other means of combating the surface water problem are diversion ditches and the use of excavated material such as a dike. Where water of either ground or surface type is a problem, a trench should not be left open longer than necessary.
Warnings of Potential Problems
Evaluation of the soil, sheeting, drainage, or other steps taken in trenching operations do not remove the need for a continuous watch on the excavation for signs of trouble. Even the most carefully evaluated measures can be in error, or conditions can change causing a failure to occur.
- Subsidence of the ground surface adjacent to the walls of a cut is the first sign of trouble. However slowly it may occur, if such subsidence continues, it is very serious.
- The third sign that failure is imminent is the spalling (surface failure) of small chunks of soil from the face of the trench wall.
- A quick bottom condition ordinarily occurs quite suddenly when water is pumped from an excavation.
- Sometimes small sand boils or mud volcanoes will give warning of instability by appearing at various points.
- On other occasions, the bottom will heave slightly and then suddenly become a bubbling mass of quicksand.
**It is important to note that most failures of trench walls happen during late winter and early spring. This is the period when soils are wet and water tables are high. Anytime there is a continued period of wet weather, whether from rain or melting snow, the danger of failing trench walls is greater.
**Trouble signs mean that IMMEDIATE action must be taken. First, braces must be installed in the trench. If the trench already has vracing, extra bracing must be used for reinforcement. Second, cracks should be sealed and covers installed. While precautionary action is being taken to prevent failure of the trench wall, all personnel and equipment not essential to the action being taken should be kept out of the trench and away from the area.
- Timber Shoring for Trenches (OSHA 1926 Subpart P Appendix C).
- Aluminum Hydraulic Shoring for Trenches (OSHA 1926 Subpart P Appendix D).
- Alternatives to Timber Shoring (OSHA 1926 Subpart P Appendix E).
Safety in the Area
**Hard Hats must be worn AT ALL TIMES while within the construction zone barriers.
**Employees shall not work within a trench at any time while it is being excavated.
Proper bracing, barricading, and shoring may be required to create a safe working environment for employees who may be working within the confines of a trench. It is one of the competent person’s duties on the job site to determine what type of system is best for the job being conducted, or if a shoring/bracing system is needed.
Workers must be able to get out of trenches quickly in the event of accidents. This is usually done so by ladders. OSHA regulations require that a stairway, ladder, ramp or other safe means of egress shall be located in trench excavations that are 4 feet (1.22 m) or more in depth, and the worker shall not be required to travel more than 25 feet (7.62 m) to reach the ladder.
**A-Frame ladders are NOT to be used as a means of egress from a trench.
Each employee in an excavation shall be protected from cave-ins by an adequate protective system designed in accordance with paragraph (b) or (c) of OSHA 1926.652 except when:
- Excavations are made entirely in stable rock; or
- Excavations are less than 5 feet (1.52 m) in depth and examination of the ground by a competent person provides no indication of a potential cave-in.
- Trenches less than 5 feet shall also be effectively protected when examination of the ground indicates the hazardous ground movement may be expected.
**Trenches less than 5 feet in depth do have the potential to cave in.
- Trenches less than 5 feet shall also be effectively protected when examination of the ground indicates the hazardous ground movement may be expected.
Protective systems shall have the capacity to resist without failure all loads that are intended or could reasonably be expected to be applied or transmitted to the system.
- Requirements for Protective Systems. (OSHA 1926.652)
- OSHA Regulation Interpretation: The Depth Shoring shall be Used.
- OSHA Construction E-Tool: Excavation.
EHS Contact Information
When employees are exposed to excavation or trenching operations that are not in compliance with these standards, they shall immediately vacate the hazardous exposure and inform the appropriate supervisor having jurisdiction over such operation of the unsafe conditions.
Contact the Department of Environment, Health and Safety at 919-962-5507 if there is a possible compliance issue with the excavation site.