14.6 Excavation, Trench Safety & Utility Locating
Key Takeaways
- Excavation is a named sub-topic in the SWRCB distribution Water Mains and Piping category.
- Cal/OSHA requires a protective system for any trench 5 feet or deeper unless it is entirely in stable rock, and a competent person must inspect daily and after every rainstorm.
- Spoil piles, equipment, and materials must be kept at least 2 feet back from the trench edge.
- A means of egress such as a ladder must be within 25 feet of lateral travel for any worker in a trench 4 feet or deeper.
- California requires notification to Underground Service Alert at 811 at least two working days before excavation, and critical crossings should be potholed before machine excavation.
Why This Section Exists
A cubic yard of soil weighs roughly 2,700 to 3,000 pounds - about the weight of a small car. A collapse does not need to bury a worker to kill them; compression of the chest by soil at waist height is enough to cause asphyxiation. Trench collapses happen without warning and are usually over in seconds, and the second fatality in a trench incident is frequently a co-worker who jumped in to help.
California excavation work is governed by Cal/OSHA, Title 8 CCR, Construction Safety Orders, Article 6, which parallels and in places exceeds the federal standard.
When a Protective System Is Required
A protective system is required for any excavation 5 feet or more in depth, unless the excavation is made entirely in stable rock. A protective system may also be required in an excavation less than 5 feet deep if a competent person examining the ground finds indications of a potential cave-in.
Additionally, a Cal/OSHA permit is required for trenches or excavations 5 feet or deeper into which a person is required to descend, and California requires an annual permit or project permit through the local Cal/OSHA district office.
Protective Systems
| System | Description | Notes |
|---|---|---|
| Sloping | Cut the walls back to a stable angle | Determined by soil type; requires room and generates more spoil |
| Benching | Horizontal steps in the wall | Not permitted in Type C soil |
| Shoring | Hydraulic, pneumatic, or timber supports that hold the walls | Speedshores, waler systems |
| Shielding (trench box) | A structure that protects workers if the wall fails; does not prevent collapse | Must extend at least 18 inches above the top of the vertical wall; workers must stay inside the box |
Soil Classification and Maximum Allowable Slope
| Soil type | Description | Maximum allowable slope (for depths up to 20 ft) |
|---|---|---|
| Stable rock | Natural solid mineral, remains intact with vertical sides | Vertical (90°) |
| Type A | Cohesive with unconfined compressive strength of 1.5 tsf or more, e.g. clay, silty clay, hardpan | 3/4 : 1 (53°) |
| Type B | Cohesive with 0.5 to 1.5 tsf, e.g. silt, silt loam, angular gravel; also previously disturbed Type A | 1 : 1 (45°) |
| Type C | Cohesive with less than 0.5 tsf, granular soils, submerged soil, soil with freely seeping water | 1.5 : 1 (34°) |
[!IMPORTANT] Previously disturbed soil is never Type A. A trench dug through the backfill of an old utility trench - which describes most water main work in an established street - is at best Type B and is very often Type C. Soil that is submerged or has water seeping freely into the trench is automatically Type C, which is exactly the condition around a water main break. Classifying a wet main-break trench as Type A because the native material is clay is a fatal error.
The Competent Person
Cal/OSHA requires a competent person - someone capable of identifying existing and predictable hazards and authorized to take prompt corrective measures to eliminate them. Duties:
- Classify the soil, using at least one visual and one manual test
- Select and inspect the protective system
- Inspect the excavation, adjacent areas, and protective systems daily before work begins, as needed throughout the shift, and after every rainstorm or other hazard-increasing event
- Remove workers immediately if hazardous conditions are found - and that authority must be real, not nominal
Access, Egress, and Housekeeping
| Rule | Requirement |
|---|---|
| Egress | A ladder, stairway, or ramp must be within 25 feet of lateral travel for every worker in a trench 4 feet or deeper; ladders extend at least 3 feet above the landing and are secured |
| Spoil piles, equipment, and materials | Kept at least 2 feet back from the edge |
| Surcharge loads | Keep excavators, trucks, and stockpiles away from the edge; vibration from adjacent traffic and equipment destabilizes walls |
| Water accumulation | Workers may not work in an excavation with accumulated water unless it is controlled by dewatering with a competent person monitoring, or a diversion system |
| Adjacent structures | Support systems required where excavation could undermine a foundation, sidewalk, or pavement |
| Falling loads | No worker beneath a suspended or raised load |
| High-visibility apparel | Required where exposed to vehicular traffic |
Atmospheric Hazards
An excavation more than 4 feet deep where an oxygen deficiency or a hazardous atmosphere could reasonably exist - near a landfill, a sewer, a gas line, or contaminated soil - must be tested before entry and monitored, with ventilation and emergency rescue equipment available.
A deep, benched water valve vault or a manhole encountered during excavation is likely a permit-required confined space in its own right, with its own entry permit, attendant, retrieval system, and atmospheric testing. An excavation and a confined space entry are two different permits and two different sets of controls, and a job can require both.
Utility Locating
California law requires notification to the regional Underground Service Alert (USA North / DigAlert South) via 811 at least two working days and not more than 14 calendar days before excavation. The process:
- Call or submit online, describing and white-lining the dig area.
- Wait the statutory period for member utilities to mark.
- Verify the marks against your own records; if the marks look wrong or are missing for a utility you know exists, do not proceed.
- Pothole (daylight) every critical crossing by hand or with vacuum excavation before using a machine. Marks locate a facility horizontally within a tolerance zone; they do not tell you its depth.
- Maintain the marks for the duration of the work, and re-notify if they become illegible.
Tolerance zone: excavate by hand or vacuum within the specified distance either side of the marked facility. Machine excavation inside the tolerance zone is how gas lines get hit.
[!WARNING] Damaging a natural gas line during a water main repair is a mass-casualty risk. If a gas line is struck: evacuate the area upwind, do not attempt to stop the leak, eliminate ignition sources, do not operate vehicles or radios in the vapor cloud, call 911 and the gas utility from a safe distance, and do not re-enter until the gas utility clears the scene. Never attempt to plug or crimp a damaged gas line.
Traffic Control
Most water main work happens in a street. A traffic control plan conforming to the California MUTCD is required, with advance warning signs, tapers of the correct length for the posted speed, channelizing devices, and a buffer space between traffic and the work area. Flaggers must be trained and equipped. The work area and the trench must be protected from vehicle intrusion, and workers must wear high-visibility apparel. Night work requires lighting that illuminates the work without blinding drivers.
A crew is repairing a water main break in an urban street. The trench is 6 feet deep, water is seeping into it from the break, and the surrounding material is old utility backfill. How must the soil be classified?
Under Cal/OSHA requirements, how far from a worker in a 6-foot-deep trench must a means of egress be located?
Underground Service Alert has marked the dig area and the crew is ready to excavate across a marked gas line. What must be done?