OSHA excavation rules for the CHST exam
The OSHA trenching and excavation rules (29 CFR 1926 Subpart P) behind BCSP® CHST questions on excavation hazards and controls, with every trigger depth in one table, the soil types, Table B-1 slopes and worked sloping math.
By the PassCited editorial team. Last reviewed . Facts checked against the sources listed below; how we check.
Short answer
Under OSHA's 29 CFR 1926.652, every worker in an excavation 5 feet or deeper needs a protective system such as sloping, benching, shoring or a shield, unless the cut is entirely in stable rock. Sloping or benching deeper than 20 feet must be designed by a registered professional engineer, and the maximum slopes are 3/4:1 for Type A, 1:1 for Type B and 1½:1 for Type C soil.
Sources: eCFR, 29 CFR 1926.652, Requirements for protective systems and OSHA, 1926 Subpart P Appendix B with Table B-1 in text form
Last checked October 27, 2026 against BCSP's CHST5 blueprint (V.2022.04.12) and the eCFR text of 29 CFR 1926 Subpart P current as of October 23, 2026.
In short
- Where it sits on the CHST: the BCSP® CHST5 outline lists "common hazards and controls associated with excavations" as knowledge item 3 in Domain 1, Hazard and Risk Identification and Control, which is 36.6% of the exam.
- The rule behind it: 29 CFR 1926 Subpart P. Section 1926.651 covers the site hazards (utilities, access, atmospheres, water, spoil, inspections). Section 1926.652 covers cave-in protection. Appendices A and B give soil types and slopes.
- The numbers to memorize: 4 feet (ladder or ramp within 25 feet of travel, air testing where a hazardous atmosphere could exist), 5 feet (protective system required), 20 feet (sloping or benching must be engineer-designed), 2 feet (spoil and equipment back from the edge), 19.5% oxygen and 20% of the lower flammable limit.
- The slopes: stable rock vertical, Type A 3/4:1, Type B 1:1, Type C 1½:1, all horizontal to vertical.
This is a study sheet built from the regulation text, not compliance advice. State Plan states can have their own rules.
Every trigger number in one table
Most CHST excavation questions turn on one of these numbers. Each row is quoted or paraphrased from the paragraph cited.
| Number | Rule | Citation |
|---|---|---|
| 15 feet | A trench is narrow relative to its length, and its width at the bottom is not greater than 15 feet | 1926.650(b) |
| 24 hours | If a utility can't mark lines within 24 hours (unless local law allows longer), you may proceed with caution and detection equipment | 1926.651(b)(2) |
| 4 feet | Trenches 4 feet or deeper need a stairway, ladder, ramp or other safe exit | 1926.651(c)(2) |
| 25 feet | No more than 25 feet of lateral travel to that exit | 1926.651(c)(2) |
| 4 feet | Test the air before entry into excavations deeper than 4 feet where low oxygen or a hazardous atmosphere exists or could reasonably be expected | 1926.651(g)(1)(i) |
| 19.5% | Below 19.5% oxygen is oxygen deficient | 1926.651(g)(1)(i) |
| 20% LFL | Prevent exposure to flammable gas above 20% of its lower flammable limit, for example with ventilation | 1926.651(g)(1)(iii) |
| 2 feet | Keep spoil, materials and equipment at least 2 feet from the edge, or use retaining devices, or both | 1926.651(j)(2) |
| 6 feet | Walkways over excavations need guardrails where they are 6 feet or more above lower levels | 1926.651(l) |
| 5 feet | Protective system required unless the cut is entirely in stable rock, or under 5 feet with a competent person finding no sign of a cave-in | 1926.652(a)(1) |
| 20 feet | Sloping or benching for excavations deeper than 20 feet must be designed by a registered professional engineer | Appendix B, Table B-1, note 3 |
| 2 feet | You may dig up to 2 feet below the bottom of a shield or trench support system, only if it is designed for the full trench depth and there's no sign of soil loss | 1926.652(e)(2)(i), (g)(2) |
| 18 inches | Where a lower vertical side is shielded or supported, the shield or support must extend at least 18 inches above it | Appendix B, Figure B-1 |
| 24 hours | "Short term" means an excavation open 24 hours or less | Appendix B(b) |
Watch the wording. The ladder rule says "4 feet or more" in depth, while the air-testing rule says "greater than 4 feet". The protective-system rule is "less than 5 feet" for the exception, so a trench exactly 5 feet deep needs protection.
Site hazards in 1926.651
These are the "hazards and controls" half of the blueprint line.
- Surface encumbrances. Remove or support anything at the surface that creates a hazard (a).
- Underground utilities. Find the estimated location of sewer, telephone, fuel, electric, water and other lines before opening the excavation, contact the utility owners, and find the exact location by safe means as you approach. Protect, support or remove lines while the excavation is open (b).
- Access and egress. A competent person designs structural ramps used only by workers. Ramps for equipment need a competent person qualified in structural design (c)(1).
- Traffic and falling loads. Workers exposed to public vehicle traffic wear high-visibility vests. No one stands under loads handled by lifting or digging equipment, and workers stand away from vehicles being loaded or unloaded (d), (e).
- Mobile equipment near the edge. If the operator can't see the edge clearly, use a warning system such as barricades, hand or mechanical signals, or stop logs (f).
- Hazardous atmospheres. Test before entry as described above and retest as often as needed when controls are in use. Where hazardous atmospheres exist or could develop, keep rescue equipment (breathing apparatus, a harness and line, a basket stretcher) readily available, and attend it when in use. Anyone entering a bell-bottom pier hole wears a harness with an individually attended lifeline (g).
- Water. No work in excavations with accumulating water unless precautions protect workers. A competent person monitors any water removal equipment (h).
- Adjacent structures. Excavating below a footing or foundation is allowed only with a support system such as underpinning, in stable rock, or with a registered professional engineer's approval (i)(2).
- Loose rock and soil. Scale loose material or use barricades on the face, and keep spoil back 2 feet (j).
The competent person's daily inspection
Section 1926.651(k) is a favorite exam topic. A competent person inspects the excavation, the adjacent areas and the protective systems:
- daily, before work starts, and as needed through the shift,
- after every rainstorm or other hazard-increasing event,
- looking for possible cave-ins, failing protective systems, hazardous atmospheres or other hazards.
If the competent person finds those signs, exposed workers are removed until the hazard is fixed. Subpart P defines a competent person as someone who can identify existing and predictable hazards and has authorization to take prompt corrective measures (1926.650(b)). The definition is about ability and authority, not a card or course.
Soil types (Appendix A)
The competent person classifies each deposit from at least one visual and one manual test (Appendix A(c)). The classes, most stable first:
| Class | What it is | Unconfined compressive strength |
|---|---|---|
| Stable rock | Natural solid mineral matter that stays intact with vertical sides | Not applicable |
| Type A | Cohesive soils such as clay, silty clay, sandy clay, clay loam; cemented soils such as caliche and hardpan | 1.5 tsf (144 kPa) or greater |
| Type B | Cohesive soil in the middle range; angular gravel, silt, silt loam, sandy loam; previously disturbed soil; dry unstable rock | Greater than 0.5 tsf (48 kPa) but less than 1.5 tsf |
| Type C | Cohesive soil at the low end; gravel, sand, loamy sand; submerged soil or soil with freely seeping water; submerged unstable rock | 0.5 tsf (48 kPa) or less |
The traps are the conditions that knock soil out of Type A. No soil is Type A if it is fissured, subject to vibration from heavy traffic or pile driving, previously disturbed, or part of a sloped, layered system with layers dipping into the excavation at 4H:1V or greater. Strong but fissured or vibrated soil drops to Type B. Layers dipping in at 4H:1V or steeper make it Type C.
Layered soil is classified by its weakest layer. Each layer may be classified separately only where a more stable layer lies under a less stable one (Appendix A(c)(4)).
The manual tests to know by name: plasticity (roll a moist sample into a 1/8-inch thread; if a 2-inch length holds from one end without tearing, it's cohesive), dry strength, thumb penetration (Type A can be indented by the thumb but penetrated only with very great effort; Type C is easily penetrated several inches), pocket penetrometer or shearvane, and the drying test (cracks as it dries mean fissures).
Protective systems (1926.652)
Sloping and benching (1926.652(b)) can be designed four ways, and the exam may ask you to name them:
- Option 1: slope at no steeper than 1½H:1V (34 degrees), the Type C slope, without classifying the soil.
- Option 2: classify the soil with Appendix A and use the slopes and shapes in Appendix B.
- Option 3: other tabulated data, approved by a registered professional engineer, with a copy at the jobsite.
- Option 4: a design approved by a registered professional engineer, in writing, with a copy at the jobsite.
Shoring and shields (1926.652(c)) follow the same pattern: Appendices A, C (timber shoring) and D (aluminum hydraulic shoring), the manufacturer's tabulated data, other engineer-approved tabulated data, or an engineer's design. You can deviate from the manufacturer's data only with the manufacturer's specific written approval (c)(2)(ii).
Rules that show up as distractors:
- Remove support members from the bottom up, release them slowly, and backfill as supports come out (e)(1)(v), (vi).
- No one stays in a shield while it is installed, removed or moved vertically (g)(1)(iv).
- No one works on the face of a sloped or benched excavation above other workers unless those below are protected from falling material (f).
- Damaged protective equipment is examined by a competent person. If its safety can't be assured, it comes out of service until a registered professional engineer approves it (d)(3).
Table B-1 slopes and worked math
Appendix B's Table B-1 gives the maximum allowable slopes for excavations under 20 feet deep, as horizontal to vertical (H:V), with OSHA's rounded angles from horizontal:
| Soil or rock | Maximum slope (H:V) | Angle |
|---|---|---|
| Stable rock | Vertical | 90 degrees |
| Type A | 3/4:1 | 53 degrees |
| Type A, short term (open 24 hours or less, 12 feet deep or less) | 1/2:1 | 63 degrees |
| Type B | 1:1 | 45 degrees |
| Type C | 1½:1 | 34 degrees |
Figure B-1 in Appendix B shows benched configurations for Type A and Type B soil and none for Type C, so a question about benching Type C soil has no Appendix B answer.
The math is one multiplication: horizontal setback per side = H ratio × depth. The top width of a sloped trench = bottom width + 2 × setback.
Worked example 1: Type B trench
A trench is 10 feet deep and 4 feet wide at the bottom in Type B soil. Using Table B-1, the slope is 1:1.
- Setback per side = 1 × 10 = 10 feet
- Top width = 4 + 2 × 10 = 24 feet
Worked example 2: same trench, Type C
Same trench, but the competent person finds water seeping from the face, which makes it Type C (1½:1).
- Setback per side = 1.5 × 10 = 15 feet
- Top width = 4 + 2 × 15 = 34 feet
Worked example 3: signs of distress
The Type B face starts to spall and ravel. Appendix B(c)(3)(ii) says the slope must be cut back to at least ½H:1V less steep than the maximum. So 1:1 becomes at least 1½:1, and the 10-foot-deep trench needs a 15-foot setback per side.
Worked example 4: exit spacing
A 4-foot-wide trench runs 6 feet deep and 120 feet long. Because it's 4 feet or deeper, no worker may travel more than 25 feet sideways to a ladder, stair or ramp (1926.651(c)(2)). Every point in the trench must be within 25 feet of an exit, so exits can be no more than 50 feet apart and no more than 25 feet from either end. Exits at 25, 75 and 115 feet from one end leave a longest walk of 25 feet, so three ladders work here. Because it's also deeper than 5 feet, it needs a protective system as well.
If you want to check slope answers fast, the excavation slope calculator and the excavation slope formula with OSHA's Table B-1 use the same ratios.
How to study it
- Learn the trigger table cold, then drill the soil-type traps. Most errors come from the Type A exclusions and the layered-soil rule.
- Practice the setback math until it's automatic; it pairs with the noise math on the CHST formula sheet with worked examples.
- Read 1926.651 and 1926.652 once in full. Our pages on 29 CFR 1926.651 specific excavation requirements and 29 CFR 1926.652 protective systems quote the eCFR text.
- Then test yourself with the CHST excavation and trenching practice questions and the construction math practice questions.
- For where excavations fit in the whole outline, see the CHST study plan in how to pass the CHST exam.
FAQ
At what depth does OSHA require cave-in protection?
At 5 feet. Under 1926.652(a)(1), every worker in an excavation needs a protective system unless the cut is entirely in stable rock, or is less than 5 feet deep and a competent person finds no indication of a potential cave-in. So a shallower trench can still need protection.
When does an excavation need a registered professional engineer?
Sloping or benching deeper than 20 feet must be designed by one (Table B-1, note 3). An engineer is also needed for tabulated data and custom designs under Options 3 and 4, for returning damaged protective equipment to service, and for approving work below an adjacent footing without underpinning.
How far back must spoil piles be?
At least 2 feet from the edge, or held by retaining devices, or both (1926.651(j)(2)).
How often must a competent person inspect an excavation?
Daily before work starts, as needed during the shift, and after every rainstorm or other hazard-increasing event, whenever workers can reasonably be expected to be exposed (1926.651(k)(1)).
Can previously disturbed soil be Type A?
No. Appendix A excludes previously disturbed soil from Type A. It is Type B unless it would otherwise be Type C.
What slope do I use if I don't know the soil type?
1½H:1V (34 degrees), the Type C slope. That is Option 1 in 1926.652(b)(1).
Check your readiness
When the trigger numbers and slope math feel solid, take the free diagnostic to see how you're doing across all four CHST domains.
Try 4 free CHST sample questions
From the PassCited question bank. Each answer is explained and cites its source. These are practice questions, not real BCSP exam questions.
Domain 1: Hazard and Risk Identification and Control
Question 1
A crew is working in a permit space that was reclassified as a non-permit space after hazards were eliminated. Partway through the shift, a worker's monitor shows a new hazard developing inside the space. What must happen first?
Show answer and source: 29 CFR 1926.1203(g)(4)
Answer: B. Each employee in the space must exit, and the entry employer then reevaluates and reclassifies the space as a permit space as appropriate
OSHA's construction confined spaces rule, 29 CFR 1926.1203(g)(4), says if hazards arise in a space reclassified as non-permit, each employee must exit. The entry employer then reevaluates the space and reclassifies it as a permit space as appropriate. Letting some workers finish, continuing under monitoring, or noting the hazard on the certification all skip the required exit.
On the job
On a wastewater treatment plant construction project, the entry employer's competent person had reclassified a new valve vault as non-permit after isolation and cleaning. Midway through the shift, a fitter's multi-gas monitor alarmed on a new hazard in the space. The coordinator ordered every worker out at once, without letting anyone finish the weld pass. Only after everyone was clear did the entry employer reevaluate the space and reclassify it as a permit space as appropriate.
Study aid, not site-specific safety advice. The official rule controls.
Answer B comes from
29 CFR 1926.1203(g)(4)(opens ecfr.gov in a new tab)“If hazards arise within a permit space that has been reclassified as a non-permit space under paragraph (g) of this section, each employee in the space must exit the space. The entry employer must then reevaluate the space and reclassify it as a permit space as appropriate in accordance with all other applicable provisions of this standard.”
Domain 1: Hazard and Risk Identification and Control
Question 2
After a heavy overnight rainstorm, a competent person inspects a trench early in the shift. She finds water pooled along one wall and fresh soil sloughing from it. Two workers already in the trench are laying pipe, and the crew lead says the morning inspection is done, so work should continue and be rechecked at the end of the shift. What does the standard require?
Show answer and source: 29 CFR 1926.651(k)(2)
Answer: D. Remove the exposed employees from the trench until the necessary precautions have been taken to ensure their safety.
OSHA's excavation rule, 29 CFR 1926.651(k)(2), requires that when a competent person finds evidence of a possible cave-in, exposed employees be removed until the necessary precautions ensure their safety. Finishing the section or rechecking later leaves workers exposed. A wall watcher removes no one. The 2-foot spoil setback keeps material from rolling in. It does not prevent wall failure.
Answer D comes from
29 CFR 1926.651(k)(2)(opens ecfr.gov in a new tab)“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.”
Domain 1: Hazard and Risk Identification and Control
Question 3
A crew is removing asbestos-containing floor tile and mastic (Class II work). No negative exposure assessment is available, so the employer issued elastomeric half-mask respirators with HEPA cartridges. One worker asks to use a tight-fitting powered air-purifying respirator (PAPR) with HEPA filters instead. The exposure conditions are such that the PAPR would give adequate protection. What must the employer do?
Show answer and source: 29 CFR 1926.1101(h)(3)(ii)
Answer: D. Provide the tight-fitting PAPR, because the worker chose it and it gives adequate protection
OSHA's construction asbestos standard, 29 CFR 1926.1101(h)(3)(ii), says the employer must provide a tight-fitting PAPR in place of the negative pressure half-mask when the employee chooses one and it gives adequate protection. Meeting the half-mask minimum does not let the employer refuse or make the worker supply the PAPR. Paragraph (h)(3)(i)(A) bars filtering facepiece respirators for asbestos.
On the job
At a warehouse reroofing job, a crew was removing asbestos-containing roofing material as Class II work, and the employer had issued half-mask respirators with HEPA cartridges. A roofer asked for a tight-fitting powered air-purifying respirator with HEPA filters instead. The EHS manager confirmed it would give adequate protection for the conditions and issued it at company expense. The manager did not point to the half-mask as the minimum or ask the worker to supply the unit.
Study aid, not site-specific safety advice. The official rule controls.
Answer D comes from
29 CFR 1926.1101(h)(3)(ii)(opens ecfr.gov in a new tab)“Employers must provide an employee with tight-fitting, powered air-purifying respirator (PAPR) instead of a negative pressure respirator selected according to paragraph (h)(3)(i)(A) of this standard when the employee chooses to use a PAPR and it provides adequate protection to the employee.”
Domain 1: Hazard and Risk Identification and Control
Question 4
On an elevated slab, a crew covers a floor hole with a sheet of plywood. The hole is in an interior work area where only workers and a small hand-propelled material cart travel. No vehicles use the area. A foreman inspects the cover and finds that it is strong enough to carry well over twice the combined weight of the workers, cart, and materials that could be on it at once. The cover sits loosely in place over the hole and is marked with the word "HOLE." Which statement best describes the cover's compliance?
Show answer and source: 29 CFR 1926.502(i)(3)
Answer: D. It is not compliant, because the cover is not secured against accidental displacement by wind, equipment, or employees when installed.
OSHA's construction fall protection rule, 29 CFR 1926.502(i)(3), requires every hole cover to be secured when installed so wind, equipment, or employees cannot displace it. This cover meets the strength and marking rules but sits loose, so it fails. The axle-load rule applies only to roadways and vehicular aisles, and a "HOLE" marking is an accepted alternative to color coding.
On the job
A foreman on a mid-rise job inspects a plywood cover over a small floor opening near a stairwell. The sheet is thick enough for the expected foot traffic and carries the word COVER in paint. It rests loose on the slab, so the foreman has the crew screw it down. Covers must be secured when installed so wind, equipment, or employees cannot displace them.
Study aid, not site-specific safety advice. The official rule controls.
Answer D comes from
29 CFR 1926.502(i)(3)(opens ecfr.gov in a new tab)“(3) All covers shall be secured when installed so as to prevent accidental displacement by the wind, equipment, or employees.”
Sources
- eCFR, 29 CFR 1926.650, Scope, application and definitions (Subpart P)
- BCSP, CHST5 Examination Blueprint, V.2022.04.12 (Domain 1, knowledge 3)
- eCFR, 29 CFR 1926.651, Specific excavation requirements
- eCFR, 29 CFR 1926 Subpart P, Appendix A, Soil classification
- eCFR, 29 CFR 1926.652, Requirements for protective systems
- eCFR, 29 CFR 1926 Subpart P, Appendix B, Sloping and benching
- OSHA, 1926 Subpart P Appendix B with Table B-1 in text form