Question 1

What are the maximum allowable slopes for excavations less than 20 feet deep in different soil types?

Accepted Answer

The maximum allowable slopes for excavations less than 20 feet deep vary by soil type and are specified in Table B-1 of Appendix B to Subpart P. The required slopes, expressed as a ratio of horizontal distance to vertical rise (H:V), are:

Stable Rock: Vertical (90°)
Type A Soil: 3/4:1 (53°)
Type B Soil: 1:1 (45°)
Type C Soil: 1 1/2:1 (34°)

These slopes represent the steepest incline allowed under the most favorable site conditions to protect against cave-ins. The actual slope used must not be steeper than these maximums.

Question 2

When is a registered professional engineer (RPE) required to design a sloping or benching system for an excavation?

Accepted Answer

A registered professional engineer (RPE) must design the sloping or benching protective system for any excavation that is greater than 20 feet deep. This requirement is detailed in a note to Table B 1 in Appendix B to Subpart P of the construction standards. For excavations 20 feet deep or less, employers may use the pre approved options in the appendix without consulting an RPE.

Question 3

What should I do if I see signs of distress, such as cracks or slumping, in an excavation wall?

Accepted Answer

If signs of distress appear in an excavation, the actual slope of the wall must be made less steep to prevent a cave in. According to Appendix B of Subpart P, the slope must be cut back to an angle that is at least ½ horizontal to 1 vertical (½H:1V) less steep than the maximum allowable slope for that soil type. All work in the excavation should cease until this corrective action is taken by a competent person.

Question 4

What are some common signs of soil distress that indicate a potential excavation cave-in?

Accepted Answer

Signs of soil distress indicate that a cave in is imminent or likely. According to the definitions in Appendix B of Subpart P, a competent person should look for the following phenomena: Development of fissures or cracks in the excavation face or adjacent ground. Subsidence (sinking) of the edge of the excavation. Slumping of material from the face or bulging/heaving of material from the bottom. Spalling, where chunks of material break off the face. Ravelling, where small amounts of material like pebbles or clumps of dirt trickle or roll down into the excavation.

Question 5

Is a steeper slope ever allowed for short-term excavations in Type A soil?

Accepted Answer

Yes, a steeper slope is permitted for certain short term excavations in Type A soil. For excavations that will be open for 24 hours or less and are 12 feet deep or less, a maximum allowable slope of 1/2H:1V (63°) is allowed. For any Type A soil excavation deeper than 12 feet or open for more than 24 hours, the standard maximum allowable slope of 3/4H:1V (53°) must be used, as detailed in Table B 1 of Appendix B to Subpart P.

Question 6

Can an excavation in Type C soil be benched?

Accepted Answer

No, benching is not a permitted protective system for excavations in Type C soil. Due to its low cohesive strength, Type C soil is too unstable for benching. As outlined in Appendix B to Subpart P, employers must use simple sloping at a 1½H:1V angle or another protective system like shielding or shoring in accordance with §1926.652.

Question 7

What are the sloping rules for an excavation made in layered soils?

Accepted Answer

When excavating in layered soils up to 20 feet deep, the protective slope must be designed based on the least stable soil layer present. For example, if a stronger soil like Type B is layered over a weaker soil like Type C, the entire excavation face must be sloped at the 1½H:1V angle required for Type C soil. Appendix B to Subpart P provides specific diagrams and rules for various layered soil configurations to ensure stability.

Question 8

If an excavation in Type B soil has a vertically sided lower portion, what protection is required?

Accepted Answer

Yes, for an excavation 20 feet deep or less in Type B soil, a vertically sided lower portion is allowed if it is protected by a shield or support system. The shield or support must extend at least 18 inches above the top of the vertical side to prevent material from the sloped portion from falling onto workers. The upper, sloped portion of the excavation must have a maximum slope of 1:1 (45°), as required by the configurations in Appendix B to Subpart P.

Sloping and benching requirements

Topics covered

Questions

What are the maximum allowable slopes for excavations less than 20 feet deep in different soil types?

What should I do if I see signs of distress, such as cracks or slumping, in an excavation wall?

When is a registered professional engineer (RPE) required to design a sloping or benching system for an excavation?

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