Unsupported Vertical Trench Cut in Stiff Clay: Analysis vs. Regulation
A 12 ft deep sanitary sewer trench will be cut through a stiff, fully saturated clay with an undrained shear strength of 800 psf and a total unit weight of 120 pcf. Groundwater is at 15 ft, so the trench bottom is dry. The superintendent proposes an unsupported vertical cut: the excavation will be open for two shifts, a 20-ton dump truck will stage 8 ft back from the crest to haul spoil, and "the clay stands fine - we've done it before." A worker must enter the bottom to set invert grade. The owner's inspector is on site and must accept or reject the plan before the crew enters. The contractor holds a fixed-price contract and has already been delayed three weeks by utility relocations.
1. Compute a short-term factor of safety for the proposed unsupported vertical cut. Does the analysis justify the superintendent's plan?
Read the worked answer
Bulk stability: For a φ = 0 clay, Taylor's stability number at a vertical face (β = 90°) is about N = c/(F·γ·H) = 0.26 at the critical failure depth.
FS = c / (0.26 · γ · H) = 800 / (0.26 × 120 × 12) = ≈ 2.1
Equivalently, γH/c = 120 × 12 / 800 = 1.8 against a critical value near 3.8–4.0.
Why 2.1 is not a green light:
- Tension crack depth z_c = 2c/γ = 2(800)/120 = 13.3 ft, which exceeds the cut depth. The whole face sits in the theoretical tension zone, so the margin depends entirely on intact, moist, undisturbed clay.
- If a crack opens and fills with water, the driving force increases and the FS drops to roughly 1.2–1.4.
- The truck adds a surcharge of about 40,000 lb spread over roughly 8 × 20 ft ≈ 250 psf near the crest, cutting the FS to about 1.6–1.8 before any cracking.
- Temporary cuts are normally held to FS ≥ 1.5. The computed margin is thin and highly sensitive to assumptions that cannot be verified in the field.
Conclusion: The analysis does not justify an unprotected vertical cut for worker entry.
2. Independent of the soil strength, what does the standard require before the worker may enter, and what protective system should be directed?
Read the worked answer
The excavation is deeper than 5 ft, so a protective system is required unless the excavation is entirely in stable rock. Soil strength does not waive the requirement.
Soil classification matters: 800 psf = 0.40 tsf, far below the 1.5 tsf needed for Type A. This is Type B at best, and Type C if there is any evidence of fissuring, previously disturbed soil, or seepage. A vertical face is not permitted in either classification.
Acceptable directions:
- Slope or bench to the Appendix B configuration (1:1 for Type B, 1.5:1 for Type C), or
- Install a trench shield/box or shoring designed for the depth and soil.
Also enforce: spoil and equipment at least 2 ft from the crest; a ladder or ramp within 25 ft of every worker; and inspection by a competent person at the start of each shift, after rain, and after any event that could change conditions.
Inspector's action: Reject the plan, stop entry, and require a listed protective system. Do not design the system yourself; do not let the schedule pressure created by the utility delay drive acceptance.
3. The cut is left open over a rainy weekend. What changes, and what should be required before re-entry?
Read the worked answer
Rain changes the problem in three ways:
- Suction is lost. The apparent cohesion that made the vertical face stand disappears as the clay wets, and the effective strength drops toward the fully softened value.
- Tension cracks fill. A water-filled crack 8–10 ft deep adds a large horizontal driving force right at the crest, which can drive the FS below 1.0 even though the bulk calculation looked fine.
- Erosion and undercutting at the toe from runoff over the crest.
Required before re-entry:
- Competent-person inspection of the full depth, both walls, and the crest, with the excavation not entered until it is cleared.
- Removal of any sloughed material and re-grading or re-benching of the slope; reinstallation of the shield if it moved.
- Verify the crest is barricaded and runoff is diverted around the excavation, not into it.
- Do not accept "it looked the same Friday." The design assumption, not the appearance, is what failed.
Document the event, the inspection, and the corrective action; the protective-system cost remains the contractor's means-and-methods risk unless a differing site condition is properly established.