Lateral earth pressure and trench safety on the PE Civil WRE exam

Lateral earth pressure is the theory in Soil Mechanics. Trench safety is the field rule in Project Sitework. The exam uses both for the same ground, whether you are loading a retaining wall or cutting a trench for a pipe, so it pays to study them together.

What you actually need to own

• Pick the state from how the wall moves. A wall that rotates or slides away from the soil sees active pressure (Ka). A wall pushed into the soil mobilizes passive resistance (Kp). A wall that cannot yield sees at-rest pressure (Ko), which falls between the two.
• The coefficients. For a normally consolidated soil, Ko = 1 − sin φ′. The Handbook's At-Rest Coefficients section also gives an overconsolidated form. For a smooth vertical wall with level backfill (Rankine), Ka = tan²(45° − φ′/2) and Kp = tan²(45° + φ′/2) = 1/Ka. Cohesion reduces active pressure by 2c′√Ka, which creates a tension-crack zone near the top.
• The pressure diagram. Lateral pressure = K × effective vertical stress. In a uniform dry soil the diagram is a triangle: Pa = ½ Ka γ H², acting at H/3 above the base. A uniform surcharge q adds a rectangle of Ka·q with its force at H/2. Below a water table, apply K to the effective stress (use γ′) and add water pressure γw·z at full value.
• The resultant. Split the diagram into rectangles and triangles. Sum the forces, then take moments about the base: ȳ = Σ(Pᵢ yᵢ)/ΣPᵢ. Point and line loads use the separate charts in the Handbook's Load Distribution from Surcharge section.
• OSHA soil types. Type A is cohesive with unconfined compressive strength of at least 1.5 tsf. Type B is between 0.5 and 1.5 tsf. Type C is 0.5 tsf or less, and also covers granular soils such as gravel, sand and loamy sand, plus submerged or freely seeping soil. Angular gravel, silt and silt loam are Type B. Fissuring, vibration, prior disturbance, seeping water, or layers dipping into the cut at 4H:1V or steeper disqualifies Type A. In layered ground, classify by the weakest layer, unless a more stable layer lies below a less stable one; then each layer may be classified on its own.
• Sloping, benching and protective systems. The maximum allowable slopes (H:V) are ¾:1 for A, 1:1 for B and 1½:1 for C. A short-term ½:1 is allowed in Type A no deeper than 12 ft. Benching is for cohesive soils. From 5 ft deep, a trench needs a protective system (sloping, benching, shoring or a shield) unless it is in stable rock. Past 20 ft, the system must be designed by a registered professional engineer. Egress is required from 4 ft.

Where people lose points

• Ka applied to water. Water pressure is not scaled by K. Running Ka through saturated total stress, or multiplying γw by Ka, undercounts the load.
• One Ka for layered backfill. Each layer takes its own Ka, so the diagram steps at each boundary.
• Active used for a wall that cannot yield. A basement or culvert wall restrained at top and bottom does not move enough to mobilize the active state, so it sees at-rest pressure.
• Resultant at H/3 regardless. H/3 holds only for a single triangle. A surcharge or a water table moves the resultant up or down.
• Slope read upside down. ¾:1 means 0.75 horizontal per 1 vertical. Top width = bottom width + 2 × depth × ratio for a symmetric cut.
• Depth thresholds. A trench under 5 ft still needs a competent person's inspection before it can go without protection. Egress allows at most 25 ft of lateral travel to a ladder, stair or ramp, so the worker midway between two ladders sets the spacing: 50 ft apart at most.

How to study it

Draw every pressure diagram before you write an equation, one shape per effect: soil, surcharge, water. Then practice finding the resultant from moments until it takes a minute. For trenches, learn the A/B/C criteria and the slopes as one table. Note that from the April 2027 exam, NCEES supplies selected subparts of OSHA 29 CFR 1926, including Subpart P for excavations. Check the specification for your exam date. Related: earthwork volumes, swell and shrinkage and well drawdown for dewatering an excavation.

Part of the Soil Mechanics area of the PE Civil WRE exam. → Start practicing free — the whole practice bank, free with an account.