Soil Mechanics

How soil pushes, compresses and fails: lateral earth pressure, consolidation and compaction, bearing capacity, settlement, and slope stability. It carries 3–5 of 80 questions on the WRE exam — a small area, but the theory under it also drives the retaining-wall, dewatering and embankment problems in Project Sitework.

What the problems look like

One governing equation, with a setup that is easy to get subtly wrong. Recurring shapes — Rankine pressure with a water table or layered backfill; the right earth-pressure state; Proctor and zero-air-voids; consolidation; bearing capacity of a footing or an embankment on soft clay; elastic settlement; and slope stability by infinite slope or stability chart.

By exam day you should be able to —

• draw the active pressure diagram for layered or partly submerged backfill and resolve the force;
• choose between active, at-rest and passive from how the wall moves;
• predict how compactive effort shifts γd,max and optimum moisture;
• compute primary consolidation settlement, including an overconsolidated clay;
• compute bearing capacity and apply the factor of safety correctly;
• pick the strength basis (undrained, drained peak, residual) for a slope and read a stability chart.

Where people lose points

• Water behind a wall: apply Ka to the effective vertical stress and add the water pressure in full; do not scale γw by Ka or run Ka through saturated total stress. In layered backfill, each layer takes its own Ka.
• Earth-pressure state: a wall braced top and bottom does not yield — it sees at-rest pressure, not active, not passive, and not fluid pressure.
• Compaction: more effort raises γd,max and lowers optimum moisture; no compaction peak crosses the zero-air-voids curve.
• Consolidation: take log10 of final over initial effective stress, divide by 1 + e0, and remember the void ratio goes down. In an overconsolidated clay, recompress up to σ′p and use Cc only beyond it.
• Bearing capacity: s_u is not q_u (q_u = 2s_u); the load on soft clay is the fill's unit weight; the factor of safety divides the capacity.
• Elastic settlement: the equation uses (1 − ν²), and the SPT-modulus correlation wants (N1)60 — corrected for hammer energy and overburden, not N60 alone.
• Slopes: the long-term check of a cut in clay is a drained, effective-stress problem — undrained is the end-of-construction check; residual strength applies only where a slide surface already exists, and a stiff-fissured clay is taken at fully softened, not peak, strength. On a chart, 1H:2V is a 63.4° face; pick the toe-circle or deep-circle curve that matches the geometry.

Have these at your fingertips

Rankine coefficients, the consolidation and bearing-capacity equations, and the slope-stability charts — plus where they live in the Handbook. From the April 2027 exam, NCEES also supplies USACE EM 1110-2-1902 (slope stability), which sets minimum factors of safety by loading case; it is not on the current list, so check the specification for your exam date.

Going deeper

• Project Sitework — retaining walls and dewatering, built on this theory
• Materials — phase relations and boring logs
• WRE design standards: the April 2027 change
• A quarter of the WRE exam isn't water

Part of the Water Resources & Environmental exam. Practice this area with exam-real problems — the tutor walks any one you miss. → Start practicing free — free with an account.