Geotechnical Engineering on the FE Civil Exam
Geotechnical engineering accounts for 9 to 14% of the FE Civil exam, making it one of the three highest-weighted content areas. This subject covers soil properties, behavior, and the design of foundations and earth structures. A strong understanding of soil mechanics principles is essential for success on this portion of the exam.
Soil Classification
The Unified Soil Classification System (USCS) is the primary classification system used in geotechnical practice. Soils are classified based on grain size distribution and plasticity characteristics:
- Coarse-grained soils: More than 50% retained on the No. 200 sieve. Includes gravel (G) and sand (S), further described by gradation (well-graded W or poorly-graded P) and fines content
- Fine-grained soils: More than 50% passing the No. 200 sieve. Includes silt (M) and clay (C), further described by plasticity (low L or high H)
- Organic soils (O): Soils with significant organic content
Know how to read a grain size distribution curve and calculate D10, D30, D60, the uniformity coefficient (Cu), and the coefficient of curvature (Cc). For fine-grained soils, use the Atterberg limits (liquid limit and plastic limit) and the plasticity chart to classify the soil.
Phase Relationships
Soil exists as a three-phase system: solids, water, and air. Key phase relationships include:
- Void ratio (e): Volume of voids divided by volume of solids
- Porosity (n): Volume of voids divided by total volume
- Water content (w): Weight of water divided by weight of solids
- Degree of saturation (S): Volume of water divided by volume of voids
- Unit weight: Total weight divided by total volume (dry, moist, saturated, and submerged unit weights)
These relationships are interconnected, and the FE exam frequently tests your ability to convert between them. Practice deriving one property from others given the specific gravity of solids, water content, and void ratio.
Effective Stress
The effective stress principle is the most important concept in soil mechanics. Total stress equals effective stress plus pore water pressure. Effective stress controls soil behavior, including shear strength and compressibility. Changes in effective stress cause changes in soil volume and strength.
Calculate the vertical stress profile in a soil deposit by summing the unit weights times layer thicknesses for total stress, then subtracting the hydrostatic pore water pressure. Below the water table, use the saturated unit weight for total stress calculations and compute pore water pressure from the depth below the water table.
Consolidation
Consolidation is the time-dependent compression of saturated fine-grained soils due to the expulsion of pore water under increased effective stress. The consolidation settlement calculation requires:
- The compression index (Cc) for normally consolidated soil or the recompression index (Cr) for overconsolidated soil
- The initial void ratio (e0)
- The preconsolidation pressure
- The change in effective stress
- The thickness of the compressible layer
The rate of consolidation is governed by Terzaghi's theory, which uses the coefficient of consolidation (cv) to determine the time required to reach a given degree of consolidation. Know how to use the time factor and degree of consolidation relationship.
Shear Strength
Soil shear strength is described by the Mohr-Coulomb failure criterion: shear strength equals cohesion plus the effective normal stress times the tangent of the friction angle. For drained conditions, use effective stress parameters (c' and phi'). For undrained conditions in saturated clay, use the undrained shear strength (Su).
Understand the difference between drained and undrained analysis and when each applies. Drained analysis is appropriate for long-term stability of foundations on clay and for all conditions in sand. Undrained analysis applies to short-term (end of construction) stability of foundations on clay.
Lateral Earth Pressure
Lateral earth pressure theories determine the horizontal pressure exerted by soil on retaining structures. Know the three conditions: at-rest (no wall movement), active (wall moves away from soil), and passive (wall moves into soil). Rankine and Coulomb theories provide earth pressure coefficients for active and passive conditions.
Foundation Basics
Bearing capacity is the maximum pressure a foundation can exert on the soil without causing shear failure. Terzaghi's bearing capacity equation provides the ultimate bearing capacity for shallow foundations using bearing capacity factors that depend on the soil friction angle. Know how to calculate the allowable bearing capacity using an appropriate factor of safety.