8.1 Soil & rock mechanics fundamentals

Soil and Rock as Engineering Materials

Engineering geology applies geologic understanding to the safe siting, design, and construction of dams, foundations, roads, tunnels, and slopes. Because the PG exam weights this subject heavily (about 18%), candidates must be fluent in the vocabulary of soil and rock mechanics. To an engineer, soil is any unconsolidated earth material that can be excavated without blasting; rock is the intact, lithified material beneath it.

Phase relationships and index properties

Soil is a three-phase system of solid particles, water, and air. The relative proportions govern behavior and are captured by index properties:

• Void ratio (e) = volume of voids / volume of solids
• Porosity (n) = volume of voids / total volume
• Water (moisture) content (w) = weight of water / weight of solids
• Degree of saturation (S) = volume of water / volume of voids
• Unit weight (gamma) = weight / volume (dry, moist, and saturated forms)

These parameters let a geologist estimate density, compressibility, and strength before laboratory testing.

Grain size and gradation

Particle-size distribution is fundamental. By the Unified Soil Classification System (USCS), gravel is larger than 4.75 mm (No. 4 sieve), sand ranges from 4.75 mm down to 0.075 mm (No. 200 sieve), and silt and clay (the "fines") pass the No. 200 sieve. Coarse fractions are measured by sieve analysis; fines by hydrometer (sedimentation). A soil is well graded when it contains a broad, continuous range of sizes (favorable for compaction and strength) and poorly graded when it is uniform or gap graded. The coefficient of uniformity (Cu) and coefficient of curvature (Cc) quantify gradation numerically.

Atterberg limits and plasticity

For fine-grained soils, consistency depends on water content. The Atterberg limits mark the boundaries between physical states:

• Liquid limit (LL) - water content at which the soil begins to flow as a liquid.
• Plastic limit (PL) - water content at which the soil begins to crumble when rolled into a 3 mm thread.
• Plasticity index (PI = LL - PL) - the range of water content over which the soil stays plastic.

A high PI signals highly plastic, clay-rich soil that is compressible and often expansive. Activity (PI divided by the clay-size fraction) helps identify problem clay minerals.

Consistency, relative density, and permeability

The same index properties feed practical field descriptors. Relative density (Dr) compares a sand's in-place void ratio to its loosest and densest states, ranking it from very loose to very dense; it strongly influences the friction angle and the resistance to liquefaction. For clays, consistency grades from very soft to hard and is tied to the undrained shear strength. Permeability (hydraulic conductivity, k) spans many orders of magnitude - clean gravels drain freely, while clays are nearly impervious - and it governs how fast pore pressures dissipate, and therefore whether loading behaves as drained or undrained. Compaction, measured in the laboratory by the Proctor test, densifies engineered fill by expelling air at an optimum moisture content, raising strength and lowering compressibility.

Unified Soil Classification System

USCS assigns a two-letter symbol. The first letter is the primary type: G gravel, S sand, M silt, C clay, O organic. The second modifier describes gradation or plasticity: W well graded, P poorly graded, M silty, C clayey, L low plasticity, H high plasticity. Coarse soils are classified by grain size and gradation; fine soils by the plasticity chart, where the A-line separates clays (above) from silts and organics (below), and the LL = 50 line divides low (L) from high (H) plasticity. Thus CL is a lean clay and CH a fat (highly plastic) clay.

Effective stress

Terzaghi's principle of effective stress underlies all soil mechanics: sigma' = sigma - u, where sigma is total stress, u is pore-water pressure, and sigma' is the effective stress carried by the grain skeleton. Effective stress - not total stress - controls strength and volume change. Rising groundwater or rapid loading increases pore pressure, lowers sigma', and weakens the soil.

Shear strength and the Mohr-Coulomb criterion

Soil fails in shear. The Mohr-Coulomb failure criterion expresses shear strength as:

tau = c' + sigma' tan(phi')

where c' is cohesion (strength independent of confining stress, largest in clays) and phi' is the angle of internal friction (frictional strength that increases with confinement, largest in dense sands and gravels). Clean sands are essentially cohesionless (c approximately 0); saturated clays under rapid (undrained) loading are analyzed with an apparent cohesion (phi = 0, undrained shear strength su). Whether a drained or undrained analysis applies depends on how fast load is applied relative to how fast pore water can drain.

Rock mass properties

Rock behavior reflects both the intact rock and the discontinuities (joints, faults, bedding, foliation) that cut it. The Rock Quality Designation (RQD) rates fracturing: it is the summed length of intact core pieces 100 mm (4 in) or longer, divided by the total core-run length, expressed as a percent. RQD above 90% is excellent rock; below 25% is very poor. Discontinuities are described by orientation (dip and dip direction), spacing, persistence (continuity), aperture, roughness, and infilling - all of which control strength, permeability, and the potential for sliding. Classification schemes such as RMR (Rock Mass Rating) and the Q-system combine these factors into design ratings. Mastering these fundamentals lets the geologist predict the settlement, bearing capacity, slope stability, and excavation behavior treated in the sections that follow.