(a) Broad soil categories and their engineering risks
Soils encountered during a site investigation are broadly divided by grain size and behaviour into coarse-grained (gravel and sand) and fine-grained (silt and clay) soils under the Unified Soil Classification System (USCS), plus organic soils (peat, organic silt/clay) as a distinct high-risk category. Each carries a different set of engineering risks:
Gravel — generally free-draining and high strength, but engineering risk arises from high permeability (difficult to dewater or grout, seepage-prone under dams/cofferdams) and from poor sampling recovery (loses fines in a driven sampler, so grading and density are often under-characterized).
Sand — strength depends heavily on relative density; loose, saturated sand is susceptible to liquefaction under seismic or cyclic loading (a major risk in coastal BC), and clean sand below the water table can flow into excavations (running/quick conditions) if not dewatered or supported.
Silt — the highest-risk category for construction operations: low permeability enough to build up excess pore pressure under load yet high enough to drain slowly and lose strength when disturbed (thixotropic/"quick" behaviour), frost-susceptible (capillary rise feeds ice lensing, causing frost heave and spring thaw weakening), and difficult to compact reliably.
Clay — low permeability, so it consolidates slowly under load, producing long-term (secondary) settlement that may not be apparent for years after construction; strength is often anisotropic and sensitive to disturbance (sensitive/"quick" clays, notably in the Champlain Sea/Leda clay deposits of eastern Canada, can lose most of their strength on remoulding and trigger retrogressive flow slides); expansive (high-plasticity) clays shrink and swell with moisture change, damaging light structures founded on them.
Organic soils (peat, muskeg) — very high compressibility and very low strength, continue to settle for years under sustained load (secondary compression dominates), and are common across much of Canada's boreal terrain; they are usually excavated and replaced rather than built on directly.
Fill (man-made ground) — treated as a separate category because its composition, compaction, and contamination status are unknown without direct investigation; uncontrolled fill is one of the most common sources of unexpected foundation settlement.
In every case the underlying engineering risk is the same in kind — inadequate bearing capacity, excessive or differential settlement, or loss of strength under a specific loading/environmental condition — but the trigger mechanism and the timescale over which it manifests differ sharply by soil type, which is exactly why identifying the soil category correctly early in the investigation drives the rest of the exploration and testing program.
(b) Soil sampling techniques and samplers
Sampling method is chosen to match the soil type and the quality of sample required (disturbed index-testing samples versus undisturbed samples needed for strength/consolidation testing):
Standard Penetration Test (SPT) split-spoon sampler (ASTM D1586) — a disturbed sample driven with a 63.5 kg hammer falling 760 mm; the blow count (N-value) is itself a widely used index of relative density/consistency, and the recovered soil is used for classification and index testing only, not strength testing.
Thin-walled (Shelby) tube sampler (ASTM D1587) — a thin-walled steel tube pushed hydraulically (not driven) into cohesive soil to recover a relatively undisturbed sample suitable for consolidation and unconfined/triaxial strength testing; unsuitable for gravelly or very stiff soils that buckle or damage the tube.
Piston sampler — a Shelby-type tube with an internal piston that maintains suction during advance, minimizing sample disturbance and loss; preferred for soft, sensitive clays where an ordinary Shelby tube would let the sample slide out or disturb structure.
Block sampling / hand-carved samples — cut by hand from a test pit or shaft and immediately waxed/sealed; the least-disturbed sample obtainable, used for critical strength testing of sensitive or fissured clay where even piston sampling is judged too disturbing.
Auger sampling (hand or power auger, e.g. bucket/continuous-flight) — rapid, low-cost, disturbed sampling in soft-to-firm cohesive soil above the water table, mainly for shallow reconnaissance and index testing.
Test-pit/trench bulk sampling — large-volume disturbed or block samples excavated by backhoe, allowing direct visual logging of stratigraphy and in-place structure; limited to depths above the water table that can be safely excavated (shoring/OH&S constraints).
Rock/soil coring (double- or triple-tube core barrel) — used where the soil grades into weathered rock or very stiff/cemented material that cannot be sampled with a driven or pushed tube; recovers a continuous core for RQD and lithological logging.
Vibrocore/direct-push (cone penetrometer, CPT) systems — the CPT itself is not a sampler but a continuous in-situ profiling tool (tip resistance, sleeve friction, pore pressure); a push-in sampler can be deployed on the same rig to recover discrete samples with minimal disturbance and no drilling spoil, increasingly the industry norm for soft-ground site characterization.
Sampler selection is governed by the required sample quality class (per ISO/CFEM disturbance categories, Class 1 being least disturbed): index testing tolerates a disturbed SPT sample, while any strength or consolidation parameter feeding a settlement or slope-stability design should come from at least a thin-walled tube, and ideally a piston or block sample in sensitive clay.
Item
Answer
2(a)
Gravel, sand, silt, clay, organic soil (peat), fill — each with distinct bearing-capacity/settlement/strength-loss risk mechanisms (liquefaction in loose sand, frost heave/quick behaviour in silt, long-term consolidation and sensitivity in clay, ongoing secondary compression in peat)
