Question 3 of 4

Question 3 (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Broad categories of soil and rock, and their engineering risks

Soils encountered in a site investigation are broadly divided by grain size and behaviour, under the Unified Soil Classification System (USCS), into coarse-grained, fine-grained and organic soils, plus fill as a distinct man-made category:

• Gravel — free-draining, high strength; engineering risk from high permeability (difficult to dewater/grout, seepage-prone under dams/cofferdams) and from poor driven-sampler recovery, which under-characterizes grading and density.
• Sand — strength depends on relative density; loose, saturated sand is susceptible to liquefaction under seismic loading (a significant risk in coastal BC), and clean saturated sand can flow into unsupported/undewatered excavations.
• Silt — builds excess pore pressure under load yet drains slowly, is thixotropic/sensitive to disturbance, frost-susceptible (ice lensing → heave, weak on spring thaw), and difficult to compact reliably.
• Clay — low permeability → slow, long-term (secondary) settlement under load; sensitive/"quick" clays (notably Champlain Sea/Leda clay in eastern Canada) can lose most of their strength on remoulding and trigger retrogressive flow slides; expansive (high-plasticity) clay shrinks/swells with moisture change.
• Organic soil (peat, muskeg) — very high compressibility, very low strength, continued secondary compression for years under load; common across Canada's boreal terrain; usually excavated and replaced rather than built on.
• Fill — unknown composition/compaction/contamination status without direct investigation; a common source of unexpected settlement.

Rock is categorized both by origin (igneous, sedimentary, metamorphic — governing mineralogy, weatherability and bedding/foliation) and, more directly for engineering purposes, by the state of the rock mass versus the intact material, since the rock mass's behaviour is usually controlled by its discontinuities rather than by intact strength alone:

• Intact rock strength (classified from very weak to extremely strong per ISRM/UCS bands) governs bearing capacity and rippability/excavatability, but rarely governs large-scale stability by itself.
• Jointed/fractured rock mass — discontinuity orientation, spacing, and condition (per RQD/RMR classification) control block instability, wedge failures in cut slopes and excavations, and localized rockfall risk, independent of how strong the intact rock itself is.
• Weathered rock/saprolite — a transitional, often highly variable and weaker material between soil and fresh rock; a common source of misclassified bearing strata if the weathering profile is not explicitly logged.
• Foliated/anisotropic metamorphic rock (schist, phyllite) — strength along the foliation is much lower than across it, creating a preferential sliding plane for slopes and excavations oriented unfavourably relative to the foliation.
• Soluble sedimentary rock (limestone, gypsum, evaporites) — karst dissolution can leave voids and irregular pinnacled bedrock, causing sudden differential settlement or sinkhole collapse not evident from a sparse borehole grid.

Across both soil and rock, the underlying engineering risk category is the same — inadequate bearing capacity, excessive/differential settlement, or a loss of strength/stability under a specific trigger — but the trigger mechanism (liquefaction, sensitivity, karst dissolution, discontinuity-controlled block failure) and the timescale over which it manifests differ sharply by material, which is exactly why correct identification early in the investigation drives the rest of the exploration and testing program.

(b) Soil and rock sampling techniques and samplers

Sampling method is matched to the material and the required sample quality (disturbed index samples versus undisturbed samples for strength/consolidation testing):

1. Standard Penetration Test (SPT) split-spoon sampler (ASTM D1586) — disturbed sample driven with a 63.5 kg hammer falling 760 mm; blow count (N-value) is itself a widely used relative-density/consistency index; sample is for classification/index testing only.
2. Thin-walled (Shelby) tube sampler (ASTM D1587) — pushed hydraulically into cohesive soil for a relatively undisturbed sample suitable for consolidation/strength testing; unsuitable in gravelly or very stiff soil.
3. Piston sampler — a Shelby-type tube with an internal piston maintaining suction during advance, minimizing disturbance; preferred in soft, sensitive clay.
4. Block sampling / hand-carved samples — cut by hand from a test pit or shaft and immediately sealed; the least-disturbed sample obtainable, for critical testing of sensitive/fissured clay.
5. Auger sampling (hand or power) — rapid, low-cost, disturbed sampling in soft-to-firm cohesive soil above the water table, for shallow reconnaissance/index testing.
6. Test-pit/trench bulk sampling — large-volume disturbed or block samples with direct visual logging of stratigraphy, limited to depths that can be safely excavated (shoring/OH&S constraints).
7. Rock coring (double- or triple-tube core barrel) — the industry-norm method for both weathered and fresh rock, recovering a continuous core for RQD, lithological logging, and strength/point-load testing; triple-tube (with an inner split liner) is preferred in weak, fractured, or highly weathered rock to maximize recovery and minimize mechanical breakage that would otherwise depress the measured RQD.
8. Oriented core sampling — a core barrel fitted with an orientation tool (scribe or electronic survey) so discontinuity dip/dip-direction can be measured from the recovered core, essential wherever rock-mass stability (not just intact strength) governs design.
9. Cone penetrometer (CPT)/direct-push systems — a continuous in-situ soil profiling tool (tip resistance, sleeve friction, pore pressure) rather than a sampler in the strict sense; a push-in sampler on the same rig recovers discrete soil samples with minimal disturbance, increasingly the industry norm for soft-ground characterization.

Sampler selection follows the required sample quality class (ISO/CFEM disturbance categories, Class 1 being least disturbed): index testing tolerates a disturbed SPT sample or a rock chip, while any strength or consolidation parameter feeding a design should come from at least a thin-walled tube in soil or an oriented, high-recovery core in rock.