Question 2 of 4

Question 2 (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 soil categories and their engineering risks

Soils are broadly divided by grain size and behaviour, under the Unified Soil Classification System (USCS), into coarse-grained (gravel, sand), fine-grained (silt, clay), organic soils, and fill as a distinct man-made category. Gravel is free-draining and strong, but its high permeability makes dewatering and grouting difficult, and a driven sampler recovers it poorly (loses fines, under-characterizing grading). Sand depends on relative density for strength; loose, saturated sand is liquefaction-prone under seismic loading, and clean saturated sand can flow into an unsupported excavation. Silt builds excess pore pressure under load yet drains slowly, is sensitive to disturbance, and is frost-susceptible (heave, spring-thaw weakening). Clay consolidates slowly, producing long-term settlement, and sensitive/quick clays (e.g. Champlain Sea/Leda clay) can lose most of their strength on remoulding and trigger flow slides; expansive clay shrinks and swells with moisture change. Organic soil (peat, muskeg) is highly compressible and weak, with continuing secondary compression for years. Fill carries unknown composition, compaction and contamination status until directly investigated.

(b) Soil sampling techniques and samplers

Sampler choice is matched to soil type and the sample quality required: the SPT split-spoon (ASTM D1586) gives a disturbed sample and an N-value density/consistency index, suitable for classification only; the thin-walled (Shelby) tube (ASTM D1587), pushed rather than driven, gives a relatively undisturbed cohesive-soil sample for strength/consolidation testing; a piston sampler maintains suction during advance for soft, sensitive clay; block/hand-carved samples from a test pit are the least-disturbed option for critical strength testing; augers give rapid, low-cost disturbed samples above the water table; test-pit bulk sampling allows direct visual logging of stratigraphy where depth permits safe excavation; and the CPT/direct-push rig can deploy a push-in sampler alongside its continuous profiling, increasingly the industry norm for soft-ground work.

(c) Broad rock categories (geological models) and their engineering risks

Rock is categorized by origin — igneous, sedimentary, metamorphic, each with a characteristic mineralogy and weathering behaviour — but engineering risk is governed more directly by the state of the rock mass than by intact strength alone. Intact rock strength (very weak to extremely strong, per ISRM/UCS bands) sets bearing capacity and rippability but rarely governs large-scale stability. Jointed/fractured rock — discontinuity orientation, spacing and condition (RQD/RMR) — controls block and wedge instability in cuts and excavations, independent of intact strength. Weathered rock/saprolite is a variable, weaker transitional material, a common source of misclassified bearing strata if the weathering profile is not explicitly logged. Foliated metamorphic rock (schist, phyllite) is much weaker along foliation than across it, creating a preferential sliding plane. Soluble sedimentary rock (limestone, gypsum) is subject to karst dissolution, producing voids and irregular pinnacled bedrock that a sparse borehole grid can miss entirely.

(d) Rock sampling techniques and samplers

The industry-norm method is rock coring with a double- or triple-tube core barrel, recovering a continuous core for RQD, lithological logging, and strength/point-load testing; the triple-tube configuration (with an inner split liner) is preferred in weak, fractured or highly weathered rock, since it maximizes recovery and avoids the mechanical breakage that would otherwise depress the measured RQD. Where discontinuity orientation matters for stability analysis (not just intact strength), an oriented core — a barrel fitted with a scribe or electronic orientation tool — lets dip and dip-direction be read directly off the recovered core. Surface or near-surface rock exposures are additionally sampled by hand specimen/chip sampling for index and petrographic testing, and by outcrop scanline or window mapping to record discontinuity statistics directly, supplementing what a limited number of boreholes can capture.

(e) Determining the engineering properties of a soil or a rock

Engineering properties are established by combining three complementary sources, since no single method supplies every parameter a design needs. Laboratory testing on recovered samples gives index properties (Atterberg limits, grain-size distribution, moisture content for soil; petrographic description, point-load index for rock) and fundamental strength/deformation parameters under controlled stress paths (triaxial and oedometer/consolidation testing for soil; uniaxial compressive strength and triaxial testing for rock) — precise, but only as representative as the sample quality allows, and limited to the volume of material actually recovered. In-situ testing (SPT, CPT/CPTu, field vane, pressuremeter for soil; borehole geophysics, packer permeability testing, and RQD/RMR/GSI rock-mass classification from core or exposure logging for rock) measures a much larger volume of ground at its actual in-situ stress state, avoiding sample-disturbance error, but generally yields an index or empirically correlated parameter rather than a fundamental one. Empirical correlations and back-analysis then bridge the two — published correlations convert an index result (SPT N, CPT tip resistance, RQD) into a design parameter, and, where a comparable structure or case history already exists on similar ground, its observed performance can be back-analyzed to calibrate or check the parameters derived from testing. A defensible design value is normally the one that is consistent across all three sources, not a single test result taken in isolation.