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) Coarse-grained soil (gravel/sand)

Drilling: solid-stem or hollow-stem auger, or mud/sonic-rotary drilling where the deposit is dense or contains cobbles that defeat an auger; casing is often needed below the water table to prevent hole collapse in clean, cohesionless material. Sampling: Standard Penetration Test (SPT) split-spoon sampler (ASTM D1586) is the industry-norm tool — it recovers a disturbed sample for classification while simultaneously producing the blow count (N-value); undisturbed sampling is rarely attempted in coarse material (a pushed thin-walled tube will not hold sand together), so where an undisturbed sample is genuinely required a frozen or block sample from a shored test pit is used instead. Testing: the SPT N-value itself is the primary in-situ index of relative density; Cone Penetration Testing (CPT, ASTM D5778) gives a continuous tip-resistance/sleeve-friction profile and is preferred where the ground will accept a pushed cone (not gravelly/bouldery ground); grain-size distribution and Atterberg-limit-adjacent index tests are run on the recovered disturbed sample. Because loose, saturated sand is liquefaction-susceptible, in-situ density is the property of greatest interest and is why SPT/CPT (density-sensitive tools) dominate this category rather than strength testing.

(b) Fine-grained soil (silt/clay)

Drilling: hollow-stem auger or mud-rotary drilling, advanced carefully to minimize disturbance of the underlying strength-sensitive material; casing/drilling mud stabilizes the hole through soft, low-strength intervals. Sampling: thin-walled (Shelby) tube sampler (ASTM D1587), hydraulically pushed (not driven), for a relatively undisturbed sample suitable for strength and consolidation testing; a piston sampler is used instead in very soft or sensitive clay, where an ordinary Shelby tube risks the sample sliding out or disturbing its structure; SPT split-spoon sampling is still run at intervals for classification and index testing even where the primary interest is strength. Testing: field vane shear testing (VST, ASTM D2573) gives an in-situ undrained shear strength directly, avoiding the strength loss that sample disturbance causes in sensitive clay; piezocone (CPTu) testing profiles undrained strength and consolidation behaviour continuously with depth via its pore-pressure channel. Strength and compressibility are the properties of primary interest here (settlement and slope/foundation stability both depend on them), which is why sample quality — not just recovery — is the controlling consideration for this material.

(c) Hard rock

Drilling: diamond coring with a double- or triple-tube core barrel, sized (typically NQ or HQ) to maximize recovery and RQD reliability in strong, sound rock. Sampling: the continuous rock core itself is the sample, logged for lithology, RQD, discontinuity spacing/orientation/condition, and weathering grade as it is recovered; oriented coring (using a scribe or borehole-camera reference) is added where discontinuity orientation, not just spacing, is needed for a stability analysis. Testing: point load index testing (ASTM D5731) is run in the field on freshly recovered core to give a fast, cheap index correlated to uniaxial compressive strength (UCS); where the rock mass's discontinuity-controlled hydraulic behaviour matters (grouting/seepage design), a packer (Lugeon) permeability test (ASTM D4630) is run in the open borehole; borehole geophysical logging (acoustic/optical televiewer) supplements core logging with continuous, oriented discontinuity data from the borehole wall. Because hard rock's strength is high and largely governed by its discontinuities rather than the intact material, classification (RQD, RMR/Q-system inputs) matters as much as any single strength number.

(d) Weak rock

Drilling: diamond coring as for hard rock, but with reduced water pressure/flush rate and a shorter core-run length to limit mechanical breakage of the weaker, often more weathered material; where the rock has weathered to a residual-soil-like consistency, drilling and sampling transitions toward the soil methods in (a)/(b) (SPT N-values become the practical index once intact recovery becomes unreliable). Sampling: core recovery and RQD are logged exactly as in (c), but expect lower recovery and more rubble/broken zones, which must be logged as such rather than forced into a continuous interpretation; where core recovery is too poor to be meaningful, large-diameter augering or a test pit/adit exposes the material directly for description and block sampling. Testing: point load index testing (ASTM D5731) remains the primary field strength index, but weak rock's point-load-to-UCS correlation is less reliable than for hard rock (more scatter, and specimens can fail along pre-existing weakness planes rather than through intact material), so a companion laboratory UCS or triaxial test on carefully selected intact pieces is more important here than for hard rock; a Schmidt hammer rebound test gives a quick, non-destructive supplementary strength index directly on exposed core or outcrop. Weathering-grade classification (fresh through residual soil) is itself a key deliverable for weak rock, since design behaviour changes markedly across that spectrum.

(e) Peat and organic material

Drilling: hand or power auger for shallow deposits, or a hollow-stem auger/sonic rig for thick or deep peat, since conventional rotary drilling flush can wash away the fibrous, low-density material before it can be sampled. Sampling: a piston sampler or a purpose-built peat sampler (e.g. a Macaulay or Russian peat corer for shallow hand sampling) is preferred to recover the material with minimal disturbance to its natural fibre structure and water content, both of which govern its compressibility; a block sample from a test pit is used where the deposit is shallow enough to expose safely. Testing: field description of the degree of decomposition (the von Post humification scale, read directly from the recovered sample) is itself a primary classification tool specific to organic soils; natural water content (routinely several hundred percent in fibrous peat) and organic content (loss-on-ignition) are determined in the laboratory on the same sample; in-situ vane shear gives an approximate undrained strength, though the fibrous structure makes the reading less reliable than in mineral clay. Because peat continues to settle for years under load (secondary compression dominates its behaviour), sample quality that preserves the natural fibre structure and water content matters more here than for any other material in this list — a disturbed or dried-out sample gives a materially wrong compressibility estimate.