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) Atterberg Limits (Liquid Limit / Plastic Limit) — ASTM D4318

Reason/conditions: run on any fine-grained (silt or clay) sample to classify it and to assess plasticity-related behaviour (shrink-swell potential, frost susceptibility, sensitivity to moisture change). Description: a remoulded sample is tested in a Casagrande cup (liquid limit — the water content at which a standard groove closes after 25 drops) and rolled into 3 mm threads (plastic limit — the water content at which the thread just crumbles); the plasticity index PI = LL−PL follows directly. How it aids the investigation: plots the soil on the USCS plasticity chart for classification (CL/CH/ML/MH), and the natural water content relative to LL/PL (liquidity index) indicates whether the in-situ soil is over- or normally consolidated and how sensitive it is to disturbance. Advantages: cheap, fast, needs only a small disturbed sample, and correlates well with compressibility/strength behaviour for preliminary design. Disadvantages: an index test only (no direct strength or stiffness value), operator-dependent, and not meaningful for coarse-grained or organic soils.

(b) Grain-Size Analysis (Sieve + Hydrometer) — ASTM D6913/D7928

Reason/conditions: run on any soil sample to determine its full particle-size distribution, most informative for coarse-grained and mixed soils. Description: a dried, weighed sample is shaken through a stack of graduated sieves (coarse fraction, down to the No. 200/75 μm sieve) and, for the fraction passing No. 200, a hydrometer test in suspension determines the finer silt/clay fraction by settling velocity (Stokes' law). How it aids the investigation: the resulting grading curve (D10, D30, D60, coefficient of uniformity/curvature) governs USCS classification, filter/drainage design, liquefaction-susceptibility screening, and compaction-curve prediction. Advantages: directly measures a fundamental, unambiguous physical property, inexpensive, and standardized worldwide so results are directly comparable across projects. Disadvantages: destroys any in-situ structure (gives no strength information), and the hydrometer portion is time-consuming (readings over 24 h) and less precise than sieving.

(c) Unconfined Compression / Triaxial Shear Strength — ASTM D2166/D2850/D4767

Reason/conditions: run on relatively undisturbed (Shelby tube or block) samples of cohesive soil to determine shear strength for bearing capacity, slope stability, or excavation support design. Description: an unconfined compression test loads a cylindrical specimen axially to failure with no lateral confinement (a quick, approximate undrained strength for saturated clay); a triaxial test confines the specimen in a cell under a controlled cell pressure and shears it axially, with pore-pressure measurement possible to obtain effective-stress strength parameters (c′, φ′) as well as total-stress (undrained) parameters. How it aids the investigation: supplies the design shear-strength parameters directly, and (for triaxial) the full stress-strain and pore-pressure response needed for effective-stress stability and settlement-rate analyses. Advantages: a controlled, repeatable stress path on a known specimen; triaxial testing can reproduce field drainage conditions (UU, CU, CD) to match the design scenario. Disadvantages: results are sensitive to sample disturbance during recovery/handling; unconfined compression is inapplicable to cohesionless or fissured/sensitive soil; triaxial testing is comparatively slow and expensive per specimen.

(d) Consolidation (Oedometer) Test — ASTM D2435

Reason/conditions: run on undisturbed samples of compressible fine-grained (especially clay) soil where long-term settlement under a new structural load is a design concern. Description: a thin specimen, laterally confined in a rigid ring, is loaded in stages (each held to full primary consolidation, doubling the load each increment) with vertical deformation and dissipation of excess pore pressure monitored over time. How it aids the investigation: yields the pre-consolidation pressure (over-consolidation ratio), compression and recompression indices (Cc, Cr), and the coefficient of consolidation (cv), which together give the magnitude and time-rate of settlement the structure will experience. Advantages: directly measures the compressibility and consolidation-rate parameters that no in-situ index test can substitute for. Disadvantages: slow (each stage may take a day or more to reach full primary consolidation, so a full test can take a week or more), and the small specimen and rigid lateral confinement do not reproduce field drainage-path geometry exactly, so field settlement rate is often somewhat faster than the lab cv predicts (attributed to natural sand/silt seams providing additional drainage paths not present in the specimen).

(e) Point Load / Uniaxial Compressive Strength (Rock) — ASTM D5731/D7012

Reason/conditions: run on recovered rock core where rock strength governs foundation bearing, excavatability, or underground opening design. Description: the point load test loads an irregular lump or core piece to failure between conical platens, converting the failure load and specimen size to an index strength Is(50); the full laboratory UCS test loads a prepared cylindrical specimen (length:diameter ≈ 2:1) axially to failure, typically with strain gauges to also obtain the elastic (Young's) modulus. How it aids the investigation: supplies the intact-rock strength and modulus values that feed rock-mass classification (RMR, GSI) and, combined with discontinuity data, the Hoek-Brown rock-mass strength criterion used for excavation and foundation design. Advantages: UCS testing gives a direct, fundamental strength and modulus value; point load testing (as the field-index companion) is fast and allows many more points to be tested for the same budget. Disadvantages: UCS specimen preparation (coring, end-grinding to the required flatness/parallelism) is time-consuming and expensive, and results represent only intact rock — excluding the discontinuities that usually govern rock-mass behaviour at full scale — while point load testing gives only a correlated (not directly measured) UCS with a wide scatter band, particularly in anisotropic or weak rock.