Question 4 of 5

Question 4 (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.

Each item is described below in the order asked.

1. (a) Cone Penetrometer Test (CPT/CPTu, ASTM D5778) — an instrumented cone is pushed continuously into the ground, logging tip resistance and sleeve friction (and, for CPTu, pore pressure) with depth. Applicable in soft-to-firm soil that will accept a pushed cone (not gravelly, bouldery, or rock ground); gives a rapid, continuous stratigraphic and strength/density profile, and CPTu dissipation tests give an in-situ estimate of permeability and consolidation rate. No physical sample is recovered, so it is normally paired with a companion borehole for classification.
2. (b) Oedometer Test (ASTM D2435) — a thin specimen of undisturbed fine-grained soil, laterally confined in a rigid ring, is loaded in stages with vertical deformation monitored over time. Applicable to compressible clay/silt where long-term settlement under a new structural load is a design concern; yields the pre-consolidation pressure (OCR), the compression/recompression indices (Cc, Cr), and the coefficient of consolidation (cv) — the parameters that govern the magnitude and time-rate of settlement.
3. (c) Usage of Shelby Tube (ASTM D1587) — a thin-walled steel tube is hydraulically pushed (never driven) into fine-grained soil to recover a relatively undisturbed sample. Applicable wherever an undisturbed specimen of clay or silt is needed for strength (triaxial/UC) or consolidation (oedometer) testing; the sample quality it preserves is what makes those downstream lab results meaningful, since a driven, disturbed sample would understate strength and overstate compressibility.
4. (d) Vane Test (field vane shear test, ASTM D2573) — a four-bladed vane is pushed into soft clay and rotated in-situ, with the torque required to shear the soil converted to an undrained shear strength. Applicable specifically to soft-to-firm, saturated fine-grained soil; gives an in-situ undrained shear strength directly, avoiding the strength loss that sample disturbance causes in sensitive clay, and is used for foundation bearing capacity and embankment/slope stability design in soft ground.
5. (e) Constant Head Test (ASTM D2434, laboratory) — a specimen of relatively permeable (granular) soil is subjected to a steady-state head difference and the resulting flow rate is measured, giving the hydraulic conductivity K directly via Darcy's law. Applicable to sand and gravel, where flow is fast enough that a steady head can be maintained and measured within a practical test duration (the falling-head variant is used instead for low-permeability fine-grained soil); the resulting K feeds seepage, dewatering, and drainage design.
6. (f) Flight Augering — a continuous-flight (or hollow-stem) auger is rotated into the ground to advance a borehole through soil, with cuttings carried up the flights (or, for hollow-stem, with the centre left open for sampling without removing the auger string). Applicable as the standard, economical drilling method in soil above the water table and in cohesive ground below it; provides a disturbed sample for visual logging/classification and a cased, open hole through which SPT split-spoon sampling or other in-situ tests are run at intervals.
7. (g) California Bearing Ratio Test (CBR, ASTM D1883) — a standard plunger is pushed into a compacted (or in-situ) soil/subgrade specimen at a controlled rate, and the resistance is expressed as a percentage of the resistance of a standard crushed-stone material at the same penetration. Applicable to subgrade and granular base materials for pavement and road design; the resulting CBR value is the direct input to empirical pavement-thickness design charts, so it is run wherever the site investigation is supporting a road, parking area, or airfield.
8. (h) Triaxial Testing (ASTM D2166/D2850/D4767) — a cylindrical specimen is confined in a cell under a controlled cell pressure and sheared axially to failure, with drainage and pore-pressure conditions controlled to reproduce the field scenario (UU, CU, or CD). Applicable to both soil and (with a modified cell) rock specimens wherever a design shear-strength parameter (c, φ in total or effective stress) is needed for bearing capacity, slope stability, or excavation support; unlike a field vane or unconfined test, it can reproduce a chosen field drainage condition and yields the full stress-strain and pore-pressure response.
9. (i) Resistivity (laboratory and in field) — in the field, an electrical resistivity survey (ASTM G57 for a single sounding; a multi-electrode array for a 2-D/3-D profile) injects current through the ground and measures the resulting potential, inverting the result into a subsurface resistivity profile used to locate the bedrock surface, map groundwater/contaminant plumes, and identify buried voids or fill boundaries between boreholes at low cost. In the laboratory, resistivity is measured directly on a soil sample (or the site's native water) to assess corrosivity — a low resistivity indicates aggressive ground that accelerates corrosion of buried steel piles, tiebacks, and pipelines, directly informing the corrosion-protection specification.
10. (j) Modified Proctor Test (ASTM D1557) — a soil sample is compacted in a mould in layers under a higher compactive effort than the Standard Proctor, producing a moisture-density (compaction) curve from which the maximum dry density and optimum moisture content are read. Applicable wherever an engineered fill will be placed under heavy loading (highway/airfield subgrade, structural fill under a high-rise) and needs a higher target density than a standard-effort specification would give; the resulting curve is the design/QA target that field density tests (item m) are compared against.
11. (k) Dilatometer Test (flat plate dilatometer, DMT, ASTM D6635) — a flat, blade-shaped probe with a thin steel membrane is pushed into the ground and the membrane is pneumatically expanded at set depth intervals, recording the lift-off and further-expansion pressures. Applicable in soft-to-stiff soil as a rapid, continuous-profile complement to CPT; converts the two pressure readings into the material index (soil type), horizontal stress index (correlated to OCR and K0, the at-rest lateral earth pressure coefficient), and constrained modulus, giving stiffness/stress-history data that CPT alone does not directly provide.
12. (l) Piezometer Analysis — a standpipe or vibrating-wire piezometer, sealed at a specific depth/stratum, is read (manually or via a data logger) over time to track pore water pressure and groundwater level at that point. Applicable at every stage from initial characterization (Question 3) through construction monitoring (Question 2); the resulting record establishes baseline groundwater conditions, detects a rising/falling trend or artesian pressure, and feeds effective-stress, seepage, and slope-stability analyses directly.
13. (m) Sand Cone and Rubber-Balloon Tests/Methods (1 Mark, ASTM D1556/D2167) — both are field density tests: a known volume of test hole is excavated in compacted fill, and the excavated soil's mass is compared to the volume of the hole, measured either by the volume of calibrated sand needed to refill it (sand cone) or by the volume displacement of a water-filled rubber membrane pressed into the hole (rubber balloon). Applicable as the standard field QA/QC check on compacted engineered fill; the resulting in-situ dry density is compared directly against the Modified Proctor (item j) maximum dry density to confirm the specified percent-compaction has been achieved before construction proceeds.