22-Agric-A2 Soil Physics and Mechanics · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A2 Soil Physics & Mechanics, National Exams May 2017 — a three-hour open-book examination; any non-communicating calculator is permitted. The cover page states that five (5) questions constitute a complete exam paper and that only the first five as they appear in the answer book are marked, that each question is of equal value, and that some questions require a written answer whose clarity and organization matter for marks. All six printed questions are worked here, because the set is a study resource rather than a timed attempt; on exam day a candidate submits only the first five, in order.
Reference texts. B.M. Das, Principles of Geotechnical Engineering, 9th ed. (weight-volume relationships, permeability, grain-size analysis, USCS classification, compaction, slope stability, well hydraulics); R.F. Craig, Craig's Soil Mechanics, 9th ed. (effective stress, seepage and flow nets, shear strength); USDA NRCS National Engineering Handbook (compaction and earthwork field practice).
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.
1) Pore water pressure, u. The pressure carried by the water that fills the interconnected void spaces of a saturated (or partially saturated) soil, transmitted through the continuous pore fluid exactly as in any static or slowly flowing liquid, and measured relative to atmospheric pressure. It can be hydrostatic (governed by the depth below a water table) or, under undrained loading, temporarily elevated as excess pore pressure.
2) Effective stress, σ′. The portion of the total stress acting on a plane within a soil mass that is actually carried through grain-to-grain contacts of the soil skeleton, given by Terzaghi's principle σ′ = σ − u. It is the effective stress, not the total stress, that governs a soil's shear strength and volume-change (compression/consolidation) behaviour.
3) Soil texture. The relative proportions, by mass, of the sand-, silt-, and clay-sized particle fractions making up a soil, determined from a grain-size (sieve/hydrometer) analysis and commonly reported using a textural triangle. Texture is a classification of particle size alone and does not by itself describe plasticity or in-situ density.
4) Degree of saturation, S. The percentage of the total void volume that is occupied by water, S = Vw/Vv × 100%, ranging from S = 0% (perfectly dry) to S = 100% (fully saturated, all voids water-filled).
5) Void ratio, e. The ratio of the volume of voids to the volume of solids in a soil mass, e = Vv/Vs. It is dimensionless and, unlike porosity, is referenced to the (constant) solid volume, which makes it convenient for tracking volume change during compression.
6) Moisture content, w. The ratio of the mass of water to the mass of solids in a soil sample (dry-mass basis), w = Mw/Ms × 100%, found by oven-drying a sample at 105–110°C to constant mass.
7) Seepage. The flow of water through the interconnected void spaces of a saturated (or partially saturated) soil under an applied hydraulic gradient, governed at the macroscopic scale by Darcy's law, v = Ki. Seepage underlies problems from simple laboratory permeameter tests up to dewatering, drainage, and the stability of earth dams and excavations.
8) Proctor density. The maximum dry unit weight (or dry density) a soil can be compacted to under a standardized compactive effort — the Standard or Modified Proctor test — determined from the peak of the compaction (dry-density vs. moisture-content) curve; the moisture content at that peak is the optimum moisture content. It is the reference value against which field compaction is specified as a percentage (e.g. 95% Proctor).
9) Phreatic surface. The free (unconfined) water table surface within a soil or aquifer, defined as the locus of points where pore water pressure equals atmospheric (u = 0); above it, in the vadose zone, pressures are typically negative (suction), and below it, positive. It forms the upper boundary condition for seepage and slope-stability analyses of unconfined systems such as earth dams and open drains.
10) Soil shear strength. The maximum shear stress a soil can sustain on a potential failure plane before it fails in shear, described by the Mohr–Coulomb criterion τf = c′ + σ′tanφ′ in terms of effective stress (or τf = su in undrained, total-stress terms for saturated clay). It is the property that governs bearing capacity, slope stability, and earth-pressure problems.
