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) At least 3 industry-norm in-situ soil testing techniques

1. Standard Penetration Test (SPT) (ASTM D1586) — a split-spoon sampler driven with a standard hammer energy; blow count N is a cheap, fast, widely correlated index of relative density/consistency, and it recovers a small disturbed sample at the same time. Advantages: low cost per point, usable in almost any soil including gravelly ground, continuous depth coverage from one borehole, and a huge body of published correlations (friction angle, bearing capacity, liquefaction triggering) exists to interpret it. Disadvantages: the N-value is only an empirical index, sensitive to hammer energy, rod length, and overburden pressure, and it is unreliable in soft, sensitive clay and in gravel (where oversize particles inflate blow counts).
2. Cone Penetration Test (CPT/CPTu) (ASTM D5778) — an instrumented cone pushed continuously, logging tip resistance, sleeve friction, and (CPTu) pore pressure. Advantages: continuous, highly repeatable profiling with depth, fast and relatively low-cost per metre, and the pore-pressure dissipation test gives an in-situ permeability/consolidation-rate estimate. Disadvantages: no physical sample is recovered (needs a companion borehole for classification/lab testing), cannot penetrate dense gravel, cobbles, or rock, and its strength/state parameters come from empirical correlations that need local calibration.
3. Field Vane Shear Test (VST) (ASTM D2573) — a four-bladed vane rotated in-situ to measure undrained shear strength directly in soft-to-firm clay. Advantages: tests the clay in its true in-situ stress state and structure, avoiding sample-disturbance strength loss, and is fast and inexpensive relative to a lab triaxial program. Disadvantages: limited to soft, fine-grained soil, gives only undrained shear strength (no stiffness or drained parameter), and needs an empirical plasticity-dependent correction factor.

(b) At least 3 industry-norm in-situ rock testing techniques

1. Point Load Index Test (ASTM D5731) — a rock specimen (core or irregular lump) loaded between conical platens to failure, converting the failure load and specimen diameter into an index strength Is(50), correlated to uniaxial compressive strength (UCS ≈ 20–25 × Is(50)). Advantages: can be run in the field on freshly recovered core with minimal specimen preparation, fast and cheap relative to a lab UCS test, allows many more points to be tested for the same budget, and works on irregular lumps that could not be machined into a lab specimen. Disadvantages: gives only an index, correlated (not directly measured) UCS with a wide scatter band; sensitive to anisotropy/foliation orientation relative to the loading axis; and does not give a stress-strain curve or elastic modulus the way a lab UCS test with strain gauges does.
2. Packer (Lugeon) Permeability Test (ASTM D4630) — a section of borehole in rock is isolated between inflatable packers and water is injected under a series of step pressures, converting the flow rate into a Lugeon value (a practical index of rock-mass hydraulic conductivity dominated by discontinuities, not intact rock permeability). Advantages: measures the rock MASS's permeability, including the discontinuity network that governs grouting requirements and seepage/uplift beneath a dam or into a tunnel — a property no intact lab specimen can represent at all. Disadvantages: slow and equipment-intensive per test interval, results are sensitive to test pressure (can hydraulically jack open discontinuities if overpressured, giving an unrealistically high reading), and the test averages over the isolated interval rather than resolving which individual discontinuity is conductive.
3. Borehole Geophysical Logging (e.g. televiewer/caliper/sonic logging) — downhole tools that log discontinuity orientation and aperture (acoustic/optical televiewer), borehole diameter (caliper, flagging breakout or washed-out zones), and sonic velocity (a proxy for rock quality/modulus) continuously with depth. Advantages: continuous, objective, oriented discontinuity data directly from the borehole wall (recovers orientation information that broken, unoriented core cannot), and rapid coverage of a long borehole length. Disadvantages: specialized equipment and interpretation expertise, requires a fluid-filled, reasonably stable, uncased borehole, and gives an indirect (velocity- or image-based) property rather than a directly measured strength.

(c) In-situ testing versus laboratory testing — advantages and disadvantages for design parameters

Across both soil and rock, the same trade-off recurs. In-situ tests (SPT/CPT/VST for soil; point load/packer/geophysical logging for rock) measure the material in its true in-situ stress state, structure, and (for rock) discontinuity fabric, and can cover far more ground per unit cost and time than an equivalent lab program — a genuine advantage where the design parameter is controlled by mass-scale behaviour (rock-mass permeability, soil density variability along a route) rather than by the intact material alone. Their shared disadvantage is that most in-situ results are index or empirically correlated parameters rather than fundamental ones, sensitive to equipment, procedure, and local calibration, and (except where a sampler is combined with the tool) no specimen is recovered for independent lab confirmation. Laboratory testing (triaxial/oedometer for soil; UCS/triaxial for rock) is the mirror image: a controlled, repeatable stress path on a known specimen gives a fundamental strength or deformation parameter directly, but the specimen is small, may be disturbed by sampling, and represents only the material actually recovered — for rock in particular, an intact lab specimen by definition excludes the discontinuities that usually govern rock-mass behaviour in the field. A defensible design value therefore comes from using in-situ results for spatial coverage and mass-scale parameters, laboratory results as the fundamental-parameter anchor, and checking the two against each other rather than relying on either alone.