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.

In-situ testing versus laboratory testing — the general trade-off

In-situ tests measure the soil in place, at its actual stress state and macro-fabric, so they avoid the sample-disturbance error that any drilled or driven sample carries, and a single push or sounding can profile a much greater length of ground per unit cost and time than an equivalent number of laboratory specimens. Their disadvantage is that most in-situ tools return an index or indirectly correlated parameter rather than a fundamental strength or deformation property, the result is sensitive to equipment calibration and operator technique, and (except where a sampler is combined with the tool) no physical specimen is recovered for visual classification or independent lab confirmation. Laboratory testing is the mirror image: it applies a controlled, repeatable stress path to a known specimen and can measure a fundamental parameter directly (undrained strength, compressibility, permeability), but the specimen is necessarily small, may be disturbed by sampling and transport, and represents only the material actually recovered — a lab program alone can miss the spatial variability an in-situ profile would reveal. A well-scoped investigation therefore uses both, treating in-situ results as continuous spatial coverage and lab results as the fundamental-parameter anchor that calibrates the correlations applied to the in-situ data.

Five industry-norm in-situ soil testing techniques

1. Standard Penetration Test (SPT) (ASTM D1586) — a split-spoon sampler driven with a standard hammer energy; the blow count (N) is a widely used, cheap, fast index of relative density/consistency, and it recovers a small disturbed sample for classification at the same time. Advantage vs. lab: low cost per point, works in almost any soil including gravelly ground where tube sampling fails, gives continuous depth coverage in one borehole. Disadvantage vs. lab: the N-value is only an empirical index (energy-, overburden-, and equipment-dependent), the recovered sample is too disturbed for any strength or consolidation test, and it is unreliable in soft/sensitive clay where a lab-tested undisturbed sample gives a far more defensible strength value.
2. Cone Penetration Test (CPT/CPTu) (ASTM D5778) — a instrumented cone pushed continuously, logging tip resistance, sleeve friction, and (CPTu) pore pressure. Advantage vs. lab: continuous, highly repeatable profiling with depth (versus discrete lab specimens at widely spaced sample depths), fast and relatively low-cost per metre, and the pore-pressure dissipation test gives an in-situ permeability/consolidation-rate estimate lab oedometer testing cannot match for speed. Disadvantage vs. lab: no physical sample is recovered (a separate borehole or push-in sampler is still needed for classification and any lab index testing), and design parameters (strength, state parameter) come from empirical correlations that must be calibrated against the site's own lab data to be trusted.
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. Advantage vs. lab: tests the clay in its true in-situ stress state and structure, avoiding the sample-disturbance strength loss that affects even a good Shelby-tube specimen tested in a lab triaxial cell, and it is fast and inexpensive relative to a full triaxial testing program. Disadvantage vs. lab: limited to soft, fine-grained soil (unreliable in stiff, fissured, or sandy/gravelly ground, where the blades cannot shear a clean cylindrical surface), gives only undrained shear strength (no stiffness or drained-strength parameter, both obtainable from a lab triaxial series), and requires an empirical correction factor for plasticity that a lab test does not.
4. Pressuremeter Test (PMT/SBPMT) (ASTM D4719) — a cylindrical probe expanded radially against the borehole or self-bored into the ground, measuring the full in-situ stress-strain curve up to the limit pressure. Advantage vs. lab: the only common in-situ test that directly measures both a deformation modulus and a limit/shear-strength parameter at essentially the true in-situ stress state, avoiding both sample disturbance and the boundary-condition idealizations of a lab triaxial cell. Disadvantage vs. lab: specialized, expensive equipment and a skilled operator, a slow test to run per point compared to SPT/CPT, and (for the pre-bored version) borehole-wall disturbance during drilling can still degrade the near-borehole soil the probe expands against.
5. Plate Load Test (ASTM D1194) — a rigid plate loaded incrementally at or near the proposed founding level, with settlement measured directly. Advantage vs. lab: measures bearing response at prototype scale in the actual ground, capturing macro-scale structure (fissures, layering) that a small lab specimen cannot represent, and gives a direct load-settlement curve rather than a parameter that still needs to be converted through a bearing-capacity or settlement formula. Disadvantage vs. lab: the stressed zone beneath a small test plate is shallow relative to a full-size foundation, so settlement measured at plate scale does not scale directly to the real foundation without an empirical size correction, and the test is slow, labour-intensive, and limited to shallow, accessible depths — a lab consolidation test remains the only practical route to a settlement parameter at depth.