16-Civ-B3 Geotechnical Design · December 2017

Question 5 of 9: When to prefer the CPT over the SPT

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. National Examinations, December 2017 — 16-Civ-B3 Geotechnical Design; three hours, open book, any non-communicating calculator. Section A holds five discussion questions worth 7 marks each of which four are marked; Section B holds four design questions worth 24 marks each of which three are marked, so the examinable total is 4 × 7 + 3 × 24 = 100 marks. All nine questions are worked below, because the set is a study resource rather than a marked script.

Reference texts. B. M. Das, Principles of Foundation Engineering, 8th–9th ed. (Cengage); B. M. Das, Principles of Geotechnical Engineering, 9th ed.; R. F. Craig / J. Knappett, Craig's Soil Mechanics, 9th ed.; Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM), 4th ed.; D. P. Coduto, Foundation Design: Principles and Practices; J. E. Bowles, Foundation Analysis and Design; ASTM D1586 (SPT), D5778 (CPTu), D2573 (field vane).

Source of design charts and assumed values (page 1, Note 6). Rankine active and passive coefficients from Das, Principles of Foundation Engineering, Ch. 7 (Eqs. 7.11 and 7.30); cantilever-wall stability procedure from Das Ch. 8 (Eqs. 8.11–8.14). Drilled-shaft adhesion factor alpha* = 0.55 and the 1.5 m surface exclusion from Reese & O'Neill (1989), tabulated in Das Ch. 12; bearing factor Nc* = 9 from Skempton (1951); block-failure check from Das Ch. 11 (Eq. 11.55). Compression index from Skempton's Cc = 0.009(LL − 10). Strain-influence factors and the C1, C2 corrections from Schmertmann, Hartman & Brown (1978) as presented in Das Ch. 5; Es = 500(N60 + 15) kPa from Das Table 5.7 (Bowles). Friction angle from the SPT via Wolff (1989), phi' = 27.1 + 0.3(N1)60 − 0.00054[(N1)60]², with the Liao & Whitman (1986) overburden correction; Vesic bearing-capacity, shape and depth factors from Das Tables 4.2 and 4.3. Every assumed value (unit weights of concrete, specific gravity for the void ratio, pile spacing, factors of safety) is stated in a callout beside the step that uses it.

Question 5: When to prefer the CPT over the SPT (7 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.

The two tests answer different questions. The SPT (ASTM D1586) drives a split-spoon sampler with a 63.5 kg hammer falling 760 mm and counts blows over the last 300 mm, at intervals of about 1.5 m, and returns a disturbed sample. The CPT (ASTM D5778) pushes a 35.7 mm instrumented cone at 20 mm/s and records cone resistance q_c, sleeve friction f_s and, in the piezocone, pore pressure u₂ every 10 to 50 mm, and returns nothing but numbers. The preference follows from that difference.

Prefer the CPT when the profile matters. A continuous record resolves layers a few centimetres thick — silt seams in a clay that control drainage, a soft band beneath a stiff crust, the top of a dense bearing stratum — all of which an SPT sampled at 1.5 m centres will miss entirely. For settlement analysis in sand, where the modulus profile is the whole answer, and for locating the founding level of piles, this resolution is decisive.

Prefer the CPT in soft to firm clays, silts and loose to medium sands. In soft clay the SPT N value is near zero and carries no useful information, while sampling disturbance corrupts the laboratory strength. The CPT reads a genuine bearing resistance, and the piezocone gives the undrained strength directly, c_u = (q_t − sigma_v0)/N_kt, with the correction for unequal end area applied through q_t. Dissipation tests at the same depth yield the horizontal coefficient of consolidation, which no borehole test provides in situ.

Prefer the CPT when repeatability and operator independence matter. The SPT is notoriously variable: energy transfer varies from about 45 to 95 per cent with hammer type and operator, so a raw N must be corrected to N₆₀ and then to (N₁)₆₀ for overburden before any correlation can be used, and borehole practice (casing, drilling fluid, bottom disturbance) adds further scatter. The CPT is a calibrated electrical measurement at a standard rate; repeat soundings on the same site typically agree within a few per cent.

Prefer the CPT for the derived quantities modern practice needs. Robertson's soil-behaviour-type charts classify the profile from q_t, friction ratio and pore-pressure parameter without a sample; E_s and the constrained modulus for Schmertmann settlement calculations come straight from q_c; liquefaction triggering is assessed from the normalised q_c1N using the NCEER/Youd–Idriss procedure, which is the method the Canadian Foundation Engineering Manual and BC practice prefer over the N-based route; and the seismic cone adds shear-wave velocity, hence G_max, for dynamic analysis. Practically, the CPT is also faster (100 m of profile in a day), cheaper per metre, produces no spoil and needs no water — a real advantage on contaminated or environmentally sensitive sites.

Where the SPT is still the right test. When the ground contains gravel, cobbles, boulders, glacial till or construction debris, the cone will refuse or the tip will be damaged, and the SPT with a solid cone shoe is the only in-situ option. When a physical sample is required for classification, Atterberg limits, chemical testing or simply to see the soil, only the borehole delivers it. When the design method or local code is calibrated on N — many pile-driving formulae, Meyerhof's direct settlement rules, and much of the historical database for a given city — the N value is what the correlation expects. When the required depth exceeds the reaction available (a truck rig can push only as hard as its ballast) or the site is inaccessible to a 20-tonne cone truck, the SPT wins on logistics. And the CPT gives no information at all about rock, so a socketed foundation still needs coring.

Conventional practice. The two are complementary, and the standard investigation on a significant Canadian project is a grid of CPTu soundings to define the stratigraphy and the strength or modulus profile continuously, tied to a smaller number of sampled boreholes with SPT and undisturbed sampling for classification, laboratory testing and local calibration of N_kt and the SBT charts.

Topic. Selecting between in-situ penetration tests, and the correlations each supports.

Key relations. N₆₀ = N E_rC_BC_SC_R/60; (N₁)₆₀ = C_NN₆₀ with C_N = (p_a/sigma'₀)^0.5; c_u = (q_t − sigma_v0)/N_kt, N_kt = 10–20; friction ratio F_r = f_s/(q_t − sigma_v0); E_s = 2 to 3.5 q_c in sand; q_c/N₆₀ ratio rising with mean grain size (Robertson).

Why this works. The cone is a small-scale, quasi-static bearing-capacity test performed continuously, so what it measures is directly proportional to the same soil properties that govern foundation performance, and at a rate slow enough to be repeatable but fast enough to be undrained in clay and drained in sand. The SPT is a dynamic test whose energy delivery is the dominant source of scatter; correcting to N₆₀ repairs part of that, but the test remains an index rather than a measurement.

Common pitfall. Comparing a raw field N with a published correlation that expects N₆₀ or (N₁)₆₀, and using q_c without the pore-pressure correction to q_t in soft clay, where the correction can be 20 per cent or more of the reading.

Source. Das, Principles of Foundation Engineering, Ch. 2 (Sections 2.7 SPT and 2.11 CPT); Canadian Foundation Engineering Manual, 4th ed., Ch. 4; Robertson & Cabal, Guide to Cone Penetration Testing.

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