16-Civ-B3 Geotechnical Design · May 2016

Question 3 of 9: Ten-storey building on a deep soft clay: site investigation and foundation choice (7 marks)

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

Notes on this paper

Paper format. Professional Engineers Ontario / Engineers Canada National Examinations, May 2016 — 98-Civ-B3 Geotechnical Design. Three hours, OPEN BOOK, any non-communicating calculator. Section A carries five discussion questions of 7 marks each (answer any four); Section B carries four design questions of 24 marks each (answer any three); 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 timed attempt.

Reference texts (98-Civ-B3 / 16-Civ-B3 Geotechnical Design).

B. M. Das, Principles of Foundation Engineering, 9th ed. — subsurface exploration (Ch. 2), bearing capacity of shallow foundations (Ch. 3), settlement of shallow foundations including Schmertmann's method (Ch. 5), retaining walls (Ch. 8), pile foundations (Ch. 11).
B. M. Das, Principles of Geotechnical Engineering, 9th ed. — shear strength (Ch. 12), lateral earth pressure (Ch. 13), slope stability including the planar-surface analysis (Ch. 15).
Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM), 4th ed. — the governing Canadian practice document for site investigation, in-situ testing, bearing resistance, deep foundations and earth-retaining structures.
R. F. Craig, Craig's Soil Mechanics, 9th ed. — effective stress, undrained versus drained behaviour, earth pressure and slope stability.
D. P. Coduto, Foundation Design: Principles and Practices, 3rd ed. — CPT correlations and settlement of shallow foundations on sand.
M. J. Tomlinson & J. Woodward, Pile Design and Construction Practice, 6th ed. — shaft adhesion in clay and driven displacement piles in sand.

Sources of design charts and assumed values (page-1 Note 6). Note 6 requires the candidate to identify the source of every design chart used and of every value assumed where the paper gives none. They are named at the point of use and collected here:

Strain-influence diagram and the C₁, C₂ correction factors (Q6) — Schmertmann, Hartman and Brown (1978), as tabulated in Das, Principles of Foundation Engineering, 9th ed., Section 5.6.
Rankine active coefficient for an inclined backfill (Q7) — Das, Principles of Geotechnical Engineering, 9th ed., Eq. (13.35).
Bearing-capacity factors N₢, Nᵤ, Nγ and the depth and load- inclination factors (Q7) — Vesic / Meyerhof as tabulated in Das, Principles of Foundation Engineering, 9th ed., Tables 3.3 and 3.4, applied to a retaining-wall base in Section 8.6.
Meyerhof bearing-capacity factor Nᵤ* for a driven pile point and the limiting point resistance (Q8) — Das, 9th ed., Section 11.9 and its interpolated Nᵤ* table; Nᵤ* = 143 at φ′ = 35°.
Adhesion factor α against cu/p₀ (Q8) — Das, 9th ed., Table 11.6 (after Terzaghi, Peck and Mesri); α = 0.68 at cu/p₀ = 0.5.
Earth-pressure coefficient K and interface friction angle δ′ for a driven high-displacement pile (Q8) — Das, 9th ed., Section 11.11: K ≈ 1.4K₀ and δ′ ≈ 0.8φ′ are assumed, and the critical-depth rule L′ = 15D is Das Eq. (11.42).
Unit weight of water γᵣ = 9.81 kN/m³ and g = 9.81 m/s² throughout; atmospheric pressure p₀ = 100 kPa.

Section A — discussion questions (7 marks each)

Question 3 — Ten-storey building on a deep soft clay: site investigation and foundation choice (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.

What the site investigation must deliver. With more than 100 m of soft clay there is no competent bearing stratum within economic reach, so the investigation cannot be aimed at "finding rock". It must instead characterise the compressible mass well enough to predict how much it will settle and how fast. The essential items are:

Stratigraphy and thickness — continuous boreholes with piezocone (CPTu) soundings to establish the full profile, the depth and thickness of any desiccated surface crust, and above all the presence, depth and continuity of any sand or silt seams, because those control the drainage path length and hence the rate of settlement.
Undrained shear strength profile $c_u(z)$ — from the field vane test with Bjerrum's plasticity correction, cross-checked against CPTu using $c_u = (q_t - \sigma_{v0})/N_{kt}$, and against unconsolidated-undrained triaxial tests on good-quality thin-walled samples. This is what the stability check needs.
Sensitivity and remoulded strength — eastern Canadian marine clays (Leda / Champlain clay) are commonly sensitive to highly sensitive, so a disturbed clay may retain only a small fraction of its intact strength; this governs constructibility, excavation support and the tolerance for pile driving.
Compressibility and stress history — oedometer (or CRS) tests giving the preconsolidation pressure $\sigma'_c$ and the overconsolidation ratio profile, the compression index $C_c$, the recompression index $C_s$, and the secondary compression index $C_\alpha$. Whether the building's stress increase stays below or crosses $\sigma'_c$ is the single most important number on the job, because it separates small recompression settlement from very large virgin-compression settlement. In a deposit this deep and this soft, secondary compression over the design life is not negligible and $C_\alpha$ must be measured, not assumed.
Rate of consolidation — the coefficient of consolidation $c_v$ from the oedometer and, preferably, from CPTu dissipation tests in situ, together with the permeability, so that the settlement-time curve can be predicted.
Groundwater regime — piezometers at several depths to establish the piezometric profile and whether it is hydrostatic, artesian or drawn down; regional drawdown from pumping would consolidate the whole deposit and add settlement independent of the building.
Unit weights and index properties — $\gamma$, water content, Atterberg limits and organic content for correlation, classification and for the effective-stress profile that everything else is referred to.
Effective-strength parameters $c'$ and $\phi'$ from consolidated-undrained triaxial tests with pore-pressure measurement, for the long-term case and for any retaining structure or basement excavation.
Chemical aggressivity — sulphate and chloride content and pH for the durability of buried concrete and steel.

The foundation. A ten-storey building on more than 100 m of soft clay is a settlement problem, not a strength problem, and the correct answer is a compensated (floating) raft, that is a raft at the base of a basement excavation sized so that the weight of soil removed approaches the weight of the completed building. If the net stress increase at foundation level is driven to zero the consolidation settlement is, in principle, eliminated, because there is no additional effective stress to consolidate under; a partially compensated raft, in which perhaps 60 to 80 per cent of the building weight is offset, reduces settlement in the same proportion and is usually the economic optimum. A ten-storey building applies roughly 130 to 150 kPa at grade, which corresponds to about 7 to 8 m of soil removed — two or three basement levels, which the building probably wants anyway.

Where full compensation is not achievable, or where differential settlement between a tower and a low-rise podium must be controlled, the raft is combined with friction (floating) piles to form a piled raft: the piles are not taken to any bearing stratum, since none exists, but act as settlement reducers that transfer part of the load deeper into the deposit and stiffen the raft. End-bearing piles are not an option here and should be explicitly ruled out in the answer.

The design checks that follow from this choice are: undrained bearing capacity of the raft, $q_u = 5.14c_u(1 + 0.2B/L)(1 + 0.2D/B) + q$, against the gross applied pressure; base heave and bottom stability of the excavation during construction, which in a soft sensitive clay is often the critical temporary condition; consolidation settlement of the full compressible thickness under the net stress increase, computed layer by layer; secondary compression over the design life; differential settlement and tilt, controlled by raft stiffness; and, if any fill is placed around the structure, negative skin friction on any piles used.

Topic. Foundation selection on a deep normally consolidated deposit; the compensated-raft concept.

Key relations. Net foundation pressure $q_{\text{net}} = q_{\text{gross}} - \gamma D_f$, driven to zero by choosing $D_f$; consolidation settlement of a normally consolidated layer $S_c = \dfrac{C_c H}{1+e_0}\log\dfrac{\sigma'_0 + \Delta\sigma'}{\sigma'_0}$; undrained raft capacity $q_u = 5.14c_u F_{cs}F_{cd} + q$; secondary compression $S_s = C_\alpha' H \log(t_2/t_1)$.

Why this works. Consolidation settlement is driven by the increase in effective stress, so if excavation removes as much weight as the building adds, the clay never sees a stress it has not already carried and the response is limited to small recompression and elastic rebound. The idea is old and well proven — it is how tall buildings were founded on the lacustrine clay of Mexico City and on the Champlain Sea clays of the Ottawa and St. Lawrence valleys.

Common pitfall. Proposing driven end-bearing piles because "the clay is soft", without asking what they would bear on. With more than 100 m of clay there is nothing to reach; piles here can only be friction piles, and a friction pile group in a deep normally consolidated clay settles as a block that stresses the clay well below the pile toes, so the group settlement can exceed the raft settlement it was meant to prevent. The second pitfall is to compute settlement from the gross pressure instead of the net pressure, which throws away the whole benefit of the basement.

Source. Das, Principles of Foundation Engineering, 9th ed., Ch. 2 (subsurface exploration), Ch. 6 (mat foundations, compensated foundations) and Ch. 11; Canadian Geotechnical Society, Canadian Foundation Engineering Manual, 4th ed., Ch. 4 (site investigation) and Ch. 11 (deep foundations).

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