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16-Civ-B3 Geotechnical Design · December 2013

Question 5 of 9: Expansive clay backfill: problems and design measures

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

Notes on this paper

Paper format. National Examinations, December 2013 — 98-Civ-B3 Geotechnical Design. Three hours, open book, any non-communicating calculator. Section A holds five 7-mark discussion questions (answer any four); Section B holds four 24-mark design questions (answer any three), so the examinable total is 4 × 7 + 3 × 24 = 100 marks. Every one of the nine questions is answered here, because the set is a study resource rather than a sitting.

Reference texts. B. M. Das, Principles of Foundation Engineering (9th ed.) and Principles of Geotechnical Engineering (9th ed.); Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM, 4th ed.) — the governing Canadian reference for foundation practice; R. F. Craig, Craig's Soil Mechanics (9th ed.); D. P. Coduto, Foundation Design: Principles and Practices (3rd ed.).

Sources of design charts and assumed values (paper Note 6). The paper requires every chart and assumed value to be identified. Bearing-capacity factors are Terzaghi’s (Das, Foundation Engineering, Table 3.1, with Nγ after Kumbhojkar 1993); the pile end-bearing factor Nq* is read from Meyerhof’s chart (Das Fig. 11.14) and cross-checked against Janbu’s closed-form expression; the adhesion factor α is Das Table 11.5 (Terzaghi, Peck & Mesri); the strain-influence distribution is Schmertmann, Hartman & Brown (1978). Assumed values — specific gravity of solids Gs = 2.65 for the Question 8 backfill, base friction δ = ⅔φ′ and base adhesion ca = ⅔c′ (CFEM §24), and a driving-parameter value K = 1.4K0 for a high-displacement driven pile (Das §11.11) — are flagged where they are used.
Check — Question 6 text and figure disagree. The printed text of Question 6 states a 1 m × 1 m square footing with γ = 20 kN/m³ and φ = 36°, while Figure 3 is drawn for a 2.5 m footing with γ = 18 kN/m³ and φ′ = 38°. The question text governs (it is the instruction to the candidate); the figure is used only for the embedment Df = 1.5 m and for the CPT modulus profile, which the text does not restate. The alternative reading is noted at the end of the answer.

Question 5: Expansive clay backfill: problems and design measures (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.

Problems. A high-plasticity expansive clay behind a 6 m wall attacks the design in five separate ways.

Swelling pressure. On wetting, an active clay expands against the restraint of the wall and develops a horizontal swelling pressure that bears no relation to $K_a\gamma z$. Measured swelling pressures of 100–300 kPa are common in a CH clay of plasticity index above about 35, which for a 6 m wall can exceed the active thrust several times over; worse, the pressure is roughly uniform with depth, so it raises the point of application and greatly increases the overturning moment. The process is seasonal and cyclic, so the wall is ratcheted outwards a little in every wet season and never recovers the movement.

Pore water and drainage. The permeability of a compacted CH clay is of order $10^{-9}$ m/s, so it will not drain. Any water that reaches the backfill — from infiltration, a leaking service or a perched table — stays there, and full hydrostatic pressure builds behind the wall. Adding $\tfrac12\gamma_w H^2$ to the earth thrust roughly doubles the total horizontal force on a 6 m wall, and the weep holes and granular drain that would relieve it in a sand backfill simply clog or never receive flow.

Shrinkage cracking. In the dry season the clay shrinks away from the wall and opens vertical tension cracks. These fill with surface water in the next storm, applying full water pressure at depth in the crack, and they also let water into the body of the fill so that the swelling cycle repeats from a wetter starting point.

Low and time-dependent strength. The drained friction angle of a high-plasticity clay is low ($\phi^{\prime}$ of 15–22° at peak, falling towards a residual of 8–12° on a polished surface), so $K_a$ is high and the long-term thrust is large. Any apparent cohesion measured on a freshly compacted specimen is lost on saturation, and the clay creeps, so the wall continues to move under a constant load.

Construction and settlement. The fill is difficult to place and compact within a narrow moisture window, it is unworkable in wet weather, and it consolidates under its own weight for years, dragging down any structure or pavement founded on it and loading the heel of the wall through negative skin friction on the stem.

Precautions and design measures. The first and cheapest measure is not to use the clay next to the wall. Excavate a wedge behind the stem bounded by a plane rising at $45^{\circ} + \phi^{\prime}/2$ from the heel and replace it with imported free-draining granular fill; the clay can then be used as general fill beyond that wedge, where it is no longer able to load the wall directly. Where imported material is genuinely unavailable, the clay can be chemically stabilised with 4–6 % lime, which flocculates the clay, cuts the plasticity index and largely destroys the swell potential; a lime-treated zone 1.5–2 m thick behind the stem is a standard solution.

Whatever fill is used, provide a full-height drainage system: a geocomposite drainage blanket or a 300–500 mm granular chimney drain against the stem, wrapped in a filter geotextile, discharging to a perforated collector pipe at the heel with positive outfall, plus weep holes at 1.5–3 m centres. Cap the backfill with a low-permeability surface seal — a 300 mm compacted clay or asphalt apron sloped away from the wall, with sealed surface drains — so that seasonal moisture change in the fill is suppressed; this is the single most effective way to stop the swell–shrink cycle. Place a compressible cushion (a geofoam or compressible fill layer 100–200 mm thick) between the clay and the stem where clay must remain in contact, so the swelling strain is absorbed rather than converted into pressure.

In the structural design, check the wall for the larger of the swelling pressure and the at-rest pressure, taken as uniform over the full height, rather than for the active pressure; include full hydrostatic pressure unless the drainage system is redundant and maintainable; use c′ = 0 with a conservative long-term φ′ (and residual values if a slip surface can form); compact the clay slightly wet of optimum to the minimum density that satisfies stability, since dry, densely compacted expansive clay swells the most; and consider replacing the gravity wall with a reinforced-soil or counterfort wall with granular reinforced zone, or a tied-back wall, so that the clay is never the load-carrying element. Finally, install inclinometers and piezometers behind the wall and inspect the drainage annually — a clay-backfilled wall is a maintained structure, not a fit-and-forget one.