Urban expansion on a marl slope above Granada
A municipality proposes 120 dwellings on a 35° north-facing slope above Granada. The slope is underlain by Tortonian marls with thin, laterally persistent conglomerate beds dipping 18° downslope; two joint sets are near-vertical. A road cut at the toe is 8 m high and shows seepage at 3 m. The upper slope has a 4 m high scarp and hummocky topography. Mean annual rainfall is 450 mm, but a 24-hour storm of 120 mm has a 25-year return period; the seismic design PGA is 0.24g. The building code requires a static factor of safety of at least 1.5 and a seismic factor of at least 1.1. A preliminary geotechnical report gives drained cohesion of 10 kPa and a friction angle of 22° for the marls, and recommends “drainage and a retaining wall.” Residents report cracks widening since the road cut was made.
1. Interpret the likely failure mechanisms and explain the relevance of each field observation.
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The most likely mechanisms are shallow translational sliding along the downslope-dipping conglomerate–marl contacts and earthflows in the saturated marls. The 35° slope is steeper than the 18° bedding, so the bedding planes can daylight at the toe where the road cut has removed support. The near-vertical joints allow rapid water infiltration and reduce rock-mass strength. Seepage at 3 m in the cut shows a perched water table; the marls will lose cohesion and gain weight when saturated. The upper scarp and hummocky topography are typical of an old landslide, so the site may contain pre-existing shear surfaces. Cracks widening after the cut indicate active downslope movement, not just settlement. The 0.24g design PGA can trigger failure by raising pore pressures and adding inertial loads. A retaining wall alone may be inadequate if it is founded on the same moving marl; drainage and slope reduction must come first.
2. Using c′ = 10 kPa, φ′ = 22°, γ = 19 kN/m³ and a 5 m deep infinite slope at β = 35°, estimate the static factor of safety for dry and saturated conditions. Can the code requirement be met? Show your calculation.
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For an infinite slope, FS = c′/(γ z sinβ cosβ) + tanφ′/tanβ. With β = 35°, sinβ = 0.574, cosβ = 0.819, so sinβ cosβ = 0.470. For z = 5 m and γ = 19 kN/m³, γ z sinβ cosβ = 19 × 5 × 0.470 = 44.6 kPa. The cohesion term is 10/44.6 = 0.224. The friction term is tan22°/tan35° = 0.404/0.700 = 0.577. Dry FS = 0.224 + 0.577 = 0.80. With the water table at the surface, use γ′ ≈ 19 − 9.81 = 9.19 kN/m³: the cohesion term becomes 10/(9.19 × 5 × 0.470) = 0.463, giving FS ≈ 1.04. Even for a 2 m deep dry slope, FS ≈ 1.14, still below 1.5. A static FS of 0.80–1.04 is unacceptable, and a seismic event would reduce it further. The code requirement cannot be met by the current geometry. Drainage, toe buttressing, slope reduction and possibly relocation are required; a retaining wall alone will not raise the factor of safety enough.
3. Recommend a monitoring and mitigation plan and state whether the development should be approved as proposed.
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I would not approve the development as proposed. The site contains an old landslide, active cracks and a toe cut in marls, and the calculated static factor of safety is well below 1.5. Approval should require a detailed geotechnical investigation: boreholes, inclinometers, piezometers, geophysical profiling and mapping of the old landslide limits. Monitoring should include surface and subsurface displacement, pore pressure, rainfall and crack width, with trigger thresholds for evacuation. Mitigation should start with drainage: intercept surface water, install deep drains and horizontal drains, and avoid discharging water into the marls. Reduce the slope angle and buttress the toe; relocate the road cut away from the slide toe. Use flexible, lightly loaded foundations or deep piles founded below the slip surface, and avoid placing dwellings on the old scarp. Revegetation can help but is not a substitute for drainage. A retaining wall may be part of the design, but only after the slope is stabilised. If monitoring shows continuing movement, the site should remain undeveloped.