Crest Loading on a 15 m Cut Slope Above a Public Road
A 15 m high cut slope at 45° in residual soil supports a proposed 30 m wide platform at its crest. The platform will impose a uniform 25 kPa surcharge for a bus depot. The slope currently has a computed factor of safety of 1.35 under the current groundwater regime; the authority requires 1.40 for the proposed condition. A 2 m thick fill layer is proposed at the crest to achieve the platform level. Ground investigation records SPT N values of 8 to 15 in the upper 5 m, increasing to 30 at depth, and a perched water table 3 m below the crest. There is a 1.5 m high masonry retaining wall at the toe, in poor condition, and a public road 2 m from the toe. Two inclinometers show 3 mm of downslope movement over six months, which the developer attributes to reading error.
1. Assess whether a factor of safety of 1.35 is acceptable and what the movement indicates.
Read the worked answer
A FoS of 1.35 is below the required 1.40 for the proposed condition, so the slope cannot be accepted as it stands. More importantly, the developer's explanation of reading error is not credible. Two inclinometers showing 3 mm of consistent downslope movement over six months indicate active or creeping movement; inclinometer readings are usually repeatable to about 0.1–0.2 mm, so 3 mm is significant. The perched water table 3 m below the crest and the poor toe wall add to the concern. The existing FoS of 1.35 may already rely on optimistic parameters. I would not accept a simple re-run with the same parameters and a 1.40 target; I would require back-analysis of the movement to calibrate strength and pore pressure.
2. What site investigation and analysis would you require before determining the slope's stability?
Read the worked answer
Investigation: (1) additional boreholes with SPT and undisturbed sampling in the upper 10 m; (2) triaxial and direct shear tests, and ring shear tests to obtain residual strength if a slip surface is present; (3) standpipe piezometers to establish the perched water table and its seasonal variation; (4) survey the existing wall and road for movement; (5) topographic and inclinometer monitoring over at least three months; (6) slope stability analysis using Bishop, Spencer or finite element methods, with sensitivity to pore pressure and strength. The 25 kPa surcharge and the 2 m fill must be included. The analysis should demonstrate a FoS of at least 1.40 for the proposed case and 1.50 for the long-term condition, depending on the authority's requirements.
3. Recommend a mitigation scheme and monitoring plan.
Read the worked answer
Mitigation: (1) remove or rebuild the poor toe wall and replace it with a toe buttress or gabion wall; (2) install horizontal drains to lower the perched water table; (3) soil nails or ground anchors through the upper 8 m, with a structural facing; (4) reduce the crest surcharge by moving the bus depot or using a lighter structure; (5) avoid the 2 m fill, or use lightweight fill. Monitoring: inclinometers, piezometers, settlement points on the road and wall, with trigger levels (e.g. 5 mm amber, 10 mm red) and a response plan. The works should be sequenced so that drainage and toe support are installed before crest loading.