Engineering & Technology · Geotechnical Engineering & Ground Systems

Slopes, retaining systems, earthworks and instability

A slope resists a tendency to move through material strength and geometry. A retaining system changes that force balance, while earthwork construction changes the material state. Neither appearance nor a single density percentage proves stability.

Review force balance, normal and shear stress, strain, density versus unit weight, fluid pressure and hydraulic head. Read graph axes and convert kN/m² to kPa, metres to millimetres and seconds to days. Ground below the surface is spatially variable; the lessons introduce every soil-specific symbol and keep saturated, hydrostatic and loading assumptions explicit.

Back to the chapter

After reading, you can

Explain how slope mechanisms, retaining-system movement, groundwater and earthwork state affect stability.

Choose a mechanism before calculating a ratio

A potential failure may involve sliding along a planar seam, a curved zone through soil, rock blocks on discontinuities, erosion or local surface movement. A limit-equilibrium calculation compares available resistance with the resistance needed for an assumed mechanism. A factor of safety is a model result for stated conditions, not a measured probability of failure.

Weight, surcharge, pore pressure and earthquake-induced forces can alter that balance. Drainage conditions can change over time: rapid loading of saturated fine soil and later drained behaviour may have different critical states. A slope that has not moved visibly during one short observation period has not demonstrated performance under every future condition.

The original graph isolates one block on one plane. Pore force reduces the effective normal force and hence the model friction, without changing the prescribed block weight. It does not represent every force acting on a real landslide or test every alternative sliding surface.

Horizontal resultant pore force normal to plane U ranges 0–50 kN. Vertical ratio available friction to downslope driving force ranges 0–1.4. Supplied block W=100 kN on a 30° plane, friction angle 35°, no cohesion or other forces, gives F=[100cos30°−U]tan35°/[100sin30°]. It decreases from 1.2128 at U=0 to .5126 at U=50 and equals .7927 at U=30. Dashed unity reference means equal model strength and demand, not an acceptance criterion for real slopes.

Retaining systems depend on relative movement

A retaining wall and its retained ground deform together. At-rest, active and passive earth-pressure conditions refer to different deformation states. Full passive resistance usually needs a different and larger displacement than an active state; it should not be assumed available without compatibility. Braces, anchors, reinforcement and foundations create additional force paths.

Water pressure is a separate contribution, not automatically reduced by a soil friction coefficient. Blocked drains or a raised water boundary can therefore change demand even if the soil description is unchanged. Global movement around or beneath a wall can govern independently of a local wall-member strength check.

Earthwork quality includes state and method

Excavating, placing and compacting material changes its fabric and stress history. Mechanical compaction commonly reduces air-filled voids; consolidation involves drainage and effective-stress change. A laboratory compaction curve relates dry density to water content for a specified material and compaction effort. Its peak is conditional, not an immutable density of the soil.

Field comparisons need the matching laboratory reference, water condition, test location and relevant construction records. A density ratio cannot reveal every weak seam, drainage defect or unsuitable inclusion. Temporary construction stages also require their own ground model; final geometry does not establish the behaviour of an earlier unsupported stage.

Test an isolated sliding mechanism and a density ratio

Assume a 100 kN block on a 30° plane, φ′=35°, c′=0, no additional forces, and pore force U acting only normal to the plane. Driving force is 100sin30°=50 kN. With U=0, available friction is 100cos30°tan35°≈60.64 kN, giving ratio 1.213.

With U=30 kN, available friction is (86.60−30)tan35°≈39.63 kN and the ratio falls to .793. Below unity, this particular static friction model cannot balance its driving force. A ratio above unity in the first case is not a project acceptance criterion or evidence that other mechanisms are adequate.

Separately assume field bulk density 2.10 Mg/m³ at gravimetric water content .15. Dry density is 2.10/1.15≈1.826 Mg/m³. Against a supplied matching laboratory maximum of 1.90 Mg/m³, relative compaction is about 96.1%. No required percentage, water range or stability criterion is supplied, so this arithmetic does not approve the earthwork.

Try the reasoning

Is a model factor of safety of 1.2 a 20% probability margin?
Show the answer and reasoning
No. It is a resistance-to-demand comparison under assumptions, not a failure probability.
Why can a drain blockage change wall loading?
Show the answer and reasoning
Pore-water pressure is a separate load contribution and depends on hydraulic boundaries and flow.
Why identify the laboratory compaction method in a density comparison?
Show the answer and reasoning
Its material, effort and water conditions define the reference maximum; another reference can change the ratio.

What to carry forward

Retain the mechanism, water state, movement compatibility and construction stage behind a stability claim.

References

Teaching resources and research supporting this lesson’s explanations and examples.