Annex A

(informative)

Potential modes of failure of slopes

Table A.1 illustrates the forms that slope failures often take when they exceed the ultimate limit state (the serviceability limit is not covered here but is just as relevant.

Definitions are given in Table A.1 from BS 6100-3 for different forms of landslip, the global definition of which is given as "03 25009 slip, landslip – movement of a mass of soil (01) or rock (03 23027) by gravity NOTE Often a rotational displacement." The reference numbers within these definitions are a system established within the BSI system to enable cross referencing and therefore are included within this table.

Failure mode	BS 6100-3 definition (all numbers in brackets refer to definitions of terms within the BS)	Sketch
Rotational – Circular	03 25010 rotational slide – rotation of a mass of soil (01) along a curved slip surface (03 27025) 03 25011 circular slide – rotational slide (03 25010) on a slip surface (03 27025) that is approximately circular	Key 1 Original profile 2 Slip surface
Rotational – Non-circular	03 25012 non-circular slide – rotational slide (03 25010) on a slip surface (03 27025) that is not wholly circular	Key 1 Original profile 2 Slip surface
Translational	03 25013 translational slide – movement of a shallow mass of soil (01) in a plane roughly parallel to the slope (01) due to a weakness on the plane For limiting equilibrium: γz sinβ cosβ = c' + (γ – mγw)z cos2β tanϕ' If c' = 0:	Key 1 Water table 2 Slip surface
Compound	03 25018 compound slide – movement of a soil (01) mass that combines the characteristics (01) of a rotational slide (03 25010) and a translational slide (03 25013)	Key 1 Slip surfaces

Failure mode	BS 6100-3 definition (all numbers in brackets refer to definitions of terms within the BS)	Sketch
Flow slides	03 25014 flow slide, mud flow – translational slide (03 25013) in saturated soil (03 23021), caused by a sudden increase in pore water pressure (03 27011), in which the soil (01) flows as a viscous fluid 03 25017 debris slide – translational slide (03 25013) of debris, forming a mantle on a slope (01) or the disturbed material at the toe of a rotational slide (03 25010), when rainfall (05 29004) or diverted surface water (01) causes downward movement of the debris Includes debris flows, see Winter et al [78]	Key 1 Scarp 2 Source area 3 Flow track 4 Bedrock 5 Superficial deposits 6 Run-out lobe
Slab slide	03 25015 slab slide - translational slide (03 25013) in which the sliding mass remains more or less intact. NOTE Usually occurring in the weathered (01) surface of a slope (01).
Block slide	03 25016 block slide – translational slide (03 25013) in which a block of relatively strong rock (03 23027) or stiff to hard clay (BS EN 12670) moves down a slope (01) as a unit	Key 1 Sliding surface
Progressive failure	Progressive failure can occur in a mass of brittle soil when it is loaded non- uniformly. Failure first develops along a rupture surface or zone within part of the soil mass and as the post-peak strains within the failure zone increase, the soil strength within the failure zone reduces from peak towards residual. Final rupture of the soil mass occurs before the failure surface has developed fully. See Potts et al [79]

Failure mode	BS 6100-3 definition (all numbers in brackets refer to definitions of terms within the BS)	Sketch
Scour	Removal of soil from the ground surface by surface water which might be flowing within a watercourse, or be in the form of floodwater or surface water run-off. Scour is a common problem for slopes, river banks or around structures. On slopes scour erosion can quickly lead to the development of gullies. Surface water run-off erosion of earthworks are also referred to as "washouts".	Key 1 Concentration of surface water flow 2 Gully 3 Washed out soil
Internal erosion	Loss of soil from a slope face as a consequence of seepage of groundwater from a preferential flow path at the slope face (often referred to as piping), or the slumping of a saturated mass of soil promoted by water seeping through a slope (commonly referred to as slumping or sloughing).	Key 1 Recharge zone, upslope 2 Piezometric pressure in confined channel 3 High permeability channel
Rock slope – plane failure	A plane failure occurs when a block of relatively strong rock or stiff to hard clay moves down-slope as a unit on a plane of weakness in the form of a fissure or joint.	Key Direction of movement Discontinuity trace Slip plane
Rock slope – wedge failure	A wedge failure is essentially three- dimensional in form and occurs when a wedge of rock or stiff clay slides bodily forward and downward on two or three well defined joint planes which intersect behind the slope.	Key Direction of movement Discontinuity trace Slip plane

Failure mode	BS 6100-3 definition (all numbers in brackets refer to definitions of terms within the BS)	Sketch
Rock slope – toppling failures	Toppling failures occur in rock slopes where discontinuities behind the face are steeply inclined.	Key Direction of movement Discontinuity trace Open fracture or void
Rockfalls	Falls occur from steeply cut faces in soils, e.g. in excavations for trenches or foundation pits when only short term stability is required. Cracks open behind the face as a result of stress relief or drying shrinkage. Failure occurs near the base of the free-standing column of soil bounded by the crack system, and the mass of soil falls forward or slides into the cut.