7.4 Risks of failure and acceptance of deformation
The risks of failure should be considered under the following headings:
- a) movement due to failure of the ground in shear;
- b) unacceptable deformation before failure is reached;
- c) loss of service due to erosion or other external causes.
The designer should consider aspects such as the potential for uncertainty within the ground model that might increase the risk of failure, the consequences of failure, and the acceptability of deformation for the structures under consideration. The design output should identify these issues and how they have been addressed in the design.
Traditionally slopes were designed using a global factor of safety to reflect the risk and consequences of failure of a slope and this approach provided a clear and simple way of increasing or decreasing the factor of safety according to particular circumstances. Designs in accordance with BS EN 1997-1:2004 are carried out using partial factors applied to loads and materials that are described in NA to BS EN 1997-1:2004. However, the designer should ensure that the risk of failure and consequences of failure have been adequately considered during the design. BS EN 1990:2002+A1 permits the variation of the relevant partial factors where the consequence of failure is either higher or lower than normal.
NOTE The traditional approach was for the designer to undertake additional design cases with unfactored parameters and use a global factor of safety considered appropriate for the uncertainties and consequences of failure of the slope.
In the case of cuttings a high safety factor is required where the results of a slip would endanger a main line railway or buildings (e.g. FoS of 1.4 rather than the commonly used target FoS of 1.3). A relatively low safety factor might be acceptable for the slopes of an excavation for a foundation structure which is to be backfilled on completion of the below-ground work, provided that a slip would not cause danger to life or to any buildings in the vicinity (e.g. FoS 1.1 or 1.2). Similar considerations apply to safety factors for embankments. One case of note is the stability of embankments during construction when founded on soft alluvium where the risk of unexpected failure is increased by the potential for porewater pressure migration along permeable laminations which is commonly mitigated by the use of a high global stability factor (e.g. FoS of 1.5) or by using normal safety factors with a pessimistic distribution of water pressure.
When considering the deformations of earthworks, it should be appreciated that earthworks can sometimes undergo large deformations without detriment to their own serviceability, although the effect of such deformation on the shear strength might be sufficient to cause failure at the ultimate limit state. In this respect, however, it is important that consideration is given to the effect of deformations on structures supported by or adjacent to the earthworks, and whether or not these deformations are likely to be progressive.
The designer should give careful consideration to mitigating the risk of overtopping or scour due to flood water. This may require adopting a risk-based approach in close co-operation with hydrologists to establish design flood levels.
7.4.2 Design life and serviceability
Design of earthworks should be undertaken to satisfy the requirements of BS EN 1997-1:2004, which requires that the ultimate and serviceability limit states should not be exceeded. Designers should also consider the durability of materials within the environmental conditions that will apply (BS EN 1997-1:2004, 2.3).
The concept of design life is not specifically addressed within BS EN 1997-1:2004, and reference should be made to BS EN 1990:2002+A1, which defines design life as "the assumed period for which a structure or part of it is to be used for its intended purpose with anticipated maintenance but without major repair being necessary".
NOTE In the UK asset owners often specify requirements associated with design life or serviceability.
The designer should consider the issues that might influence ultimate and serviceability limit states of the earthworks in developing the proposed solution and determining an appropriate design methodology.
BS EN 1997-1:2004, 12.6 requires the serviceability limit state design to assess deformation; the asset owner's specification, or the intended purpose of the earthworks, should determine the criteria for allowable deformation. Seasonal movements due to the swelling and shrinkage of soils and movements due to external influences other than loading from the earthworks itself should be considered as part of the serviceability limit state design.
The concept of design life was developed for the design of structures to be constructed from materials that deteriorate with time, and hence the structure may be designed based on an assumed rate of deterioration; however, traditional earthworks are constructed from natural materials (soil or rock) that are not expected to deteriorate noticeably over the timescale of an engineering project (which is short in geological terms). Deterioration of natural earthworks materials should be avoided during the life of the works by:
- choice of materials acceptable for each class of fill and compaction control during construction, i.e. by the earthworks specification; for example selected granular fill at structures should not include argillaceous rock which will not be durable in such a setting, the rate of deterioration can be rapid but is difficult to predict and hence not suited to the design life concept;
- the maintenance regime adopted, particularly of drainage, vegetation and the activity of others using the earthwork (both human and animal);
consequently it is not usually appropriate to design earthworks of natural material that will fail a limit state after a defined period of time.
COMMENTARY ON 7.4.2
This situation changes when earthworks includes artificial materials such as gabions, geosynthetics, soil nails, embedded piles or structures that form part of the earthwork solution. For these combined forms of earthwork the structural element can be designed to a design life to reflect the durability of the engineered elements. However, the significance of the element of the works to the overall stability of the earthworks may vary, e.g. a geotextile separator is likely to have a less direct impact on stability than oversteepened earthworks constructed of reinforced earth.
For highways schemes a common approach has been to assume a design life of 60 years for earthworks which are considered as any form of slope less than 70° slope face angle and can incorporate steeper minor earth retaining structures of up to 1,5 m vertical retained height (compared to the conventional 120 years for a structure). However, assessing the relative contribution of an element of the earthworks is often difficult and can distort the decision making process if simple rules are applied. It is important that the designer considers how the earthworks will behave, consequences of failure and the design life of other elements of the scheme; for example, for a soil nailed slope of 60° face angle adjacent to a major highway, the corrosion protection of the soil nails would have to be assessed giving consideration to the potential for differential corrosion at the particular site if that could lead to progressive failure of the system.
An employer's requirement for "maintenance free earthworks" is usually unrealistic, since in most cases it is likely to be more meaningful to specify a requirement for "the design and construction of the works to be completed to achieve serviceable status of the earthworks to a design life of 60 years and be major maintenance free for the first 25 years."
7.4.3 Effect on neighbouring structures
COMMENTARY ON 7.4.3
Buildings close to embankments and cuttings can be damaged by lateral soil deformation or heave. Excavation for road cuttings or foundation structures can cause vertical and horizontal deformation in the ground surrounding the excavation which can damage buildings or buried services. Upward soil movements beneath a deep basement excavation can cause damage to adjacent structures or to tunnels at a considerable depth.
As in the case of stability considerations, the effects of deformation are time-dependent, possibly requiring many years before the full effects become manifest. It will usually be found that the critical factor is the serviceability limit state of structures supported by the earthworks or affected by them, rather than that of the earthworks themselves.
Where the critical factor is the serviceability limit state of structures supported by the earthworks or affected by them, rather than that of the earthworks themselves, the calculations to determine the serviceability limit state should be made by conventional methods applicable to structures but based on data obtained from predicted ground deformations.
7.4.4 Impact on existing slopes
In cases where earthworks will be constructed in the vicinity of existing slopes either in the form of natural slopes or earthworks, the designer should assess the potential impacts on those existing slopes.
Examples can include widening existing earthworks, forming earthworks on an existing slope, or undertaking earthworks in close proximity of other slopes. In all these cases the earthworks activities can change the loading on the existing slope or modify the surface water and groundwater flow paths, both of which could be detrimental to the stability of the existing slope and therefore require consideration within the design. Where the existing slope is identified as being of poor stability then the design of the new earthworks will require special consideration to ensure that the resulting structure is adequately stable. Other related information is provided in 7.6.11 regarding embankments on sloping ground and Clause 11 on earthworks asset management and maintenance.
7.4.5 Stabilization of existing unstable slopes
Where works are planned to stabilize an existing unstable slope (either a natural or an earthwork slope) as well as satisfying the requirements of this standard (including the information at Clause 11), the design should require each aspect to be considered in greater detail since the construction works could exacerbate existing problems or create new problems on a slope of marginal stability.
The desk study, ground investigation and monitoring need to be planned to enable the problems of the existing slope to be adequately understood. Where possible the design should give consideration to other previous schemes that have been successful in similar ground conditions.
On existing landslips (especially in fine grained soils) it is preferable to determine both the groundwater profile and the rate of deformation in order to understand how these factors are linked to seasonal weather conditions, and assess the likelihood of a significant acceleration in movement if conditions deteriorate. Drainage is normally an important component of slope stabilisation works; however, these should be designed in recognition of ongoing slope movements as these can damage drainage runs leading to acceleration of the movements.
Understanding of the deformation history and the stress state of the soils will be particularly important in soil types where a significant reduction in shearing resistance can be expected (see 7.2.4) and design with residual parameters may be appropriate. It is often advantageous to plan the remedial works to achieve a gradual improvement in stability to reduce the risk of accelerated deformation during construction.
For major landslips, the design should include an element of risk management since complete stabilization under all conditions might not be realistic.
There are publications on the design of slope stabilization measures which should be referred to (e.g. Bromhead ); however, in this situation the previous experience of the design team will be particularly important.