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EN 1997 Eurocode 7 introduces in the verifications regarding structural and geotechnical limit states design approaches that vary for different combinations of groups partial coefficients for actions,  for material strength and overall strength of the system.

Each EU member state issues the National Annex (NA) or detailed specifications for the application of the directives contained in EN 1997.

For example, the first approach is used in the UK and Portugal, the second approach in most European countries (Germany, Slovakia, Italy, etc.) for the calculation of the bearing capacity and the third approach in the Netherlands and in most European countries for the calculation of slope stability.

The specifications give the values ​​of the partial factors to be used and indicate approaches to be adopted in the design phase for the different works (bearing capacity, anchors, bulkheads, retaining walls, etc.).

 

DESIGN APPROACHES

2.4.7.3.4.2 Design Approach 1

 

1.Except for the design of axially loaded piles and anchors, it shall be verified that a limit state of rupture or excessive deformation will not occur with either of the following combinations of sets of partial factors:

 

Combination 1: A1 “+” M1 “+” R1

Combination 2: A2 “+” M2 “+” R1

 

where “+” implies: “to be combined with”.

 

NOTE: In Combinations 1 and 2, partial factors are applied to actions and to ground strength parameters.

 

2.For the design of axially loaded piles and anchors, it shall be verified that a limit state of rupture or excessive deformation will not occur with either of the following combinations of sets of partial factors:

 

Combination 1: A1 “+” M1 “+” R1

Combination 2: A2 “+” (M1 or M2) “+” R4

 

NOTE 1: In Combination 1, partial factors are applied to actions and to ground strength parameters. In Combination 2, partial factors are applied to actions, to ground resistances and sometimes to ground strength parameters.

NOTE 2: In Combination 2, set M1 is used for calculating resistances of piles or anchors and set M2 for calculating unfavourable actions on piles owing e.g. to negative skin friction or transverse loading.

 

3.If it is obvious that one of the two combinations governs the design, calculations for the other combination need not be carried out. However, different combinations may be critical to different aspects of the same design.

 

 

2.4.7.3.4.3 Design Approach 2

1.It shall be verified that a limit state of rupture or excessive deformation will not occur with the following combination of sets of partial factors:

 

Combination: A1 “+” M1 “+” R2

 

NOTE 1 In this approach, partial factors are applied to actions or to the effects of actions and to ground resistances.

NOTE 2 If this approach is used for slope and overall stability analyses the resulting effect of the actions on the failure surface is multiplied by γE and the shear resistance along the failure surface is divided by γR;e.

 

2.4.7.3.4.4 Design Approach 3

1.It shall be verified that a limit state of rupture or excessive deformation will not occur with the following combination of sets of partial factors:

 

Combination: (A1* or A2†) “+” M2 “+” R3

 

*on structural actions

†on geotechnical actions

 

NOTE 1 In this approach, partial factors are applied to actions or the effects of actions from the structure and to ground strength parameters.

NOTE 2 For slope and overall stability analyses, actions on the soil (e.g. structural actions, traffic load) are treated as geotechnical actions by using the set of load factors A2.

 

 

The table 3.1. below shows which of partial factor are used in each design approach, depending on the type of structure being designed.

 

Table 3.1 - Ultimate limit state, design approach (*on structural actions,+ on geotechnical actions)

 

 

Table 3.2 - Shows the relative magnitude of the key parameters when using Combination and using Combination 2

 

 

Table 3.3 - Shows the relative magnitude of the key parameters when using Design Approach 2

 

 

Table 3.4 - Shows the relative magnitude of the key parameters when using Design Approach 3

 

Spread foundations

 

6.1 General

1. The provisions of this Section apply to spread foundations including pads, strips and rafts.

2. Some of the provisions may be applied to deep foundations such as caissons.

 

6.2 Limit states

1. The following limit states shall be considered and an appropriate list shall be compiled:

- loss of overall stability;

- bearing resistance failure, punching failure, squeezing;

- failure by sliding;

- combined failure in the ground and in the structure;

- structural failure due to foundation movement;

- excessive settlements;

- excessive heave due to swelling, frost and other causes;

- unacceptable vibrations.

 

6.3 Actions and design situations

1.Design situations shall be selected in accordance with 2.2.

2.The actions listed in 2.4.2(4) should be considered when selecting the limit states for calculation.

3.If structural stiffness is significant, an analysis of the interaction between the structure and the ground should be performed in order to determine the distribution of actions.

 

6.4 Design and construction considerations

1. When choosing the depth of a spread foundation the following shall be considered:

- reaching an adequate bearing stratum;

- the depth above which shrinkage and swelling of clay soils, due to seasonal weather changes, or to trees and shrubs, may cause appreciable movements;

- the depth above which frost damage may occur;

- the level of the water table in the ground and the problems, which may occur if excavation for the foundation is required below this level;

- possible ground movements and reductions in the strength of the bearing stratum by seepage or climatic effects or by construction procedures;

- the effects of excavations on nearby foundations and structures;

- anticipated excavations for services close to the foundation;

- high or low temperatures transmitted from the building;

- the possibility of scour;

- the effects of variation of water content due to long periods of drought, and subsequent periods of rain, on the properties of volume-unstable soils in arid climatic areas;

- the presence of soluble materials, e.g. limestone, claystone, gypsum, salt rocks;

 

2. Frost damage will not occur if:

- the soil is not frost-susceptible;

- the foundation level is beneath frost-free depth;

- frost is eliminated by insulation.

 

3. EN-ISO 13793:2001 may be applied for frost protecting measures for building foundations.

 

4.In addition to fulfilling the performance requirements, the design foundation width shall take account of practical considerations such as economic excavation, setting out tolerances, working space requirements and the dimensions of the wall or column supported by the foundation.

 

5. One of the following design methods shall be used for spread foundations:

- a direct method, in which separate analyses are carried out for each limit state. When checking against an ultimate limit state, the calculation shall model as closely as possible the failure mechanism, which is envisaged. When checking against a serviceability limit state, a settlement calculation shall be used;

- an indirect method using comparable experience and the results of field or laboratory measurements or observations, and chosen in relation to serviceability limit state loads so as to satisfy the requirements of all relevant limit states;

- a prescriptive method in which a presumed bearing resistance is used (see 2.5).

 

6. Calculation models for ultimate and serviceability limit state design of spread foundations on soil given in 6.5 and 6.6 respectively should be applied. Presumed bearing pressures for the design of spread foundations on rock should be applied according to 6.7.

 

6.5 Ultimate limit state design

6.5.1 Overall stability

1. Overall stability, with or without the foundations, shall be checked particularly in the following situations:

- near or on a natural or man-made slope;

- near an excavation or a retaining wall;

- near a river, a canal, a lake, a reservoir or the sea shore;

- near mine workings or buried structures.

 

2. For such situations, it shall be demonstrated using the principles described in Section 11, that a stability failure of the ground mass containing the foundation is sufficiently improbable.

 

6.5.2 Bearing resistance

6.5.2.1 General

1. The following inequality shall be satisfied for all ultimate limit states:

Vd ≤ Rd               [6.1]

2.Rd shall be calculated according to 2.4.

3.Vd shall include the weight of the foundation, the weight of any backfill material and all earth pressures, either favorable or unfavorable. Water pressures not caused by the foundation load shall be included as actions.

 

6.5.2.2 Analytical method

1.The sample analytical calculation for bearing resistance given in Annex D may be used.

2.An analytical evaluation of the short-term and long-term values of Rd shall be considered, particularly in fine-grained soils.

3.Where the soil or rock mass beneath a foundation presents a definite structural pattern of layering or other discontinuities, the assumed rupture mechanism and the selected shear strength and deformation parameters shall take into account the structural characteristics of the ground.

4.When calculating the design bearing resistance of a foundation on layered deposits, the properties of which vary greatly between one another, the design values of the ground parameters shall be determined for each layer.

5.Where a strong formation underlies a weak formation, the bearing resistance may be calculated using the shear strength parameters of the weak formation. For the reverse situation, punching failure should be checked.

6.Analytical methods are often not applicable to the design situations described in 6.5.2.2(3)P, 6.5.2.2(4)P and 6.5.2.2(5). Numerical procedures should then be applied to determine the most unfavorable failure mechanism.

7.The overall stability calculations described in Section 11 may be applied.

 

6.5.2.3 Semi-empirical method

1.The sample semi-empirical method for bearing resistance estimation using pressuremeter test results given in Annex E is recommended.

 

6.5.2.4 Prescriptive method using presumed bearing resistance

1.The sample method for deriving the presumed bearing resistance for spread foundations on rock given in Annex G is recommended. When this method is applied, the design result should be evaluated on the basis of comparable experience.

 

6.5.3 Sliding resistance

1. Where the loading is not normal to the foundation base, foundations shall be checked against failure by sliding on the base.

2. The following inequality shall be satisfied:

Hd ≤ Sd + Epd        [6.2]

3.Hd shall include the design values of any active earth forces imposed on the foundation.

4.Rd shall be calculated according to 2.4.

5.The values of Rd and Rp;d should be related to the scale of movement anticipated under the limit state of loading considered. For large movements, the possible relevance of post-peak behaviour should be considered. The value of Rp;d selected should reflect the anticipated life of the structure.

6.For foundations bearing within the zone of seasonal movements of clay soils, the possibility that the clay could shrink away from the vertical faces of foundations shall be considered.

7.The possibility that the soil in front of the foundation may be removed by erosion or human activity shall be considered.

8.For drained conditions, the design shear resistance, Rd , shall be calculated either by factoring the ground properties or the ground resistance as follows;

Rd = V'd tan δd                        (6.3a)

or

Rd = (V’d tan δk ) / γR;h                (6.3b)

 

Note In design procedures where the effects of actions are factored, the partial factor for the actions (γF ) is 1,0 and V’d = V’k in equation (6.3b).

9. In determining Vd', account shall be taken of whether Hd and V'd are dependent or independent actions.

10. The design friction angle δd may be assumed equal to the design value of the effective critical state angle of shearing resistance, ϕ'cv;d , for cast-in-situ concrete foundations and equal to 2/3 ϕ'cv;d for smooth precast foundations. Any effective cohesion c' should be neglected.

11. For undrained conditions, the design shearing resistance, Rd , shall be calculated either by factoring the ground properties or the ground resistance as follows:

Rd = A' cu;d                (6.4a)

or

Rd = (Acu;k) / γR;h        (6.4b)

 

12. If it is possible for water or air to reach the interface between a foundation and an undrained clay subgrade, the following check shall be made:

Rd ≤ 0,4 Vd (6.5)

 

13. Requirement (6.5) may only be disregarded if the formation of a gap between the foundation and the ground will be prevented by suction in areas where there is no positive bearing pressure.

 

6.5.4 Loads with large eccentricities

1.Special precautions shall be taken where the eccentricity of loading exceeds 1/3 of the width of a rectangular footing or 0,6 of the radius of a circular footing. Such precautions include:

- careful review of the design values of actions in accordance with 2.4.2;

- designing the location of the foundation edge by taking into account the magnitude of construction tolerances.

2. Unless special care is taken during the works, tolerances up to 0,10 m should be considered.

 

6.5.5 Structural failure due to foundation movement

1.Differential vertical and horizontal foundation displacements shall be considered to ensure that they do not lead to an ultimate limit state occurring in the supported structure.

2.A presumed bearing pressure may be adopted (see 2.5) provided displacements will not cause an ultimate limit state in the structure.

3.In ground that may swell, the potential differential heave shall be assessed and the foundations and structure designed to resist or accommodate it.

 

6.6 Serviceability limit state design

6.6.1 General

1.Account shall be taken of displacements caused by actions on the foundation, such as those listed in 2.4.2(4).

2.In assessing the magnitude of foundation displacements, account shall be taken of comparable experience, as defined in 1.5.2.2. If necessary, calculations of displacements shall also be carried out.

3.For soft clays, settlement calculations shall always be carried out.

4.For spread foundations on stiff and firm clays in Geotechnical Categories 2 and 3, calculations of vertical displacement (settlement) should usually be undertaken. Methods that may be used to calculate settlements caused by loads on the foundation are given in 6.6.2.

5.The serviceability limit state design loads shall be used when calculating foundation displacements for comparison with serviceability criteria.

6.Calculations of settlements should not be regarded as accurate. They merely provide an approximate indication.

7.Foundation displacements shall be considered both in terms of displacement of the entire foundation and differential displacements of parts of the foundation.

8.The effect of neighboring foundations and fills shall be taken into account when calculating the stress increase in the ground and its influence on ground compressibility.

9.The possible range of relative rotations of the foundation shall be assessed and compared with the relevant limiting values for movements discussed in 2.4.9.

 

6.6.2 Settlement

1.Calculations of settlements shall include both immediate and delayed settlement.

2.The following three components of settlement should be considered for partially or fully saturated soils:

- s0 : immediate settlement; for fully-saturated soil due to shear deformation at constant volume, and for partially-saturated soil due to both shear deformationand volume reduction;

- s1 : settlement caused by consolidation;

- s2 : settlement caused by creep.

3.The sample methods for evaluating settlements s 0 and s 1 given in Annex F may be applied.

4.Special consideration should be given to soils such as organic soils and soft clays, in which settlement may be prolonged almost indefinitely due to creep.

5.The depth of the compressible soil layer to be considered when calculating settlement should depend on the size and shape of the foundation, the variation in soil stiffness with depth and the spacing of foundation elements.

6.This depth may normally be taken as the depth at which the effective vertical stress due to the foundation load is 20 % of the effective overburden stress.

7.For many cases this depth may also be roughly estimated as 1 to 2 times the foundation width, but may be reduced for lightly-loaded, wider foundation rafts.

 

Note: This approach is not valid for very soft soils.

 

8.Any possible additional settlement caused by self-weight compaction of the soil shall be assessed.

9.The following should be considered:

- the possible effects of self-weight, flooding and vibration on fill and collapsible soils;

- the effects of stress changes on crushable sands.

10. Either linear or non-linear models of the ground stiffness shall be adopted, as appropriate.

11. To ensure the avoidance of a serviceability limit state, assessment of differential settlements and relative rotations shall take account of both the distribution of loads and the possible variability of the ground.

12. Differential settlement calculations that ignore the stiffness of the structure tend to be over-predictions. An analysis of ground-structure interaction may be used to justify reduced values of differential settlements.

13. Allowance should be made for differential settlement caused by variability of the ground unless it is prevented by the stiffness of the structure.

14. For spread foundations on natural ground, it should be taken into account that some differential settlement normally occurs even if the calculation predicts uniform settlement only.

15. The tilting of an eccentrically loaded foundation should be estimated by assuming a linear bearing pressure distribution and then calculating the settlement at the corner points of the foundation, using the vertical stress distribution in the ground beneath each corner point and the settlement calculation methods described above.

16. For conventional structures founded on clays, the ratio of the bearing capacity of the ground, at its initial undrained shear strength, to the applied serviceability loading should be calculated (see 2.4.8(4)). If this ratio is less than 3, calculations of settlements should always be undertaken. If the ratio is less than 2, the calculations should take account of non-linear stiffness effects in the ground.

 

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