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Eurocode 7

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

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.

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.

2.4.7.3.4.3 Design Approach 2

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 gE and the shear resistance along the failure surface is divided by gR;e.

2.4.7.3.4.4 Design Approach 3

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.

| 1 | 2 | 3 | | | | Combination 1 | Combination 2 | | | | | General | A1+M1+R1 | A2+M2+R1 | A1+R2+M1 | A1(A2+)+M2+R3 | | Slope | A1+M1+R1 | A2+M2+R1 | E1+R2+M1 | E2+M2+R3 | | Piles and anchor-ages | A1+M1+R1 | A2+M1+R4 | A1+R2+M1 | A1(A2+)+M2**+R3 |

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

| A1 | M1 | R1 | A2 | M2 | R1 | | | | | Permanent actions (G) | Unfavorable | gG | 1,35 | | | 1,0 | | | | Favorable | gG,fav | 1,0 | | | 1,0 | | | | | Variable actions (Q) | Unfavorable | gQ | 1,5 | | | 1,3 | | | | Favorable | gQ,fav | 0 | | | 0 | | | | | Coef.of shearing resistance (tanf) | gf | | 1,0 | | | 1,25 | | | | Effective cohesion (c') | gc' | | 1,0 | | | 1,25 | | | | Undrained strength (cu) | gcu | | 1,0 | | | 1,4 | | | | Unconfined compressive strength (qu) | gqu | | 1,0 | | | 1,4 | | | | Weight density (γ) | gg | | 1,0 | | | 1,0 | | | | Resistance (R) | gR | | | 1,0 | | | 1,0 | |

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

| A1 | M1 | R1 | | | | | Permanent actions (G) | Unfavorable | gG | 1,35 | | | | Favorable | gG,fav | 1,0 | | | | | Variable actions (Q) | Unfavorable | gQ | 1,5 | | | | Favorable | gQ,fav | 0 | | | | | Material properties(c) | gM | | 1,0 | | | | Material resistance (Rv) | gRv | | | 1,4 | | | Sliding resistance (Rh) | gRh | | | 1,1 | | | Earth resistance against retaining structures | gRe | | | 1,4 | | | ....in slope | | | 1,1 | | |

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

| A1 | A2 | M2 | R3 | | | | | Permanent actions (G) | Unfavorable | gG | 1,35 | 1,0 | | | | Favorable | gG,fav | 1,0 | 1,0 | | | | | Variable actions (Q) | Unfavorable | gQ | 1,5 | 1,3 | | | | Favorable | gQ,fav | 0 | 0 | | | | | Coeff.of shearing resistance (tanf) | gf | | | 1,25 | | | | Effective cohesion (c') | gc' | | | 1,25 | | | | Undrained strength (cu) | gcu | | | 1,4 | | | | Unconfined compressive strength (qu) | gqu | | | 1,4 | | | | Weight density (γ) | gg | | | 1,0 | | | | Resistance (R) (except for pile shaft in tension) | gR | | | | 1,0 | | | Pile shaft resistance in tension | gR,st | | | | 1,1 | |

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

Spread foundations

6.1 General

6.2 Limit states

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

  2. loss of overall stability;

  3. bearing resistance failure, punching failure, squeezing;

  4. failure by sliding;

  5. combined failure in the ground and in the structure;

  6. structural failure due to foundation movement;

  7. excessive settlements;

  8. excessive heave due to swelling, frost and other causes;

  9. unacceptable vibrations.

6.3 Actions and design situations

6.4 Design and construction considerations

  • 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;

  • the soil is not frost-susceptible;

  • the foundation level is beneath frost-free depth;

  • frost is eliminated by insulation.

  • 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).

6.5 Ultimate limit state design

6.5.1 Overall stability

  • 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.

6.5.2 Bearing resistance

6.5.2.1 General

Vd ≤ Rd              [6.1]

6.5.2.2 Analytical method

6.5.2.3 Semi-empirical method

6.5.2.4 Prescriptive method using presumed bearing resistance

6.5.3 Sliding resistance

Hd ≤ Sd + Epd        [6.2]

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).

Rd = Ac cu;d (6.4a)

or

Rd = (Ac cu;k ) / γR;h (6.4b)

  1. 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)

  1. 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

  • 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.

  • 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

6.6 Serviceability limit state design

6.6.1 General

6.6.2 Settlement

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

  • s1 : settlement caused by consolidation;

s2 : settlement caused by creep.

Note This approach is not valid for very soft soils.

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

  • the effects of stress changes on crushable sands.