9.5 Determination of earth pressures
(1)P Determination of earth pressures shall take account of the acceptable mode and amount of any movement and strain, which may occur at the limit state under consideration.
(2) In the following context the words "earth pressure" should also be used for the total earth pressure from soft and weathered rocks and should include the pressure of ground-water.
(3)P Calculations of the magnitudes of earth pressures and directions of forces resulting from them shall take account of:
- the surcharge on and slope of the ground surface;
- the inclination of the wall to the vertical;
- the water tables and the seepage forces in the ground;
- the amount and direction of the movement of the wall relative to the ground;
- the horizontal as well as vertical equilibrium for the entire retaining structure;
- the shear strength and weight density of the ground;
- the rigidity of the wall and the supporting system;
- the wall roughness.
(4) The amount of mobilised wall friction and adhesion should be considered as a function of:
- the strength parameters of the ground;
- the friction properties of the wall-ground interface;
- the direction and amount of movement of the wall relative to the ground;
- the ability of the wall to support any vertical forces resulting from wall friction and adhesion.
(5) The amount of shear stress, which can be mobilised at the wall-ground interface should be determined by the wall-ground interface parameter δ.
(6) A concrete wall or steel sheet pile wall supporting sand or gravel may be assumed to have a design wall ground interface parameter δd = k·φcv;d , k should not exceed 2/3 for precast concrete or steel sheet piling.
(7) For concrete cast against soil, a value of k = 1,0 may be assumed.
(8) For a steel sheet pile in clay under undrained conditions immediately after driving, no adhesive or frictional resistance should be assumed. Increases in these values may take place over a period of time.
(9)P The magnitudes of earth pressures and directions of resultant forces shall be calculated according to the selected design approach (see 220.127.116.11), and the limit state being considered.
(10) The value of an earth pressure at an ultimate limit state is generally different from its value at a serviceability limit state. These two values are determined from two fundamentally different calculations. Consequently, when expressed as an action, earth pressure cannot have a single characteristic value.
(11)P In the case of structures retaining rock masses, calculations of the ground pressures shall take account of the effects of discontinuities, with particular attention to their orientation, spacing, aperture, roughness and the mechanical characteristics of any joint filling material.
(12)P Account shall be taken of any swelling potential of the ground when calculating the pressures on the retaining structure.
9.5.2 At rest values of earth pressure
(1)P When no movement of the wall relative to the ground takes place, the earth pressure shall be calculated from the at rest state of stress. The determination of the at rest state shall take account of the stress history of the ground.
(2) For normally consolidated soil, at rest conditions should normally be assumed in the ground behind a retaining structure if the movement of the structure is less than 5×10-4×h.
(3) For a horizontal ground surface, the at rest earth pressure coefficient, K0, should be determined from:
The formula should not be used for very high values of OCR.
(4) If the ground slopes upwards from the wall at an angle β ≤ φ' to the horizontal, the horizontal component of the effective earth pressure σ'h;0 may be related to the effective overburden pressure q' by the ratio K0;β, where
The direction of the resulting force should then be assumed to be parallel to the ground surface.