2.4 Geotechnical design by calculation

2.4.1 General

(1)P Design by calculation shall be in accordance with the fundamental requirements of EN 1990:2002 and with the particular rules of this standard. Design by calculation involves:

  • actions, which may be either imposed loads or imposed displacements, e.g. from ground movements;
  • properties of soils, rocks and other materials;
  • geometrical data;
  • limiting values of deformations, crack widths, vibrations etc.;
  • calculation models.

(2) It should be considered that knowledge of the ground conditions depends on the extent and quality of the geotechnical investigations. Such knowledge and the control of workmanship are usually more significant to fulfilling the fundamental requirements than is precision in the calculation models and partial factors.

(3)P The calculation model shall describe the assumed behaviour of the ground for the limit state under consideration.

(4)P If no reliable calculation model is available for a specific limit state, analysis of another limit state shall be carried out using factors to ensure that exceeding the specific limit state considered is sufficiently improbable. Alternatively, design by prescriptive measures, experimental models and load tests, or the observational method, shall be performed.

(5) The calculation model may consist of any of the following:

  • an analytical model;
  • a semi-empirical model;
  • a numerical model.

(6)P Any calculation model shall be either accurate or err on the side of safety.

(7) A calculation model may include simplifications.

(8) If needed, a modification of the results from the model may be used to ensure that the design calculation is either accurate or errs on the side of safety.

(9) If the modification of the results makes use of a model factor, it should take account of the following:

  • the range of uncertainty in the results of the method of analysis;
  • any systematic errors known to be associated with the method of analysis.

(10)P If an empirical relationship is used in the analysis, it shall be clearly established that it is relevant for the prevailing ground conditions.

(11) Limit states involving the formation of a mechanism in the ground should be readily checked using a calculation model. For limit states defined by deformation considerations, the deformations should be evaluated by calculation as described in 2.4.8, or otherwise assessed.

NOTE Many calculation models are based on the assumption of a sufficiently ductile performance of the ground/structure system. A lack of ductility, however, will lead to an ultimate limit state characterised by sudden collapse.

(12) Numerical methods can be appropriate if compatibility of strains or the interaction between the structure and the soil at a limit state are considered.

(13) Compatibility of strains at a limit state should be considered. Detailed analysis, allowing for the relative stiffness of structure and ground, may be needed in cases where a combined failure of structural members and the ground could occur. Examples include raft foundations, laterally loaded piles and flexible retaining walls. Particular attention should be paid to strain compatibility for materials that are brittle or that have strain-softening properties.

(14) In some problems, such as excavations supported by anchored or strutted flexible walls, the magnitude and distribution of earth pressures, internal structural forces and bending moments depend to a great extent on the stiffness of the structure, the stiffness and strength of the ground and the state of stress in the ground.

(15) In these problems of ground-structure interaction, analyses should use stress-strain relationships for ground and structural materials and stress states in the ground that are sufficiently representative, for the limit state considered, to give a safe result.