6.7 Foundations on rock; additional design considerations
(1)P The design of spread foundations on rock shall take account of the following features:
- the deformability and strength of the rock mass and the permissible settlement of the supported structure;
- the presence of any weak layers, for example solution features or fault zones, beneath the foundation;
- the presence of bedding joints and other discontinuities and their characteristics (for example filling, continuity, width, spacing);
- the state of weathering, decomposition and fracturing of the rock;
- disturbance of the natural state of the rock caused by construction activities, such as, for example, underground works or slope excavation, being near to the foundation.
(2) Spread foundations on rock may normally be designed using the method of presumed bearing pressures (see Annex G). For strong intact igneous rocks, gneissic rocks, limestones and sandstones, the presumed bearing pressure is limited by the compressive strength of the concrete foundation.
(3) The settlement of a foundation may be assessed on the basis of comparable experience related to rock mass classification.
6.8 Structural design of spread foundations
(1)P Structural failure of a spread foundation shall be prevented in accordance with 188.8.131.52.
(2) The bearing pressure beneath a stiff foundation may be assumed to be distributed linearly. A more detailed analysis of soil-structure interaction may be used to justify a more economic design.
(3) The distribution of bearing pressure beneath a flexible foundation may be derived by modelling the foundation as a beam or raft resting on a deforming continuum or series of springs, with appropriate stiffness and strength.
(4)P The serviceability of strip and raft foundations shall be checked assuming serviceability limit state loading and a distribution of bearing pressure corresponding to the deformation of the foundation and the ground.
(5) For design situations with concentrated loads acting on a strip or raft foundation, forces and bending moments in the foundation may be derived from a subgrade reaction model of the ground, using linear elasticity. The moduli of subgrade reaction may be assessed by a settlement analysis with an appropriate estimate of the bearing pressure distribution. The moduli may be adjusted so that the computed bearing pressures do not exceed values for which linear behaviour may be assumed.
(6) Total and differential settlements of the structure as a whole should be calculated in accordance with 6.6.2. For this purpose, subgrade reaction models are often not appropriate. More precise methods, such as finite element computations, should be used when ground-structure interaction has a dominant effect.
6.9 Preparation of the subsoil
(1)P The subsoil shall be prepared with great care. Roots, obstacles and enclosures of weak soil shall be removed without disturbing the ground. Any resulting holes shall be filled with soil (or other material) to replicate the stiffness of the undisturbed ground.
(2) In soils susceptible to disturbance, such as clay, the sequence of excavation for a spread foundation should be specified to minimise disturbance. Usually it is sufficient to excavate in horizontal slices. In cases where heave is to be controlled, excavation should be in alternate trenches, the concrete being cast in each trench before excavating intermediate ones.