They are based on the use of a rectangular stress block for the soil bearing pressure. CIRIA Report 516 gives the following recommendations, based on research by Brinch Hansen as incorporated in Eurocode 7. įor calculating the allowable maximum ground bearing pressure, BS8002 states that engineers should refer to BS8004 and Terzaghi & Peck (1967) but unfortunately neither of these actually gives guidance on foundations subjected to eccentric and inclined loads. Where design is based on the undrained shear strength of clay (c u), M should be 1.5, or greater for a clay which requires large strains to mobilise its peak strength. the design value of Φ' should be used, incorporating a mobilisation factor M of 1.2 or based on the critical state soil strength. For this calculation the soil properties used to calculate the bearing capacity should be derived on the same basis as those used to calculate the earth pressures - i.e. 4.2.2.1) and not exceeding the ultimate bearing capacity from BS8004 ( cf. The allowable maximum ground bearing pressure under a retaining wall should be checked in accordance with standard soil mechanics theory, taking into account the inclination and eccentricity of the applied loads ( cf. For convenience, presumed allowable bearing values from BS8004 Table 1 are reproduced below. 4.2.2.2 requires them to be based on a mobilisation factor M of between 2 and 3. Where earth pressures in a cohesive soil are estimated on the basis of undrained shear strength, BS8002 Cl. The bearing pressure on the soil under the toe of the wall should not exceed the allowable bearing pressure recommended by BS8004, taking into account soil conditions, groundwater level and also foundation depth and breadth. The coefficient K p for passive pressure based on Rankine theory (smooth wall face, level soil surface) is tabulated below. K a = cos²(Φ' - α)_Ĭos²α ²Ĭoefficients for Rankine active pressure are tabulated below The values of K a in the tables are for the horizontal component of the active pressure and are calculated from the formula Values are tabulated for δ = 0, δ = 20 º (the maximum value for a smooth wall surface) and also for tanδ = 0.75tan Φ', which is the maximum permitted by BS8002 for a rough wall surface.
This gives very similar answers to the charts in BS8002 but the tables have been extended to cover walls with sloping backs. Maximum values of δ based on this limit are tabulated below.Ĭoefficients for the horizontal component of active earth pressure based on Coulomb’s theory (as extended by Mayniel and Muller & Breslau) are tabulated below. The design wall friction δ depends on the wall surface and the situation being considered but tanδ must not exceed 0.75 × design tan Φ'. For convenience, design values of Φ' for M =1.2 are tabulated below. In BS8002, the design shear strength of the soil is based on the tangent of the friction angle reduced by a mobilisation factor M. NOTES FOR EARTH RETAINING WALL DESIGN TO BS8002