TABLE OF CONTENTS In the design of retaining structures safety factors are applied on one hand in order to account for the uncertainty in applied load, soil strength parameters, groundwater conditions and soil geometry, on the other hand to provide a margin of safety against failure, and to keep deformations with in acceptable limit. Most design of retaining walls are based on either working stress method (total factor of safety) or limit state method (partial safety factor). The current development of safety in geotechnical engineering in Europe based on the partial safety concept is illustrated briefly in section 1.2. Therefore, this section will concentrate on the historical development of safety concept in geotechnical engineering and comparison of different approaches.
Fig. 4.29 illustrates the different methods employed in retaining wall design, where the shaded part of the pressure diagram shows those forces contributing to the resisting and mobilising moments after applying the corresponding factor of safety. These methods are:
Factor of safety on passive earth pressure: This method is based on Terzaghi's finding that passive pressure require more movement to be mobilised than did active pressure. Hence, the passive pressure used in design is reduced by a factor ? p (Fig. 4.29a). Values of ? p between 1.5 and 2.0 are usually employed (CP2 1951; EAB 1994). For normally consolidated soft soil ? p > 2 is also possible
Factor of safety on net total passive pressure: The design condition in this method...
