Design Philosophy

For the designer, the primary boundary conditions are stability and allowable settlement, both as a function of time. This chapter therefore approaches the design process from these two aspects; the intention is not to trivialize the other aspects, but rather to simplify the subject. In the present chapter, ‘safety’ is not a specific concept, as it is in Chapter 3; the word ‘safety’ is here used in its generally accepted meaning.

An earth structure requires additional space because a difference in level as compared with ground level has to be spanned – above (in the case of filling) or below (in the case of excavation) – mostly by means of a slanting surface or slope. The extra space occupied is partly determined by the properties of the subsoil; the slope angles which can be achieved depend on the strength and deformation properties of the foundation, and, in the case of filling, of the filling material. In addition, aspects such as maintenance and management may affect the design. We have in mind factors such as the possibility of mowing by machine, the need for a maintenance platform, etc.

The designer’s task is to devise acceptable solutions for safely transferring the load to the subsoil. The load usually consists of the earth structure’s own weight. A load may also result from the function of the structure itself, such as for example traffic on roads and depots at industrial sites. An excavation is the equivalent of a negative load. In addition, in a country such as the Netherlands, the load exerted by the groundwater flow must not be underestimated. This load occurs is areas where there is permanent polder pumping, or in areas affected by temporary high sea and river water levels or by temporary pumping. These examples show that the concept of load must therefore be understood at its widest.

Every load on soil in principle upsets the prevailing pattern of the in situ stress. The science of mechanics teaches us that a change in the stresses leads to deformation. If the soil is not capable of absorbing the increased stresses, very large deformations may occur, possibly leading to collapse.

In addition, it is the importance of a particular earth structure which usually dictates the safety margin in the design. The design technique applied, in combination with the nature and scope of the soil investigation, is the key factor with respect to safety.

The primary focus in building a structure on subsoil of low bearing capacity and high compressibility is the control of stability and deformation. There are two important questions in this connection: is the structure stable in all circumstances and are the deformations in the structure allowable?

For any given load, the stability or equilibrium of an earth structure is determined by the shear strength of the soil. The shear strength, in its turn, depends on the magnitude of the strength parameters, such as cohesion and the angle of internal friction and the effective stress level. The methods which have been developed to make it possible to build safe earth structures are intended to:

·        Restrict the shear stress; for example by gradually bridging the height differences by means of a gradual slope, which may or may not be combined with a berm or carefully raising the stress level by phased filling;

·        Increase the shear strength in situ; for example by soil replacement or soil improvement.

In principle, it is always possible to construct a safe earth structure on highly compressible subsoil using traditional filling materials such as