The materials that comprise the lower parts of the road and which form the subgrade are all geomaterials — particulate solids with pore spaces occupied by a combination of water and air in varying proportions. The solid particles are, for the most part, crystalline. They are derived, ultimately, from geological sources. Individually the grains have considerable strength which means that the mechanical response (strength, stiffness, resistance to development of rutting) of an assembly of particles is a primarily a consequence of the way the individual grains interact with one another and not of their own properties.
The primary contribution to mechanical property derives from the ease or difficulty with which one particle can be moved adjacent to another particle. This ease or difficulty is controlled by many factors which can, broadly, be grouped into three: physical characteristics of the grains, arrangement of the grains and the fluid conditions in the pores. The list of factors under each heading would be very long, but the following aims to highlight some of the more important:
• Physical characteristics of the grains:
о Particle shape; о Particle mineralogy; and о Particle roughness.
• Arrangement of the grains:
о Size and size distribution of the grains; and о Packing of the grains.
• Fluid conditions in the pores: о Fluid pressure in the pores;
о Surface tension effects in the pores between fluids; and о Water adsorption to mineral surfaces.
When a stress is applied to a granular or soil material, the stress has to be carried across the assemblage of grains via the inter-particle contact points. These contacts will be subjected to both normal and shear stresses. Both can cause compression that is recoverable and slippage between the particles at the contact or damage and wear to the contact. Recoverable compression of the contacts will contribute to the stiffness behaviour of the whole material while slip and damage will contribute to plastic deformation. In addition the assembly of particles will re-arrange itself by sliding and rolling of particles — also contributing to the stiffness and plastic deformation behaviour of the whole. Changing the shape and nature of the contacts and changing the packing of particles will all, therefore, have an impact on strength, stiffness and resistance to plastic deformation.
As the force carried at an inter-particle contact point increases, the laws of friction dictate that (unless the contact point fails in some way) there will be greater resistance to shear loading. Thus a greater compressive stress applied to an assembly of grains allows the whole material to gain shear strength and resistance to shear deformation which is characterised by the apparent angle of frictional resistance, ф’ . The greater compression of the particle contacts also makes further compression more difficult leading to the phenomenon of a non-linear stress-dependent modulus, so often observed in granular materials. Section 9.4 introduces some of the models of mechanical behaviour that are used to replicate these behaviours.
Adding water under pressure to a pore will cause all the particles around the pore to become loaded so that some of the force that previously was carried across the adjacent inter-particle contact points will now be carried by the pore fluid (Fig. 9.1).
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T Fig. 9.1 Schematic of inter-particle forces. (i) an assembly of particles is subjected to some external normal, o’, and shear, t, stresses, which are carried through the assembly at the contact points as shown by the black bars; (ii) when the pore space between particles A and B is pressurised by a fluid at pressure, u, particles A and B experience a pressure on them (illustrated only for A, not illustrated for B) which reduces the inter-particle force, fn, and makes shear, fs, more easy to take place because of reduced friction at the contact between the particles |
This is the reason behind the effective stress equation, Eq. 1.1, which is further described in this chapter at Eq. 9.19 and following. Because some force is now carried through the pores, the inter-particle forces acting at the contacts are reduced and, therefore, due to the frictional effects, the shear strength, stiffness and resistance to permanent deformation are also reduced.
If water is retained in the pores due to surface tension effects, then the opposite will occur with a suction being applied to the adjacent particles. This causes the inter-particle contact forces to increase and the shear strength, stiffness and resistance to permanent deformation will all rise. These influences of water on mechanical performance are the subject of Section 9.5 and the required models are given in Section 9.6.
