33.2: Terzaghi's Effective Stress Principle

Rocks disintegrate with weathering. In the processes of sedimentary geology when rivers discharge sediments into a pool, various grades of soil grain are deposited at different distances from the point of discharge. The ancient 'soft rock' termed the Cambridge gault clay in the city of Cambridge in England is a 20 m thick layer of silt or clay size soil grains, deposited on the bed of an ancient ocean. It now outcrops around Girton, just west of Cambridge. Above it, when the ancient warm sea bed was far away from points where rivers were depositing sediment, coral could grow. This coral formed the chalk rock that extends far across Europe. Locally near Cambridge, it outcrops in the Gog Magog Hills and along a line through Newmarket, Royston and Luton. There are ripples in the chalk that rise up to form the North Downs and dip in the South Downs. This chalk rock forms the White Cliffs of Dover. Vertical cracks form when lateral pressure falls in this chalk. These cracks can leave a heavy cliff face resting on a foundation of gault clay that, when it fails, lets the cliff face fall into the English Channel.

Anywhere that a sediment of strong durable grains accumulates as a uniform aggregate of sand or gravel that can be excavated for use in construction, such aggregate will be mixed with water and cement powder as a component of mortar in brickwork, or in structural concrete reinforced with mild steel bars. Where natural ground is soft enough, ungraded soil can be excavated, hauled to site and spread out in layers to be compacted with rollers to build up large road or dam embankments. In natural or compacted soft ground the strong soil grains form an effectively stressed aggregate structure in which forces are transmitted from grain to grain through the ground. In a volume of ground part of the volume is occupied by solid soil grains and the pore space between grains in saturated ground contains incompressible pore water. Whenever soft ground is loaded there will be pore pressure gradients. Terzaghi's primary consolidation theory analysed transient flow of ground water and surface settlements.

Terzaghi’s effective stress principle applied to sand recognises that sand needs to have strength if a slope of sand is not to slump. A slope can only be stable if there are intergranular compressive stresses. The total compressive stress that is applied normal to a particular plane, σ, is equal to σ' + u, where σ' is an effective compressive stress normal to a plane and u is pore water pressure. This is Terzaghi’s effective stress principle.

This principle is illustrated by the behaviour of sand in the two bottles shown on the section titled Angle of Repose and also below. The air can easily move from pore space to pore space in the sand aggregate, but the water will take time to do so.

These two bottles show that if there is time for the pore water pressure to diffuse (i.e., there is drainage), the same slope at repose is achieved with and without the presence of water. If there is not enough time, liquefaction will occur, as could happen in the rocking of a bulk carrier, i.e., a merchant ship designed to transport unpackaged bulk cargo such as metal ores.