Consolidation mechanism

Consolidation is a process by which soils decrease in volume.

According to Karl Terzaghi "consolidation is any process which involves

decrease in water content of a saturated soil without replacement of water by air.”

Reduction in volume takes place by expulsion of water under long term static loads. It occurs when stress is applied to a soil that causes the soil particles to pack together more tightly, therefore reducing its bulk volume.

When this occurs in a soil that is saturated with water, water will be

squeezed out of the soil.

WHAT IS CONSOLIDATION?

When a saturated clay is loaded externally,

WHAT IS CONSOLIDATION?

saturated clay

GL

the water is squeezed out of the clay over a long time (due to low permeability of the clay).

WHAT IS CONSOLIDATION?

This leads to settlements occurring over a long time,

time

sett

lem

en

t

which could be several years.

-Lowering of the ground water table

-External static loads from structures

-Self weight of recently placed soils

-Desiccation

Consolidation may occur due to following reasons

The consolidation of a soil deposit can be divided into 3 stages :

(1) Initial consolidation (2) Primary consolidation (3) Secondary Consolidation

STAGES OF CONSOLIDATION

When a load is applied to a partially saturated soil, a decrease in volume occurs due to expulsion and compression of air in the voids.

A small decrease in volume also occurs due to compression of solid particles .

The reduction in volume of the soil just after the application of the load is known as initial consolidation.

INITIAL CONSOLIDATION

After initial consolidation, further reduction in volume of soil mass occurs due to expulsion of water from voids. This is known as primary consolidation.

In fine grain soils, the primary consolidation occurs, over a long time.

On the other hand, in coarse grained soils , the primary consolidation occurs rather quickly due to high permeability .

PRIMARY CONSOLIDATION

The reduction in volume continues at a very slow rate even after the excess pore water pressure developed by the applied pressure is fully dissipated and the primary consolidation is complete.

This additional reduction in the volume is known as secondary consolidation.

Secondary consolidation becomes important for certain types of soil, such as peats and soft organic clays.

SECONDARY CONSOLIDATION

Consolidation Vs. CompactionProcess of reduction

of volume under sustained, static loading.

It causes reduction in volume of a saturated soil due to squeezing out of water from soil.

It is a rapid process of reduction of volume by mechanical process such as rolling, tamping.

The volume of partially saturated soil decrease because of expulsion of air from the voids at the unaltered water content.

Consolidation Vs. CompactionIt is a process which

occurs in nature when the saturated soil deposits are subjected to static load caused by the weight of the buildings & other structures.

It is artificial process which is done to increase the density of the soil to improve its properties before it is put to any use.

Impact of Temperature on the Soil Consolidation Process

Several works in the field show that a temperature increase induces firstly an increase of the permeability and secondly, for normally consolidated states, an irreversible contraction and a modification of the yield limit. These results point to the fact that a temperature increase could lead to an increase of the settlement rate and an increase of the final value of settlement.

This figure shows two kinetics of consolidation process obtained at 25 and at 60°C