CONSOLIDATION OF SOIL

Prepared By-Ankush JainJoydeep Biswas S. Girish Arun S.

PREREQUISITES :

• Pore water pressure(μ)- The stress carried by the pore water is known as pore water pressure.

Pore water pressure refers to the pressure of groundwater held within a soil or rock, in gaps between particles (pores). • Effective Stress(σ’) - The stress carried by the

solid particles or the solid portion of the soil is known as effective stress. It is directly transmitted from particle to particle.

It is also defined as intergranular pressure which is responsible for change in volume of solids

WHAT IS CONSOLIDATION??

When a saturated clay is loaded externally,

saturated clay

GL

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

PROCESS OF CONSOLIDATION

Saturated Clay is subjected to an

external pressure

Pressure applied is initially taken by the water in the

pores resulting in excess pore water

pressure

If drainage of water is permitted the water flows out due to excessive

pressure

Due to this a portion of applied

stress is transferred to soil

skeleton

This causes a reduction in excess

pore water pressure.

Gradual transfer of load takes place

from pore water to soil skeleton

Gradual compression

occurs with the flow of water out of

the soil mass.

This process is called

consolidation.

FACTORS FOR CONSOLIDATION

• Due to external static load from structures .• Due to self weight of soil such as recently placed

fills.• Due to lowering of ground water table .• Due to desiccation.

SETTLEMENT IN SOIL CAN BE DIVIDED INTO THREE STAGES

Settlement (S)

Immediate settlement

Primary Consolidatio

n

Secondary Consolidatio

n

IMMEDIATE SETTLEMENT

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. There may also occur compression of solid particles, this happens in fully saturated soils .

PRIMARY CONSOLIDATION

• After initial consolidation , further reduction in volume occurs due to expulsion of water from voids.• The excess pore water pressure is decreased.• This depends on the permeablity of soil , and

hence it is time dependent.

SECONDARY SETTLEMENT

• The reduction in volume still continues at a very slow rate even after excess hydrostatic pressure is fully dissipated. • Such reduction is attributed to plastic

readjustment of the soil particles.• It is generally very small.

TERZAGHI THEORY OF CONSOLIDATION

• The process of consolidation of a clay-soil-water system may be explained with the help of a mechanical model as described by Terzaghi and Frohlich (1936).

• The model consists of a cylinder with a frictionless piston as shown in Fig. The piston is supported on one or more helical metallic springs. The space underneath the piston is completely filled with water.

• The springs represent the mineral skeleton in the actual soil mass and the water below the piston is the pore water under saturated conditions in the soil mass.

• When a load of p is placed on the piston, this stress is fully transferred to the water (as water is assumed to be incompressible) and the water pressure increases. The pressure in the water is u = p

• This is analogous to pore water pressure, u, that would be developed in a clay-water system under external pressures. If the whole model is leak proof without any holes in the piston, there is no chance for the water to escape. Such a condition represents a highly impermeable clay-water system in which there is a very high resistance for the flow of water

• If a few holes are made in the piston, the water will immediately escape through the holes. With the escape of water through the holes a part of the load carried by the water is transferred to the springs. This process of transference of load from water to spring goes on until the flow stops.

• when all the load will be carried by the spring and none by the water. The time required to attain this condition depends upon the number and size of the holes made in the piston. A few small holes represents a clay soil with poor drainage characteristics.

• When the spring-water system attains equilibrium condition under the imposed load, the settlement of the piston is analogous to the compression of the clay-water system under external pressures.