Module 4:Preloading and vertical drains Lecture 10...

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Module 4:Preloading and vertical drains Lecture 10:Introduction to preloading and vertical drains

The Lecture Contains:

Preloading and vertical drains

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Pre, loading and vertical drains

Preloading

● Increases the bearing capacity ● Reduces the compressibility of weak ground

Achieved by placing temporary surcharge on the ground. Surcharge generally more than the expected bearing capacity.

In cohesion less soil and gravel lowering water table

Most effective soft cohesive ground.

The process may be speed up by vertical sand drains. Vertical drains are installed in order to accelerate settlement and gain in strength of soft cohesive soil. Vertical drains accelerate primary consolidation only. As significant water movement is associated with it.Secondary consolidation causes only very small amount of water to drain from soil; Secondary settlement is not speeded up by vertical drains.

Only relatively impermeable soil benefit from vertical drains.

Soils which are more permeable will consolidate under surcharge. Vertical drains are effective where a clay deposit contain many horizontal sand or silt lenses.

Pre loading VerticalReduce total and differential settlement speed up to settlement process

Economy in foundation systemdo not reduce the amount of deformationunder a given load.

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Pre loading and vertical drains

Figure 10.1

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Figure 10.2


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Figure10.3

Pre loading allows cheaper spread footings.

Pre loading allows savings on foundation costs.


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Figure 10.4

Vertical drains accelerate settlements and do not reduce final movements.


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Methods of Preloading vertical drainage:

Cylindrical sand drains: Sand drains consisted simply of boreholes felled with sand. The holes may be formed by driving, jetting and/or angering. Its diameter is 200 to 450mm and would be spaced 1-5 to 6m appart.

A large diameter sand, in a fine grained soil, enables rapid consolidation of surrounding material and also provide vertical compressive reinforcement

Geosynthetic drains:

Figure 10.5


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Advantages:

Easy and Rapid installation

Mode of uniform material, easily stored and transported tensile strength of the strips helps to preserve containing low costs .

Type Core material Filter material Dimension (mm)

PVC PVC none 100

Geodrain PE Cellulose

Mebradrain PP PP or PES

Acidrain PE PES 100

Col band PES none 100

As the radial consolidation theory assumes the drain has a circular section on equivalent drain has to be calculated for strip drains. An equivalent sand drains diameter can be fund based on equal void area.

Equal void area Koerner (1986)

B = width of strip

t = thickness of strip.


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= void area /total c/s area of strip.

porosity of surd drains.

Equal circumference


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Preloading and vertical drains

Vertical drains

There is a large settlement consequent upon the charge of void ratio from e to

Figure 10.6

When the preload is removed the soil undergoes swelling from B to C and void ratio increases from

When building is erected, the same intensity of pressure is applied but now the settlement is a function to reloading.

So settlement of building reduces considerably. In direct pre loading, the time for consolidation may run into years because of low permeability of clay and long drainage path. To reduce the time for consolidation vertical drains with drainage blanket on top are used.


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Figure 10.7


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Figure 10.8

The flow within the zone is combination of redial flow towards the sand and vertical flow towards the free draining boundary. The average degree of consolidation is

Assuming uniform vertical drain at the surface, Degree of consolidation due to redial drainage. Barron (1948)


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Where,

Radial time factor


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Pre, loading and vertical drains

Square Pattern & triangular pattern

Figure 10.9

Considering the influence area of each drain to be circular, we have

Coefficient of radial consolidation

In field problem, is small compared to neglected

For a time t and the time factor can be calculated.


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Figure 10.10


Module 4:Preloading and vertical drains Lecture 10...

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