Sunday, May 19, 2019

Foundation of structure and soil stabilization : all you need to know

Welcome to Nepal Engineering Services.

Foundation is a part of structure that withstands the weight of the structure, other different types of loads transmitting into the subsurface beneath the ground level. It acts as the legs of the structure . As legs support our body, foundation supports the structure and its dynamics. The soil of the foundation should not fail in shear and the settlement should be within limit.

Foundations are generally classified as below:

1) Shallow foundation: The foundation in which depth of the foundation is less than its width is called shallow foundation. They transmit load to relatively small depth. They are used if the loads of structure are relatively less. They are of following types:

A) Strip footing:-
Strip footing is generally used for load bearing walls. So it is also called wall footing. In this type of footing length is very very greater than width of the footing.

B)Isolated/Spread footing
This type of footing supports individual columns. The slab of the isolated footing may be circular, rectangular, square in plan. The footing generally is trapezoidal but it may be cubic or cuboidal.

C) Combined footing:
This type of footing supports two or more columns in a row. This type of footing is used when thae area required is limited. This type of footing is used in providing expansion gap between building.

D) Cantilever/Strap footing:

 In this type of footing, two footings are connected by beams.There is no soil reaction in this type of footing. 

E) Mat footing:

 This type of footing supports all the columns and wall load of the structure. This type of footing is used if the allowable soil pressure is low. It reduces the soil settlement. 

2) Deep column:

 In this type of footinf, the depth is much greater than width of footing. This is used in condition of soil where the soil near the ground is not capable to support a structure.

Factors affecting the choice of foundations:
1) Function of structure
2) Subsurface soil condition
3) Service life
4) Risk level
5) Safety factor
6) Economic condtion
7) Types of superstructure

The requirements of foundation are:
1) The differential settlement of soil must be within safe limit.
2) Foundation should be safe against shear strength and soil rupture.
3) Lateral soil pressure should not affect the footings.

Improvement of foundation soil:
The improvement of soil usually indicates in increasing the shear strength of soil and decreasing the compressibility of soil reducing by reducing the airvoids of the soil. The purposes of soil improvement are :
1) Increasing the shear strenght of soil
2) Decreasing the air voids
3) Decreasing the permeability of soil
4) Decreasing compressibility

The methods of soil stabilization are:
1) Mechanical compaction
2) Dynamic Compaction
3) Preloading
4) Sand and stone columns
5) Use of admixtures
6) Injection of suitable grouts

1) Mechanical compaction: The application of mechanical energy like rollung,temping and vibration in order to increase the density of soil is called mechanical compaction. It aims at reducing air voids and decreasing permeability. For cohesive soul, the compaction should be done around the optimum moisture content so that maximum dry density is obtained. In cohesionless soil vibration is done for compaction. The purposes of mechanical compaction are:
a) to reduce permeability
b) to prolong durability
c) to control swelling and shrinkage
d) to reduce liquifaction chance

2) Dynamic compaction : Dynamic compaction are generally used for
deeper layers of soils.
The types of compaction for deeper layer of soil are:
i) Vibroflotation
ii) Dropping of heavy weight
iii) Blasting

i) Vibroflotation:
It is used for compacting granular soil . A
vibrofloat is a cylindrical tube containing water jets at top and botom and
with a rotating eccentric weight, which develops a horizontal vibratory motion. This method is very effective for increasing density of sand deposit for depths up
ii)Dropping of heavy weight:
Loose soils are compacted by repeated dropping of a heavy weight on the ground surface so as to cause compaction up to sufficient depths. This
method can be used for compacting cohesive as well as cohesion less soils. A heavy concrete or steel blook weighing 600 kN and up to a height of 45 m, from which height it is allowed to fall
freely on to the ground surface. The process is repeated as to cover the entire area.

iii) Blasting

Explosives buried under the soil are used to densify loose soils. In this
method, holes are bored and explosives are placed in them. The holes
are filled back with soil. These explosives when detonated cause shock waves in
the ground causing densification of loose surrounding soil.

In this method large mass of load is kept on the soil before construction for long period of time. The purpose of preloading is:
i) To make enough settlement so that later the settlemnet is permissible.
ii) To increase the bearing capacity of soil

Disadvantages of preloading method
i) The time of consolidation is long
ii) It requires large space.


Grouting is the process by which fluid like materials, is injected in to the void spaces of the underground soil and is allowed to solidify, therefore reducing the void
space and hence increase the load caring capacity of the soil or to reduce
permeability or both.

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