Some useful notes on Pile Cap Design

Definition of Pile Cap: A pile cap refers to a thick concrete mat that is situated on concrete or timber piles driven into soft or unsteady ground to develop a proper & secure foundation. It normally builds up section of the foundation of a building, mostly a multi-story building, structure or support base for heavy equipment.

It is frequently found that one pile can’t bear the entire load enforced on the column. Therefore, the column requires over one pile to bear the load. Here, a pile cap plays an important role to disperse the column load to these piles uniformly.

In this stage of design, precautions should be undertaken to disperse the load to the piles evenly by placing the center of gravity of the column to match up with the center of gravity of the pile cap.

To make sure that the load is transmitted from the column to the pile, the pile steel reinforcement should be expanded inside the pile cap with minimum 600 mm so that the load is transmitted with the bond among concrete and steel.

The pile caps are designed as a rigid foundation and ensure that the piles bear equivalent loads from the column in order that the pile cape thickness should be designed to withstand the punching stresses and the tension in top and bottom.

Shape and Size of Pile Caps:

1. The shape and plan dimensions of the pile cap are based on the number of piles in the group and the gapping among each pile.
2. These pile caps outlines & minimizes the plan area for uniform arrangement of piles about the load.
3. The pile cap should overlap exterior piles by minimum 150mm but should not be too much, usually not in excess of the diameter of the pile diameter.

It reduces the plan area as well as the cost, at the time of arranging adequate length to the followings:

1. Affix the tension reinforcement with a large radius bend
2. Provide sufficient cover to the reinforcement
3. Meet the tolerances of the construction

Depth of Pile Cap - The depth of pile cap is influenced by the following factors :

1. Shear strength of pile cap (beam and punching shear)
2. Shrinkage and swelling of clay

3. Frost attacks
4. Pile anchorage
5. Water table and soluble sulphates
6. Maintain bolt assemblies for steel columns

Shear is considered as most important factor for selection of depth of pile cap.

Normally a pile cap is much deeper as compared to a pad footing having similar dimensions, since it is prone to greater concentrated reactions, and accordingly to much greater bending moments and shear forces.

However, the increased depth assigns greater rigidity to the pile cap that results in spreading the load uniformly to all piles.



Read more
~~~~~~~~~~~~~~~~~~~~~~~~
Published By
Rajib Dey
www.constructioncost.co
~~~~~~~~~~~~~~~~~~~~~~~~

Compaction Effects On Soil Structure

With compaction, the air is ejected from the existing voids in the soil. Compaction plays an important role in construction field since it make the engineering characteristics of soil better In the construction field to a great extent. In this exclusive civil engineering article, you will learn the impact of compaction on various characteristics of the soil.

It is found that the soil gets solid at the time of compaction. To make the compaction process smooth, some amount of water is included with the soil and the water content at which the highest dry density of soil can be sustained is defined as optimum moisture content.

Therefore, when the amount of water added remains under the optimum moisture content then it is defined as dry of optimum compaction. If the amount of water added remains in excess of the optimum moisture content then it is defined as wet of optimum compaction.

How does compaction influence the soil Properties? : The following properties of soil are affected with compaction -

1. Permeability
a. Compaction minimizes the existing voids in the soil and as a result permeability is also decreased.
b. At a specific density, for the similar soil sample, permeability becomes higher for soils which are compacted to dry of optimum as compared to those compacted to wet of optimum.

2. Compressibility
a. The Compressibility of compacted soil differs on the basis of the amount of pressure enforced.
b. For low-pressure range, compressibility is high for soils which are compacted to wet of optimum as compared to soil compacted to dry of optimum.
c. In the same way, for high-pressure ranges, compressibility is high for soils which are compacted to dry of optimum as compared to soil compacted to wet of optimum.

3. Shear Strength
a. Shear strength of soil compacted to dry of optimum remains in excess of those compacted to wet of optimum at lower strains.
b. At greater strain, soil compacted to wet of optimum will contain greater shear strength. o The shear strength of compacted soil is also impacted by the type of compaction, drainage conditions and type of soil.

4. Soil Structure
a. Soils compacted to dry of optimum have flocculated structure because of the attraction among soil particles caused by low water content.
b. Soils compacted to wet of optimum have scattered structure owing to the repulsive force among soil particles caused by high water content.

5. Swelling of Soil
a. When the soil is compacted to dry of optimum, the soil requires water and it swells quickly by getting in touch with water.
b. When water is compacted to wet of optimum, the soil particles are headed in a dispersed manner and no swelling happens.
c. To get rid of swelling, the compaction of soils should be made to wet of optimum.

6. Shrinkage of Soil
a. The scope of shrinkage is high for the soil compacted to wet of optimum than dry of optimum.
b. For dry of optimum compaction, soil particles are in random orientation and they remain in durable condition.
c. For wet of optimum, soil particles remain in parallel orientation and they become inconstant that streamlines the packing of particles producing shrinkage.

7. Pore Water Pressure
a. Pore water pressure is extreme for those soil whose water content is high. So, soils compacted to wet of optimum compaction will expose more pore water pressure as compared to soil compacted dry of optimum.

8. Stress-strain Behavior of Soil
a. Soils compacted to dry side of optimum will bear more stress for little strain and as a result, stress-strain curve of this type of soil is much sheerer and elastic modulus is high. Due to this, brittle failure may happen.
b. In the same way, soils compacted to wet of optimum will create more stress even for smaller stress. Therefore, Stress-Strain curve, in this case, is much flatter and plastic-type failure happens at a larger strain. These type of soils contain low elastic modulus.



Read more
~~~~~~~~~~~~~~~~~~~~~~~~
Published By
Rajib Dey
www.constructioncost.co
~~~~~~~~~~~~~~~~~~~~~~~~