Table of Content for Pile Foundation-
- Advantages of Pile Foundation
- Classification of Piles
- Load Carrying Capacity- Static Method, Dynamic Method, In-situ Penetration Test and Pile Load Test.
- Negative Skin Friction
The load of the structure is transferred to the earth by foundations. The foundations are stable until the soil beneath the footing has enough strength to accommodate the stresses arising due to the load of the structure.
The strength of the soil depends upon the type of soil, gradation of soil, depth of strata, degree of compaction, depth of water table etc. It is a general trend that the strength of the soil increases with depth.
When the soil near the earth surface does not have enough strength, deep foundations are made to transfer the load to deeper strata. Pile foundation, piers, well foundation and caisson foundation are examples of the deep foundation.
Deep foundation having diameter lesser than 0.6m is called pile. Pier has a diameter of more than 0.6m and is often referred to as ‘Drilled-Pier’.
Advantage of Pile Foundation-
- Pile foundation have a lesser differential settlement.
- Pile Foundation can transfer the load to strong strata through deep water.
- It helps in sustaining structures which have horizontal loadings such as earth pressure and wind load.
- It is advantageous to use a pile foundation when it is estimated that earth can wash-out, erode or scour from beneath the shallow foundation.
- Pile foundation is advantageous for structures subjected to uplift pressure.
- For expansive soils such as black cotton soil, pile foundation can be used to transfer the load below the active zone.
- Pile foundation can be used for collapsible soils that are subjected to the moisture change.
Classification of Piles-
The piles can be classified in following manner-
Classification on the basis of Material-
- Steel piles are thick steel pipes or rolled H-section provided with an epoxy coat and a shoe at the bottom.
- Timber piles are of straight, sound and defect-free timber. Timber piles are also provided with steel shoes at both ends. Splicing is done by steel sleeves.
- Concrete piles are made up of either pre-cast concrete or in-situ concrete. Pre-cast piles can be prestressed. Cast in-situ piles are made by pouring concrete into the borehole. The cast-in-situ concrete pile can be cased or uncased.
- Composite piles are made up of two materials. They are rarely used as the two materials don’t join properly.
Classification on the basis of Mode of Load Transfer-
- End Bearing Piles transfer the load directly to the soil below it.
- Friction pile transfer the load to the soil along its’ depth through friction. The third type of pile is the combination of the above two.
Classification on the basis of Method of Installation–
- Driven piles are driven into the earth by means of striking on their top. The striking can be done by single acting hammer, double acting hammer, drop hammer, diesel hammer or vibratory pile driver. Driven piles can be pre-casted or cast in-situ.
- Bored piles are the ones that are casted into the bore-hole. Bored piles can have straight edges or bulbs. The bored piles with bulbs are called under-reamed piles.
- Screw Pile are screwed into the soil.
- Jacked Pile are jacked into the soil by means of hydraulic jacks.
Classification on the basis of Use-
- Load bearing pile transfer the load of the structure to the earth.
- Compaction piles are used to densify the soil. These are driven piles. When a pile is driven into the soil, the pile pushes the soil sideways. This process densifies the soil in the nearby area of the pile.
- Tension piles are used for structures subjected to the uplift pressure. These piles hold the structure into its position and maintain the vertical stability of the structure.
- Sheet piles are used for retaining the earth/water. They are commonly used in hydraulic structures to prevent seepage.
- Fender piles are sheet piles used to protect the waterfront from the impact of ships.
- Anchor piles are used as anchorage for sheet piles. It provides horizontal stability to the sheet piles.
Classification on the basis of Displacement of Soil-
- Displacement pile is the pile that displaces the soil around it from its original position. All the driven piles are displacement piles because the earth around the pile is densified due to lateral movement.
- Non-displacement pile doesn’t displace the soil around it. All the bored piles are non-displacement piles.
Load Carrying Capacity of the Pile Foundation-
The load-carrying capacity of the pile can be assessed theoretically as well as practically. The methods are as follows-
1. Static Method for Pile Foundation-
The ultimate load-carrying capacity of the pile is equal to the sum of end bearing strength of the soil and the friction developed along with the depth of the pile. This method gives a more accurate result. The load-carrying capacity of the pile is calculated as-
2. Dynamic Method for Pile Foundation-
The resistance offered to the pile when driving it through the soil can be correlated to the strength of the soil and thus the load-carrying capacity of the pile. Since the load-carrying capacity is correlated and hence it is not much reliable.
The empirical formulas used for this method are Engineering News Record Formula, Modified Engineering News Record formula, Hiley formula, and Danish Formula.
3. In-situ Penetration Test for Pile Foundation-
The in-situ standard penetration test gives fairly reliable result. The penetration number (N) can be used to calculate the strength of the soil. Cone penetration test can also be used.
4. Load Test for Pile Foundation-
It is the most reliable method. A test pile is subjected to the load and settlement is observed. The bearing capacity is then calculated at the maximum permissible settlement.
The load-carrying capacity of the pile is-
- 2/3 of the load applied at 18mm settlement or 2% of pile diameter whichever is lesser. And,
- 50% of the final load at which the total displacement equal 10% of the pile diameter in case of uniform diameter piles and 7.5% of bulb diameter in case of under-reamed piles.
Method of Pile Load Test-
The load is conducted on a test pile after the rest period of 3 days for sandy soil and 1 month for soft clay after casting. The test pile is subjected to loading through the reaction girder. A gauge is installed and set at zero to check the settlement of the pile.
The load is applied in the increments of 20% to the allowable load and this rate is maintained till the settlement is 0.1 mm per hour for sandy soil and 0.02mm for clayey soil or a maximum of two hours (As per IS 2911-2010).
The gauge readings are recorded after a fixed time interval. The loading is gradually decreased after reaching the allowable load and reverse readings are recorded. The final rebound is recorded after 24 hours when final loading has been removed.
The net pile settlement is calculated as-
A Pile Load Test Report Format has been attached here and also at the end of this article.
Negative Skin Friction in Pile Foundation-
When the soil surrounding the pile settles more than the pile, it generates a downward drag. This downward drag is called negative skin friction.
The negative skin friction imposes additional stresses on the pile. This additional stress reduces the load-carrying capacity of the pile and may even cause the failure of the pile foundation.
Hence, the net ultimate load-carrying capacity of the pile due to negative skin friction is-
A protective coating or sleeve can be provided to the pile to protect it from negative skin friction.
Click Here for IS 2911-1979.
Click Here for PILE LOAD TEST REPORT FORMAT.