Deep foundations

DEEP FOUNDATIONS

The design and construction of Deep Foundations is a challenging job for a civil engineer. 'A foundation is said to be a Deep Foundation, when its depth is more than the width of the foundation. Deep foundations transmit the load of a super structure through weak soils to strong soil beds or rock beds available at great depth.

Uses: When the soil available at a reasonable depth of less than, say, 3 to 6 meters is not having the desired bearing capacity, deep foundation is used.

Types of Deep Foundations

1. Piles, 2. Piers and 3. Well

 

1. PILE FOUNDATIONS

Pile

In some cases, the soil at a site may have a very low bearing capacity for great depths (6 m or more). It may be impracticable to improve the strength of such soils by compaction. In such cases, a pile foundation is used to transmit the weight of the structure to a stratum of good strength or to rock.

Pile is a R.C.C. column member (or a timber column member). It is driven into the ground to a suitable depth. It transfers the load on it to a deeper and harder layer of the soil or rock.

Generally, part of the load on the pile is taken by friction offered by the surrounding soil. The remaining part of the load is transmitted to the hard stratum up to which it is sunk.

Piles are installed in groups by driving by hammer or by any other suitable means. The pile groups may be subjected to vertical loads or horizontal loads or a combination of vertical and horizontal loads.

Advantages:

A pile occupies less space. It is not liable to big settlements.

1.Usage of Piles

1. When loads to be transmitted to the sub-soil are very large and concentrated.

2. Providing Mat foundation may not be economical.

3. When foundation soil is loose and hard stratum is available at about 10 m depth.

4. When considerable rise or fall of sub-soil water level occurs seasonally.

5. When the structure is tall and heavy, but deep bed of sandy soil is available.

6. When there are possible future constructions of deep sewers / canals close to the site.

7. When the foundation is to be carried below the maximum possible scour depth.

8. To prevent any excessive settlement of the soil.

9. The top soil is of expansive nature like Black Cotton Soil, etc.

10. Sea-shore or river bed constructions or foundations in moist areas.

 

2. CLASSIFICATION OF PILES

Piles are classified on the basis of

1. Material Compositions

2. Installation Methods

3. Functions or Uses

4. Ground Effects

1. Classification Based on Material Compositions

Ground Based on Material Composition, piles are classified as Steel Piles, Concrete Piles, Timber Piles and Composite Piles.

(i) Steel Piles

Steel piles are made in three forms: Rolled Steel H-Section Piles, Box Piles and Tube Piles. There can be no restriction on length due to high strength of steel.

However, steel piles may be affected by corrosive agents like salt, moisture or acid. To prevent the corrosion of piles, the thickness is increased or encased in concrete or chemical coating with paint is applied.

See Fig. 11. In this, H-Section Pile having wide flanges is used. The pile projects slightly above the Ground Level and functions as a Column. Due to its small cross-section, it can be driven into the soil easily.

Box Pile is rectangular or octagonal. It is filled with concrete. When H-Section pile is difficult to be driven, box file is preferred. Tube Pile can be sunk into the ground easily. It is filled with concrete.

(ii) Concrete Piles

Concrete Pile is stronger and more durable than steel piles. It is not subjected to decay by termite like timber piles. Concrete piles may be either Cast-in-situ or Pre-cast.

Cast-in-situ Concrete Piles

These piles are cast in the site itself. Standard types of cast-in-situ piles are: Simplex Pile, Raymond Pile, etc.

Simplex Cast-in-situ Concrete Pile: See Fig. 12.

A bore is dug into the ground by inserting a Casing. The reinforcement made of Main Reinforcement Rods and Lateral Reinforcement Rods (called Stirrups) are placed into the ground. The bore is filled with cement concrete.

In the case of Simplex Pile, the casing is withdrawn from the bore after placing the concrete as shown in Fig. 12. Hence, it is called Uncased Cast-in-situ Concrete Pile.

Raymond Pile: In this, the casing is kept in position inside the ground. Concrete is then poured. Casing is not withdrawn. Hence, it is called Cased Cast-in-situ Pile.

Pre-cast Concrete Piles

These piles are cast in a yard and transported to the site. Concrete pile is pre-cast to specified lengths and shapes with reinforcement. The reinforcement enables the pile to resist the bending moment developed during lifting and transportation.

Pre-cast piles may be square, circular or octagonal in section. Fig. 13 shows a Pre-cast Square Pile with corners chamfered.

A Solid Cast Iron Shoe is provided to prevent the lower end from breaking, if it strikes a boulder underground. The pile shoe should be coaxial with the pile. It is firmly fixed to the concrete.

The depth to which the pile has to be driven is determined by preliminary borings. This will prevent over-driving of piles.

(iii) Timber Pile

Timber Pile is the oldest type of pile made from tree trunks. The length of a timber pile should not exceed 20 times it top width to avoid buckling. The life of timber piles may increased by treating them with preservatives. At the bottom of the pile, an iron shoe is provided. At the top, a steel plate is fixed.

(iv) Composite Pile

It is made up of two different materials driven one above the other. Steel and concrete combination or timber and concrete combination of composite piles are in use. Timber pile or steel pile is provided below the ground water level. Concrete pile is provided above the timber or steel piles.

2. Classification Based on Installation Methods

Based on installation techniques, piles are classified as Cast-in-situ piles and Driven piles. Cast-in-situ Piles: These piles are explained above in detail. Driven Piles: Driven piles may be made of Concrete, Steel or Timber. These are explained above in detail.

3. Classification Based on Functions or Uses

(i) End-bearing Piles: See Fig. 14. If the top soil is very weak, piles are used to transmit the load of the superstructure directly to the underlying bed rock or hard stratum. Such piles are called End-bearing Piles. Endbearing piles are resting on a very hard stratum.

The soil through which these piles have passed are not assumed to resist the loads. The soil provides only lateral support to the piles.

Two or more piles support one Column. These piles form one group, provided with a common concrete top, called Pile Cap. Column of the structure is resting on the  Pile Cape cap. The pile cap distributes the load equally to the piles.

End-bearings piles may be Concrete Piles, Steel Sheet Piles and Timber Piles.

(ii) Friction Piles: See Fig. 15. If the bed rock does not exist at a reasonable depth below the Ground Level, the load is transferred by friction along the pile length. Such piles are called Friction Piles.

In these piles, Skin Friction (frictional Stratum resistance) is developed between the surface of the piles and the sand particles surrounding them. Skin Friction transfers the load along the length of the piles.

Since friction piles do not rest on hard stratum, they are also known as Floating Piles.

Uses

• Friction piles are used where deep bed of sand is available.

• The frictional resistance of the surrounding soil against the downward movement of the pile can be increased by providing a longer pile, a greater diameter for the pile, a rough lateral surface to the pile and by providing piles in group.

Friction piles may be Cement Concrete Piles, Steel Piles and Composite Material Piles.

(iii) Under-reamed Piles

In Expansive Soils such as black cotton soil, very soft clay, filled-up earth, etc., building cracks due to relative ground movements. This differential settlement is caused by alternate swelling and shrinking of the soil due to changes in its moisture content.

To avoid differential settlements, the structure is anchored to a depth where the volumetric change of soil due to Lateral seasonal variations is negligible. This can be Stirrup economically obtained in shallow as well as deep layers of expansive soil by using Under-reamed Pile.

Load is transferred to hard strata of high bearing capacity. In fact, the building structure is anchored to the ground by Under-Reams using under-reamed piles.

See Fig. 16. Under-reamed Pile is a cast-in-situ pile. It is reinforced by Main Vertical Rods and Lateral Stirrups.

These piles are provided with Bulb-shaped Enlargement Under-ream near its bottom end. If the pile is subjected to heavy loads, more than one bulb can be provided.

A pile having only one bulb near its bottom is known as Single Under-reamed Pile. It can be used successfully for one and two storey buildings.

Piles having two bulbs are called Double Under-reamed Piles. With one additional bulb, bearing capacity is increased by 50%.

So, by increasing the number of bulbs, very high capacity piles (called Multiple Underreamed Piles) can be constructed for supporting multi-storey buildings and heavy structures.

4. Classification Based on Ground Effects

Compaction Piles

Compaction piles are used to compact granular soil, thus increasing their density and in turn bearing capacity. In granular soils, there is a tendency for compaction. In clays, heaving of ground surface often results. Piles used to compact soils are called Compaction or Displacement Piles. These piles displace substantial volume of soil during installation.

Non-displacement Piles

Driven piles installed in pre-drilled holes are called Non-displacement Piles. These piles are used to prevent the movement of earth slopes and to safeguard the foundation from damage due to scour.

 

3. PIER FOUNDATIONS

Piers are Large Diameter Massive Shafts with or without Broad Base at the bottom. Deep holes are drilled into the ground. Piers are installed by placing R.C.C. concrete-in-situ. Hence, piers are also known as Drilled Piers.

Broad Based Pier or Belled Pier: See Fig. 17.

Drilled piers are provided with a Broad Base at the bottom of the straight shaft. These are known as Broad Based Pier or Belled Pier. The bell is angled as shown.

Advantage: Vibration is not caused as in the driven pile installation.

Uses: Equipments used in the construction of drilled piers produce less noise. Therefore, piers are quite suitable for areas near hospitals and educational institutions. Drilled piers are used in the area where it is difficult to install pile foundation.

 

4. WELL FOUNDATION

See Fig. 18. Well foundation is hollow inside, resembling a well. It has no top or bottom cover.

A Well Curb of steel or R.C.C. is constructed in a yard. It is placed where the well has to be sunk.

The soil from inside the curb is dug. The Cutting Edge is sharp for knifing into the soil. The well is gradually driven down till the required depth of reaching a hard stratum. The soil inside the well is removed and the well is made to sink.

After installing the curb in place, a Masonry Well Steining is constructed above it.

Then, the bottom of the well is plugged with concrete. It is known as Bottom Plug. It is bowlshaped so as to have inverted arch action. The hollow portion above the bottom plug is filled with sand. It is called as Top Plug. Well is covered with Well Cap.

Shapes of wells in foundation are: Circular, Rectangular, Twin circular, Double-D, etc. See Fig. 19.

Advantages and Uses

• Well foundation is provided in soils which are sandy and soft for great depths. This foundation is meant for heavily loaded structures on low bearing capacity soils.

• Well foundations are used under-water such as bridges, docks, etc. Bridges/docks have to resist large lateral forces and are located in shallow running water in rivers with heavy scour. It is believed that the famous Taj Mahal is built on brick wells.