Upload Now
ADVERTISEMENTS:
The method of construction of pile foundations depends on the type of pile, whether the pile is a bored pile or a driven pile.
Method # 1. Bored Pile:
In the case of a bored pile, a circular hole is drilled into the soil/rock up to the required depth, with the help of rotary or percussion drilling tools, similar to those used for soil exploration. Temporary casing of minimum 1-m length is generally used in the bore hole. Additional length of temporary casing may be used depending on the condition of soil strata and groundwater level. Drilling fluid or drilling mud of suitable consistency may also be used instead of temporary casing for stabilizing the sides of the bore hole. The drilling mud is a bentonite suspension with montmorillonite clay having exchangeable sodium cations.
Bentonite used in the drilling mud must have the following properties:
ADVERTISEMENTS:
i. Liquid limit = 300%-450%.
ii. Maximum sand content = 7%.
iii. Density of bentonite solution = 1.12 g/cc.
iv. Differential free swell < 200%.
ADVERTISEMENTS:
v. pH of bentonite suspension < 11.5.
If a bore hole is stabilized by drilling mud, the bottom of the bore hole is cleaned carefully before concreting is taken up. Temporary casing is not required except near top if the drilling mud is used.
Once the bore hole of required depth is made, the reinforcement cage is placed in position and concreting is done. As per IS – 2911 (Part I/Sec II)-1979, the minimum area of reinforcement is 0.4% of the cross-sectional area of the pile. The minimum clear cover is 40 mm. The minimum clear spacing between longitudinal reinforcements is 100 mm. The minimum diameter and spacing between lateral ties and spirals is 6 and 150 mm, respectively.
The concrete used should have a minimum slump of 100 mm when the concrete in the pile is not compacted. The slump should not exceed 180 mm in any case. The minimum grade of concrete to be used for piling should be M-15. When concrete is not exposed to sulphates, the minimum cement content should be 300 kgf/m3. For concrete exposed to sulphates, the minimum cement content should be in accordance with IS – 456-1978. When concreting under water or drilling mud, 10% additional cement over that required for the designed mix of concrete for the required slump should be used subject to a minimum of 370 kgf/m3.
ADVERTISEMENTS:
Concreting is done uninterrupted using the tremie method. The tremie pipe is submerged in the concrete already placed, without permitting the concrete to fall through free water into the bore hole. In exceptional cases, when interruption in concreting is permitted, this interruption should not be more than 1 or 2 h in duration and the tremie should not be taken out of the concrete during this period.
The top of the concrete in a pile is brought above the cutoff level to permit removal of laitance (weak concrete) before the pile cap is constructed to ensure proper embedment of the pile into the pile cap. The cutoff level is the level where the installed pile is cut off to support the pile caps or beams or any other structural component at that level. Manual chipping above the cutoff level is permitted after three days. Pneumatic tools should not be used up to seven days of casting the “pile.
Following are the advantages and disadvantages of bored cast in-situ piles:
Advantages of Bored Cast In-Situ Piles:
ADVERTISEMENTS:
a. Soil can be inspected and checked with the previously conducted soil exploration data.
b. Length of piles can be easily modified depending on the actual soil strata encountered during construction.
c. There is no limitation on the maximum length of bored piles. This is because the installation of the bored piles do not require driving for installation and hence the bored piles do not encounter resistance by soil/rock during installation unlike in driven piles. These piles can be installed up to greater lengths and larger diameters, as may be required from design requirements, compared with driven piles.
d. The diameter of bored piles can also be enlarged at the bottom, up to three times the shaft diameter, which enables an increase in the end-bearing resistance of the pile. End enlargements of up to 2-3 times the shaft diameter are possible in clays or soft rocks. This enlargement is possible in clayey soils but not in granular soils.
ADVERTISEMENTS:
e. There is no damage to adjoining piles or structures or service lines as no driving of the pile is involved. Less driving force is required for making bore holes into the soil than for driving a pile.
Disadvantages of Bored Cast In-Situ Piles:
a. Boring methods may loosen sandy or gravelly soils.
b. Concreting under water is difficult.
c. Concrete cannot be inspected during or after the construction.
Method # 2. Driven Piles:
The equipment for driving and installation of driven piles, known as pile driving rig, consists of a movable steel or timber structure designed for handling, pitching and driving the piles in the correct position and alignment. A hammer, operating in the guides or leaders of the rig, is used to provide the required energy for driving the piles.
Different types of hammers are available, designated by type and size, and are described as follows:
i. Drop hammer: It is made of cast iron, weighing 1-1.4 t, falling freely through a height of 3-9 m. The hammer is raised by a winch and allowed to fall freely on the pile top. The drop hammer is the oldest type of hammer. The rate of application as well as the efficiency of the blow is low and, hence, it is rarely used these days.
ii. Single-acting steam hammer: In this case, the hammer is attached to the piston of a steam cylinder. The hammer weighs about 1.8 t, and the height of fall is about 1 m. Although the height of fall is less, the rate of application of blows is very fast, about 50-65 blows per minute.
iii. Double-Acting Steam Hammer:
In this case, the hammer is raised by the steam pressure up to a required height and then the steam pressure is applied on the other side of the piston so that the blow is applied under steam pressure with more impact energy.
iv. Diesel Hammer:
A diesel hammer consists of a ram, a fuel injection system and an anvil at its lower end. The ram is first raised manually and the fuel is injected near the anvil. When the hammer drops on the anvil, it compresses the fuel and the fuel is ignited simultaneously. The pressure, developed by compression and combustion of fuel, pushes the pile downward and the ram upward. The energy imparted varies from 1 to 4 tm, depending on the design of the diesel hammer.
v. Vibratory Hammer:
A vibratory hammer, or pile driver, consists of two shafts, known as exciters, rotating in opposite directions. The horizontal components of the centrifugal forces generated by the exciters cancel each other, while the vertical components add together, causing driving of the pile. The method is used where the vibration and noise of conventional pile driving equipment cannot be permitted.
The efficiency of hammer blow (ƞ) as recommended by IS – 2911 (Part I/Sec III)-1979 (R 1997) is given in Table 20.3, where Ls is the stroke length.
vi. Pile Driving by Water Jet Method:
When piles are to be driven through soft soil or a thin hard layer, the water jetting technique, similar to the wash boring method used in soil exploration, can be used. In this, a pipe is attached to the side of a pile and water is pumped down through the pipe. The water jet softens the soil and scours the material. The water jet and the hammer can be used together for pile driving in stiff clay.
vii. Partial Auguring Method:
In this method, a power auger is used to drill the hole for part of the pile length. The pile is then inserted into the hole and driven by hammer blows for the remaining length. Batter piles are generally installed by the partial auguring method to ensure that the alignment is properly maintained.
Following are the advantages and disadvantages of driven cast in-situ piles:
Advantages of Driven Cast In-Situ Piles:
a. They can be driven to a predetermined set.
b. Pile lengths are easily adjustable.
c. An enlarged base can be formed that helps in achieving high end-bearing resistance in granular soils.
d. Piles with closed ends can be driven to avoid the ill-effects of groundwater.
Disadvantages of Driven Cast In-Situ Piles:
a. Driving of piles would affect the safety and stability of the nearby structures and service lines.
b. Concrete cannot be inspected during or after the construction of the pile.
Method # 3. Screw Pile:
A screw pile consists of a cast iron or steel shaft terminating into a helix or screw base. The pile shaft may be hollow or solid. The pile is installed by screwing the pile into the ground by using an electric motor. Screw piles are useful in soft clay or loose sand, where the large contact area of the screw helps in increasing the load capacity of the pile through skin friction resistance.
It consists of a hollow cylindrical shaft, in the form of a hollow steel pipe of diameter 5-100 cm, with one or more helical plates welded outside at different heights in the form of plates of various thicknesses and diameters. The screw pile is provided with a 45° end cut to assist the driving of the pile.
The “thread” or helix on the screw pile allows it to be turned into the ground with speed and accuracy and without the noise and vibrations associated with a pile driver. The addition of the helix also increases the bearing capacity and pull-out resistance, making screw piles a good option for deep foundations, such as for transmission towers, as well as lighter load projects such as decks or fences.
Screw piles are the fastest, most flexible and environmentally friendly foundation options available. Installing a screw pile does not require any pre-excavation, and there is no need to dispose of tailings. There is no waiting time for the concrete to cure, and there are no noisy pile drivers or cumbersome cement trucks to deal with.
Comments are closed.