The design and construction of deep foundations for transferring the weight of the super- structure through soft or weak soils, to deep load bearing strata is a challenging job for a civil engineer. Piles, piers and caissons are the most common types of deep foundations. For any system the mechanism of deriving support from the soil or rock below and adjacent to the foundation is similar. However, each system differs in its method of construction.
1. Pile Foundations
Piles are slender structural members normally installed by driving by hummer or by any other suitable means. The piles are usually placed in groups to provide foundations for structures. Piles may be classified according to their material composition, installation method, group effect and their function as a foundation.
(i) Classification Based on Materials
Under this classification, piles may be further classified as timber, steel, concrete or composite piles. Timber piles are the oldest types of piles made from tree trunk. The maximum length of pile is 20 m. The life of timber piles may be increased by treating them with preservatives (Fig. 15.2(a)).
Steel piles consist generally of either pipe piles or rolled steel H-section piles. Because of high strength there can be no restriction on length but steel piles are affected by corrosive agents such as salt, acid, moisture and oxygen. In order to prevent steel piles from corrosion, the thickness is increased, encased in concrete or chemical coating is applied (Fig. 15.2(b)). Concrete piles are precast to specified lengths and shapes with reinforcement. The reinforcement is provided to enable the pile to resist the bending moment developed during lifting and transportation. Concrete piles are also cast in-situ (Fig. 15.2(c)).
Figure 15.2 Classification based on materials
(ii) Classification Based on Installation Methods
Based on installation techniques piles are classified as driven piles and cast-in-situ piles. Driven piles may be concrete, steel or timber. Concrete piles are classified as driven pre-cast concrete piles, and bored cast-in-situ concrete piles. Driven precast concrete pile is the one casted in a casting yard subsequently driven to the required location.
Driven cast-in-situ pile is formed within the ground by driving a closed bottom casing and subsequently filling with concrete in the hole so formed with adequate reinforcement. Bored cast in-situ pile is formed within the ground by excavation or boring with or without the use of a temporary casing and subsequently filling it with plain or reinforced concrete.
(iii) Classification Based on Ground Effects
Piles are also used to compact soils and such piles are referred to as displacement or compaction piles. These piles displace a substantial volume of soil during installation. In granular soils, there is a tendency for compaction, whereas in clays heaving of the ground surface often results. Driven piles installed in pre-drilled holes are also called as non-displacement piles. Piles are also used to prevent the movement of earth slopes and to safeguard the foundation from damage due to shock.
(iv) Classification Based on Functions
Where the top soil is soft or too weak to support the super-structure, piles are used to transmit the load to the underlying bed rock, such piles are called end-bearing piles or point bearing piles. If the bed rock does not exist at a reasonable depth below the ground surface, the load is transferred through friction along the pile shaft such piles are called friction piles. Transmission towers, off-shore platforms, and basement mats are subjected to uplift forces and piles are used to resist the uplift forces, which are called uplift piles or tension piles. In order to resist horizontal and inclined forces in water and earth retaining structures batter piles are used. Application of piles for providing anchorage to sheet piles are called as anchor piles (Fig. 15.3).
Figure 15.3 Classification based on function
2. Pile Groups
Where piles are used for foundation support, they are always used in a group. This requirement is essential so as to assure that the imposed structural load lies within the support area provided by the foundation. As per the building codes at least three piles should be used to support a major column and two piles to support a foundation wall.
A pile cap is provided near the ground encompassing all the top ends of piles. Pile caps are almost invariably made of reinforced concrete. The axial and the lateral load carrying capacity of a pile group is significantly affected by a pile cap. In order to keep the stresses in the pile cap to a minimum the piles should be arranged in the most compact geometric form. Typical arrangement of one pile group is shown in Fig. 15.4.
Figure 15.4 Typical pile group
3. Drilled Piers
Drilled piers are structural members of relatively large diameter massive struts constructed of concrete placed in a pre-excavated hole.
They are also called bored piles, large-diameter piles, foundation piers and drilled caissons. The shaft can be enlarged at the base resulting in a belled or under-reamed pier.
The common type of drilled pier is the straight shafted type (Fig. 15.5(a)). The shaft is taken through the upper soil layers and the end is placed on the firm ground or rock. Drilled piers which are provided with a broad base (called a bell) at the bottom of the straight shaft are referred to as belled piers. The bell may have a shape of a dome or it may be angled (Fig. 15.5(b)). The third type is the extended straight shaft or socketed pier in which the straight shafts are extended into the underlying rock layer (Fig. 15.5(c)).
Figure 15.5 Types of drilled piers
4. Caissons
Caissons are structural boxes or chambers that are sunk in place through ground or water. The sinking is systematically done by excavating below the bottom of the unit which thereby descends to the final depth. These have large cross-sectional area and hence provide high bearing capacity. Two types of caissons, viz., open caisson and monolith caisson are shown in Fig. 15.6.
Figure 15.6 Two types of caissons
5. Well Foundation
Well foundation is a type of caisson. It is constructed either on dry ground or over an artificially formed island. The curbs are pitched in the current position and then sunk into the ground to the desired level by grabbing the soil through the dredge holes formed by the masonry or concrete the steining. In India, this procedure of initial sinking is referred to as the caisson method. Well foundations have all the advantages of open caisson. Figure 15.7 shows a typical cross section of a well foundation.
Figure 15.7 Typical section of a well foundation
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