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1.1 Pile foundations
Pile foundations [11] 魔女之家攻略are the part of a structure ud to carry and transfer the load of the structure to the bearing ground located at some depth below ground surface. The main components of the foundation are the pile cap and the piles. Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity The main types of materials ud for piles are Wood, steel and concrete. Piles made from the materials are driven, drilled or jacked into the ground and connected to pile caps. Depending upon type of soil, pile material and load transmitting characteristic piles are classified accordingly. In the following chapter we learn about, classifications, functions and pros and cons of piles.
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1.2 Historical
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In the early days of civilisation[2], from the communication, defence or strategic point of vie
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w villages and towns were situated near to rivers and lakes. It was therefore important to strengthen the bearing ground with some form of piling.
Timber piles were driven in to the ground by hand or holes were dug and filled with sand and stones.
In 1740 Christoffoer Polhem invented pile driving equipment which rembled to days pile driving mechanism. Steel piles have been ud since 1800 and concrete piles since about 1900.
The industrial revolution brought about important changes to pile driving system through the invention of steam and diel driven machines.
More recently, the growing need for housing and construction has forced authorities and development agencies to exploit lands with poor soil characteristics. This has led to the development and improved piles and pile driving systems. Today there are many advanced techniques of pile installation.
1.3 Function of piles
As with other types of foundations, the purpo of a pile foundations is:to transmit a foundation load to a solid ground to resist vertical, lateral and uplift load.
A structure can be founded on piles if the soil immediately beneath its ba does not have adequate bearing capacity. If the results of site investigation show that the shallow soil is unstable and weak or if the magnitude of the estimated ttlement is not acceptable a pile foundation may become considered. Further, a cost estimate may indicate that a pile foundation may be cheaper than any other compared ground improvement costs.
In the cas of heavy constructions, it is likely that the bearing capacity of the shallow soil will not be satisfactory, and the construction should be built on pile foundations. Piles can also be ud in normal ground conditions to resist horizontal loads. Piles are a convenient method of foundation for works over water, such as jetties or bridge piers.
苹果手机死机1.4 Classification of piles
1.4.1 Classification of pile with respect to load transmission and functional behaviour
End bearing piles (point bearing piles)
Friction piles (cohesion piles )
Combination of friction and cohesion piles
1.4.2 End bearing piles
The piles transfer their load on to a firm stratum located at a considerable depth below the ba of the structure and they derive most of their carrying capacity from the penetration resistance of the soil at the toe of the pile (e figure 1.1). The pile behaves as an ordinary column and should be designed as such. Even in weak soil a pile will not fail by buckling and this effect need only be considered if part of the pile is unsupported, i.e. if it is in either air or water. Load is transmitted to the soil through friction or cohesion. But sometimes, the soil surrounding the pile may adhere to the surface of the pile and ca
us "Negative Skin Friction" on the pile. This, sometimes have considerable effect on the capacity of the pile. Negative skin friction is caud by the drainage of the ground water and consolidation of the soil. The founding depth of the pile is influenced by the results of the site investigate on and soil test.
1.4.3 Friction or cohesion piles
Carrying capacity is derived mainly from the adhesion or friction of the soil in contact with the shaft of the pile (e fig 1.2).
Figure 1-1 End bearing piles | Figure 1-2 Friction or cohesion pile布置会场 |
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1.4.4 Cohesion piles
The piles transmit most of their load to the soil through skin friction. This process of driving such piles clo to each other in groups greatly reduces the porosity and compressibility of the soil within and around the groups. Therefore piles of this category are some times called compaction piles. During the process of driving the pile into the ground, the soil becomes moulded and, as a result los some of its strength. Therefore the pile is not able to transfer the exact amount of load which it is intended to immediately after it has been driven. Usually, the soil regains some of its strength three to five months after it has been driven.
