Piles, Piers and Caissons

Deep foundations

1. Pile foundations A pile is as a column inserted in the ground to transmit the structural loads to a lower level of subsoil or a stable stratum. They are used when soils of suitable bearing capacity at great depths such that using traditional deep foundations would become uneconomical The natural low bearing capacity of the subsoil. Heavy point loads of the structure exceeding the soil bearing capacity. Presence of highly compressible soils near the surface e.g. filled, ground and underlying peat strata. Subsoils such as clay, which may be capable of moisture movement or plastic failure High water table.

2. Classification of piles according to the mode of load transfer

3. Classification according to the method of installation A. Driven piles: These include the timber piles, steel piles, and precast piles driven into the soil using various driving techniques, without excavations. B. Driven cast in-situ piles : These involve driving (using an auger) a steel tube with its end closed by a detachable conical tip and at required depth, concrete is pumped through the tube to detach the tip, and the auger is removed along with the soil, while concrete is being cast.   C . Bored cast in-situ piles: A bore is dug in the soil to a required depth and it is concreted with or without reinforcement and with or without a casing.

4. Types of piles Timber piles: These are made of treated timber and they are used for short-term requirements. They cannot carry large loads, and their load carrying capacity is limited to the strength of the wood. Before driving, the timber head is covered with a zinc coating to avoid brooming Brooming

5. II. Precast concrete piles: These are cast at a yard close to the site or transported to site. They are usually square, octagonal or circular in shape

6. III. Cast-in-situ concrete piles: These are also called bored piles because they are constructed by boring a hole in the soil and casting concrete in-situ, usually with reinforcement.

7. v. Composite piles: These are made by combining two portions of piles each of a different material e.g. timber and concrete. They are used when the required pile length is greater than that achievable by boring. Their disadvantage is the difficulty in forming a proper joint between the two portions of the piles. For this reason, they are rarely used.

8. VII. Steel piles: These may be rolled sections such as the H-shape(commonest) or fabricated shapes (like box shape ). Steel H piles are installed by drop hammer or static pressure, after welding a driving shoe at the tip to avoid end damage.

9. Methods of pile installation Drop Hammers (Drop hammering ) A hammer with approximately the weight of the pile is raised at a suitable height in or along a guide and released to strike the pile head. There are three types of drop hammers; Single acting steam of compressed air hammer, Double-acting pile hammer and Diesel/internal combustion hammer

10. B. Vibrating/Vibratory hammering These use a variable speed oscillator attached to the top of the pile. It consists of two counter-rotating weights which induce vibrations enough to cause a push and pull effect at the pile head, sufficient to break through soil layers. They are commonly used on sandy or gravelly soil.

11. C. Jetting piles (water jetting) These aid pile penetrations into sand or sandy gravel soils. The effect is limited in firm to stiff clay soils containing much coarse gravel, cobbles or boulders.

12. D. Jack-in piling This is a silent (noise free) method used for installing precast piles into the ground with minimum head damage. The pile is clamped and injected into the ground using hydraulic cylinders

13. E. Boring methods Boring can be done by auguring or percussion drilling and when the required bore depth is achieved, it is concreted with or without reinforcement and with or without a casing. Boring Lowering Casing

14. Lowering rebar Placing treamie

15. Boring mechanisms

16. Drilled Pier Foundations Piles and piers serve the same function with no great difference between them, except for their method of installation. A pile is installed by driving and a pier by excavation. A pile cast in-situ with a diameter greater than 0.75m (2.5ft) in the USA is called a drilled pier or drilled shaft, and in other countries apart from the USA, a large-diameter bored cast-in-situ pile. Types of drilled piers May be described in four types similar in construction techniques but different in design assumptions and mechanism of load transfer.

17. Straight-shaft end-bearing piers Their load support mechanism is similar to end bearing piles and the soil around the pier perimeter is assumed to contribute no support to the exerted load.

18. B. Straight-shaft side wall friction piers These pass through overburden soils and are assumed to carry none of the load and penetrate far enough to a bearing stratum to develop design load capacity by side wall friction.

19. C. Combination of Straight-shaft side wall friction and Straight-shaft end-bearing piers Both side wall friction and end bearing capacity contribute to the carrying of design load. When the load is carried into a rock layer, this pier is referred to as a socketed pier or a drilled pier with a rock socket

20. D. Belled or Undreamed Piers These are piers with a bottom bell or under ream and a greater percentage of the load is assumed to be carried by these.

21. Caisson Foundation These are prefabricated enclosed structures which may be round, square or rectangular in shape. They are partially or fully filled with concrete and constructed in-situ or precast and towed to the site. Function They have the same function as piles. They can also be sunk through the water or soft soils to provide a fry working place for other activities. Caissons for bridge foundations are usually cellular reinforced concrete structures with circular, rectangular shapes. They are wholly or partially constructed at a high level and sunk in stages to the desired level through internal excavation assisted by the application of a kentledge .

22. Types of Caissons Box Caissons (floating caissons) These are precast concrete boxes that are open at the top and closed at the bottom. They are cast on land and towed to the site to be sunk slowly by filling the inside with sand, gravel and water or concrete. The bottom is at times made of wood to make it float during towing.

23. B. Open caissons (well foundations) These are open at the top and bottom. Upon reaching the final depth , concrete seal of 2-5m thick is cast through water (if present) to seal the bottom. After the seal has cured, water present inside is pumped out and the space filled with concrete.

24. C. Pneumatic caissons (compressed air caissons) In this type, the top is sealed and compressed air is used to prevent water from entering the inside working chamber. This will help in sinking and concreting in dry conditions. On reaching the required depth, the chamber is filled with concrete. Pneumatic caissons are used in difficult subsoil conditions below water. They are usually designed to form part of the finished structure.

25. Monolithic Caissons These are usually rectangular in plan and are divided into a number of voids or wells through which the excavation is carried out. They are similar to open caissons but are larger and have greater self-weight and wall thickness, making them suitable for structures which have to resist considerable impact forces.

26. CHUDLEY, R., & GREENO, R. (2006). ADVANCED CONSTRUCTION TECHNOLOGY. Edinburgh: Pierson Prentice Hall. Murty , V. (2002). Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering. CRC Press. References