Precast prestressed spun concrete piles are closed-ended tubular sections of 400 mm to 600 mm diameter with maximum allowable axial loads up to about 3 000 kN.
Pile sections are normally 12 m long and are usually welded together using steel end plates. Pile sections up to 20 m can also be specially made.
Precast prestressed spun concrete piles require high-strength concrete and careful control during manufacture.
Casting is usually carried out in a factory where the curing conditions can be strictly regulated.
Special manufacturing processes such as compaction by spinning or autoclave curing can be adopted to produce high strength concrete up to about 75 MPa. Such piles may be handled more easily than precast reinforced concrete piles without damage.
This type of piles is generally less permeable than reinforced concrete piles and may be expected to exhibit superior performance in a marine environment. However, they may not be suitable for ground with significant boulder contents. In such cases, preboring may be required to penetrate the underground obstructions.
Spalling, cracking and breaking can occur if careful control is not undertaken and good
driving practice is not followed
Precast reinforced concrete piles are not common nowadays.
These piles are commonly in square sections ranging from about 250 mm to about 450 mm with a maximum section length of up to about 20 m. Other pile sections may include hexagonal, circular, triangular and H shapes. Maximum allowable axial loads can be up to about 1 000 57kN.
The lengths of pile sections are often dictated by the practical considerations including
transportability, handling problems in sites of restricted area and facilities of the casting yard.
These piles can be lengthened by coupling together on site.
Splicing methods include welding of steel end plates or the use of epoxy mortar with dowels.
This type of pile is not suitable for driving into ground that contains a significant amount of boulders or corestones.
Large-displacement piles Advantages and Disadvantages
Large-displacement piles include all solid piles, including precast concrete piles, and steel or concrete tubes closed at the lower end by a driving shoe or a plug, i.e. cast-in-place piles.
Advantages of Displacement Piles
Material of preformed section can be inspected before driving.
Steel piles and driven cast-in-place concrete piles are adaptable to variable driving
Installation is generally unaffected by groundwater condition.
Soil disposal is not necessary.
Driving records may be correlated withinsitu tests or borehole data.
Displacement piles tend to compact granular soils thereby improving bearing capacity and stiffness.
Pile projection above ground level and the water level is useful for marine structures and obviates the need to cast insitu columns above the piles.
Driven cast-in-place piles are associated with low material cost.
Disadvantages of Displacement Piles
Pile section may be damaged during driving.
Founding soil cannot be inspected to confirm the ground conditions as interpreted from the ground investigation data.
Ground displacement may cause movement of, or damage to, adjacent piles, structures, slopes or utility installations.
Noise may prove unacceptable in a built-up environment.
Vibration may prove unacceptable due to presence of sensitive structures, utility installations or machinery nearby.
Piles cannot be easily driven in sites with restricted headroom.
Excess pore water pressure may develop during driving resulting in false set of the piles, or negative skin friction on piles upon dissipation of excess pore water pressure.
Length of precast concrete piles may be constrained by transportation or size of casting yard.
Heavy piling plant may require extensive site preparation to construct a suitable piling platform in sites with poor ground conditions.
Underground obstructions cannot be coped with easily.
For driven cast-in-place piles, the fresh concrete is exposed to various types of potential damage, such as necking, ground intrusions due to displaced soil and possible damage due to driving of adjacent piles.
Piles can be classified according to the type of material forming the piles, the mode of load transfer, the degree of ground displacement during pile installation and the method of installation.
Pile classification in accordance with material type (e.g. steel and concrete) has drawbacks because composite piles are available. A classification system based on the mode of load transfer will be difficult to set up because the proportion of shaft resistance and endbearing resistance that occurs in practice usually cannot be reliably predicted. In the installation of piles, either displacement or replacement of the ground will predominate.
A classification system based on the degree of ground displacement during pile installation, such as that recommended in BS 8004 (BSI, 1986) encompasses all types of piles and reflects the fundamental effect of pile construction on the ground which in turn will have a pronounced influence on pile performance.
Such a classification system is therefore considered to be the most appropriate.
Piles are classified into the following four types :
(b) Small-displacement piles, which include rolled steel sections such as H-piles and open-ended tubular piles.
However, these piles will effectively become largedisplacement piles if a soil plug forms.
(c) Replacement piles, which are formed by machine boring, grabbing or hand-digging. The excavation may need to be supported by bentonite slurry, or lined with a casing that is either left in place or extracted during concreting for re-use.
(d) Special piles, which are particular pile types or variants of existing pile types introduced from time to time to improve efficiency or overcome problems related to special ground conditions.
There are a number of foundation types available for geotechnical engineers.
1. Shallow foundations
Shallow foundations are the cheapest and most common type of foundations (Fig.1).
Shallow foundations are ideal for situations, when the soil immediately below the footing is strong enough to carry the building loads. In some situations soil imme-diately below the footing could be weak or compressible. In such situations, other foundation types need to be considered.
Fig.1. Shallow foundations
2. Mat foundations
Mat foundations are also known as raft foundations. Mat foundations, as the name implies, spread like a mat. The building load is distributed in a large area (Fig.2).
Fig.2. Mat foundations
3. Pile foundations
Piles are used when bearing soil is at a greater depth. In such situations, the load has to be transferred to the bearing soil stratum (Fig.3).
Fig.3. Pile foundations
4. Caissons
Caissons are nothing but larger piles. Instead of a pile, a group few large caissons can be utilized. In some situations, caissons could be the best alternative (Fig.4).
Fig.4. Caissons
5. Foundation selection criteria
Normally, all attempts are made to construct shallow foundations. This is the cheapest and fastest foundation type. The designer should look into bearing capacity and settle-ment when considering shallow foundations.
The geotechnical engineer needs to compute the bearing capacity of the soil immediately below the footing. If the bearing capacity is adequate, settlement needs to be computed. Settlement can be immediate or long-term. Immediate and long-term settlements should be computed (Fig.5).
Fig.5. Different foundation types
The Fig.5 shows a shallow foundation, mat foundation, pile group, and a caisson. A geotechnical engineer needs to investigate the feasibility of designing a shallow foundation due to its cheapness and ease of construction.
In the previous situation, it is clear that a weak soil layer just below the new fill may not be enough to support the shallow foundation. Settlement in soil due to loading of the footing also needs to be computed.
If shallow foundations are not feasible, then other options need to be investigated. Mat foundations can be designed to carry large loads in the presence of weak soils. Unfortunately, cost is a major issue with mat foundations. Piles can be installed as shown in the figure ending in the bearing stratum. In this situation, one needs to be careful of the second weak layer of soil below the bearing stratum.
Piles could fail due to punching into the weak stratum (Fig.6).The engineer needs to consider negative skin friction due to the new fill layer. Negative skin friction would reduce the capacity of piles (Fig.7).
Due to the new load of the added fill material, weak soil layer 1 would consolidate and settle. Settling soil would drag the piles down with it. This is known as negative skin friction or down drag.
Fig.6. Punching failure (soil punching into the weak soil beneath due to pile load)
Pile foundations have been used as load carrying and load transferring systems for many years.
In the early days of civilisation, from the communication, defence or strategic point of view 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 resembled to days pile driving mechanism. Steel piles have been used 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 diesel 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.
