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The advantages and disadvantages of large-displacement piles are summarised in Table 4.1.

Table 4.1 – Advantages and Disadvantages of Displacement Piles

Advantages Disadvantages

Large displacement piles

(a) Material of preformed section can be inspected before driving.

(b) Steel piles and driven cast-in-place concrete piles are adaptable to variable driving lengths.

(c) Installation is generally unaffected by groundwater condition.

(d) Soil disposal is not necessary.

(e) Driving records may be correlated with insitu tests or borehole data.

(f) Displacement piles tend to compact granular soils thereby improving bearing capacity and stiffness.

(g) 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.

(h) Driven cast-in-place piles are associated with low material cost.

(a) Pile section may be damaged during driving. (b) Founding soil cannot be inspected to confirm the

ground conditions as interpreted from the ground investigation data.

(c) Ground displacement may cause movement of, or damage to, adjacent piles, structures, slopes or utility installations.

(d) Noise may prove unacceptable in a built-up environment.

(e) Vibration may prove unacceptable due to presence of sensitive structures, utility installations or machinery nearby.

(f) Piles cannot be easily driven in sites with restricted headroom.

(g) 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.

(h) Length of precast concrete piles may be constrained by transportation or size of casting yard.

(i) Heavy piling plant may require extensive site preparation to construct a suitable piling platform in sites with poor ground conditions.

(j) Underground obstructions cannot be coped with easily.

(k) 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.

Small displacement piles

(a) As (a), (b), (c), (d), (e) and (g) for large- displacement piles.

(b) Cause less ground disturbance and less vibration.

(a) As (a), (b), (d), (e), (f), (i) and (j) for large- displacement piles.

4.2.2 Precast Reinforced Concrete Piles

Precast reinforced concrete piles are not common nowadays in Hong Kong. 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

kN. 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 commonly adopted in Hong Kong include welding of steel end plates or the use of epoxy mortar with dowels. Specially fabricated joints have been successfully used in other countries, e.g. Scandinavia.

This type of pile is not suitable for driving into ground that contains a significant amount of boulders or corestones.

4.2.3 Precast Prestressed Spun Concrete Piles

Precast prestressed spun concrete piles used in Hong Kong 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.

Precast prestressed spun concrete piles have been successfully employed in Hong Kong for many projects in the past. 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 (see Section 8.2.5 for more details).

4.2.4 Closed-ended Steel Tubular Piles

The use of box-section steel piles is not common in Hong Kong but steel tubular piles are becoming increasingly popular, particularly for marine structures.

Steel tubular piles have high bending and buckling resistance, and have favourable energy-absorbing characteristics for impact loading. Steel piles are generally not susceptible to damage caused by tensile stresses during driving and can withstand hard driving. Driving shoes can be provided to aid penetration.

For corrosion protection, steel tubular piles installed in a marine environment may be infilled with reinforced concrete to a level below the seabed and adequate for load transfer between reinforced concrete and steel tube. The steel tube above such level can be considered as sacrificial and ignored for design purposes.

4.2.5 Driven Cast-in-place Concrete Piles

Driven cast-in-place concrete piles are formed by driving a steel tube into the ground to the required set or depth and withdrawing the tube after concrete placement. The tube may be driven either at the top or at the bottom with a hammer acting on an internal concrete or compacted gravel plug. A range of pile sizes is available, up to 600 mm in diameter. The maximum allowable axial load is about 1 400 kN. The maximum length of such piles constructed in Hong Kong is about 30 m.

Proprietary systems of top-driven, cast-in-place piles have been used in Hong Kong. In this method, the steel tube is provided with a loose conical or flat cast-iron shoe which keeps the tube closed during driving. Light blows are usually imparted to the tube during extraction, thus assisting concrete compaction.

For bottom-driven, cast-in-place piles with an expanded base, the tube does not have to withstand direct impact and can be of a smaller thickness. Also, the piling rig does not need to be as tall as rigs for other driven cast-in-place piling systems. When pile driving is completed, the tube is held against further penetration and the bottom plug is driven out by the hammer within the tube. An enlarged pile base is formed using 'dry' mix concrete, with a water/cement ratio of approximately 0.2, which is rammed heavily with the internal hammer.

4.3 SMALL-DISPLACEMENT PILES