By Dr. Heng Li [bshengli@polyu.edu.hk] Tel: 2766 5879

1. References

R. Chudley, Construction Technology, Volume 1 to 4, 2^nd Edition, Longman M.J.Tomlinson, 1994, Pile Design and Construction Practice, 4^th Edition, E&FN Spon

2. Learning Objectives

Students should master the following issues after the lectures,

This lecture covers the following:

3. Method of Constructing Hang-Dug Caissons

The construction of a hand-dug caisson is carried out in the following stages:

Stage 1
The excavation is carried out by hand. The spoils are loaded on to the skip. The skip is lifted out and its contents discharged to the side of the shaft. In favorable ground conditions, excavation is carried out in stages of about 1m or less. However, in soft soils, the depth should be no more than that which can be excavated and lined during a working shift.

Stage 2
On completion of each stage of excavation, a tapered steel formwork is lowered into the shaft. The formwork is set up and suitably braced.

Stage 3
The formwork is then filled with Grade 20 concrete so as to provide a minimum of 75mm thick concrete lining to the shaft.

Vertical reinforcement in the form of hairpins between the rings is essential. Hoop reinforcement in the concrete lining is not recommended.

Stage 4
The formwork is left in placed while the next stage of excavation is carried out. This provides support to the fresh lining and the surrounding ground during excavation.

Each concrete ring overlaps the preceding one. This provides a means of feeding concrete into the form and also seals against the ingress of water or soil.

If required, dewatering is carried out by pumping.

Stage 5
When the desired depth has been reached, limited belling of the shaft may be carried out in suitable ground conditions.

Stage 6
The caisson is completed by placing a reinforcement cage in the shaft and concreting the shaft. Concreting should proceed without interruption until the entire shaft is concreted.

Concreting should be cast to a height such that laitance on top may be removed on completion of concreting, leaving sound concrete at cut-off level.

The caisson may be reinforced over its full length or over a short length at the top, depending on the load carried by it. This reinforcement should extend at least 1m into the caisson cap.

Stage 7
When the caisson is completed, its head is trimmed so that about 75mm extends into the caisson cap. A caisson cap, similar to a pile cap, is then cast over the top of the caisson.

4. Steel Piles

The most common types of steel piles are:

Steel piles have the advantages of being robust, light to handle, capable of carrying high compressive loads when driven on to a hard stratum, and capable of being driven hard to a deep penetration to reach a bearing stratum or to develop a high skin-frictional resistance, although their cost per metre run is high compared with precast concrete piles. They can be readily cut down and extended where the level of the bearing stratum varies; also the head of a pile which buckles during driving can be cut down ad re-trimmed for further driving. They have a good resilience and high resistance to buckling and bending forces.

4.1 Steel Tube Piles
Steel tubes piles are fabricated by welding a plate which has been rolled into a continuous helix. The diameter of the piles varies from 250 to 600mm.

Tube piles are usually driven by an internal drop hammer. Alternatively, they may be vibrated into the ground. Tube piles are usually bottom driven with an open end. However, in some cases, a flat plat is welded to the toe of the pile. If such an end plate is used, a plug of earth or dry concrete should be placed inside the tube pile. The internal drop hammer drives the pile by striking this plug of concrete. Driving open ended tube piles is similar to driving a casing.

On completion of driving, a capping plate is welded on.

4.2 Steel H-section Piles
H-section piles are in the form of wide-flanged steel sections and rolled in accordance with standard. The displacement of the soil is small compared with other types of displacement piles. Hence, this type of pile is called a ‘small displacement’ pile.

H-section piles may be driven by any type of hammer, but the head of the pile should be protected by a helmet.

H-section piles do not require any pile shoes. In special cases, the toe of the pile is strengthened by welding stiffening plates. A pile thus stiffened is useful for punching through thin layers of rock or boulders.

As in all steel piles, the head of the pile is treated and embedded in the concrete pile cap.

4.3 Steel Screw Piles
The screw pile consists of a hollow or solid shaft, to which is fitted a large diameter helical blade. The function of the screw is to provide a large bearing area at the foot of the pile and, if required, to resist uplift. The shaft may be of mild steel and the helical blade of cast iron, welded mild steel or cast steel.

Screw piles may also be formed entirely of reinforced concrete. The diameter of the helical blade may vary from 600mm to 3m. A boss of conical shape is usually provided below the screw to facilitate driving.

The pile is screwed into the ground by applying torque to the head of the pile shaft. The head of the pile shaft is square, and torque is applied by means of a powerful winch and cable.

When the desired depth is reached, the pile head is trimmed and then embedded in the concrete pile cap. Steel screw piles are used where the ground consists of silt, soft clay or other soil of very low bearing capacity.

However, steel screw piles are no longer very popular because other types of piles are faster to install and more economical.

4.4 Equipment for Pile-driving

Piling Hammers

4.4.1 Drop Hammer
The simplest form of piling hammer is the drop hammer. It is simply a heavy weight that is allowed to drop freely on the head of the pile. The drop weight is usually operated between guide rails.

Today, since their slow rate of operation and inconsistent delivered energy, drop hammers are seldom used to drive foundation piles. They are sometimes used to drive piles for small projects and in remote areas.

If a drop hammer is used, the inspector should ascertain that the specified weight has been furnished along with special equipment that may be required, such as an automatic trip.

4.4.2 Single-Acting Hammer
Single-acting hammers are powered by compressed air or steam pressure, which is used to raise the hammer ram for each stroke. These hammers are best suited to driving timber or precast concrete piles, since the drop of each blow of the hammer is limited in height and is individually controlled by the operator.

The single-acting hammer is also suitable for driving all types of pile in stiff to hard clays, where a heavy blow with a small drop is more efficient and less damaging to the pile than a large number of lighter blows.

4.4.3 Double-Acting Hammer
Double-acting hammers can be powered by steam, but they are usually powered by compressed air, which is used both to raise the ram and to accelerate its fall. They have light rams and operate at a relatively high speed. They are not as effective in driving foundation piles as single-acting hammers, and they are used principally for driving sheet piles or underpinning piles.

5. Limitations

5.1 Bored Piles

• Concrete in shaft liable to squeezing or necking in soft soils where conventional types are used.

• Special techniques needed for concreting in water-bearing soils.

• Concrete cannot be inspected after installation.

• Enlarged bases cannot be formed in cohesionless soils.

• Cannot be extended above ground level with special adaptation.

• Low end-bearing resistance in cohesionless soils due to loosening by conventional drilling operations.

• Drilling a number of piles in group can cause loss of ground and settlement of adjacent structures.

5.2 Steel Piles