Piling System I

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

 

1. References

R. Chudley, Construction Technology, Volume 1 to 4, 2nd Edition, Longman

2. Learning Objectives

Students should master the following issues after the lectures,

  1. Choice factors for selecting different piling systems.
  2. Bored piles
  3. Caissons
  4. H-piles
  5. Limitations of each systems.

The material will be covered by two lectures. The first lecture covers the following:

The choice factors

Bored Piles

Caissons (Part 1)

Emphasis of the study should be given on the choices factors, the limitations of each system, and the applicable situations of each piling system.

3. The Choice Factors

3.1 Reasons for Piling

  • Where weak strata over lies firm stratum, piles can be used to reach the firm stratum by by-passing all the weak/stratum.

  • Where concentration of loading occurs, it is best dealt with by piling because it is most economical to transfer a load directly from the point of application to the bearing stratum.

  • When uplift to building may occur, friction piles may be used to overcome the uplifting force.

  • When the loading is so high that other foundation methods would not be appropriate. Piles supported on rock create the greatest bearing capacity.

  • When the ground floor slab has to be carried above the ground, e.g. on a sloping site.
3.2 Factors Affecting Choice of Piles
  • Depth to be reached.

  • Loading requirements.

  • Soil conditions regarding strength, corrosion and ground movement, etc.

  • Environmental restrictions e.g. noise and air pollution control.

  • Access of site – may limit the use of long precast pile.

  • Congested or open site – may limit the use of piling rigs.

  • Headroom restriction e.g. under a bridge flyover.

  • Effect on adjoining buildings, if adjoining buildings are unstable, excessive vibrations is to be avoided.

  • Piling plant and equipment available.

  • Reliability of types of piles and the expertise and familiarity of the specialist sub-contractor.

  • Time available for completion of the piling contract.

  • Cost per unit length of pile.

 

4. Statutory & Q.C. for Piles

In Hong Kong, in-situ replacement piles have come into increasing use in recent years for stability of foundations. To prevent possible pile failures, statutory and quality control become necessary.

4.1 Statutory Control (HK)

The statutory control of all piling work in Hong Kong is laid down clearly in the Building (Construction) Regulations 1995, Chapter 123, Section B, Clause 35.

In general, the foundation of a building should safely sustain and transmit to the ground all dead load, wind load and imposed load of the building without impair the stability of the building and any adjoining buildings.
Clause 38 particularly concerned with caissons which include bored piles exceeding 900mm in diameter. The one that we should pay particular attention is clause 38(5) which states that "where any doubt exists as to the capacity of any caisson to sustain, adequately and without undue settlement, the load for which it has been designed the caisson shall be tested:-
  1. By means of core drilling of the completed in-situ concrete.

  2. By any other method to the satisfaction of the Building Authority, in which case, the Building Authority shall determine the standard of acceptance to be adopted.
4.2 Pile Testing
The main objective of forming a test pile is to confirm that the design and formation f the chosen pile type is adequate.
4.2.1 Coring Test
This test is used to check the strength of the concrete and the intersection between the caisson base and the rock stratum. It can be used to detect various defects occurring in concrete, e.g. honeycombing, segregation, voids, cracks, etc.
  
4.2.2 Loading Test
A loading test is made usually for one or other of the following reasons:
  • To determine the load-settlement relationship, particularly in the region of the anticipated working load.

  • To serve as a proof test to ensure that failure does not occur before a load is reached which is a selected multiple of the chosen working load.

  • To determine the real ultimate bearing capacity as a check on the value calculated from dynamic or static formulae, or to obtain information that will enable other piles to be designed by empirical methods.
4.2.3 Integrity Tests
They are used to check the soundness of the caisson shaft and the quality of the concrete. They are generally very rapid, cause minimal disruption to the site and are relatively inexpensive so that large numbers of piles on a site can be economically examined. There are two main groups of tests: Sonic Test and Vibration Test.
Sonic Test

It is based on measuring the propagation time of a sonic signal between two vertical tubes cast into the pile during construction.

Vibration Test
In this test, an electro-dynamic vibrator is placed on the head of the caisson, applying a constant sinusoidal force of 50N within the range 20-5000 Hz. The head of the pile simple moves up and down as the frequency as the vibrator where the velocity of this movement is measured by the transducer for analysis.
  
Bored Piles
There are various methods of installing bored piles. Two methods of providing this are:-

by using a steel casing, for example, with small diameter bored piles; and

by using drilling mud (bentonite) to support the excavation, for example, with large diameter bored piles.

5.1 Small Diameter Bored Piles with Temporary Casing

Small diameter bored in-situ pile range in diameter from 300 to 950 mm and is designed to carry loads of up to 1500 KN.

Crane mounted or lorry mounted rotary drilling rigs or a percussion boring tool is used to excavate these piles. The stages of installation using a tripod or shear leg percussion boring tool are described below.

Stage 1
The percussion tool, consisting of the tripod or shear leg, a winch and the cutter, are set up. A starter hole is then made by dropping the cutter from the raised position.

Stage 2
The first section of casing, which has a cutting edge, is placed in the starter hole. This section of casing is known as the cutter casing. The casing, which varies in length from 1 to 1.4m, can be extended by screwing on additional lengths.

Stage 3
Additional lengths of casing are screwed on to the cutter casing. A driving cap is placed on top of the casing, and the casing is driven ahead of boring. The casing is driven by using the weighted head of the percussion tool which acts as a drop hammer.  

Stage 4
Soil is bored out from within the casing by means of the percussion tool cutter.

Stage 5
Stages 3 and 4 are repeated and the boring and casing extended to the required depth.

Stage 6
A steel reinforcement cage is placed in the borehole and a high slump concrete is then placed into the temporary casing. The concrete should be placed by means of a tremie pipe or by trunking.

Stage 7
The temporary casing is extracted when concreting is completed.

This type of pile with temporary support can be very long, and the length can be varied to suit different site conditions. The pile can be constructed in restricted headroom, or confined sites, and within centimetres of existing structures and old sewers without damaging their stability.

The problems associated with small diameter piles are the limitations in the diameter of the pile (maximum diameter 600mm), the difficulty of positioning the reinforcement cage correctly, and the possibility of causing settlement to adjacent structures.

5.2 Large Diameter Bored Piles Supported by Bentonite

Large diameter bored in situ piles range in diameter from 1000 to 3600mm, and are designed to carry loads of up to 30,000KN.

Truck or crane mounted rotary drilling rigs or grab rigs with or without casing oscillators are used to excavate these piles.

The four stages in the installation of these piles using a crane mounted rotary drilling rig are described below.

Stage 1
A short length of casing is pitched in the required position. Excavation is carried out within the casing by means of a helical auger (or grab), and the casing is then inserted into the ground. This short length of casing prevents surface water and debris from entering the borehole. It also prevents the collapse of the loose surface soil at the mouth of the borehole, and the loss of the bentonite through the loose surface soil.

Stage 2
The borehole is filled with bentonite suspension from storage tanks. When mixed with the correct amount of water, bentonite exhibits thixotropic properties. Boring proceeds through the bentonite, which is fed continuously into the hole during boring.

Stage 3
Boring stops when the desired depth is reached. The auger is then withdrawn. A reinforcement cage is lowered through the bentonite and concrete is placed through a tremie pipe. The bentonite, which is displaced by the concrete, is pumped back into storage tanks. The bentonite can be strained to remove soil particles, and then re-used.

Stage 4
On completion of the concreting, the tremie pipe is removed and the short casing is withdrawn. Large diameter bored piles, the maximum diameter of which can be 3.6m, the pile length can be varied to suit site conditions, and piles can be as long as 60m.

However, care must be taken to ensure that the reinforcement cage is correctly positioned to provide adequate cover to all the reinforcement.

5.3 Equipment for Installing Bored Piles

Rotary Drilling Rigs and Augers

The drilling rig may be mounted on a mobile crane, or a truck. The drilling rig consists of an auger, mounted on a telescopic or extendable kelly bar. The kelly bar is a square hollow steel shaft which is generally about 7.5m long, but can be extended if required.

Crane and Grab Rigs and Casing Oscillator
Grabs consist of clamshell buckets which are opened and closed by means of wire ropes, or hydraulically. They are suspended from mobile cranes.

The grab is dropped on to the soil in an open position and is then closed. It is then raised to the surface and emptied at the side of the shaft.

The temporary casing is driven into the soil by means of an oscillator, which is hydraulically powered. The oscillator clamps itself to the casing and, by a combination of rotating and pushing, forces the casing into the ground. The oscillator is also used to withdraw the casing on completion of the concreting of pile.

Topic

Pile Type

Small Diameter Bored Piles

Large Diameter Bored Piles

Noise Pollution

Very little

Very little

Possibility of installing in restricted headroom

Yes, possible

No, not possible

Possibility of ground heave

No risk

No risk

Possibility of ‘necking’ or ‘waisting’

May occur

"Necking’ or ‘waisting’ may occur

Possibility of forming a large diameter pile

Not applicable

Yes, up to 3.6m

Ease of quality control of the pile

No, difficult to control

No, difficult to control

Special equipment required

No special equipment required

No special equipment required

Possibility of varying the pile length on site

Yes, possible

Yes, possible

6. Hand-Dug Caissons

In Hong Kong, ‘Hand-dug caisson’ is a term generally used for what other would called a ‘Hand-dug pile’. A hand-dug caisson is a cylindrical shaft formed in the ground, with openings at both top and bottom. The shaft is excavated in stages by hand and the wall of the shaft is lined with 75 to 100mm thick in-situ concrete as excavation progresses. If the water table is high, the thickness of lining may be increased to 150 or 200mm at depths of 30m.

In Hong Kong, hand-dug caissons are often used instead of large diameter cast-in-situ piles. The diameter of the caisson ranges from 800 to 3000mm. It is common practice to excavate the shaft to a depth of 45m. This may include 20m or more, below the water table.

6.1 Why Hand-dug Caisson is Popular in Hong Kong

Various Soil Conditions
The geological conditions in Hong Kong varies drastically from sloped granolithic sites near the foot of the Victoria Mountain in Hong Kong Island to some of the sandy-silt areas in Yuen Long. Hand-dug caisson construction is more flexible and it can adjust the method of excavation easily from simply hand-digging for soils to rock drilling.

Comparative Low Plant or Equipment Cost
Both bore piling construction and pre-cast pile driven construction will require the use of heavy machines. However, Hand-dug caisson construction requires only simple equipment. Thus the plant cost for this particular construction method is comparatively low.

Availability of Manual Labour
For each caisson, two workers are usually required. One on the ground level receiving the steel bucket for discharge while the other is working inside the caisson. The cost of caisson is the cheapest among all types of foundation.   

Construction Period
The time required to complete the excavation by the use of boring machine is very much dictated by the efficiency as well as the number of the machines available.

6.2 Caisson Foundation

The four aspects of caisson foundations which must be considered are: Design, Acceptance, Performance and Construction Safety.

Design
In designing a caisson, generally, there are two main points needed to be considered. Firstly, the size and depth of the caisson so that the expected loads will not cause settlement. Secondly, the economy of caisson construction in different situations.

Acceptance
Acceptance of a caisson is a type of quality assurance using testing methods and it is to determine whether or not the caisson has been constructed according with the design assumptions.

Performance
The performance of a caisson in service is its settlement behavior during and after the application of construction and service loads.

Construction Safety
The main draw back of the use of hand-dug caisson is construction safety. During the year 80-86, there are 27 fatal accidents where 30 workers were killed and 85 non-fatal accidents where 97 workers injured. The causes of the accidents are:-

fall of person

falling object

electrocution

gassing

in rush of mud or water

The Building Regulation (Construction) and The Construction Site (Safety) Regulation have laid down some requirements which are related to caisson.

6.3 Equipment for Forming Hand-Dug Caissons

The equipment commonly used in forming hand-dug caissons are:-
  • A pair of timber frames supporting a steel runway beam above the caisson.
  • A steel drum skip to allow tilting for discharge.
  • A hoisting system consisting of an electrically operated chain block.
  • An electric fan-blower which supplies air directly to the bottom of the caisson.
  • An electric water pump for pumping out any ground water.
  • Hand and/or pneumatic tools for excavation work.
  • A tapered steel formwork, for the concrete lining to the caisson.
Safety equipment would also include spare parts for the fan-blower, a first aid kit and equipment to administer oxygen.