Piles - Design of Steel H Piles

Discuss the design considerations for steel H-pile foundation
(by S S LAM 11/91)
1 INTRODUCTION
Because of the very limited land resources in Hong Kong,

it becomes unavoidable that structures have to be
constructed on sites with unfavourable ground conditions.
The use of pile foundation as supports to these structures
seems inevitable in order to transmit loads from the
structure through weak subsoil strata down to a load
bearing stratum. Among various types of pile foundations,
steel H-pile foundation has been widely used.
In carrying out a design for a steel H-pile foundation,

designers should give considerations to a number of
factors such as the geological conditions of the site, the
capacity of individual pile, the overall capacity of the
pile group and the corrosion protection of the pile.
2 SITE INVESTIGATION
For a pile foundation design, a carefully planned site

investigation is crucial to obtain design information such
as the subsurface stratigraphy, ground water condition and
soil properties to assist in the selection of pile types,
determination of the design load and the estimation of the
pile length. Attention must be paid to the
characteristics of the soils above the bearing strata,
including the existence of obstructions, waterbearing or
caving soils, soils that may heave when displaced, soils
that may consolidate and produce negative skin friction,
and other factors that may influence pile installation and
performance.
Apart from exploring the subsurface conditions, designers

should be aware of any site constraints that may hinder
the piling operation. For example, when the site is in
close proximity to a number of buildings, the pile driving
operation may be restricted because the vibration
generated during the operation may have a detrimental
effect to the adjoining buildings. If such hinderance
exists, measures shall be formulated during the design
stage to cope with the situation.
3 DESIGN FOR INDIVIDUAL PILE
To start with the design for a H-pile foundation,

designers should first carry out an assessment of the
loads on the piles. Basically, loads from the
superstructure include the weight of the structure,
various form of live loads and the lateral loads such as
due to the effect of wind. Besides, the designers should
pay attention to the effect of the negative skin friction
on the pile which is due to the possible consolidation of
the soft subsoil inside which the piles are embedded.
Having considered all possible kinds of loads and their
combinations, the designers can work out the working load
on the piles.
Based on the working load, the cross sectional area of the

pile required can be determined. Then, a suitable class
of H-pile can be selected for used. At this stage, the
designers should note that the working load on the pile
should not exceed a certain percentage of the yield
strength of the pile. In BS 8004 : Foundation, the
suggested figure is 30%. This provision aims at reducing
the risk of damaging the pile due to the high dynamic and
repetitive loading acting on the pile head by the hammer
during the pile driving process.
In Hong Kong, the extent of installing an H-pile is

governed by the set value of driving the H-pile into the
ground which is measured in terms of the amount of
penetration of the pile per 10 blows of the driving
hammer. This set value is usually calculated by the Hiley
formula which is a dynamic pile-driving formula commonly
used in Hong Kong. Since it is only an empirical formula
relating the resistance of the bearing stratum to the
energy required for driving the pile, it is important that
designers should conduct an assessment in the design stage
on the appropriateness of applying this formula so as to
ensure that the H-pile can be installed to give the
required capacity with the required factor of safety.
4 GROUP ACTION OF THE PILE FOUNDATION
The supporting capacity of a group of piles can, in many

situations, be considerably less than the sum of the
capacities of the individual piles comprising the group.
In all cases, the elastic and consolidation settlements of
the group are greater than those of a single pile carrying
the same working load as that on each pile within the
group. This is because the zone of soil or rock which is
stressed by the entire group extends to a much greater
width and depth than the zone beneath the single pile.
Even when a pile group is bearing on rock, the elastic
deformation of the body of rock within the stressed zone
can be quite appreciable if the piles are loaded to their
maximum safe capacity. Group action in pile foundations
has resulted in many recorded cases of failure or
excessive settlement, even though loading tests made on a
single pile have indicated satisfactory performance.
One way of reducing the risk of block failure of the piles

is by controlling the spacing of the piles. By allowing
sufficient space between the piles, the degree of
overlapping of the stressed zone of each pile can be
reduced and thus the reduction of the stress intensity in
a particular zone. BS 8004 recommends that the spacing
between the piles should be at least a perimeter of the
pile.
Designers should give due consideration in designing the

pile foundation to account for the group action of the
piles so as to avoid block failure of the pile foundation.
5 CORROSION PROTECTION OF THE H-PILE
The effects of corrosion on the life of steel H-piles are

now regarded as being less significant than was thought in
the past.
When the pile is driven into undisturbed natural soil, the

type and amount of corrosion will be so small that it will
not affect significantly the strength or life of the pile
to support structures. The reason is that undisturbed
soils are so deficient in oxygen that they will not
sustain the process of corrosion.
In contaminated ground where corrosion of steel may be

higher than normal, some protection over the length of
pile above and to a certain depth below the water table is
necessary. The designer should determine the extent and
type of protection that should be used in order to ensure
that the pile can function properly throughout its design
life.
6 CONCLUSIONS
H-pile foundation is one of the commonest type of pile

foundation used in Hong Kong for supporting structures.
In carrying out the design for an H-pile foundation,
designers should pay attention to its suitability to be
used for a particular site, the performance of individual
piles forming the foundation and the overall performance
of the pile group. Besides, measures should be taken, if
necessary, to protect the piles against corrosion to
guarantee their proper functioning throughout their design
life.

Piles - Design of Steel H Piles

Uploaded by

Copyright:

Available Formats

Piles - Design of Steel H Piles

Uploaded by

Document Information

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Piles - Design of Steel H Piles

Uploaded by

Copyright:

Available Formats

What are the main factors to consider in the design of an individual steel H-pile?

What are the main factors to consider in the design of an individual steel H-pile?

How does group action affect the capacity of a pile foundation?

How does group action affect the capacity of a pile foundation?

Discuss the design considerations for steel H-pile foundation

(by S S LAM 11/91)

Because of the very limited land resources in Hong Kong,

In carrying out a design for a steel H-pile foundation,

For a pile foundation design, a carefully planned site

Apart from exploring the subsurface conditions, designers

3 DESIGN FOR INDIVIDUAL PILE

To start with the design for a H-pile foundation,

Based on the working load, the cross sectional area of the

In Hong Kong, the extent of installing an H-pile is

4 GROUP ACTION OF THE PILE FOUNDATION

The supporting capacity of a group of piles can, in many

One way of reducing the risk of block failure of the piles

Designers should give due consideration in designing the

5 CORROSION PROTECTION OF THE H-PILE

The effects of corrosion on the life of steel H-piles are

When the pile is driven into undisturbed natural soil, the

In contaminated ground where corrosion of steel may be

H-pile foundation is one of the commonest type of pile

You might also like