Steel Piles Installation.

Steel piles can be installed by a variety of methods and equipment.There are three types of installation equipment, which operate by impact, vibration or by jacking.

Each has particular advantages and disadvantages, and the final choice is, in most cases, a balance between speed and economy of installation. A further deciding element is the increasing concern for noise and vibration control to which the industry has responded with the development of new installation equipment such as hydraulic hammers.

The installation of steel piles is a specialist activity, calling for considerable knowledge and experience of handling piles and hammers to achieve an acceptable placement within specified tolerances of position and level. Guidance on the practical limits that can be achieved in position and level for driven steel piles is available from the FPS (Federation of Piling Specialists) and in the TESPA (Technical

European Sheet Piling Association) publication  Installation of steel sheet piles.

Guidance is also included at the back of the new ICE Specification for Piling and Embedded Retaining Walls and in the Corus Piling Handbook.

The designer should refer to these documents before carrying out a design, because the advice given will often affect the details at connections to the pile cap in the structure.

There is much to gain from matching the stiffness of the pile to the hammer and to the anticipated soil resistance at the site to achieve satisfactory driveability and to ensure achievement of the required design penetration.

In addition, following research work over the last 30 years, there is a developing understanding of the benefits of measuring the soil resistance during driving as a check on the designer’s predicted compressive axial static capacity (see the ICE

Specification for Piling and Embedded Retaining Walls).Although there are some caveats to be applied to this practice, it is indisputable that  both designer and installer gain from using dynamic analysis of pile driving and that this is of ultimate benefit to their clients in many ways. For instance, it is a QA tool to permit the evaluation of piles that have an unexpectedly high resistance at a higher level; it can lead to fewer and shorter piles being acceptable; it can save construction time by avoiding delays; and it provides an equitable means of resolving disputes about specifications that turn out to be unrealistic due to inadequate site investigation or inaccuracy in design prediction methods.

The relationship between the static and dynamic capacity is now better under- stood, and in many soils and rocks there is very little difference between them.

However, it is still sound practice to carry out static load testing to check the design penetration because in some soils pile capacity has been known  to decrease after driving. Test piles should therefore have both static and dynamic measurements in order to relate the two, and then dynamic pile analysis can be used to check the project piles.

1 Determining bearing capacity
The safe load capacity of driven steel piles should be verified.The only certain way to do this is by static test loading. Unfortunately, to do this for every pile is neither practical nor economic for most projects.

Alternative ways of load testing piles are by stress wave analysis using the CAPWAP program21
and a dynamic pile analyser.

Pile head settlement is most accurately determined by static test load. Dynamic formulae give no indication of this but stress wave analysis will give an indication the technique is rapidly improving.

Static test loading
Two forms of test loading are in general use, but the maintained load method is considered the more accurate.Details of both test methods are given in BS 8004 clauses 7.5.5 and 7.5.6.

Excessive conservatism has been found in current practice and in the currently used specifications for load testing piles, which has been compounded by unrealistic design assumptions on the soil parameters that are used in pile resistance prediction methods to determine a ‘working load’. This has led to costly overdesign of
piling.

The amount of kentledge or tension resistance should always be in excess of the estimated ultimate bearing capacity of the pile.A factor of 1.5 should be a minimum, since loading to twice the ‘working’ or ‘serviceability’ load is the accepted requirement, and the working load is normally about half the ultimate capacity in both  LSD and ASD procedures.

In the maintained increment load test (MLT), kentledge or adjacent tension piles or soil anchors are used to provide a reaction for the test load applied by jack(s) placed over the pile being tested. Figure 29.8 shows a typical arrangement.The load is increased in defined steps, and is held at each level of loading until all settlement has either ceased or does not exceed a specified amount in a stated period of time.


Test load arrangement using kentledge
Fig. 29.8 Test load arrangement using kentledge

A plot of load versus head settlement (Fig. 29.9) provides an understanding of pile head performance, which is an essential requirement for ensuring compatibility between superstructure and foundation, but it reveals only part of the picture of overall pile performance.

Load–settlement plot
Fig. 29.9 Load–settlement plot

Elastic shortening can be significant in steel piles, especially those which are long and fully stressed in grade S355 quality. Such shortening is of no consequence provided it is anticipated and allowed for in design and evaluation.

2 Pile-driver calibration
To overcome the need to test load every pile, careful records taken during the driving of a subsequently satisfactorily test-loaded pile can be used to evaluate later piles. This system is reliable when the hammer, pile size, pile length, and geology remain unchanged for subsequent piles. However, as soon as any one of these elements is changed, further test loading and recalibration is required.

3 Dynamic formulae
These were an early attempt to relate the energy output of the impact hammer and the pile penetration per blow of the hammer to the static resistance of the pile.There are a number of such formulae available and they date from the 1920s but all have proved to be unreliable and are no longer used.

4 Dynamic stress wave analysis
Dynamic methods have gained wide acceptance and are embodied in the new ICE Specification for Piling and Embedded Retaining Walls. Direct on-site computer analysis of pile gauge readings provides information on pile toe resistance, pile shaft resistance, pile shaft integrity, and actual energy input to the pile head.

This form of evaluation provides valuable information as the pile is driven, which is an ideal arrangement. However, it still relies on relating dynamic resistance and ‘set’ to static resistance, and care must be taken to ensure the results are not mis-leading.Redrive checks at ‘set’ are essential, and a cross-check by static loading may be advisable at the start of a large contract.

Redrive checks must be carried out to ensure that the resistance of the pile does not change after driving is completed.This is most likely to occur if pore water pressures in the soil are altered by the action of driving the piles.

5 Design stresses during driving
A driven pile is usually driven to a resistance of twice the working load or to ultimate bearing capacity to demonstrate an adequate factor of safety. BS EN 12699 embodies offshore practice where it states that the maximum dynamic stress in the shaft during driving should be limited to be less than 90% of yield.

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