Stage 2 Final Stage Fixed Earth Free Earth
11.16 Installing combined HZ or high modulus walls 52
Installation of sheet piles
Piling Handbook, 8th edition (revised 2008)
Installation of sheet piles
Installation of sheet piles
Chapter 11/1
11.1 Introduction This chapter provides an introduction to the modern methods of installing sheet piling taking into account the equipment available for safe working practice.
A knowledge of the characteristics of the steel and the section are not enough to guarantee good results prior to installation and this chapter briefly describes the practical information to be considered to ensure proper product installation. It also indicates how pile driveability can be predicted following a thorough evaluation of the ground conditions.
This chapter also contains information on pile driving equipment which is current at the time of writing and includes impact hammers, vibratory pile drivers, hydraulic pressing and special systems. Brief descriptions of driving methods, ancillary equipment and guideline procedures to assist in the adoption of good practice when installing sheet piles are also included.
Finally some common installation problems are illustrated and special aspects of driving briefly outlined.
11.2 Driving methods
11.2.1 General Whilst it is recognised that, in common with most civil engineering projects, a measure of flexibility is desirable to meet site conditions, every precaution must be taken to maintain the necessary standards of safety whilst giving the required alignment and verticality of the installed piles.
Therefore principal consideration must be given to access of plant and labour and working positions for handling the piles and threading the sheets together. The length of the piles and height from which they can be pitched and driven safely and accurately is also important
Whenever possible sheet piles should be driven in pairs. The first sheet piles in a wall must be installed with great care and attention to ensure verticality in both planes of the wall. Control of the sheet pile installation must be maintained during both the pitching and driving phases of the installation process.
There are two principal pile driving methods available to installers, pitch and drive and panel driving. The features, advantages and disadvantages of each method are described below.
11.2.2 Pitch and drive method
This method requires equipment to control the verticality of the pile during installation so that piles can be pitched and driven one by one. The pitching operation can be carried out close to ground level meaning that operatives are potentially at less risk and downtime in windy conditions can be reduced.
Piles can be installed to final level by this method (necessary when using the Japanese presses with single piles) or left at a higher level
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Installation of sheet piles
Chapter 11/2
to backdrive using panel driving techniques with other, generally heavier, hammers to speed up production or drive accurately in deeper more difficult strata.
This method is the simplest way of driving piles but is only really suited to loose soils and short piles. For dense sands and stiff cohesive soils or in the case of possible obstructions, pitch and drive is not recommended.
In recent years, the method has become more favoured by installers as purpose built equipment is now available to adequately control the pile during installation. In the right conditions, productivity is maximised.
Fig 11.2.2
It is more difficult to control forward lean using the pitch and drive method because the leading lock has less resistance than the trailing or connected lock as a result of soil and interlock friction.
Although the piling may commence from a true vertical position, the top of the piles will have a natural tendency to lean in the direction of driving. This will get progressively worse if not countered. When driving long straight sections of wall with a planned pitch & drive method it may be advisable, with the Engineers consent, to allow for supplying pre-fabricated tapered correction piles for use at approximately fifty metre intervals. This
Installation of sheet piles
Chapter 11/3 is important to consider when using the Japanese pressing machines because it may not be possible to revert to a panel backdriving system to avoid or correct the forward lean problem.
With pitch and drive, the free leading interlock is constantly in danger of rotation in plan which increases the deeper the free end penetrates the ground as it is unsupported during the driving operation. When a pile rotates during installation, friction develops in the connected locks making driving progressively more difficult.
11.2.3 Panel driving Piles may be threaded together above the ground in a support frame to form a panel prior to driving. In this situation, both interlocks are engaged before any driving takes place and this balancing of the friction forces ensures maximum control and accuracy. The piles are then driven in stages and in sequence into the ground. Sequential driving enables verticality to be
maintained.
Sheet piles should be installed using the panel-driving technique to ensure that good verticality and alignment is achieved and to minimise the risk of driving difficulties or declutching problems.
This technique is important for maintaining accuracy when driving long piles or driving into difficult ground
As a whole panel of piles has been pitched there is no need to drive all piles fully to maintain progress of the piling operations.
During driving, the tops of adjacent piles can be kept close together meaning that the stiffness of the piles is maintained across both connected locks allowing the pile toe to be driven through soil of greater resistance without undue deviation.
If obstructions are encountered, individual piles can be left high without fear of disruption to the overall efficiency of the installation process. Engineering decisions can then be taken to attempt to remove the obstruction or drive piles carefully either side of the obstruction before trying once more to drive or punch through it if further penetration is necessary.
Panel driving is the best method for driving sheet piles in difficult ground or for penetrating rock - which is unlikely to be possible with the pitch and drive method. Piles are usually paired up or neighbouring sheets levelled up at the head before commencing the hard driving operation with a heavier hammer. Care should be taken when piles are firstly pitched and installed in singles and driven in the first stage with a vibrohammer. It is easier to execute two stage driving in pairs if the piles are pre-ordered and installed in crimped pairs. Difficulty of pairing up in the panel is avoided in this way and safer more efficient operation of impact hammers can be ensured.
Piling Handbook, 8th edition (revised 2008)
Installation of sheet piles
Chapter 11/4
5. 1st panel part driven.
1. Pitch, align and
plumb 1st pair. 2. Drive 1st pair carefully
& accurately pitch rest ofpanel.
4. Drive remainder of panel - working back towards 1st pair.
3. Ensure last pair are accurately positioned
&plumbed, drive last pair.
7. 1st panel driven to final level in stages. Last pair of 2nd panel plumbed&
driven accurately.
The lower frame is usually left in position after removal of the upper frame until driving is sufficiently progressed for it to be removed.
8. 1st panel completed.
2nd panel part driven.
3rd panel pitched.
Last pair of 2nd panel becomes 1st pair of 3rd panel.
6. 2nd panel pitched.
Last pair of 1st panel become 1st pair of 2nd panel. Gates supported by through bolting to last driven pair.
Fig 11.2.3
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Chapter 11/5
11.2.4 Staggered driving
It is essential that the heads of adjacent piles or pairs are kept close together to maximise the pile performance when driving in hard conditions. This means that the installer should keep moving the hammer from one pile to another in sequence to advance the toe of the piling with less risk of damage or refusal. This technique is known as staggered driving. It is not recommended that piles are advanced more than 2 metres beyond neighbouring piles unless driving conditions are relatively easy for the pile section and equipment used.
11.2.5 Cofferdam and closure installation techniques
When installing cofferdams or high modulus walls, accuracy is essential – particularly where it is necessary to pitch a pile of significant length into both adjacent pile interlocks to close a gap.
If the gap tapers it will be very difficult to interlock and drive the closure pile successfully. Therefore the panel driving method is the favoured method for installing structures of this type.
CIRIA SP95 gives sound guidance on what needs to be
considered when installing sheet piles for cofferdams and dealing with the issues that need to be addressed to be able to carry out other operations in the dry. With any sheet pile project, the risk of declutching should be minimised especially when men are required to work in dewatered cofferdams.
When joining walls or closing to fixed positions, panel installation methods are obligatory to maintain safe working conditions. It is necessary to avoid the risks and potential disaster caused by de- clutched or damaged piles when planning, designing and executing the works.
The panel driving technique is also best for the control of wall length and creep by using appropriate guide walings. This may be important when dimensions are critical. Curved walls can also be set out using this method with curved walings to suit.
11.3 Driving systems and types of hammer
The choice of a suitable driving system is of fundamental importance to ensure successful pile installation with due regard to the safety of operatives and environmental disturbance.
The three basic driving methods are:
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Installation of sheet piles
Chapter 11/6
Impact driving
This is the best method for driving piles into difficult ground or final driving of piles to level in panel form. With a correctly selected and sized hammer it is the most effective way of completing deep penetration into hard soils in most conditions.
The downside is that it can be noisy and not suitable for sensitive or restricted sites
Vibrodriving
This is usually the fastest and most economical method of pile installation but usually needs loose or cohesionless soil conditions for best results.Vibration and noise occurs but this can be kept to a minimum provided the right equipment is used and the site is not too sensitive
Pressing
Otherwise known as silent vibrationless hydraulicjacking.
Machines of various types are now widely used. This method is very effective in clay soils but less so in dense cohesionless ground unless pre-augering or jetting techniques are used.
This is the most effective method to use when installing sheet piles in sensitive locations where piling would have not been considered in the past.
11.3.1 Mixed driving methods
Specialist plant and equipment is now available that may combine methods such as pressing and vibrodriving by use of a telescopic leader rig fitted with a high frequency vibrodriver. The pressing, which in this case is carried out by lowering the mast of the rig using hydraulic rams, is known as ‘crowding’ and is normally limited to a force of 15 to 30 tonnes
11.3.2 Impact hammers
There are several types of impact hammer available to suit the particular requirements of a site. Most impact hammers will involve a piston or ram and an anvil block with a driving cap which spreads the blow to the pile head. The machines are usually supported by a heavy frame or chassis and normally need leg guides set up to fit snugly to the pile section being driven to maintain a vertical position during operation. Alternatively the hammers can be set up to be supported and aligned by a leader rig. It is very important that, because of the height and
slenderness of these types of hammer, the hammer is prevented from rocking or swaying when delivering powerful blows to the piles.
Installation of sheet piles
Chapter 11/7 The principle differences between hammers are the size and mechanism for delivering the blow from the ram. Some hammers deliver the blow freely under gravity, others are able to accelerate the fall of the ram and are described as double acting. In all cases the effectiveness of driving will depend on the power and efficiency of the blow.
Modern hammers are in widespread supply and, provided they are adequately maintained, can be expected to totally outperform the older types of pile hammer. Therefore the impact hammer types described below are those that are most commonly in use.
Descriptions and detail of older types such as diesel hammers can be found in previously published installation guides. Hydraulic hammers totally outperform diesel hammers in terms of efficiency, are more environmentally acceptable, and are less likely to damage the head of the pile when transmitting the driving force.
11.3.3 Transmitting the blow to the pile
Any pile section can be set up to be driven with a suitable impact hammer. However it is not only important to size the hammer correctly but it is imperative that the driving cap and / or anvil plate fits well and is correctly sized to suit the pile section being driven - especially on wide piles or pairs of piles. The hammers should not be used to drive piles of different widths without changing the fittings. The central axis of the ram should always align with the centre of the driven pile section in plan and the blow spread evenly over the full cross sectional area of the pile.
11.3.4 Refusal criteria – hard driving
It is crucial to set refusal criteria for hard driving with impact hammers. A penetration of 25 mm per 10 blows should be considered as the limit for the use of all impact hammers in accordance with the hammer manufacturer's recommendations.
Under certain circumstances a penetration of 1 mm per blow could be allowed for a few minutes. Longer periods of time at this blow rate will cause damage to the hammer and ancillary equipment and may also result in damage to the pile head.
11.3.5 Hydraulic single acting hammers
These hammers are suitable for driving pairs ofZ orU-piles in all ground conditions. They are usually too wide to fit on single piles.
As the hammers can be adapted with heavy block ram weights they are particularly suitable for prolonged driving into thick clay strata.
This type of hammer consists of a segmental ram guided by two external supports; the ram is lifted by hydraulic pressure to a pre-
Piling Handbook, 8th edition (revised 2008)
Installation of sheet piles
Chapter 11/8
set height and allowed to free-fall onto the anvil or driving cap.
The weight and the height of drop of the ram can be varied to suit the pile section and the site conditions
Ram weights are usually set up in 3, 5, 7 or 9 tonne modes although some up to 14 tonnes are available to suit driving ofHZ, high modulus and box pilesections. The drop height is variable up to approximately 1.2 metres. At maximum ram weight and stroke height a blow rate of 40 blows/minute can be achieved when used in automatic sequence.
For driving in stiff clays it is always preferable to use a heavy ram, with short stroke to minimise pile head damage and noise emission levels.
The hammer controls are precise, and used correctly this type of hammer can achieve 75-90% of rated output energy.
11.3.6 Hydraulic double-acting hammers
These hammers can be used on single or pairs of piles. They are particularly suited to drive U-piles or heavyZ piles with reinforced shoulders in hard driving situations and with rapid blow action can be used effectively to penetrate very dense sands gravels and rock This type of hammer consists of an enclosed ram which is lifted by hydraulic pressure. On the downward stroke, additional energy is delivered to the ram, producing acceleration above that from gravity alone and powerful blows to strike the anvil or driving cap
which is purpose built to fit the pile section.
When set up for use with sheet piles, these hammers will deliver a maximum energy/blow of 15 kNm to 90kNm with a blow rate up to 150 per minute. The electronic control system ensures optimum control of the piling process.
The ram weight of the machines suitable for standard sheet pile sections range from 1.2t to 6.5t
Bigger machines are available for driving large non- standard pile sections andHZ piles for high modulus systems and offshore projects. The total weight of the hammer ranges from approximately 2.5t to 20t.
The machines are usually rope suspended from a crane and because even the lighter machines are very powerful, effective driving systems are available at significant reach using large crawler cranes.
Under normal site conditions it is usual to select a ram weight that is in the range 0.75 to 2 times the weight of the pile plus the driving cap.
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11.3.7Control and settings
These hammers can usually be operated on different settings to suit the pile and ground conditions. For instance a heavy ram weight ratio hammer on wide piles can be used with a low setting to suit driving in clay and smaller hammers on a rapid blow setting can be used to drive single piles in dense sandy soils. Equipment to provide digital readout of energy and blow count, for driving records and control, is available to be fitted to most machines.
11.3.8 Impact hammers and driving stresses
The driving stresses in the pile, when using impact hammers, are likely to be greatest at the head of the pile. This is known as the peak head stress value (σp). The mean driving stress (σm) is estimated by dividing the driving resistance (soil resistance + friction) by the cross-sectional area of the pile.
The peak driving stress can be estimated using the following formula:-
σp=σm.[( ) -1]
whereξrepresents the efficiency of the blow from the piling hammer (e.g. 1 = 100% efficiency, 0.75 = 75% efficiency, etc).
Where the impact hammer has a low efficiency (for instance, diesel hammers may rate at 30%-40% efficiency) then the yield stress of the steel section may be exceeded by the peak driving stress causing buckling at the pile head.
Also note that for highly efficient hydraulic hammers which usually operate at 85 to 95% efficiency, the hammer energy may be transmitted effectively to the toe of the pile. It is therefore important that the pile continues to penetrate the ground when driving for a sustained period because toe damage can occur when the penetration rate is low or refusal sets are exceeded.
11.3.9 Vibratory pile drivers
These hammers are usually the quickest and most effective equipment for driving piles in loose to medium dense cohesionless soils. They are particularly useful for extracting piles or withdrawing the pile being driven in order to take corrective action.
11.3.10Mechanism and use
Vibratory driving works by reducing the friction between the pile and thesoil. The vibrations imparted to the pile temporarily disturb the surrounding soil causing minor liquefaction, which results in a