1 Introduction
[1] Steel pipe piles are mostly used for marine works; jetties quay walls etcetera. On land prefabricated concrete piles or in-situ concrete piles are more popular. For steel pipe piles, the different types are as follows;
- Vertical piles, sometimes to resist bending forces [specially popular in earth quake zones]
- Raker piles, in sets of 2, resisting longitudinal forces only, but as a set to resist horizontal forces
- Predrilled piles if hard strata have to be penetrated
- Piles with rock sockets, mostly to increase resistance to tension in dolphin piles.
Note; Large diameter bore piles through steel casing, mostly used in bridge foundations.
[2] Steel pipe are normally fabricated off site in cans with lengths of 12 m1 or 18 m1 [coated or uncoated] and most times they have to be transported over sea to reach their destination. Sometimes, if fabrication can be done in the area, piles may be spliced to larger or final length and brought directly to the [floating] piling rig by barges. In this case coating can be done at the fabrication shop.
Note: Coating of piles, only up to 3 m below seabed
[3] Normally the cans are coated first and then welded to their final length on site, and the piles transported to the [temporary] loading quay and loaded by crane on a transport barge. This will bring the piles in batches of “say” 6 pieces to the [floating] piling rig.
A cheaper method, however, is to transport floating by welding plates or using pneumatic plugs at the ends.
[4] At the piling rig the pile is lifted from the barge, brought in a vertical or raker position and driven into the seabed / river bed with a suitable method and piling hammer.
Note: Throughout the process there is chance on coating damage, therefore handling methods should get proper attention in the work preparation process; nylon or polypropylene slings and nylon rollers on the pile guides should be used, where possible.
[5] After the pile is installed and super structure is finished, pile touch-up will have to take place and cathode protection system, if required, will be installed. Sometimes special protection jackets will be required to protect the splash zone against corrosion.
2 Corrosion
[1] General
Main problem of steel in marine [seawater] conditions is corrosion, when in contact with oxygen.
This corrosion will concentrate in the so called “splash zone”
To overcome the corrosion problem following measures are normally taken
- allow for [say 3 mm] corrosion allowance on the wallthickness [wt]
- Coating of the piles, but coating damages can never be avoided
- Cathodic protection of the piles, sacrificial or impressed current, but this only works below the low water level.
- Install an extra protection collar in the splash zone area by bituminous wrapping or collars in steel, UV resistant plastics or oyher materials and grouting the ‘annulus’
[2] Considerations
Often specifications double up on the requirements and for example corrosion allowance is combined with a splash zone protection collar.
Further it is useless to specify an expensive coating system in combination with a cathodic protection system. The CP systems takes normally a damage factor of 20%
The splash zone is the most critical area with regard to corrosion; corrosion is less severe in area’s constantly under seawater level.
Sometimes it makes sense to increase the corrosion allowance in the splash zone only
The corrosion below the mud line is minimal; normally there is no need to coat the part of the pile below sea bed, although some consultants specify it.
The most common coating is a preparation by abrasive blast to “near white’ metal [SA 21/2 or SSP 5] and apply 400 micron d.f.t. coal tar epoxy in 2 layers
[3] Corrosion in seawater [BS 8004 : 1986 - section 10]
Above BS gives following corrosion rates for each exposed face:
| Position | Corrosion rate | Remarks |
| Splash zone Tidal Zone Permanently under water
Below sea bed |
0.10 to 0.15 mm per year 0.08 mm per year 0.08 mm per year
0.01 to 0.02 mm per year |
Depends on exposure [calm sea to rough sea]Cannot be protected by CP If not protected by CP
Ditto |
3 Steel pipe pile supply
3.1 General
When buying piles through our PL department, following is important:
- Fabrication method; spirally welded or longitudinal welded
- Dimensions; diameter and wall thickness
- Length of individual pipe pieces [cans], mostly 12 meter each.
- Total quantity, spare quantities to be included
- Steel quality [ minimum yield & chemical composition]
- Specification, test requirements and mill certificates
- Coating requirements [uncoated, primed or fully coated]
- Pipe ends bevelled or straight
- Delivery time or schedule
- Place of delivery [Ex works, FOB or C&F are most common]]
- Payment condition, preferable LC or 30 days after delivery of goods and certificates, whichever is later.
3.2 Fabrication method
[1] Longitudinally welded pipe piles are most common; they are bent to shape on special rollers in cans of 3 to 6m1, first welded longitudinally and then automatically but welded at the mill. Normally they have a delivery time [ex mill] of 3 months.
[2] Spirally welded piles have shorter delivery times. Some Clients object the use of this type of piles, based on prejudice [old time fabrication methods of the past did not give proper alignment on the welds].
There is a limit to the wall thickness diameter relation to fabricate these piles. This depends on the type of equipment in the mill, the thicker the wall the more difficult to bend the pipe.
Generally the upper limit is on 19 mm [3/4"] for a pile diameter over 30″ [762 mm]
3.3 Pipe dimensions
[1] Pile dimension are given in “Outside diameter [OD]” and “Wall Thickness [WT], mostly based on inches, as follows:
[2] Pile dimensions as most common for pipe piles:
| OD | WT | Weight per m1 | Paint surface | WT / OD relation |
| 24″ 30″
36″ 42″ 48″ |
3/8″ 1/2″ 1/2″
5/8″ 3/4″ 5/8″ 3/4″ 1″ 5/8″ ¾” 1″ ¾” 1″ 1 1/4″ |
143 kg191 kg 239 kg
300 kg 358 kg 356 kg 427 kg 572 kg 398 kg 477 kg 636 kg 572 kg 763 kg 954 kg |
2.00 m2 p m1 2.40 m2 p m1
2.80 m2 p m1 3.20 m2 p m1 3.80 m2 p m1 |
1.5 %2.1 % 1.6 %2.1 %2.4 %
1.7 % 2.1 % 2.7 % 1.6 % 1.8 % 2.5 % 1.6 % 2.1 % 2.6 % |
[3] WT / OD relation as per above varies from 1.5 % [normal driving] , 2.0 % to 2.5 % [very heavy driving].
Also vertical piles receiving vertical [bending] loads have normally a WT / OD relation above 2%.
Normally pipe piles with WT/OD relation of less then 1% can be used for [temporary] casings for [vertical] large bore piles in soft soils and use of piling collars [strengthening of the pile toe] are to be considered then. This will be necessary if piles are driven in stiffer under layers and will avoid deformation of the pile bottom. If the pile bottom will be deformed to a say oval shape, the drill cannot pass any more. This also applies, of course, for [raker] pipe piles with rock sockets
3.4 Steel qualities and steel grades
[1]The basis for the steel quality [steel grade] in all international specifications for pipe piles is the minimum yield point, which hasa relation to Min Tensile strength and elongation
[2]Following are most common steel grades:
| Min Yield pointN/mm2[MPa] | Min Tensile Strength N/mm2 [MPa] | Min Elongation% | BritishBS 4360 | AmericanASTM A 572 | EuronormEN 10248 |
| 270 360 420 | 420 490 520 | 25 22 19 | Gr 43A Gr 50A NA | Gr 40 or X 40 Gr 50 or X 50 Gr 60 or X 60 | S270GP S355GP S430GP |
[3] Steel grades with a higher quality, will give problems on welding, need for preheating etc
[4] Sometimes the steel copper contents is increased to improve the corrosion resistance of steel
[5] Sometimes it is necessary to check the lamination of the steel plates at the mill.
3.5 Factory testing/ mill certificates/
[1] The pipe fabricators, will get their steel plates from steel mills, who will guarantee the quality of their plate [mechanical & chemical properties, plate thickness and lamination] through mill certificates.
[2] The fitting and welding process can be checked by 10% to 100% ultra sonic testing [cheapest and most common] or 10% to 100 % X-ray [expensive]. These are all ‘Non Destructive’ tests
[3] The dimensional tolerances [circular shape and straightness] will be checked as well to confirm to applicable specification of the project.
[4] At the end, the final supplier, should provide an affidavit of compliance, backed up with all test results available and as agreed in the purchase order.
[5] All above needs to be clear before a purchase order can be concluded; obtaining the required quality certificates, without prior agreement will be difficult and troublesome.
It is essential that this is left to our professional buyers, even if we intend to buy local.
3.6 Transportation to site
[1] If the piles are bought on “Ex works loaded on our transport” bases, we have to collect the pipes [the supplier will load the piles] and bring them to site or harbour for onward transport, all at our costs.
Sometimes it can be advantageous to use inland water transport, because when the pipes are large in diameter, you cannot load much piles on a trailer .
[2] FOB supplies [Free On Board] means that the supplier will deliver the pipes in the ship [included loading] on a place,date and ship nominated by us.
The alternative is FAS [Free Alongside Ship], in that case we have to pay the loading costs.
The onwards cost for sea freight are for our [the buyer] account.
[3] For C+F deliveries [Cost & Freight], the supplier will deliver them in the Harbour designated by us on or before the date as agreed in the purchase order. We[the buyer] are to arrange and pay the unloading, harbour fees, clearing costs [clearing agent], import duties [if applicable] and onward transport to site.
[4] Following documents amongst others[?] will [sometimes] be required to arrange importation of goods;
- Bill of Lading by shipping company
- Customs invoice
- Packing list
- Certificate of Origin
[5] If goods are to be imported duty free, it is common that the goods are consigned to the Owner or Client, since they are the party possessing the Custom Duty Exemption. Sometimes the duty free good have to appear on a so called “Master list” , on which the items to be imported are listed. This “Master list” needs approval by the government authorities, before it becomes effective. This sometimes takes a long time and preparation of this “Master list” has to be done very early in the project.
[6] Once custom authorities have cleared the goods, they can be transported to site. All official documents pertaining the importation, must be carefully filed for future reference.
The transportation of the pipe piles is preferable by barges, unless the local road net work is well established
[7] The cost for sea freight on volume is always calculated as square volume of the pipes. For a 0.914 m diameter pipe the volume will be 0.914 x 0.914 = 0 .83 m3 per meter .
For sea freight cost calculation, the transporters will look at the higher figure, volume or weight.
If above pile is heavier then 0.83 ton p m the weight has to be paid.
If the weight is below 0.83 per ton, the volume has to be paid.
This principal is known as ” freight-ton”
4 On site Preparation of piles
4.1 General
[1] The most common method is that the 12 or 18 m1 steel cans arrive on site uncoated.
These cans will then be coated , welded to their final length and welds tested, welds then to be coated as well and the piles transported to the [temporary] quay, loaded on the barge and transported to the piling barge.
[2] Sometimes it can be decided that the cans will be fully coated at source, but this will lead to lots of transport damage, double handling and will often delay the delivery of the cans with 1 to 2 months. Repairing coal tar epoxy coating is not easy and expensive
It can be considered to only apply a holding primer at source and do the final coat at site, reducing the degree of repair. The other disadvantages, however will remain.
[3] Complete pile sections often have a weight of over 20 tons, requiring large equipment for handling and transporting. It is advisable to locate the pile preparation yard close to the water front, allowing the piles to be rolled into the water and by bringing them to the piling barge ‘afloat’ [after closing the pile ends of course]
[4] If not transported afloat, the pile transportation should be done by pipe dollies, but the crane loading the piles on the transport barge will often be larger then 100 tons, due to the required reach of the crane.
It can be considered to transport the pile in half pieces and do the final weld [and coat] on that the loading quay
[5] Gantry cranes are very useful tools on the pile preparation yard, omitting the need for heavy and more expensive crawler cranes
4.2 Coating of piles
[1] In normal cases the coating of piles are a 400 micron coal tar epoxy on a surface preparation by abrasive blast to “near white ‘ metal. More expensive coating system are waist of money, and cathodic protection systems will take care of eventual painting damage, which are unavoidable. The additional splash zone protection is also a clear requirement then, but not always specified. Argument could be that the Client will constantly repair the coating in the splash zone, but this is not always feasible in case of moderate swell, while it is difficult to control.
[2] Abrasive blasting, to remove rust and mill scale, can be done in following grades:
| Description | Swedish Standard | USA Standard | Remarks |
| Commercial blast Near white metal White metal | SA 2 SA 21/2 SA 3 | XXX SSPC 10 SSPC 5 | Unsuitable for marine conditions Normal practice Almost impossible to achieveNeeds [air] conditioned space. |
[3] Another requirement is the roughness of the preparation, mostly requiring a minimum 50 micron profile. When applying the coating, surface temperature should not be more then 60 Degrees C. The need for shading and or protection against rain and dust is obviously a must
Normally the surface preparation requirements are stated in paint suppliers technical leaflets. Amercoat 78 HB [Ameron] is an often used type of paint, although all major paint suppliers can supply coal tar epoxy.
[4] For abrasive blasting operations a 15 m3 / min [600 cfm] compressor for 2 nozzle is required, together with blasting pots, blasting gun and protective clothing for the operator. As mentioned, the blasting area should be fully enclosed, to avoid large amounts of dust to spread over the site. This will allow to blast and coat approx 200 m2 per 10 hr day.
A crane, crawler or gantry will be required to handle the cans. Small gauge rail is necessary to bring the cans inside and out of the blasting shed.
Blasting material can be blasting grit [30 kg/m2 / once recycled] or steel shot, which can be recycled at least 5 times.
[5] Coal tar epoxy is a 2 component paint, which can be applied in one 400 micron layer [Dry Film Thickness]. It is however recommended to apply a holding primer first [in the blasting shed] quickly to avoid the blasted steel to corrode again. Specially in tropical climates, the steel will corrode very fast.
By spraying the protective coat in 2 layers there will be more control on over spraying. Although theoretical coverage is approx 2 m2 per litre [dft 400 micron], 1.5 m2 per litre is a more practical figure. For thinner, incl cleaning of tools, allow 0.20 liter per m2.
Two different colours are normally used, brown for 1st layer and black for 2nd layer
4.3 Welding of piles
[1] Two splicing systems are considered:
- Conventional system, by lining up cans on steel beams, fitting the cans together and welding the pile manually by welding rod and [400A]diesel welder
- Automatic welding using roller system and automatic welding columns; so called SAW or Submerged Arc Welding.
[2] Conventional manual welding is time consuming [1 to 2 man days per weld or even more], and requires a good skill [Class 6G] from the welder. Specially in under developed regions it can be a problem to find good welders. Subsequently more time could be required to repair and re-inspect the welds
This only to be considered for small quantities, since set up costs are relatively low.
[3] Automatic welding is preferable for big quantities although set up costs [investment] can be high. It will take 2 to 3 hrs to make a proper weld, incl fitting and the chance on defects is minimum.
[4] Testing of welds [from cheap to expensive]can be done as follows
- Visual inspection, with experience, improper welds can be detected easily. Further the so called “Hi-Lo” and misalignment are being done visually.
- Dy-penetration tests, by spraying the weld with red dy [ink] and consequently chalk powder over the weld, so cracks will show.
- Ultra sonic [100%], but it depends again on the skill of the inspector, since there is no proper record
- X ray [10% or 100%], making photo films over the full depth of the weld. All defects will be shown and the photo’s can remain on file.
- During X -ray the work has to stop and the welding yard vacated; therefore this should be done in evenings.
4.4 Site Transport and Handling
[1] As mentioned try to locate the pile welding yard close to the waterfront or loading quay, thus reducing handling and transportation costs.
[2] If the same crane can do the pile handling at the fabrication yard as well as loading the transport barge, a considerable saving can be made. In combination with small gauge rail ["small spoor"] extra savings are possible.
[3] If we work with gantry cranes for pile handling the gantry cranes can extend over the transport barge [Excavated basin] or piled structure] for immediate loading.
Also tower cranes can be considered for smaller diameter piles.
[4] If piling yard is on distance, use dollies to transport the piles. Consider to do one weld at the loading quay then.
[5] A transport barge, loaded with a daily production, and tugboat will bring the piles to the piling barge.
[6] Cheapest method is fabrication along waterfront and rolling the completed piles into the water, after closing both ends. This can be by pneumatic seals or welding steel plates, but removing these can be time consuming.
4.6 Temporary Quay
[1] When no existing infrastructure is available, one has to construct a temporary quay at a suitable place, protected from the swell and with a non shallow [approx 4 m1 deep] water front [if available].
[2] This can consist of a [backfilled] retaining wall of sand/ rubble filled scrap containers [cheap], sheet piles or a steel structure on steel pipe piles, with a short approach trestle [expensive].
[3] When we start working on a ‘virgin’ beach the difficulty is the shallow water, requiring dredging of [narrow] approach channels and or large quantities of fill. Possible erosion and siltation has to be taken into account.
[4] In case of jetties in a ‘design & construct’ case, it is possible to introduce a rubble mound causeway in the design and incorporate a temporary quay in it.
5 Driving of Piles
5.1 General
[1]Pile driving from a crane barge or a jack up barge, depending on sea conditions. The expected sea behaviour therefore should be known before hand;
Also the barge dimension is governed by the expected waves and related down time as well as the crane size required to handle the pile and/ or hammer
[2]The pile dimensions and required penetration will generally follow from the detailed design, although most times the pile diameter will already be chosen in the basic [tender] design in case of design and construct projects.
[3]In combination with available soil data, a piling hammer will be selected and consequently the required crane lifting capacity will follow.
Specially on raker piles, this requires a proper assessment and it will also depend on the envisaged work method [fixed leader or flying leader with strong template].
[4] If we do a design+ construct project, we can adopt the design to the floating equipment, which we expect to have available in the area at the envisaged award date. For large quantities it serves to have higher mobilization costs in order to achieve a better productivity.
5.2 Soil and pile predictions
[1] Without soil information we can not design a pile neither can we predict the piling performance, The soil information will mostly be determinedthrough SPT borings and [Dutch] Cone Penetration Tests [CPT's]
[2] Making pile driving predictions is work for specialists. Basically these pile driving analyses predict the blow counts on various penetration levels and compression [pressure] in the pile during pile driving.
[3] Sometimes [often] designers give pile dimensions and required penetration levels, without taking into account the drivability of the pile. Imagine a theoretical pile size to be driven down with a heavy hammer, where by the yield stresses occurring into the pile is higher then the allowable yield stress.
[4] It is important to have the drivability checked; As a rule of thumb, piles can be driven up to SPT’s of 60 to 70; if above 70, pre drilling will be required .
5.3 Piling Hammers
[1] In time piling hammers have developed from falling weights, steam hammers, air hammers, diesel hammers into hydraulic hammers/ vibrators.
Hammer capacities is defined by the energy per blow in kNm or KGM, considerable amounts of energy are lost due to helmet, adaptor and pile.
Pile predictions make assumptions for these energy losses.
[2] Diesel hammers work on the principal of diesel engines. The fuel [diesel] explodes under compression and the rated energy doubles in theory, according manufacturers. This is not true and adjustments have to be made to 70 % of what the supplier states.
The most popular piling hammers are the Delmag hammers, the type indicating the piston weight. For a D100 the piston has a weight of 10 tons.
They can be adjusted by reducing fuel supply. The hammer will work on maximum energy at 42 blows per minute. The hammer starts by lifting the hammer with a secondary lifting wire and letting it fal a few times; it needs to warm up. Also pistons with hydraulic starting devices exist
[3] Hydraulically operated hammers have an increased impact because the hammer is lifted and pushed down hydraulically. Therefore the energy is fully adjustable and can be registered as well. By electing the proper helmet, the efficiency is much better than diesel hammers, while the weight will be much less. These hammers always have separate power packs, consisting of a hydraulic pump and electrical or diesel operated engine.
They can also be used for pile extraction and under water and do not lose capacity if on raker piles
They are very expensive, compared to diesel hammers, but have double efficiency at similar ram weights/ hammer weights and cause less noise.
[4] Steam hammers are a bit out dated, owns a Menck MRBS 3,000. The rated energy per blow of 45,000 kgm , but due to the construction of the helmet and the adaptor, the weight is high [120 tons] and the energy loss is high.
Further a steam boiler is required I s o a power pack, although it can also operate on air [compressors]
[5] Air hammers [BSP or Atlas Copco] are mostly used for low capacity application, such as straight web sheet piles or pipe piles up to 12″diameter in soft soils.
[6] Pile vibrators, hydraulically or electrically operated are available in different capacities. They can be used for initial pile pitching and for, of course, sheet piles. The can not penetrate hard layers [sediment stone]
[7] Blow count is the measurement of the number of blows per 250 mm of pile penetration. A normal blow count is in the order of 50. Blow counts for final set over 100 should be avoided, since the hammer [diesel] will overheat and get damaged.
[8] Pile helmets transfer the piston impact to the pile, these are more important for concrete piles [easier to damage] then for steel piles. They absorb some of the peak impacts to a longer lasting and lower impact [poly-penco discs] , to avoid yielding of the pile head. Helmets can also function as adaptors, to suit different pile diameters.
5.4 Fixed Leader
[1] A fixed leader means that the piling leader and indirectly the hammer are fixed to the piling crane or a sheer leg [not often used] forming one unit.
[2] Fixed leaders can have a length from say 20 m1 to say 60 m1and the leader must be stiff enough to guide the pile and support the hammer.
This automatically implies the need for a relatively heavy crane with 3 hoisting drums, hydraulic sliding table, specially when raker piles have to be driven
[3] The method is extremely fast on [near] vertical piles, up to 10 piles per [10 hrs] day
There is a capacity limitation on raker piles over 1: 4 rake, while speed reduces with the increase of the piling angle.
Normally in case of raker piles, production will be 3 to 5 piles per [10 hrs] day
5.5 Flying leader + Template
[1] In case of a flying leader, the piling crane [or sheer leg] first places the pile in the template and there after the [flying] leader/ hammer. For vertical piles off shore cages can be used; these are shorter and easier to place on the pile.
[2] Disadvantage is the lack of speed, due to the need for 2 crane movements. Also the flying leader has to be placed over the pile tip, requiring a long boom. Also lifting long piles in the template can be problematic and requiring long booms, unless the template can open at one side.
[3] Flying leader with fixed [to barge] template is a system where by in case of rakers the swell should be close to “zero”. The template otherwise starts to ‘hang’ on the pile by the barge movement. This swell limitation is less important for vertical piles
It is slower due to separately lifting pile and hammer but ideal for smaller quantities.
Production will be averaging 2 in case of raked piles and 3 to 4 in case of vertical piles per day.
[4] Flying leader in free standing template; by pitching 2 or 3 piles at a time, process can be speeded up, compared to the above. Handling the template and driving the temporary support pile will lead to time losses again
This system is less vulnerable to swell .
Note: For pile driving a 6 point mooring system is required, unless working with a free standing template, but still recommended
5.6 Pile head preparation
[1] After driving the pile to the required penetration or achieving the required blow count [more normal], the pile head has to be cut to the required level and the connection to the capping beam[ concrete or steel] should be prepared.
In deeper water temporary pile bracing will be required
[2] To prepare the pile head a scaffold has to be fixed to preferable a pair of piles. Gas cutting and/or welding equipment [diesel welder] can be placed on the platform , in order to do the pile head preparation without assistance of the crane barge. Alternatively, in calm weather a small barge eventually with a small crane can be used.
[3] In case of concrete plugs, shear rings or weld studs are placed inside the pile, as well as the bottom shutters for the concrete plug and the rebar cage.
[4] Also temporary supports for an eventual in situ concrete connection can then be welded to the pile.
5.7 Cathodic Protection [C P ]
[1] Cathodic protection systems only work below the [sea] water level. For the splash zone additional protection will be required. Corrosion is an electrical process, which occurs when oxygen is available in the seawater or open air
[2] The most common method is the impressed current system. This means that a piece of metal [mostly aluminium, sometimes zinc] is welded to the piles in such a way that electrical current will pass through the connection [mostly welded]
[3] Since aluminium is a less ‘noble’ metal then steel the sacrificial metal will start corroding instead of the steel. Normally the required life time is between 20 to 30 years, requiring 200 to 300 kg of aluminium on an average 30′ pile [10 kg p year]
[4] The impressed current system is a bit more complicated;. Through a system of transformers, rectifiers and anodes, the corrosion electrical current is reversed to go into the pile thus omitting any corrosion process. Although the investment is normally less, the maintenance and continuous attention for the system will be more expensive on the long run
5.8 Splash zone protection and touch up
[1] As already mentioned the CP system will not work above water, while in the spash zone most paint damage will occur. On top of that the corrosion will be dramatically more severe than in other underwater area’s.
[2] Normally one has to assume that their will be a final touch up before substantial completion and the Client will maintain afterwards. This sounds OK , but in realty it will not be done.
[3] A proper splash zone protection is by multilayer [bituminous] wraps around the piles. ‘Retrowrap’ and ‘Denso sea shield’ are trade marks for systems, which are often used .
[4] Also an oversized thin steel casing could be used, where by the annulus is grouted.
Both systems will need diving support, which is problematic in heavy swells.
5.9 Jackets
[1] Jackets are mostly used for piled structures getting considerable horizontal loads, such as Riser Structures, Breasting and Mooring Dolphins.
Basically they consist of a 3 or 4 pipe sleeve structure, stiffened by bracings. They can be standing on the sea bottom [long jackets], or freestanding [short jackets. Piles are driven through the oversized sleeves, mostly slightly raked to max 1:5.
[2] The jacket system is an expensive solution, requiring extremely heavy equipment [mostly sheer legs] to position the jacket, while there is relatively a lot of extra steel required compared to normal piled structures.
5.10 Pre-drilling
[1] In for example the Gulf area on many locations cap rock layers exist on top of the softer sand layers. To penetrate these layers pre-drilling has to take place through an oversized temporary casing incl pile shoes. This can be by Kelly bar equipment in case of [almost] vertical piles, but in case of raker piles, reverse circulation drilling [such as Wirth] is preferred.
[2] In case of intermediate hard layers, the pile will be driven to a reasonable blow count [say 15 blows per 30 cm] and there after predrilled by RCD drilling equipment. Some specification limit the pre-drilling below the pile shoe to say 6 meter. There after, having removed the “plug” and internal soil, the pile driving can resume with less resistance.
[3] If there is a chance on pre drilling being necessary, the flying leader method is mandatory.
When working in shallow water , the pile could protrude above the upper template so much that there is a stability problem for raker piles, when the top mounted RCD drilling rig is installed. It might be necessary to apply field splices then.
5.11 Socket drilling / tension anchors
[1] In some cases, the overburden is so thin that for vertical bearing piles, the toe stability is insufficient and must be fixed to the underlying rock
In other cases, the raker piles are loaded on tension [Breasting or Mooring Dolphins] and the overburden is insufficient to achieve sufficient anchorage
In the above situations the piles must be anchored to the underlying rock by rock anchors or tension anchors.
[2] The most common method is to drill [reverse circulation]an undersized the hole underneath the pile toe, place a rebar cage and fill [tremie] the pile bottom with concrete. In this case shear rings are in the inner pile toe will be required. On the other hand it can be obvious to have a pile strengthening collar at the bottom of the pile and this will act as a shear ring. As an alternative a steel beam can be used i.s.o a rebar cage
[3] Another option for tension piles is a method where one or a few smaller 100 to 150 mm holes are drilled in the rock by a DTH hammer. By inserting pre-tensioning strands and stressing the cables a tension anchor is formed. This method can be done at a moment that the dolphin is completely or partly concreted, thus saving time on the total schedule.
6 Pile testing on driven piles
[1] Piles can be designed on:
- Compression – most common, vertical piles and raker piles
- Tension – for example in breasting and mooring dolphins, mostly raker piles
- Bending – mostly vertical piles only
Factor of safety will be between 2 to 3 [BS 8004], depending on the confidence of the designer of the available subsoil date. In practice this means that it is mostly between 2.5 and 3
[2] The Safe Working Load [SWL] is the allowable load, used in the design. The Ultimate Load or Capacity for compression piles is described as the force requiring the pile toe to settle 10% of it’s diameter.
For a standard 36″ pipe pile a SWL can be in the order of 300 tons giving in the worst case a required UL capacity of 900 tons.
[3] Specifications can require following to demonstrate the suitability of the pile;
- Static load test, using concrete ballast and/ or a combination of tension pile
- Calculation by dynamic pile formulae and wave equation analyses.
- Horizontal load tests, for vertical piles on bending
- Load tension tests, measuring skin friction only.
Note: Never accept a specification where raker piles should be tested, except for[ may be] tension.
[4] Static Load Tests
The most common test, by creating a cant ledge structure, ballasted by concrete blocks and pushing the pile down by hydraulic jack. Sometimes tension piles can be used to reduce the required ballast. This static load test can be up to ultimate capacity, or to a load with a reduced safety factor.
The problem in our type of work is that these tests are time consuming, mostly by the time to discuss and agree the test load results. The expensive piling equipment is idle then. Also the mobilization or fabrication of say 400 m3 of concrete blocks in isolated regions and the need for temporary support piles, is costly.
[5] Dynamic pile formula’s in the old days, were based on blow counts only. There was an unknown factor of energy loss in hammers and driving caps. Modern systems [PDA pile driving analyse] now can measure the forces and acceleration resulting from the blows on top of the pile, making this method more reliable. However consultants still require these measurements to be correlated to a real static load test
[6] A relatively new system is the “Statnamic” pile test. The principal is a ballast weight and the creation of an explosion on top of the pile. Readings of downward force, pile head movement will give immediate results of expected ultimate and safe working load. It avoids lengthy processes of loading and re- loading and consequent discussions on the interpretation of the result. The method still requires international recognition and inclusion in International standards.
[7] Horizontal pile tests can be very simple, by pressing 2 adjacent vertical piles away from each other by hydraulic jack. Also design loads will generally be low. By measuring load and displacement, conclusions can be made.
[8] Tension tests also are more simple, needing 4 support piles, a steel frame, pulling head and a hydraulic jack to pull up the pile.
[9] The so called ‘Osterberg‘ cell measures loads in the bottom of the pile, but is only to be used for cast in situ bore piles.
Maaf nih, tablenya agak berantakan so tunggu update terbaru atau edit sendiri ya…