Pile foundations are used to transfer the loads from superstructures to deeper, more stable layers of soil or rock. The durability and protection of pile foundations are crucial to ensure their long-term performance and prevent potential failure.
1) Durability and protection of timber piles
- Timber piles permanently below ground water level have an indefinite life.
- There are numerous examples of timber piles that are more than 2000 yrs old being found in excavations below the water level.
- While timber does not decay from fungal attack if the moisture content is kept below 20%.
- It is impossible to maintain it in this dry condition when buried in the ground above water level.
- Precaution against fungal attack must be commenced at the time the timber is felled by properly stacking at proper place.
- Suitable methods of preserving timber for piling work involve pressure impregnation with creosote or a solution of copper sulphate and potassium dichromate (celcure).
- The adoption of preservations treatment does not give indefinite life above ground level, it may be preferable to adopt a composite pile having a concrete upper section.
- For timber piles in river and marine structure, the most destructive agency which can occur in piles immersed in brackish or saline water is attack by Molluscan or crustacean bores.
- It may result into complete destruction of the piles.
Following methods of protecting timber piles against attack by bores may be adopted.
- Tipping stone around the piles (this protect only the length covered by the stores).
- Sleeving the timber with galvanized iron, copper of aluminum sheeting. Encasing the piles, jacketing the piles with pre cast concrete tubes and filling the space between the timber and the tubes with cement grout.
- Coating the piles with cement sand mortar, applied with a spray gun (eg. the ‘Shotcrete’ or ‘Gunite’ process).
2) Durability and protection of Concrete piles
- Properly mixed concrete compacted to a dence impermeable mass is one of the most permanent of all constructional material and give little cause of concern about its long-term durability in a non aggressive environment.
- However concrete can be attacked by sulphate and sulfuric acid occurring naturally in soils, by corrosive chemicals which may be present in industrial waste in fill materials and by organic acids and carbon dioxide present in ground water as a result of decaying vegetable matters.
- Attack by sulphates is a disruptive process whereas the action of organic acids or dissolved carbon dioxide is one of leaching.
- Attack by sulphuric acid combines features of both process.
- The severity of attack by soluble sulphates must be assessed by determining the soluble sulphate content and the proportions of the various cataions present in an aqueous extract of the soil.
- These determinations must be made in all cases where the concentration of sulphate in a soil sample exceeds 0.5%.
3) Durability and protection of Steel piles
- Corrosion of iron or steel in the electrolyte provided by water or moist soil is an electro-chemical phenomenon in which some areas of the metal surface act as anodes and other areas act as cathods.
- Pitting occurs in anodic areas, with rust as the corrosion product in cathodic areas.
- Air and water are normally essential to sustain corrosion but bacterial corrosion can take place in the absence of oxygen i.e. in anaerobic conditions.
- Anoerobic corrosion is caused by the action of sulphate – reducing bacteria which thrive below the sea or river bed in polluted waters, particularly in relatively impermeable silts and clays.
- Where steel piles are buried in fill or disturbed natural soil, the thickness of metal in a bearing pile should be such that the steel section should not over stressed due to wastage of the metal by corrosion over the period of the useful life of the structure.
- Maximum rates of corrosion may be taken as 0.08mm / year.
- In contaminated ground where corrosion of steel may be higher than normal, some protection over the length of pile above and a depth of 0.6m below the water table can be given by two coats of coal-tar-pitch paint applied cold to a metal surface.
- Other protective measures in contaminated disturbed ground includes jacketting the pile with concrete or filling the shafts of hollow piles with concrete capable of carrying the full load, where water table is shallow the pile can be extended down to a depth of 0.6m below water level to protect the steel of the pills. .
i) Steel piles for marine structures
- Steel piles supporting jetties, offshore platforms, and other river or marine structures must be considered for protection against corrosion in five separate zones. These are as follows
4) Methods of Contract
- Contract procedure – Basic types of contractual arrangement under which piling may be undertaken.
- Railway is responsible for deciding the type or alternative type of pile, the working loads, and the allowable settlement under test load.
- Railway specifies the material to be used, the working stresses, fabrication methods and penetration depths.
- Tenders are invited on the basis of a detailed specification and drawings, accompanied by a site investigation report, and a site plan showing existing surface levels, proposed regrading levels, and the operating levels for the piling rigs.
- Railway invites tenders for one or an alternative system of piling from specialist contractors.
- The invitation to tender is accompanied by a pile layout showing individual pile loads or column and wall loadings, and by a detailed specification including such items as materials, working stresses, performance under load test and other criteria of acceptability.
- The contractor decides on the required type (or alternative type) of pile, the diameter and the penetration depth for the specified working loads, and bases his tender on his own estimates of performance.
- Site information as described for Method 1 should also be supplied.
- Has the advantage that the responsibility of each party is clearly defined.
- The Contractor has the responsibility only of selecting the most efficient type of plant to do the job and to install the piles in a sound manner complying with the specification.
- The method has the disadvantage that the knowledge and experience of the Contractor may not be full utilized, since Railway may not always select the most suitable pile for the job.
- In exercising his responsibility or deciding on the pile diameters and penetration depths, Railway may instruct the Contractor to install preliminary test piles before making final decisions on the dimensions of the working piles.
- Railway supplies a drawing to the tendering Contractor showing the wall and column layout of the structure together with the loadings; the site information as described for Method 1 is also supplied.
- No specification is issued and the Contractor is expected to submit a brief specification with his tender, and to guarantee the successful performance of the piles.
- Provides the widest choice of piling systems and utilizes the experience of the Contractor to the fullest extent, but greater care is needed in defining responsibility.
- In particular, Railway must specify precisely his requirements for performance under loading tests, both on preliminary and working piles.
- While the Contractor is responsible for selecting the type, diameter and penetration depth of the piles, he should be requested to submit his calculations for these selections for the approval of the Railway.
- The statement concerning working loads on columns, walls or individual piles should make it clear as to whether or not the loads have been factored in compliance with the Codal Provisions.
- Method is unsatisfactory in most respects.
- It is usually stated in the tender invitation or it is implied that the Contractor assumes responsibility for all aspects of the work.
- The Contractor must decide whether or not load testing is required and the criteria for successful performance under test.
- The method can work satisfactorily if Railway invites tenders only from those firms who have the necessary experience, and can be relied on to act in the best interests of the Railway.
- However, problems can arise when, because of unforeseen variations in the ground conditions, the Contractor is obliged to increase substantially the penetration depth, or to increase the number of piles or even to abandon a particular system.
- These problems inevitably lead to claims by the contractor, and Railway may find it difficult to accept them since it reflects on our ability in selecting the Contractor.
vii) Items of responsibility which must be defined in the conditions of contract
- The site investigation is undertaken by the Railway before inviting tenders for the piling.
- Railway should include all relevant details in the site investigation report in the tender documents.
- The facilities provided by the main contractor, or to be included in the piling contract, should be stated.
- These include such item as access roads, hardstandings for piling plant, storage areas, fencing, watching, lighting, and the supply of electrical power and water.
- Hardstandings (working platforms) for large piling plant may need to be of substantial construction and Railway should state the form in which they will be provided including the level of the platform in relation to the pile commencing surface and cut-off level.
- Underground services and obstructions can be a contentious item.
- It is normally Railway’s responsibility to locate all known buried services and other obstructions to pile installation.
- It is unfair to the Contractor for the Railway to disclaim all responsibility for the accuracy of the location plan, and to expect the contractor to accept the consequences of damage to service.
- The clause in the conditions of contract covering underground obstructions needs to be carefully worded to be fair to the interests of all parties.
5) Piling specifications
- A few matters which require particular attention are listed below:
a) Setting out
- The responsibility for setting out is clear if the piling contractor is the main contractor.
- If the specification does not define the responsibility for setting out, the piling sub-contractor must have a clear understanding with the main contractor on this matter.
b) Ground heave
- In the case of the Method 1 type of contract Railway, in specifying the type and principal dimensions of the pile, must accept responsibility for the effects of ground heave.
- However, if the contract is of the Method 2 category the matter is not so clear, and piling contractors are reluctant to accept responsibility for ground heave, either for remedial work to risen piles, or for repairing damage to surrounding structures.
- It should be clearly defined.
c) Loss of ground due to boring
- The responsibilities for these are similar to those for ground heave.
d) Noise and vibration
- The Contractor is responsible for selecting the plant for installing piles and is therefore responsible for the effects of noise and vibration .
- If the local authority has regulations limiting noise emissions these should be stated in specification.
e) Piling programme
- If Railway wishes to install the piles for the various foundations in a particular sequence to suit the main construction programme he should state the sequence in the specification, since it may not be the most economical one for the piling contractor to follow.
- Tolerances in plan position, vertical deviation from the required rake, and deviation in level of the pile head, should be specified.
g) Piling records
- Railway should specify the form in which he requires the Contractor to submit records .
h) Cutting down pile heads
- The specification should define whether it is the main contractor’s or the piling contractor’s responsibility to remove excess lengths of pile projecting above he nominal cut-off level.
i) Method of measurement
- The method of measuring pile length as installed should be defined clearly in the specification.
j) Removal of spoil
- The respective responsibilities for the removal of spoil from bored piles, the removal of cut-off lengths of pile, trimming off laitance and ground raised by ground have, and the disposal of used bentonite slurry, should be covered in the specification.
To ensure the durability and protection of pile foundations, it is important to consider the local site conditions, the type and quality of materials used, and the design and construction methods employed. Regular inspection and maintenance can also help to identify and address any potential issues before they escalate into significant problems.