The Pile Foundation Systems Engineering Essay
01.)Industrial outlook of the local economy from the point of view of 2 companies.
As students of BITS Pilani Dubai it is necessary that we understand the needs and condition of the local economy as most of us will be getting placed in companies operating within the region. To understand the local economy, i have gathered information from the local,news papers, global economic forums, the reports available from the internet about the growth, investment and trade details of the united arab emirates.
The economy of Abu-dhabi is booming in more ways than one. It is economy which is primarily still oil centric. The government of abu-dhabi is very cash rich and took the step of intensive investing in the infrastructure development especially after the recession. This was a very intelligent step as this kept alive the cash flow within the economy. The problem the government faced was that they could not affor to invest in infrastructure such as bridges and flyovers as these do not fetch any cash i.e. are not money generating investments. These dead investments are important but not in the time of recession as it was essential for the government to even keep alive it’s own cash flow which was exactly the mistake the government of dubai had done. That is why the government proposed the 3 nuclear power plants, the formula 1 track, the Ferrari theme park, the presidential palace, sheikh khalifa port; all adding up to a staggering 80 billion dollars over a period of 5 years. The scheme has fortunately worked so far as reflected by the growth data of the gulf region. The only economy doing better than Abu-Dhabi within the gulf is Saudi-Arabia. Industrty wise also the economy is doing moderately well. Prices of most industrial consumables are falling sharply which has fuelled the demand. The real-estate sector is losing its index as the prices are still high compared to the demand though very cheap compared to the prices of similar properties in india. There is a sharp decline expected in the rates of properties. This is because of the low demand but the high supply. The drop in prices comes as a good news to the local economy.
The economy of Dubai is not in a very secure state and cannot be called “Healthy”. Trying to cash into the real-estate boom, dubai government funded a host of civil projects, hike in prices of which was purely out of speculation. Eventually when recession hit, the prices long with the demand dropped to a level where there has been no significant ROI (return on investment) for the dubai government. This coupled with the absence of oil within the economy spelled doom for the dubai government. The only good news from the government of dubai was the widely acclaimed success of the Dubai Metro making it easier for the local residents to travel and also making travel within the city considerably cheaper. Domestic manufacturing within dubai is almost absent and thus any market if ever, has its eyes focused on the economy of abu-dhabi. The intensive borrowing from banks to invest in real estate put the dubai in the current state. Result as we see today is that the government has almost stopped investing in infrastructure. The government to increase its liquid cash reserves was forced to sell “bonds” of the dubai government. The future of the economy is bright if this cash is spent on projects which can revive the cash flow of the Dubai government. Otherwise these bonds will only be “more borrowing” which will lead to “more debt” eventually bringing a downfall of the local growth.
The other emirates:
The other emirates are not major centres of demand as there is no population, no major sea port, no domestic manufacturing, no trade or any natural resources. This has lead to slow growth and isolation. There is still scope of growth and development but only through intensive investment in coming years.
SWOT ANALYSIS OF JINDAL SAW GULF L.L.C
SWOT stand for strength weakness opportunity and threats. It is one of the most basic tools of management to analyse an idea or project or product. It defines clearly where the object of interest is strong and what can be improved. This helps the end user to clearly understand a line he/she can work along to turn the object better than it initially was. For a greater understanding of the project i have done a SWOT analysis of the Project Jindal Saw Gulf L.L.C.
SWOT analysis is a strategic planning method used to evaluate the Strengths, Weaknesses, Opportunities, and Threats involved in a project or in a business venture. It involves specifying the objective of the business venture or project and identifying the internal and external factors that are favorable and unfavorable to achieve that objective. The technique is credited to Albert Humphrey, who led a convention at Stanford University in the 1960s and 1970s using data from Fortune 500 companies.
A SWOT analysis must first start with defining a desired end state or objective. A SWOT analysis may be incorporated into the strategic planning model. Strategic Planning has been the subject of much research.
Strengths: characteristics of the business or team that give it an advantage over others in the industry.
Weaknesses: are characteristics that place the firm at a disadvantage relative to others.
Opportunities: external chances to make greater sales or profits in the environment.
Threats: external elements in the environment that could cause trouble for the business.
Identification of SWOTs is essential because subsequent steps in the process of planning for achievement of the selected objective may be derived from the SWOTs.
-The only company in the region to manufacture the product.
- Previous experience in the field as Jindal SAW is the largest DIP producer in INDIA.
- Cheaper gas and electricity available in the region compared to India and the European union.
- Abu-Dhabi being the head of IRENA will emphasis more on sustainable products.
- Well established contacts in the region of the director, due to a previous venture with the sheikh.
-Expensive labour accommodation, visa charges.
-High labour salary.
-Inclination of the local government towards European products.
-Land only available for rent and not for purchase in Abu-Dhabi. In Jebel Ali freezone there is a huge shortage of electricity.
-The major stake being given to a local.
-Building its own labour camp.
-Exporting the product to other nations around.
-The recession leading to lower prices of commodities and labour.
-The product from UK is expensive because of the immense freight.
-Exploiting the high rates of DIP from UK to capture local market and expand.
-A similar plant in KSA looking for expansion.
-A further breakdown in the global economy leading to 0 demand.
-increase in steel prices.
-Electro steel establishing a similar plant in UAE.
Hence from the SWOT analysis we can see that the plant has many strengths across varying fields:
Geographical: The plant is located in an area where demand is much more than the capacity of the plant and the demand is being met by purchasing the DIP from UK. Because of the high freight, labour, tax, fuel and electricity costs in the UK the product is very expensive. This can be used to its benefit.
Costing: The cost incurred for fuel and electricity is minimal due to the region being oil and gas rich. This is a very huge advantage as the plant has its own induction furnace, meaning the gas consumption of the plant will be immense.
Experience: The employees and the entire team of JINDAL SAW GULF LLC is experienced with ductile iron pipe manufacturing, selling and marketing. This means it will be easier to establish, gain and maintain a good quality and that for the marketing team it will be easier to sell what they know. Also it will help as it will be easier for the process team to curb the cost.
A compressor is a machine which does the work of compressing the required matter with the specific technology. Compressors are very common in our lives and most machines will not function without it. Our Air conditioners, Cars, vacuums etc all have compressors.
The specification of a compressor is mentioned with 3 things mainly which are:
-FAD or Free Air Delivery
-Pressure required of the compressed air.
-Power rating of the prime (i.e motor or engine) in the comporessor
-The number of compression stages
-Cooling method (air, water, oil)
-Drive method (motor, engine, steam, other)
-Lubrication (oil, Oil-Free where Oil Free means no lubricating oil contacts the compressed air)
FAD stands for Free Air Delivery, it is the amount of Atmospheric Air
(Free Air) that can be sucked in by the Compressor at inlet conditions( Suction Side ) of
-Atmospheric Pressure at 1 atmosphere.
-Atmospheric Temperature at 20°C/15°C.
-Relative Humidity at 0% (100% Dry Air) &
-The motor RPM at 100% of its rated value
The Free air delivery can be generally mentioned in 2 units:
-m3/min or cubic meter per min
-CFM or cubic feet per min.
1 m3/min = 35.3 CFM
There are 3 main kinds of compressors namely:
-Rotary or screw type compressor.
Rotary type compressors are positive type displacement compressors. A rotary screw compressors use two helical screws, known as rotors, to compress the gas. In a dry running rotary screw compressor, timing gears ensure that the male and female rotors maintain precise alignment. In an oil-flooded rotary screw compressor, lubricating oil bridges the space between the rotors, both providing a hydraulic seal and transferring mechanical energy between the driving and driven rotor. Gas enters at the suction side and moves through the threads as the screws rotate. Clearances between the threads decrease and compress the gas. The gas exits at the end of the screws.
The effectiveness of this mechanism is dependent on precisely fitting clearances between the helical rotors and the chamber for sealing of the compression cavities.
Typically, they are used to supply compressed air for general industrial applications. Trailer mounted diesel powered units are often seen at construction sites, and are used to power air operated construction machinery.
In an oil-free compressor, the air is compressed entirely through the action of the screws, without the assistance of an oil seal. They usually have lower maximum discharge pressure capability as a result. However, multi-stage oil-free compressors, where the air is compressed by several sets of screws, can achieve pressures of over 150 psig, and output volume of over 2000 cubic feet (56.634 cubic meters) per minute (measured at 60 °C and atmospheric pressure).
Oil-free compressors are used in applications where entrained oil carry-over is not acceptable, such as medical research and semiconductor manufacturing. However, this does not preclude the need for filtration as hydrocarbons and other contaminants ingested from the ambient air must also be removed prior to the point-of-use. Subsequently, air treatment identical to that used for an Oil-flooded screw compressor is frequently still required to ensure a given quality of compressed air.
Diagram of a rotary screw compressor
In an oil-flooded rotary screw compressor, oil is injected into the compression cavities to aid sealing and provide cooling sink for the gas charge. The oil is separated from the discharge stream, then cooled, filtered and recycled. The oil captures non-polar particulates from the incoming air, effectively reducing the particle loading of compressed air particulate filtration. It is usual for some entrained compressor oil to carry over into the compressed gas stream downstream of the compressor. In many applications, this is rectified bycoalescer/filter vessels. In other applications, this is rectified by the use of receiver tanks that reduce the local velocity of compressed air, allowing oil to condense and drop out of the air stream to be removed from the compressed air system via condensate management equipment.
Standard oil-flooded compressors are capable of achieving output pressures over 200 psig, and output volumes of over 1500 cubic feet per minute (measured as Free Air Delivery at 60 °C and atmospheric pressure).
As the name indicates, this type of Screw Air Compressor are driven by Electric Motor. These are modified stage of Rotary Air Compressor. The Rotary Screw Air Compressor is a Single Stage, Oil Flooded, Rotary Screw Compressor driven by Electric Motor through a Coupling. The Compressor Casing accommodates a pair of Male and Female Helical Rotors, machined with highest precision and mounted on Rolling Element Bearings.
The Male Rotor has four Helical Lobes which mesh with six Flutes of the Female Rotor. The speed of Male Rotor is 1.5 times higher than Female Rotor. The Male Rotor is driven by Coupling through Step-Up Gears. The Male Rotor Lobes rotate into the Female Rotor Flutes, the Air is trapped in the Inter lobe Spaces and smoothly Compressed until the Lobe-Flutes reach the outlet port. Pulsation free air is delivered by Screw Air Compressor, due to continuous Compression takes place in all the Lobe-Flute spaces.
My Experience with Compressors:
While working in Supercement Manufacturing Co. L.L.C. i was given the task of finalizing the technical details of compressors which the Company needed. I was provided with the following data:
FAD - 8000 feet3/min Pressure for penning application: 12 bar.
Pressure for other applications: 8 bar.
This seemed confusing as no other detail was provided such as the breakdown of the FAD between the 2 pressures. I contacted the person and was told that 33 m3/min was the 12 bar application and the rest 177 m3 of it was 8 bar application. There are 3 major companies in the field of compressors, all of which i called to enquire if they could supply the specified capacity.
The 3 companies were :
A preliminary meeting was arranged with all three companies to establish if they could supply the required specifications and in how much time. All companies took about a week of working days to come up with the first quote. The quote of all three companies was more than 2.2 million AED and the salesman declared that the quotation was the best price for the best technical quote. The size of the compressors offered was 2000 CFM = 56.657 cubic meter, the number of compressors recommended were 4.
After days of discussion as this was not a small order, i rejected the proposal as i was convinced that the offer was something they had given to me knowing that the customer will not be well versed with the details of a compressor. I used my engineering knowledge to justify my rejection.
Plant layout with compressed air requirement.
It didn’t make sense to purchase such big compressors for such a huge plant. Reason being that the bigger the compressor, the more demand you are catering to, but also that the travel by that air from the compressor outlet to the point of usage increases. The longer the pipe, the more number of bends will be encountered by the air. Hence the air will lose energy in terms of the pressure drop.
The other major dis-advantage of the provided solution was that the piping in this case would increase significantly. If we were to purchase smaller compressors, the piping would not be long as there would be not much wasted in inter connections with other equipments. The piping will then be only for that localized area.
If the piping is to cater to such a huge inflow of air, the piping diameter would increase, leading to an increase in the cost of the piping which we have to consider while purchasing the compressor.
For a bigger compressor the dis-advantage is that storing a stand-by compressor would be too costly as we would have to purchase a 2000 CFM compressor to keep the plant up and running.
Hence the entire is becoming inefficient because of the continuous loss in pressure energy. The piping length and diameter increase significantly and the spares also become costly.
Because of these reasons i could reject the offer provided by the suppliers who all agreed that the logics were right. They were asked by me to provide a smaller size.
About another week later i was provided with compressors of 33 cubic meter FAD = 1165 cubic feet of FAD. The compressors rating was 160 Kilo Watts. This size seemed to be in a reasonable range catering to most of the problems pointed out earlier. The number of compressors offered were 8. It was decided that the compressors will be installed at each point of use so that the loss incurred during travel and purge are minimized. The quote came down to 2.2 million AED automatically as this size was more commercially purchasable and was not a special compressor because of its size.
We negotiated for a price of 1 million. After heavy negotiations we were quoted a price of 1.6 million AED.
Just to be sure i searched over the internet if the solution was sufficiently suiting to our requirement and i came across the Variable compressor which seemed to make sense for such a huge demand as for this demand there were bound to be fluctuations. After a month of discussion and negotiations we were back to trying and finalizing the technical aspects. We considered it a good idea to include a variable compressor to be able to supply the air between the multiples of 33, i.e.
If the demand is from 0-33, the single variable compressor would start up.
If the demand is from 33-66, one fixed and one variable compressor would work.
If the demand is from 66-99, two fixed and one variable compressor would work.
This was if the demand is say 50 cubic meter, the fixed compressor would supply 33 cubic meter and the variable compressor would supply the other 17 cubic meter while consuming lesser power than the full rating.
On the other hand if the compressor were to be fixed, the 2 compressors would work to deliver 66 cubic meter of air where the requirement is only 50 cubic meter. This way too the system becomes more efficient.
Ingersoll then tried to convince me that the variable speed compressors of Ingersoll Rand are much more advanced than the others.
They assured me that the number of starts and stops in the case of there variable type compressor are infinite meaning that a fluctuation in start and stop will not decrease the life of the compressor.
They also introduced me to the technology of HPM or Hybrid Permanent Magnet which they claimed was the most sophisticated compressor available in the market. The motor itself was variable and had absolutely no bearings. This made the compressor very efficient in energy consumption and operation as well.
“The Nirvana HPM system is not only ultra-efficient and reliable, but is also designed to enable limitless starts and stops to meet demand, without the risk of damaging itself. This type of motor technology is often used in elevators, where safety, efficiency, and reliability are vital concerns. Frequent starts and stops is a problem for conventional induction VSD compressors — doing so causes heat problems and insulation breakdown. When they do start up, these compressors require a power surge of up to twice full-load current in order to overcome initial inertia. This generates excessive heat and limits the operating pattern of the compressor. Induction VSD compressors also run unloaded when demand is below minimum, reducing efficiency and driving up energy costs. The Nirvana HPM, on the other hand, requires less power at start-up, never operates at more than full-load amps, and shuts down immediately at minimum speed to avoid wasted energy. Nirvana ensures constant pressure throughout the entire operating range.”
On the day before we were due to finalize the deal with Ingersoll Rand, we were having a meeting with the Ingersoll Rand representative to re-confirm the scope of supply along with the terms of payment. It is then that a Mr Anil from the company asked us why we are not opting for Centrifugal compressors. It was yet another setback in my effort to settle the issue but at the same time proved out to be the best solution.
Advantages of Centrifugal Compressor:
The reasons provided for centrifugal being a superior technology are as follows:
Lesser Spare Parts Required
Greater life Cycle( of up to 30 years)
Lesser Specfic Energy
Centrifugal compressors are used throughout industry because they have fewer rubbing parts, are relatively energy efficient, and give higher airflow than a similarly sized reciprocating compressor (i.e. positive-displacement). Their primary drawback is that they cannot achieve the high compression ratio of reciprocating compressors without multiple stages. Centrifugal fan/blowers are more suited to continuous-duty applications such as ventilation fans, air movers, cooling units, and other uses that require high volume with little or no pressure increase. In contrast, multi-stage reciprocating compressors often achieve discharge pressures of 8,000 to 10,000 psi (55 to 69 MPa). One example of an application of centrifugal compressors is their use in re-injecting natural gas back into oil fields to increase oil production.
Centrifugal compressors are often used in small gas turbine engines like APUs (auxiliary power units) and smaller aircraft gas turbines. A significant reason for this is that with current technology, the equivalent flow axial compressor will be less efficient due primarily to a combination of rotor and variable stator tip-clearance losses. There are few single stage centrifugal compressors capable of pressure-ratios over 10:1, due to stress considerations which severely limit the compressor's safety, durability and life expectancy.
Additionally for aircraft gas-turbines; centrifugal flow compressors offer the advantages of simplicity of manufacture and relatively low cost. This is due to requiring fewer stages to achieve the same pressure rise. The fundamental reason for this stems from a centrifugal compressor's large change in radius (relative to a multi-stage axial compressor); it is the change in radius that allows the centrifugal compressor to generate large increases in fluid energy over a short axial distance.
Compare Centrifugal Compression technology with other machines such as reciprocating, screw, and positive displacement compressors and the advantages are clear.
No wearing parts requiring regular replacement
Oil filter elements and seal gas filter elements are easily replaced on line.
Require regular maintenance such as replacement of piston rings, gland packing and valve plates
Results in high operating expenses and significant machine downtime
100% oil free
Prevents oil contamination of system
Oil filters must be installed at discharge
Potential for oil carryover to foul the process
Pulsation free and require no dampers
May be subjected to suction and discharge pulsations
Require the use of large pulsation dampers to reduce pressure fluctuations
Feature inlet guide vane control, plus by-pass for smooth gas delivery under all conditions
The use of cylinder unloading for stepped flow control can result in complicated process control due to sudden changes in capacity
Capable of handling substantially higher volumes of gas in one or two small casings for smaller overall package
May have 4 or 6 cylinders requiring more space for installation
Require only a pad suitable for supporting the static weight of the package
Require a large and deep foundation to handle heavy weight and unbalanced forces
Precautions must be taken to prevent transmission of vibration to other equipment
-The specific energy of a Centrifugal compressor is about 4.8-5.5 Kw per m3/min. Whereas for all other compressors it starts at 5.7 and goes up to 6.2.
-The Air from a centrifugal compressor is oil-free. This is a huge advantage for the particular application because, during the peening application, if the air has even a small concentration of oil, the coating there after of the epoxy will not be of an acceptable quality as the cohesion between the epoxy and the metal surface wont be complete. On the other hand, the air from a centrifugal compressor is oil free meaning the epoxy coating will have a better finish and rejection rate will also be lesser.
-The rotary compressors also have a term called the service factor. This is a secret that most salesman will never share with you. The service factor is where the rated capacity of the motor is more than the one given on the broachers. This is so that the motor never runs at its full capacity and thus granting an extended life to the equipment. But this causes a problem to the end user as he is not taking into account this extra power consumption while making a decision based on this energy. The common service factor is from 1.15 to 1.20 which means the power rating of the motor if told to be 200 Kw will actually be 230 or240 Kw.
-The life of a centrifugal compressor is not less than 20 years without any major replacement. On the other hand a Rotary compressor needs replacement of its components such as the screw element (which is 30% of the cost of the compressor) once every 7-8 years depending on operation which means that Depreciation is much more in Rotary type as well as the maintenance cost is more than that of a centrifugal compressor.
Disadvantages Of a Centrifugal Compressor:
There are some disadvantages as well of using a Centrifugal compressor which are as follows:
No turn down
Requires Cold Water or Chillers
Efficient only on continuous 100% load.
These Dis-advantages were overcome by the following facts
The operation of the plant already requires a supply of cold water and hence there will be a cooling tower which will also cater to the need of the compressor.
Turn down is the capacity of the compressor of acting variable exampl a general turn down for variable compressors of Ingersoll is 30% which means that it can consume around 30% energy and supply 30% of the maximum air supply. The turn down of the centrifugal compressor is only 70% meaning that it can go down to a consumption of 70% of the maximum supply and consume about 70% of the max consumption. 70% though a less turndown is first of all better than a fixed compressor. Also the fact that the company once achieves 80-90% of its production demand by the sales, will require 100% of the air requirement.
While working at Super Cement i came across the concept of a Life cycle of a product. This concept is mainly used during the purchase of a product. The concept dictates that while purchasing any product we should keep a host of criterion in mind instead of only the purchasing cost. For example, purchasing a product A which is price x and has a life of y years is not advisable compared to the product B which might cost 3x but have a life of 4y years.
This concept proved helpful while purchasing compressors as the life of a rotary compressor is 10-15 years depending upon operation and maintenance. But the life of a centrifugal compressor is about 25-30 years and the maintenance is also cheaper. The price on the other hand is only 1.5 times that of the price of a rotary compressor.
Finally we settled for the centrifugal compressor due to the advantages over the dis-advantages though it is a bit of a risk as the company personally does not has any experience with centrifugal compressors. But through research and testimony of other people who have encountered the technology before, it was decided to purchase the compressor.
PILING AND FOUNDATION
A deep foundation is the kind of foundation different from shallow foundations by the depth they are drilled into the ground compared to the sea level. For many reasons a geotechnical engineer would suggest without compromise, a deep foundation over a shallow foundation. Some of the common reasons are:
Lesser Soil bearing capacity
Very heavy loads
Site constraints such as lesser/restricted space for making foundations
There are varying terminologies used to describe different types of deep foundations including piles, drilled shafts, caissons and piers. The naming terms may vary between engineering disciplines and firms. Deep foundations can be made out of timber, steel and reinforced concrete. Deep foundations can be installed by either driving them into the ground or drilling a steel shaft and filling it with concrete, mass or reinforced. In the gulf, primarily these bore holes are filled with a cage of steel first and then concrete is poured into them. The usual grade of concrete used is 40 N SRC where SRC means Sulphate Resistant Concrete. This is so that the concrete can protect the inner layers of re-enforcing steel from sulphate attack from the soil so that the quality or strength of the Pile or foundation does not decrease over time.
Pictures of piling from site.
Pipe piles being driven into the ground.
Prefabricated piles are driven into the ground using a drill. The wooden pile is made from trunks of tall trees. Concrete drills are available in square, octagonal, and round cross-sections. They are reinforced with steel and are often pre-compacted.
Steel piles are either pipe steel piles or some sort of beam steel section (like an I pile or an H-pile). Historically, wood piles were spliced together when the design length was too large for a single pile; today, splicing is common with steel piles, though concrete piles can be spliced with difficulty.
Driving piles, as opposed to drilling shafts, is advantageous because the soil displaced by driving the piles compresses the surrounding soil, causing greater friction against the sides of the piles, thus increasing their load-bearing capacity.
Pile foundation systems:
Foundations built on piles often have groups of piles connected by a pile cap (a large re-enforced concrete block into which the heads of the piles are embedded) to distribute loads which are larger than one pile can manage to bear. Pile caps and isolated piles are typically connected with grade beams to tie the foundation elements together; lighter structural elements bear on the grade beams while heavier elements bear directly on the pile cap.
A monopile foundation uses a single, generally greater-diameter, foundation structural element to support all the loads (weight, wind, static, dynamic etc.) of a large above-surface structure with a high weight.
A large number of monopile foundations have been utilized in recent years for economical construction of offshore wind farms in shallow-water subsea locations. For example, the Horns Rev wind farm from 2002 in the North Sea near Denmark utilizes 100 large monopies of around 4 meter diameter with a depth of around 30 meters into the seabed, while the Lynn and Inner Dowsing Wind Farm off the coast of England went online in 2008 with over 100 turbines, each suppoted on a 2.5 meter radius monopile foundation which go to a depth of about 20 meters each.
The typical construction process for a wind turbine subsea monopile foundation in sand includes driving a large hollow steel pile, of some 4 m in diameter with approximately 2-inch-thick walls, some 25 m deep into the seabed, through a 0.5 m layer of larger stone and gravel to minimize erosion around the pile. A "transition piece (complete with pre-installed features such as boat-landing arrangement, cathodic protection, cable ducts for sub-marine cables, turbine tower flange, etc.)" is attached to the now deeply-driven pile, the sand and water are removed from the center of the pile and replaced with concrete. An additional layer of even larger stone, up to 0.5 m diameter, is applied to the surface of the seabed for longer-term erosion protection.
Picture 2 of piling.
Also called caissons, drilled shafts, drilled piers, Cast-in-drilled-hole piles (CIDH piles) or Cast-in-Situ piles. Rotary boring techniques offer larger diameter piles than any other piling method and permit pile construction through particularly dense or hard strata. Construction methods depend on the geology of the site. In particular, whether boring is to be undertaken in 'dry' ground conditions or through water-logged but stable strata - i.e. 'wet boring'.
Boring is done until the hard rock or soft rock layer is reached in the case of end bearing piles. If the boring machine is not equipped with a rock auger, then socketing of the hard rock layer is done with the help of a heavy chisel which is dropped from a height of about 1.5 metres (depends on the weight of the chisel and design requirements) by suspending it from a tripod stand attached to a winch crane. The socketing is carried out until the desired depth within the rock layer has been attained. Usually, the required depth within the rock layer is considered to be equal to the diameter of the pile in hard rock layers and is taken to be equal to 2.5 times the diameter of the pile in soft rock layers.
'Dry' boring methods employ the use of a temporary casing to seal the pile bore through water-bearing or unstable strata overlying suitable stable material. Upon reaching the design depth, a reinforcing cage is introduced, concrete is poured in the bore and brought up to the required level. The casing can be withdrawn or left in situ.
'Wet' boring also employs a temporary casing through unstable ground and is used when the pile bore cannot be sealed against water ingress. Boring is then undertaken using a digging bucket to drill through the underlying soils to design depth. The reinforcing cage is lowered into the bore and concrete is placed by tremmie pipe, following which, extraction of the temporary casing takes place.
The reinforcement cage may need to be lapped with another cage if the depth of the pile exceeds 12 metres as that is the standard length of reinforcement bars of diameter 16mm and above.
In some cases there may be a need to employ drilling fluids (such as bentonite suspension) in order to maintain a stable shaft. Rotary auger piles are available in diameters from 350 mm to 2400 mm or even larger and using these techniques, pile lengths of beyond 50 metres can be achieved.
Such piles commonly fail due to the collapse of the walls of the shaft resulting in the formation of a reduced section which may not be able to bear the loads for which it had been designed. Hence at least a third of piles in projects with a large number of piles are tested for uniformity using a "Pile Integrity Tester". This test relies on the manner in which low intensity shock waves are affected as they pass through the pile and are reflected to judge the uniformity and integrity of the pile. A pile failing the integrity test is then subjected to a pile load test
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