A comparative study of the use of replacement pile foundations versus displacement pile foundations - Assignment Example

?Table of Contents Table of Contents Executive summary 3 Introduction 4 ification of piles 4 Pile ification according to load transmission method 5 End bearing piles 5 Friction/cohesion piles 5 Cohesion piles 6 Friction piles 6 Friction and cohesion piles 6 Pile classification according to the type of material 7 Timber piles 7 Concrete pile 7 Steel piles 7 Composite piles 8 Pile classification according to effect on soil 8 Driven/displacement piles 8 Bored/replacement piles 9 Merits and limitations of the principal types of piles 9 Advantages of preformed displacement piles 10 Disadvantages of preformed displacement piles 10 Advantages of cast in displacement place piles 10 Disadvantages of cast in place displacement piles 11 Advantages of cast in place replacement piles 11 Disadvantages of cast in place replacement files 11 Cost estimations on pile foundations 12 Material 12 Size 13 Standards 13 Equipment 13 Labour 14 Pile installation methods 14 A comparative study of the use of replacement pile foundations versus displacement pile foundations Executive summary This paper explores the use of pile foundations in the construction industry. It defines and identifies different classifications of piles and their applications. This paper seeks to make a comparison between replacement and displacement pile foundations. It presents a detailed analysis of the advantages and disadvantages of these types of pile foundations as used in relations to their characteristics. Discussion of the various aspects considered when laying the foundations also lies within the scope of this paper. The paper emphasizes the cost consideration for laying the different classes of pile foundations in architectural structures. Finally, the paper presents a discussion of the various methods applied in laying of the highlighted classes of pile foundations. Introduction Pile foundations are the structural parts that are involved with carrying and transferring the structural loads to the ground at a depth that is below the surface of the ground. The pile foundation mainly consists of the piles and the pile caps. Piles are long members that are also slender and carry out the function of transferring the loads to deep soils which have high load bearing capacities. Piles are mainly constructed using concrete, wood and steel. They are classified according to various standards which are dependent on the type of soil, material used to construct the pile and the characteristics of the piles to transmit loads. Piles are fixed to the ground through such processes as drilling, jacking, or driving them to connect to the pile caps. In basic point of view terms, piles can be broadly classified into two groups; displacement piles or driven piles and replacement or bored piles. Displacement piles are formed prior to being fixed to the ground through whatever mechanism, be it driving, jacking, screwing or hammering to the ground. Replacement piles are formed after a hole is dug into the ground in which the pile is formed. There are various other modes of classifying piles such as the mode of the load bearing material capacity. This paper will however focus on the replacement vs. the displacement piles. A comparative study of the use of these two types of piles will be given (Berezantsev, 1961). Classification of piles Piles are classified according to various means. This part of the paper will look at the various classification methods. Piles can be classified according to the method used for load transmission and the resulting functional behavior (Whitaker, 1970). Pile classification according to load transmission method In this group of piles, there are three subdivisions; end or point bearing piles, friction or cohesion piles and a combination of both friction and cohesion piles. End bearing piles These types of piles perform load transfer to the firm stratum that is located way deep into the structural base. Most of their carrying capacity results from the resistance to penetration of the soil below them. The figure below shows a representation of this pile. (Abebe and Smith, 2011) Friction/cohesion piles In this type of piles, the carrying capacity is derived from the friction of the soil that is in contact with the surface if this pile. Cohesion piles Load transmission in these types of piles is via skin friction. When piles are driven in this manner (in groups), there is a remarkable reduction in the soil compressibility and soil porosity within those groups. Piles of this category are referred to as compaction piles. Friction piles The mode of load transfer in these piles is through skin friction. Driving these piles does not result to appreciable compaction of the soil. The piles are commonly referred to as “floating pile foundations”. Friction and cohesion piles These types of piles extend from the prior discussed types. When they extend from the end bearing pile, in a condition of not hard enough stratums, the piles get driven to deep enough lengths, to encounter frictional resistance. At times, they are varied by having larger bearing areas. Pile classification according to the type of material In this manner, the piles fall into four categories: timber, concrete, steel and composite piles. Timber piles These piles have been there since time immemorial and are basically used in areas which have a lot of timber. Timber is very useful in long cohesive piles and piles that are required below embankments. There are various standard lengths that should be set for the timber. The condition of the timber should also be in such a way that it is immune from insect attacks as they can be easily vulnerable. In driving timber piles, care should be taken on the direction and firmness of the ground as it tends to damage the timber. Concrete pile Concrete piles fall into precast concrete and cast in place concrete. The precast concrete is so in order to be able to easily connect with each other to the required lengths. It is necessary to reinforce the concrete so that they can persevere with the driving stress (Tomlinson, 1987). Steel piles These types of piles are suitable where long lengths are required. They have a lesser cross sectional area. They have an extra advantage in that they can be increased or reduced in size at will through cutting and welding. Composite piles These piles are made from a variety of materials. A good example is where both concrete and timber are used. Timber can be used below the level of water and concrete applied at water levels. The figure below shows a depiction of the composite piles. Pile classification according to effect on soil Driven/displacement piles When the piles are being driven to the grounds, there is radical movement of soil as the pile penetrates the ground. There also might be a vertical component of the movement in this process. The figure below depicts this kind of piles. Bored/replacement piles These piles are also referred to as non displacement piles. They are inserted into the ground by first creating a void. Piles are then formed through various processes like casting of concrete into these voids. Some grounds such as clay support this form due to the fact that the ground can hold itself without support. Some grounds are not stable, like gravel, and would require support, during creation of the void. Various replacement piles exist, as follows: augered piles, “cable percussion drilling”, “large diameter under reamed”, “pre cast concrete unite”, “drilled in tubes” and mini piles. Merits and limitations of the principal types of piles As previously discussed, the main types of piles are replacement and displacement. There are pros and cons of these two types of piles that might have an impact on deciding on the simplest type of pile to use. In looking at the merits and the limitations of the various types of piles that do exist, there will be some subdivisions. The displacement piles will be grouped into preformed and “cast in place” piles whilst the replacement piles will only be in “cast in place” Advantages of preformed displacement piles 1. It is possible to inspect the quality and nature of soundness before driving them to the ground 2. They are not susceptible to necking or squeezing 3. Their construction is not interfered by ground water 4. They can be able to withstand pressures such as high bending stress and tensile stress. 5. It is possible to drive these piles to long lengths Disadvantages of preformed displacement piles 1. The piles that have an unjointed nature cannot be easily varied in length 2. Sometimes breakages occur as they are being driven 3. It is not economic especially when the mode of design is dependent on the driving stress instead of the working stress 4. There is a lot of noise and vibration during the driving process 5. Soil displacement during the driving process can have an ill effect on the structures that are adjacent 6. It is not possible to drive the piles in low head room circumstances Advantages of cast in displacement place piles 1. They have an easily adjustable length 2. In areas where there is ground water, they can be driven with a closed end, 3. It is possible to enlarge their bases 4. Their design is dependent on the working conditions 5. The use of internal drop hammer reduces the noise and vibrations Disadvantages of cast in place displacement piles 1. In the case where temporary tubes are used, necking becomes a possibility 2. It is not possible to inspect concrete after it is installed 3. when tubes need to be extracted, length becomes limited 4. the process of displacement can damage the nearby structures 5. there is too much noise and vibration which might be unacceptable due to the posed environmental hazards Advantages of cast in place replacement piles 1. they have a variable length 2. it is easy to compare the soil that has been removed with design data 3. it is possible to carry out some penetration tests in boreholes 4. boulders and other kinds of obstructions can be broken by drilling tools 5. it is possible to have the piles go to very log lengths 6. they have managed noises and vibrations during the construction process 7. they do not possess ground heave Disadvantages of cast in place replacement files 1. in soft soils, the piles can squeeze or neck 2. there is a requirement for extra expertise in the concreting process when dealing with grounds that bear water 3. it is not possible to inspect the concrete after installation 4. it is not possible to form enlarged bases in soils that can collapse 5. at times, the process of boring may lead to instability in the structures that are adjacent Cost estimations on pile foundations Piling work is the most critical factor in cost factor determination. The take of process for this work mainly involves the process of determining the type of piles that are to be used, the procedures to be followed during installation, the length requirements for the piles and the number of piles that will be used for the whole process. The process of pricing of the piling work can either be done through the number required or considerations of length. In the matters of the driven pile, it is very essential that the driving hammer that will be used to be also considered. In dealing with bored piles, presence of water in the area where the pile is being installed has a very big effect on the overall price of the pile. Water tends to significantly increase the cost of the installation process of the piles (Popescu et al., 2003). It is necessary to consider the movements of the equipments at work tat are used drive or excavate the piles at the site. When there is a high degree of movement, there tends to be less productivity in installation of the piles. This is due to the fact that the equipment tends to spend more time while moving instead of doing the actual work. Estimation of costs for the piling process also includes the cost of mobilizing and demobilizing the drilling equipments. The construction and installation of pile foundations can be costly compared to normal foundations. The element of cost should be considered first before beginning any construction. There are many fgactors that should be considered when calculating the estimated cost of construction of a pile foundation. The cost for constructing these foundations are affected by various factors Material There are different materials used for the construction of these foundations. The desired strength of the foundation determines the material to be used. The costs of materials vary and in the end determine the total cost of the foundation. Timber piles are cheaper than those made from either concrete or steel. Composite piles are mainly used when the cost of constructing a foundation needs to be cut down. These piles have been identified as being able to attain considerable lengths at comparatively low costs. Size The size of the pile to be used in a foundation can immensely affect the total cost of the foundation. Factors of stress and load of a material will be gauged to determine the size of pile required for a foundation. Timber piles are able to reach any length but their stress and the maximum load capacity is extremely low compared to other material. In case where great depth is desired the choice of material will be steel. This is because steel can go up to greater depths than the other materials. In such a case, the engineer cannot be able to determine the cost of the foundation. The size shall dictate the cost of the materials to be used. Standards The standards set out by the British code of practice have to be adhered when constructing a pile foundation. These standards impact the cost of construction as a company has to construct structures conforming to the required standards. A construction company cannot decide to cut cost by using cheap material with low load capacity. This would amount into a breach of the set construction standards. These standards dictate the load and stress capacity required in all types of foundations. When costing, standards have to be considered and followed. Equipment The choice of equipment to be used for the construction of foundations is also a cost factor. The cost of construction can be greatly affected if wrong kind of equipment is used. When constructing a foundation in which great depths are necessary, use of heavy digging equipment might be essential. The cost of running the equipment always has a direct impact on the total cost for the construction. The equipment to be used should be considered in the costing aspect. When wrong choice of equipment is used a project can delay and the costs escalate. Labour The choice of labour force hired will determine the level of wages and salaries to be paid out. The skill level of the personnel to be used should be put into the cost consideration. The number of employees required for a project will determine the cost of labour which needs to be used. The wage levels vary according to the skills which an individual possesses. It is therefore, necessary to consider the type of labour force which a company desires to employ for the construction. The numbers and skill levels of employees should be considered together when costing for a pile foundation. Since these structures require high levels of finesse, the expertise of employees should also be of relatively high quality Pile installation methods Methods of Pile Installations  Introduction Installation processes and methods are both key factors n the design phase of pile installations. This part of the paper focuses on two techniques of pile installations; through boring using mechanical auger and through the use of a pile hammer. It is important that installation equipment and methods be carefully identified and selected during the phase of design. This is as a way of preventing the piles from getting damaged. The key factors to consider while using the pile hammer installation technique include: The pile size and weight Driving resistance that needs to be overcome for the design penetration to be attained. The space available as well as the visible headroom How available the cranes are Present noise restrictions on site Methods of Pile driving These methods can be grouped into four; dropping weight, explosion, vibration, jacking and jetting. Drop Hammers In the use of this installation technique, a hammer whose weight is similar to that of the pile gets lifted to a considerable height and is then lowered to hit the head of the pile. This simple hammer is used in line with test piling and test frames where the use of a compressor or steam boiler to drive only a few piles is likely to be uneconomical. The two main kinds of drop hammers are the compressed air hammers/ single acting steam hammer and the double acting pile hammer. The single acting steam hammer contains a heavy cylinder into where compressed air put raising it up the piston rod. On attaining a height that is reachable to the operator or controller, the steam gets cut off making the cylinder freely fall off on the helmet of the pile. The double acting hammer on the other hand is driven through the use of compressed air or steam and does not require a piling- frame. It can be joined to the pile top using leg guides whereby the pile is guided through the use of a timber – framework. When used together with pile- frames, the hammer has back guides that are bolted to it to support the leaders. The short leg guides are the ones used to prevent any hammer movement to the pile top from taking place. These types of hammer are mainly used for driving sheet pile. Diesel hammers These are also grouped into two; single and double-acting. During the use of the single acting hammer is through a ram which is lifted by an explosion at the cylinder base. In a double acting hammer on the other hand, the ram is lifted creating a vacuum in a different annular-chamber. The output here is almost double that of the single acting hammer. When the conditions are favorable, this type of hammer brings about efficiency and quality in terms of pile—driving power. The hammer is not however recommended for all ground types.  Pile driving using vibrations  These hammers referred to as vibratory-hammers are powered using electricity or hydraulic energy. They contain eccentric masses that contra rotate within- a- housing that is joint to the top of the pile. The vibration amplitude is enough to fragment the skin frictions occurring on the pile sides. The use of these hammers is mainly suitable for areas with gravelly or sandy soil.  Jetting Water jetting is mainly used to help penetrate piles into sandy gravel or sand. The effect that this method has on other types of soils such as stiff clays is very minimal. Boring Techniques Continuous Flight Auger (CFA) Equipment contains a base carrier that is movable and that has a flight auger that is hollow stemmed. The flight auger is continuously rotated into the earth until a required piling depth is attained. The pile is formed by pouring concrete through the flight- auger during its withdrawal from the earth. A protective cap is fitted onto the auger at the central base tube then later rotated onto the earth using a hydraulic motor that operates on a hauler joint to the pole or mast. When it reaches the needed depth, concrete that is highly workable is then drilled through the hollow stemmed auger. It is the pressure under which this concrete is pumped that removes the protective cap. Auger rotation in one direction and similar to the rotation during boring, expels the soil vertically during the removal of the auger. As this happens, concrete is poured in the created space to form a pile. The auger rotation should match the concrete flow during this process to prevent the hole sides from collapsing an occurrence that is likely to cause voids in the concrete. This method is most effective when working on soft soil as it allows for the installation of different bored piles with varying diameters and which can easily penetrate through many and varying soil conditions.  Attaining full success and complete efficiency in the functioning of the auger however requires that the soil be self supporting and not contain cobbles, boulders and roots (Meyerhof, 1976) Underreaming This is a special feature of piles that are auger bored which is at times used to enable exploitation of the load bearing base. When this method is being employed, the soil has to be capable of standing open in order to be able to withstand and also be able to employ this technique. The best types of soils which are applicable in this idea are stiff soils and clays such as the London clay. In order to come up with the under ream, the tool that is used for under reaming is engaged in a closed position and fitted into the straight section of the shaft of the pile. It is then expanded at the base of that pile and thus creates the under ream in normal circumstances after the installation process and prior to casting of the pile. Conclusion Pile foundations continue to be extensively applied in the construction industry to provide structural strength to buildings and other architectural structures. The installation of pile foundations aims at transferring structural load to the ground. While there are two principle classes of the piles based on the effect they have on the soil, other methods of classification present numerous classes of pile foundations. Piles can also be classified by the method of load transmission and the material used to make the piles. The displacement and replacement piles display various merits and limits based on the method of installation of a pile. These limits and merits form play a crucial role when deciding the right method of installation to use in a structure. The cost consideration can be termed as the other crucial factor implicated in the choice of a pile foundation (Popescu et al., 2003). Wrong choice of pile foundation can lead to a structural disaster when a structure becomes overloaded. REFERENCES Berezantsev et al (1961) Load bearing capacity and deformation of piled foundations Proc. 5th Int Conf Soil Mechanics and Foundation Engineering, Paris, vol.2 pp.11 - 12 Meyerhof, G G (1976) Bearing capacity and settlement of pile foundations, Proceedings, American Society of Civil Engineers 102(GT3), pp 195-228 Tomlinson, M J (1987) Pile design and construction practice 3rd Ed, Viewpoint Publications, Palladian Publications Ltd. Whitaker, T (1970) The design of piled foundations Oxford : Pergamon Popescu, C. Ovararin, N. and Phaobunjong, K. (2003). Estimating building costs Volume 11 of Civil and environmental engineering, New Orleans: CRC Press. Read More