CFA Piles in Granular Soils - Literature review Example

CFA Piles in Granular Soils: A Literature Review Table of Contents Table of Contents 2 Introduction: 3 2.Literature Review: 3 3.Conclusion: 8 References 9 1. Introduction: CFA or continuous flight auger piles is a foundation technology that has become more popular over the recent years. Owing to the speed and economical advantages of construction of these piles, engineers are highly benefitted from using CFA piles in their projects. However there are limitations of these constructions as well particularly when the conditions are not favorable. Hence several practical considerations are necessary to be considered by engineers as and when they make use of these piles (Brown 2005). The favorable geotechnical conditions for use of CFA piles include soil conditions such as medium to very stiff clay soils, weak limestone, residual soils, and medium dense to dense silty sands and well-graded sands, and cemented deposits. However as far as granular soils are conditions they fall under the unfavorable geotechnical conditions for use of CFA piles (Geotechnical Engineering Circular (GEC) No.8 Design and Construction of Continuous Flight Auger Piles Final 2007). The present study focuses on the historical development of the equations used for the design of CFA piles in granular soils. 2. Literature Review: The size of typical continuous flight auger piles is 30 to 100 cm in diameter. It is mostly used for purposes of installation of methods and equipments, considering the advantages they provide for their speed and cost effectiveness. CFA piles are also known as augered cast-in-place or augercast piles in the United States for their practice. These piles can be distinguished from others through their constructions since the concrete that is used in the piles, which is many times a mix of sand and cement, is located “through the hollow center of the continuous flight auger drill string as the augers are withdrawn and then the reinforcement is placed into the wet fluid mix after the casting operation is complete” (Brown n.d.). Thus the pile becomes a structural element reinforced with concrete designed accordingly to the need of the construction and the nature of the soils. When lengths of 10 to 30 m are used for the CFA piles, they have been found to be most cost effective. The construction is effective when it is completely done in soils, although these piles may be used in weak rocks as well. The piles can be drilled and finished with very high speeds, which makes it possible for the constructor to install several piles within a short period of time. The advancements associated with design and construction of the CFA piles have been based on two major factors in the present times. These include: (1) the process of drilling and casting being monitored and guided by electronic controls; and (2) providing improved capabilities through drilling equipments that are more powerful (Brown n.d.). When rotary piles are considered, use of powered cutting augers is done such that the piles may be advanced to the necessary strength. Clay and soft rocks are mainly chosen for the purpose. However, granular soils or materials may also be used considering the fact that “their economic viability drops if large volumes of granular material need to be piled” (Kaul 2010). When the conditions of the ground are mixed, CFA piles are found to be most suitable, reflecting their development and increase in use over the recent years. However while designing and construction are considered, it is also essential to realize that “their efficiency can drop significantly in ground with significant obstructions” (Kaul 2010). The main advantages of using CFA piles include (Kaul 2010): Absence of noise and vibration Cost effectiveness and high speed of installation Presence of groundwater not affecting the piles or the process Can perform under low headroom conditions Can operate in a wide range of soil conditions Can operate within small clearances from obstructions The range of conditions of the soil in which CFA piles can be installed can be represented through the following equation (Kaul 2010): ć = 0, ǿ soils with certain exceptions; ć, ǿ = 0 soils except for those penetrations that are in excess of around 8 m in hard clay or soft rocks. It has also been observed that the performance of CFA piles has significantly improved as a result of introducing measures of quality control, which include electronic sensors and visual displays that can having ability to record the depth of the auger, the rate of flow of the concrete, torque, penetration, and rates of withdrawal, pressure of concrete, and other related factors. However there are certain significant disadvantages as well that are listed as follows (Kaul 2010): Limitations in the depth of operations Risks of ingression of groundwater in between the piles Rising costs in cases where temporary casing may be required Circular cross-section resulting in lower efficiency of the structure When granular soils are considered in particular, ingression of the groundwater is a major concern for the design and construction of the CFA piles. It has the ability to create loss of soils. However a good seal of the water is possible to achieve if the area can be jet-grouted in between and behind the piles before the process of excavation initiates, thereby preventing the loss of soil (Kaul 2010). The designs of piles are in general based on laboratory tests. Shear parameters are considered for calculations of the bearing ability of the piles and their resistances. These parameters include Φ, c, cu, based on static formulae. The use of these parameters is exception considering the fact that these may be used for the calculation or verification of resistance of the shaft that include the following factors (Holeyman 2001): Defining the lower limit of the resistance of the shaft for piles that are bored in granular soils Calculation of the friction of the shaft at times when the tests for soils performed in situ are not sufficient or there are doubts with the results or in cases where the conditions of the soils are not well represented Calculation of the friction of the shafts of tension piles. The penetration of the CFA piles into soils is dependent on a wide range of torques. But the effect created by the ground may vary due to the level of compression of the soils which may be “similar to an equivalent sized displacement pile to soil disturbance” (Newson 2003). It has been obtained through studies that such disturbance in the soils may be avoided even if the rates of penetration at the augering are lower than what is generally recommended. As far as the CFA auger is concerned, it has been obtained that it offers advantages of transportation of the soils as it enables reduction of the rate of boring, and hence ensures that the auger does not cut out or become spoiled. As a result of this, greater flexibility is obtained for the CFA piles allowing them to perform effectively in a wide range of conditions of the soil (Newson 2003). The construction of the CFA piles is done by augering a bore hole on a continuous basis. The diameter is small in size as already mentioned earlier. The augering is done into the earth allowing injections of “cementitious grout through the hollow stem of the auger as it is withdrawn, after which reinforcing steel is inserted into the grout” (O’Neill et al 1999). The cost effectiveness of the process comes since the speed of the process if very high. In order to minimize any form of defects associated with the process of production in introduction of the CFA piles in granular soils, it is essential that significant monitoring is followed on the grout and on the pressure that is applied on the column of the grout. Studies have obtained that with suitable monitoring, it is possible for the CFA piles to be effective and prove to be reliable for the purpose of foundation load bearing elements. One of the studies by researchers focuses on the construction of CFA piles in the Houstan area considering granular soil conditions of the region. Load tests were conducted in this research to determine probable failures in compilation of the property profiles and results (O’Neill et al 1999). If the face value of the process is considered, the installation of CFA piles can seem to be a simple process. However when the piles are required to be drilled, the process can prove to be highly complex. Thus the CFA piles have significant effects on the ground as well comprising granular soils. In this case the visual inspections of the bore become difficult as the process remains unseen with CFA. Hence researchers have been required to draw conclusions based on experiences of the past, considering the likely outcomes of installation of piles in certain conditions of soil, such as the granular soils. As researchers could obtain from their studies, there is an “appropriate relationship between drilling resistance and pile strength” (Mure et al 2002). If a certain condition of the ground is found to be difficult, then it is likely that the importance of the of research in that site will increase, enhancing the need for further detailed understanding on the design and construction of CFA piles in granular soils (Mure et al 2002). 3. Conclusion: It could be obtained from the above literature review that early researchers have been concerned with the topic of CFA piles construction in granular soils, with sincere interest being given in developing the equations for the design and construction of the piles. It could be obtained that there are certain assumptions and consequences of such design and construction, which clearly reflect on the fact that the use of CFA piles in granular soils is less suitable than other soil conditions. However the negative impacts can also be avoided through effective monitoring measures, which researchers obtained from their studies. Nonetheless, the use of CFA piles in granular soils has significantly developed over the recent years, increasing the scope of further research in the area. References Brown, D. (n.d.) Recent Advances in the Selection and Use of Drilled Foundations. Malcolmdrilling. [Online]. Available at: http://www.malcolmdrilling.com/files/Brown_Recent%20Advances%20in%20the%20Selection%20and%20use%20of%20Drilled%20Foundations%20(GI%202012).pdf [Accessed 5 July 2014]. Brown, D.A. (2005) Practical Considerations in the Selection and Use of Continuous Flight Auger and Drilled Displacement Piles. Geotechnical Special Publication, No.132, pp.1-11. Geotechnical Engineering Circular (GEC) No.8 Design and Construction of Continuous Flight Auger Piles Final (2007) FHWA. [Online]. Available at: https://www.fhwa.dot.gov/engineering/geotech/pubs/gec8/03.cfm [Accessed 4 July 2014]. Holeyman, A.E. (2001) Screw Piles - Installation and Design in Stiff Clay. Florida: CRC Press. Kaul, K. (2010) Cut-and-Cover Metro Structures: Geo-Structural Design: An Integrated Approach. Florida: CRC Press. Mure, N. et al (2002) Just-in-Time Continuous Flight Auger Piles Using an Instrumented Auger. NIST. [Online]. Available at: http://fire.nist.gov/bfrlpubs/build02/PDF/b02122.pdf [Accessed 8 July 2014]. Newson, T.A. (2003) BGA International Conference on Foundations: Innovations, Observations, Design and Practice : Proceedings of the International Conference Organised by British Geotechnical Association and Held in Dundee, Scotland on 2-5th September 2003. London: Thomas Telford. O’Neill, M.W. et al (1999) Axial Performance of Continuous-Flight-Auger Piles for Bearing. CTR. [Online]. Available at: http://library.ctr.utexas.edu/pdf1/3940-2.pdf [Accessed 7 July 2014]. Read More