Part I – CFA Pile Installation
[University]
Abstract
This paper is an investigative report on the use of Continuous Flight Augers or CFA in granular soils. In order to guide us in our investigation, this parts that are interrelated to each other. We will discuss the history of CFA’s and its application. This paper will also touch the common formulas used in its application and how it was derived. By taking a critical look into the formulas, we should be able to determine its effectiveness and should be able to identify flaws if there are any. We should also be able to identify the parameters that the formulas would be applicable in the standard tests that are made with CFA piles.
Introduction
In general, piling has been extensively used for foundation engineering even before the earliest times. Ulitskii (1995) claimed that pile construction method has been used by ancient Romans. He hinted that pile would have borrowed its roots from the word “pilium” which means “a heavy legionnaire’s lance”. Piles are long or slender members that are placed beneath the foundation to increase the bearing capacity of the soil. Piles are installed to support structures especially when deep foundation is required in areas where the soil’s bearing capacity is not sufficient to support the loads, settlement of the soil exceeds tolerable limit and whenever shallow foundation is economically disadvantageous. There are several ways of installing piles. It can be driven, drilled or installed in a pre-drilled hole. The focus of our study is installation of piles using Continuous Flight Augers or CFA piles.
Continuous Flight Augers has gained popularity in usage because of its quiet and vibration free operation as compared to other piling methods. Considering these qualities, it is preferred in areas that are populated usually in cities and other urbanized environments. A study by Van Impe (2003) revealed that 50 percent of the pile market in the world utilizes CFA piles and is constantly increasing (Impe, 2003 as cited in Mandolini et al, 2005). The challenge though is due to the fact that CFA equipment is rather bulky and transporting it may cause damage to transportation facilities such as highways and bridges. Never the less, CFA pile installation can be considered as the future of pile technology. CFA piles are installed with the use of an auger and a drilling mechanism. Basically, the process is drilling the auger into the ground in a depth specified in the plan. While the auger is being gradually lifted, fresh concrete is pumped in its place simultaneously. The auger used in this equipment is hollow in the middle which acts as a pathway of the concrete that is being pumped into the hole in the soil. Reinforcements are placed immediately after the auger is removed (Brown, Dapp, Thompson & Lazarte, 2007).
CFA Piles Bearing Capacity
In situ Testing
Three pile installation techniques were tested and compared in the Naples area to check for the bearing capacity of CFA Piles. Samples were load-tested until failure occurred. Static load testing is a procedure where in loading is applied using a calibrated hydraulic jack and measurements are directly taken after the application of the loads. The methods used in the pile installations were bored, driven and CFA. As described in Mandolini et al (2005), the methodology is as follows:
20 trial piles are all cast in situ concrete piles (bored, driven and CFA); the diamater (d) ranges between 0.35 m and 2 m; the length (L) between 9.5 m and 42 m; the ratio L/d between 16 and 61. The experimental values of Qs were obtained at a displacement of the pile head equal to 10% d (Mandolini et al, 2005).
Using the formula Qs=πd24qb+πdL(qs), where Qs is the bearing capacity, d is the diameter, qb is the unit base resistance and qs is the average skin friction. The data that was obtained showed that CFA piles have a bearing capacity (Qs) of 37 times its weight. As compared with other techniques, bored piles showed the lowest bearing capacity of only 12 times its weight while drilled piles got 73 times its weight as its bearing capacity (Mandolini et al, 2005).
CPT Testing
Another method of testing the capacity of CFA piles was conducted in Prince Albert, Saskatchewan Canada in August 18, 2008. The method used in testing is Cone Penetration Testing methods or CPT. This test procedure was adopted by ASTM D 3441 in 1974 and were outlined in ASTM D-5778 in 1995. The procedure for testing is as follows:
A standard pieziometer was used with a 600 tip, 10cm2 base area and a 150 cm2 friction sleeve was used in this project.
The readings that were observed were tabulated and analyzed using soil profiling method of Robertson (1990) and it showed that the measured bearing capacity (Qm) is aproximately close with the predicted bearing capacity (Qp) of the CFA pile (Robertson, 1990 as cited in Pardoski, n.d.). Qm is derived using the readings of the CPT test while Qp is derived using this formula, Rult=Rs+Rt=Csrsdz+Atrt where:
- Rult is the ultimate bearing capacity of the pile,
- Rs is the summation of load carried by the pile shaft,
- Rt is the load carried by the pile toe,
- Cs is the cirumferential area of the pile shaft,
- rs is the unit shaft resistance over depth dz
- At is the pile toe area and
- rt is the unit toe resistance
Comparison of the Formula Derivation Between the Two Processess
The formula used in deriving the bearing capacity of CFA piles used by Mandolini et al (2005) and Pardoski (n.d) is almost similar. Both equations account for the base resistance and the shaft resistance of the pile which is basically the most important aspect of the formula. Shaft friction plays an important role in the design of CFA piles. According to Gavin et al (2009), “At a typical maximum allowable pile head settlement of 25 mm, more than 71 % of the pile resistance was provided by shaft friction. Mandolini’s formula is quite direct. This formula is advantageous in situ testing formulation since the actual bearing capacity of the CFA pile can be measured and readily compared. In situ testing also gives higher reliability since it accounts of the environment and actual operation and installation of CFA piles.
On the other hand, Pardoski’s formula considers several factors before it was derived. The unit shaft resistance over depth (rs) and the unit toe resistance (rt) is a quantity that her derived from the following relationships; rs=∝Su where ∝ is the adhesion coefficient and Su is the undrained shear strenght, rt=NcSu where Nc is the bearing capacity factor set at 9. Pardoski’s formula accounts for contributing factors that is important in experimental analysis of CPA pile installation. This method is aparently useful in analysis of CFA piles capacity for the purposes of reasearch and mapping areas to be considered in the installation of CFA piles. The CPT method that he applied is an economical way of testing CFA piles without considerable waste of material.
Conclusion
The revelation that the measured bearing capacity of the pieziometer is close to the predicted bearing capacity of the CFA pile has a major relevance in this study. It implies that the CPT test is a reliable way of testing the bearing capacity of piles which is a possible economical solution rather than in situ testing. One important consideration in installing CFA piles is the nature of soil. In situ testing provides the needed knowledge in determining the actual soil environment which is necessary in the analysis of the CFA pile bearing capacity. Though the CFA pile lags behind drilled pile in bearing capacity, the ease and efficiency of this procedure are important factors that can be seriously considered. As discussed previously, CFA pile installation creates a quiet and vibration free operation as compared with other pile intallation methods. These and other desirable factors is the reason why CFA pile methods are gaining popularity and might in the future become the most used method for installing piles.
Reference List
ASTM. (2013). ASTM D1143 / D1143M - 07(2013). Retrieved 2013, from http://www.astm.org/: http://www.astm.org/Standards/D1143.htm
Brown, E. D. (2007, April). Design and Construction of Continuous Flight Auger Piles. Retrieved October 2013, from http://www.fhwa.dot.gov/: http://www.fhwa.dot.gov/engineering/geotech/pubs/gec8/gec8.pdf
Budhu, M. (2010). Soil Mechanics and Foundation, 3rd Edition. John Wiley & Sons Inc.
Gavin, K. C. (2009, June). The shaft capacity of CFA piles in sand. Journal of Geotechnical and Geoenvironmental Engineering,, 135 (6), 790-798.
Mandolini, A. R. (2005). Pile Foundation: Experimental Investigations, analysis and design. Retrieved October 2013, from http://www.geochina-cces.cn/: http://www.geochina-cces.cn/download/2005%E5%A4%A7%E9%98%AA%E4%B8%BB%E9%A2%98%E6%8A%A5%E5%91%8A/Mandolini%20A.%20et%20al.%20Pile%20foundations%20-%20Experimental%20investigations%20analysis%20and%20design..pdf
Pardoski, K. (n.d.). Derivation of CFA pile capacity in a silty clay soil using CPTu. Retrieved October 2013, from http://www.cpt10.com/: http://www.cpt10.com/PDF_Files/3-39pardapc.pdf
Rogers, D. (n.d.). Fundamentals of Cone Penetrometer Test (CPT) soundings. Retrieved October 2013, from http://web.mst.edu/: http://web.mst.edu/~rogersda/umrcourses/ge441/CPT-Soundings.pdf
Ulitskii, V. (1995). History of Pile Foundation Engineering. Soil Mechanics and Foundation Engineering.
Part II – CFA Pile Operation in Wolverhampton
Careful considerations should be done when making drilling and boring operations to insure the health and safety of the public. Wolverhampton, being a populated city in the West Midlands, England have Health and Safety legislations that needs to be considered before a building permit is granted. The process that the designer needs to undertake in order to ensure that he has complied with the Health and Safety Regulations of Wolverhampton is as follows:
- Creating a detailed plan to be submitted to the Building Control Officer. The plan should include all details and specifications and the scope of work.
- Drilling and boring operations may cause dust and noise polution. As what we have discussed in Part I of this paper, CFA Pile Methods can be used to minimize the noise and vibration caused by drilling operations. The work area needs to be enclosed in order for dust particles to stay contained and does not contaminate the environment. In the actual drilling operation, constant wetting of the soil being drilled should be strictly monitored to avoid dissipitation of dust and other harmful particles.
- The designer should also consider the Traffic Hazard that might be caused by the drilling operation. It would help to coordinate with the City council for a possible issues that may arise that may cause traffic jams and pedestrian inconveniences. Depending on the size and area of the drilling operation, a close coordination with Wolverhampton’s authorities is necessary to prevent any accidents and other health and safety hazards.
