Abstract
This dissertation paper is based on the development of the equations used for the design of CFA piles and more specific on granular soils. Tests that may be used in obtaining the required geotechnical parameters used in the designing of the equations are also reviewed. The tests based approaches are to allow contractors and designers to come up the best piles required for a certain project due to the parameters identified. Assumptions and errors resulting in the equation design are also covered. Present. Step-by-step method formulations of equations that are used for designing CFA piles are presented.
Key WordsCFA, drilled displacement piles, CPT, PDT and testing.
Dissertation style research
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This dissertation paper is focused on conducting a literature review and also summarizing of the historical developments that are behind the equations used for designing CFA piles and majorly on granular soils. Various tests that describe the measures taken into account when obtaining the desired geotechnical parameters that will be used in designing CFA equations. Also during this dissertation, comments on any assumption/s made and the errors present during design strategies will be accounted for.
The emphasis of this paper will be made on granular soil being a type of non-cohesive soil. Any Pile driven through the ground has many effects or it affects the surrounding soil considering it depends on the soil’s relative density. The soil occurrence in loose soils compacted making a depression on the ground surrounding the pile. In other types of soils such as denser soils, the compactness is minimal. This causes the ground to heave due to the soil being displaced in upwardly manner. In the latter type of soil, driving technique is much preferred to the boring technique. The reason attributed to this is the compaction occurring during driving which increases end-bearing capability.
Let’s take a look why CFA piles are most preferred to others. There seems to be no impact shock, vibrations, or low noise and it allows piling. CFA piles can also be adapted to use in sensitive urban areas as well re-development areas. This choice of pile can also be utilized in wide ranges involving cohesive and non-cohesion type of soils and to make it even better, can be utilized with or without water-bearing strata. While using CFA piles, the semi- displacement that results from the lateral soil compression, it increases the capacity of the final load bearing (Grieder, 2011).
CFA types of piles are drilled foundations in which the pile/s is drilled to a final depth via a one continuous process. It may require the continuous flight of an auger design. CFA pile’s capacity does fall in between a driven and drilled pile. During its designing, it is found that, its load behavior is similar to that of drilled and of driving piles. During this dissertation, it shall be shown that the total capacity will be, Qf=Qb+Qs
Also during its designing, side resistance are mobilized with minimal displacements hence; the allowable side shear that greater than the drilled pile is achieved. Also design for the end bearing suitable for the soil condition is also taken into account (Grieder , 2011). Skin friction in granular soils is minimal due to the low friction 'shell' formed around the shell. Tapered piles may be used to overcome this issue as a result of friction. Tapered piles reduce the occurrences of the problematic gap since they work on the soil that is re-compacted from the blows it makes.
The assumptions that are made during the designing of CFA piles are present, hence should be taken into consideration. The designer will have to understand the empirical relationships needed to be applied and that, it is from those empirical methods that the two values of Qb and Qs shown during the formulation of designing equations, reaches peak values both estimated at the depth of twenty and ten diameters respectively. Another assumption made during the designing is that, the skin friction parameter should never surpass the value of 110kN/m² and also, its base resistance doesn’t exceed a value of 11000kN/m. Echoing what has been mentioned earlier in this dissertation, piles used on non cohesion or granular soils for that matter, are based on the empirical formula relationships. The historic bit of the onset of the equations is shown below.
During the designing, one needs to find the CFA’s pile carrying capacity (ultimate), given by the equation,
Qf=Qb+Qs
Where Qb = base resistance found from the bearing capacity equation formulated by Terzaghi’s and Peck’s (1948). It happened to be the first prediction of the settlement footings on a non-cohesive soil by the use of a test known as standard penetration on blow counts (That’s the N values). It is as shown below,
qf=1.3 cNc+pd Nq+0.4 gBNg
Since the pile depth is as big as compared to the diameter, the term 0.4 gBNg is then ignored. The term on the left, 1.3 cNc reduces to Zero, due to the nature of the soil (non-cohesiveness).
In designing the equation that will be used in determining the net or total unit base resistance, takes the form below.
qnf=qf=pd(Nq-1)
Hence; the net total base resistance is illustrated as shown below.
Qb=pd(Nq-1)Ab
Where, pd= Effective overburden pressure usually at the pile base level.
In finding the total unit skin friction (for the pile shaft), Terzaghi’s and Peck’s (1948) concluded that, qs or f as some would refer it to, is expressed as shown below .
qs=Ks.sy'.tand
Where
sy'= The average vertical effective stress
d= Angle of wall friction.
Ks= The Coefficient factor of the Earth’s pressure.
Teng (1969) proposed a development of the above equation taking into account the settlement of 25 mm measurement that was earlier based on the equation proposed by Terzaghi and Peck (1948). The modified equation new look was of the is form
qs=35Ncor-3B+0.32B2Rw2Fd kN/m2
Where, qs is the net limited pressure bearing (for the settlement of 25mm in kN/m2), Ncor the value of corrected standard penetration, B is the footing width in m.
Rw2= The water table correction factor
Fd = Depth factor (also given by (1+DfB)≤2.0)
Df= The foundation’s depth (meters).
Meyerhof (1956) proposed a development of Teng’s equation slightly different from the one shown above,
qs=12 NcorRw2Fd Only if B≤1. 2m. Then,
qs=8NcorB+0.32B 2Rw2Fd Only for B ≥1.2m in the provision that,Fd =(1+0.33DfB)≤1.33)
Bowles (1996) developed another equation based on the fact that, the experimental results emanating from Teng and Meyerhof were seemingly too conservative. The resultant Teng’s modified equation is shown below.
qs=53(Ncor-3)B+0.32B 2Rw2Fd
Bowles equation had another form but only with a limit.
qs=12.5NcorB+0.32B 2Rw2Fd Only for B ≥1.2m was the equation.
The equation was developed with a provision that, if it occurred in any way that tolerable settlement was more than 25 mm, the bearing pressure ( safe bearing pressure) computed via the designed equations would be easily increased linearly by an equation deduced below.
qs'=S'25qs
Where,
qs'= Is the ultimate safe bearing pressure (required for a settlement S' in mm) as shown in the equation and qs=is the ultimate safe bearing pressure (only this time, for a settlement of 25mm).
There are tests that describe the designs to be used to obtain the geotechnical parameters. A few of them shall be revisited. Standard penetration test, is one of them which basically identifies the simple in-situ ( N value ) , the number of blows taken to drive a bar of 50mm and of diameter 300mm into the base (bore hole base) .
The values from the above STP test would be very useful in knowing the burden stress involved as shown by Schmertmann (1975) and Meyerhof (1976). Schmertmann correlated N-values of the SPT test and came up with the following equation.
Effective overburden stress = material's weight above the boreholes basebase - weight of water
Meyerhof’s (1976) brought up a relation between the frictional resistances and values of N. He recommended the N-values to be first normalized with respect to the overburdening effective stress. The normalization of N is as shown below,
Normalized N, =Nmeasured×0.77log(1920/ Avarege verticle stress (sy'))
There is also the load testing and the various methods it entails. Load testing the resistance factor which will be involved in the designing of the CFA piles. Load testing comprises of Dynamic Pile Testing (PDT) technically, a conventional Load Frame (Top Load Static Load Tests), Statnamic testing and lastly O-cell Testing (Osterberg Cell Testing). Top Load Static Load testing generally comprises of compressive loads to a maximum of 6.5 MN. It also tensions loads with a maximum of 5.0 MN. Other noticeable entailments are hydraulic Jack and strain gauges for transferring the load down the pile shaft. O-Cell testing on the other hand involves no pile reaction and has a higher (as compared to other methods) test loads to a maximum of 150 MN.
The next test is the Cone penetration test (CPT). It involves two; End-bearing resistance and the Shaft resistance. End-bearing resistance measurements occur when the capacity of the piles end-bearing are taken to be equal as compared to the cone resistance qc. In this measurement, there are the variations in the measured cone resistance. This calls for another method referred to Van der Veen's. A method of averaging stating that, qb (The average cone resistance) calculated over depth considerations is equated to 3 pile diameters that are situated above in relation to the one pile diameter found below the base level belonging to the pile.
The second and last part of CPT testing involves the Shaft resistance. This illustrates how the skin friction is calculated and hence assists to design the desired equations. Skin friction can also be calculated as shown from the previous first part that involves CPT. Calculations are starting from the values involving the local side friction making use of the empirical relationship as shown below.
In any depth,
qs=Sp.qc
Sp Is the coefficient dependent on the pile type.
This dissertation paper hence the historical developments behind the equations used for designing CFA piles on granular. Various tests that describe the measures taken into account when obtaining the desired geotechnical parameters used in designing CFA equations were revisited at shown effectively. It was found that CFA pile is indeed economical since the load testing involved is economical as well and that it can be done quickly.
Works Cited
Fang, H.-Y., 1990. Foundation Engineering Handbook. 1st ed. New York: Springer Publishing.
Grieder, J., 2011. Application and Use of Continuous Flight Auger Piles in Western Canada. [Online] Available at: http://www.geotechnical.ca/Events/Docs/110322-Grieder-GSE%20Lunch%20Presentation.pdf[Accessed 7 Nov 2013].
Khuram, R., 2010. Pile and Pile group. [Online] Available at: http://www.authorstream.com/Presentation/rizwankhurram-509648-presentation1/[Accessed 7 Nov 2013].
Murthy, V., 2002. Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering.. 2nd ed. New York: Marcel Dekker Inc..
T. Lunne, P. R. a. J. P., 1997. Cone Penetration Testing in Geotechnical Practice. 1st ed. London: Powell. Blackie Academic & Professional.
Terzaghi, K. P. R. a. M. G., 1996. Soil mechanics in engineering practice, Third Edition. New York: John Wiley & Sons Inc..
