Determination of bearing capacity of bored pile using SPT number N and undrained shear strength

In this paper, the determination of bearing capacity of bored pile using the SPT number and undrained shear strength is presented. The advantages of this method are simple and always feasible, especially for bridge and pier designing in case of expressway projects constructed on soft soils. The 22TCN 272-05 standard is used to calculate bearing capacity of bored pile including skin friction and end bearing capacities. An applicable form for calculating the bearing capacity of bored pile is made. An example is presented and the applied result is shown.


INTRODUCTION
Recently, bored pile has found widespread applications in many geotechnical problems such as high building, foundation, bridge and etc.The most important part has been how to calculate bearing capacity of pile used to design accurately, effectually.Efforts have been made to enrich the methods for determining the bearing capacity of bored piles.It can be calculated by using SPT, CPT, physical soil properties [1,2].However, it is clear that the bearing capacity of bored piles is not always determined because of lack of some parameters.In addition, some equations are complicated with several components.Therefore, in this paper, a simple, feasible and easy method is presented to calculate the bearing capacity of bored piles based mainly on SPT number and undrained shear strength.The procedure follows the 22TCN 272-05 standard [3].This standard is translated from AASHTO LRFD Bridge design specification 2010 [4].

Skin friction capacity
The a -method, based on total stress, may be used to relate the adhesion between the pile and clay to the undrained strength of clay.The nominal unit skin friction, in kPa, may be taken by: qs = a Su Su = mean undrained shear strength (kPa)

 = adhesion factor applied to Su
The adhesion factor,  , is assumed to vary with the value of the undrained strength, Su as specified in Table 1.The nominal skin friction of piles in cohesionless soils, in kPa, as provided in Reese and Wright (1977), may be taken as: where N is the SPT blow number Therefore, the skin friction capacity of pile, Qs in kN, is:

Qs = qs As
where As = surface area of pile shaft (m 2 )

End bearing capacity
For axially loaded shafts in cohesive soil, the nominal unit tip resistance, qp, by the total stress method as provided in O' Neill and Reese (1999)  The value of Su should be obtained within a depth of 2.0 diameters below the tip of the shaft.If the soil within 2.0 diameters of the tip has Su < 24 kPa, the value of Nc should be multiplied by 0.67.
If Su > 96 kPa with diameter > 1.9 m and the settlement of piles is not evaluated, we use qp = qpFr in which

Allowable bearing capacity
To calculate the allowable bearing capacity of bored pile, the resistance factors should be taken as tabulated in

Group effect
The effect of pile group can be considered by group effect factor as shown in Table 3 depending on the distance between piles (D is pile diameter).

RESULTS OF APPLICATION
The above procedure for calculating the bearing capacity of bored pile is applied to one boring log as presented in Figure 1.This is one of the boreholes in Cao Lanh -Vam Cong high way project.The depth of the borehole is 80m.According to steps for calculating bearing capacity of bored pile as illustrated above, an excel form is established as given in Table 4.As results,

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with pile of 64m deep from the bottom of pile cap, the ultimate bearing capacity is 15,017 kN.By using resistance factors of 0.65 for cohesive soil and 0.55 for cohesionless soil in case of side resistance and 0.55 for cohesive soil and 0.50 for cohesionless soil in case of end resistance, the allowable bearing capacity is 8,543 kN.The designed distance between piles is equal to 3 times pile diameter, hence the group effect factor is 0.70.Therefore, the allowable bearing capacity of pile is finally 4,342 KN.

CONCLUSIONS
This paper presents the method to calculate the bearing capacity of bored pile using the SPT number N and undrained shear strength Su.The calculation procedure follows the 22TCN 272-05 standard.
Comparing to the other methods, this method has several strong points as follows: - The formulas are simple, easy and feasible -Method of calculation has a high reliability

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The input data obtained from field tests describes accurately soil conditions in fact In case of multi-layer soil with sand layer, it is difficult to take undisturbed sample to conduct laboratory tests for determining bearing capacity; hence, this method is used.

DISCUSSION
It has been known that the bearing capacity of bored pile is usually determined based on physical properties of soil.However, sometimes, we have not enough information, that means soil properties, for calculating.To overcome this problem, the above method is used because the input data are always determined.Moreover, in case of a lack of undrained shear strength Su or it has not been determined yet; the empirical equation proposed by Skempton (1957) and Ladd (1971) shall be taken as: qp = NcSu ≤ 4000 in which Nc = 6[1+0.2(Z/D)]≤ 9 where D = diameter of drilled shaft (m) Z = penetration of shaft (m) Su = undrained shear strength (kPa)

Figure 1 .
Figure 1.The boring log

Table 1 :
Values of  for determination of side resistance in cohesive soil(Reese and O'Neill, 1988)

Table 2 Table 2 :
The resistance factors in cohesive and cohesionless soils

Table 4 .
The result of calculation of bearing capacity of bored pile