Part E Worked examples
E8 DERIVATION OF BASE AND SHAFT COMPONENTS FROM A COMPRESSION
PILE LOAD TEST
E8.1 Introduction and data
The ULS design pile load capacity is obtained from a single pile load test using three methods of interpreting the pile load tests to failure.
Method A considers the maximum pile shaft capacity by inspection;
Method B uses Chin (1972) to derive both ultimate shaft and base capacities; and
Method C uses the results of instrumentation which distinguishes between components of shaft and base resistance.
The pile has a 1.02 m diameter shaft and a 2.06 m diameter base. The total length of the pile is 22 m.
E8.2 Method A calculations
Figure E8.1a shows the load-settlement points measured in the pile test, for which the ultimate resistance was 14.2 MN at a settlement/shaft diameter ratio of δ / D = 10%. The pile shortening line is also shown on Figure E8.1a. This line is the elastic compression of the pile assuming that the total imposed load results in compression of the whole length of the pile shaft. The intersection of this pile shortening line with the pile load-defl ection curve is at approximately 6.2 MN (δ= 5 mm) and it is this load that may be assumed to approximately equal to the shaft friction of the pile. By inspection, it is therefore considered that the shaft resistance is about 6.2 MN, so the base resistance is inferred to be 8.0 MN.
E8.3 Method B calculations
Figure E8.1b shows the same data on a ‘Chin plot’.
From this it could be inferred that the shaft capacity is about 8.3 MN and the total ultimate capacity (at very large displacement) 16 MN, giving a base capacity of 7.7 MN.
E8.4 Method C calculations
During the loading cycle, the test pile was
instrumented at locations along the shaft and base.
The load cells showed a shaft resistance of 6.7 MN and a base resistance of 7.5 MN at δ / D = 10%. The results for the shaft and base resistances are shown in Figure E8.1c.
In order to calculate the design resistance of the pile from the measured resistance in a pile load test, Paragraphs 7.6.3.2(6)P and 7.6.3.2(10)P must be used.
PART E WORKED EXAMPLES
141
a) Under-reamed pile, load-settlement curve
b) Chin plot
c) Instrumented test pile, load-section curve 16
Figure E8.1 Design of a compression pile, assessing shaft and base capacity from loading test
Paragraph 7.6.3.2(6)P considers the characteristic resistance of the pile based on both the load test results and the number of load tests carried out (see E7):
Rck = Rcm/ ξ
where Rck= the characteristic, k, resistance of the compression, c, pile test(s) Rcm= the measured, m, resistance from the compression pile tests (only
one test was carried out here)
ξ= a factor depending on the number of pile load tests and, where there is more than one test, whether the minimum Rcmor average Rcmis being considered (Table 7.1).
Paragraph 7.6.3.2(10) allows the design resistance to be derived from the characteristic resistance:
Rcd= Rck/ γ.
The partial factors (γ) are the equivalent of the ULS Case C factors presented in Paragraph 2.4.2(14)P for use with ground properties. Values of γ are provided for total pile capacity, shaft capacity and base capacity (Table 7.2).
The value of γis different for different types of pile installation (conventional bored, continuous flight auger or driven). This example uses bored piles.
The design capacity of the pile is now calculated for Method C.
For a single pile load test the value of ξis 1.5 (Rcmfor average and minimum conditions are obviously the same). The measured pile resistance was 14.2 MN, hence the characteristic resistances are:
Rck= Rcm/ ξ Rck= 14.2 / 1.5
= 9.47 MN total resistance Rsk= 6.7 / 1.5
= 4.47 MN (subscript ‘s’ for shaft) Rbk= 7.5 / 1.5
= 5.0 MN (subscript ‘b’ for base).
The designer may then chose between two approaches:
either Rcd= Rck/ γt
= 9.47 / 1.5
= 6.31 MN (γt= 1.5 on total resistance) or Rsd= Rsd+ Rbd
= 3.44 + 3.13
= 6.57 MN on combined shaft and base where Rsd= 4.47 / 1.3
= 3.44 MN (γs= 1.3 on shaft) + Rbd = 5.0 / 1.6
= 3.13 MN (γb= 1.6 on base).
In this instance the capacity derived by resolving the total resistance into shaft and base components is some 4% larger than the resistance of the pile when no distinction is made between shaft and base, providing a more economic design which is in compliance with the code.
E8.5 Method comparisons
The results for the three Methods are summarised in Table E8.1. In the table, the displacement that is appropriate to the total resistance in each Method is stated. It is clear that the displacements for Methods A and C are the same (the measured resistance) while Method B is for a larger displacement. In order to calculate the design pile resistance, the shaft and base resistance is expressed as a fraction of the total resistance. This breakdown between shaft and base resistance is then used with the measured ultimate compression resistance to obtain the design resistance of the pile.
Table E8.1 Method comparison
Method A: Method B: Method C:
By inspection Chin method By measurement
Shaft resistance (MN) 6.2 8.3 6.7
Base resistance (MN) 8.0 7.7 7.5
Total resistance (MN) 14.2 16.0 14.2
at δ= 10% D δ= large at δ= 10% D
Ratio shaft/total 0.437 0.519 0.472
resistance
Ratio base/total 0.563 0.481 0.528
resistance
Measured ultimate 14.2 14.2 14.2
compression resistance, Rcm(MN)
Characteristic ultimate 9.47 9.47 9.47
compression resistance, Rck(MN)
Rck= Rcm/ 1.5 (Table 7.1)
Characteristic shaft 9.47 ×0.437 9.47 ×0.519 9.47 ×0.472
resistance, Rsk(MN) = 4.14 = 4.91 = 4.47
Characteristic base 9.47 ×0.563 9.47 ×0.481 9.47 ×0.528
resistance, Rbk(MN) = 5.33 = 4.56 = 5.00
Case C design shaft 4.14 / 1.3 4.91 / 1.3 4.47 / 1.3
resistance, Rsd(MN) = 3.18 = 3.78 = 3.43
Case C design base 5.33 / 1.6 4.56 / 1.6 5.00 / 1.6
resistance, Rbd(MN) = 3.33 = 2.85 = 3.13
Case C design total 6.51 6.63 6.56
resistance, Rcd(MN)
In this example, the division between shaft and base resistance was varied between the three Methods. Nevertheless, the resulting design total
resistances for ULS Case C differed by less than 2%, due partially to the even distribution between base and shaft resistances.
It is important to note that although there is some uncertainty in the assessment of the measured ratioof shaft to base resistance, the calculation uses the measured totalresistance, which is clearly determined in the test to be 14.2 MN.
PART E WORKED EXAMPLES