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Degree of utilization

Design is unacceptable if the degree of utilization is > 100%

Design is unacceptable if the degree of utilization is > 100%

© The correlation factors given in the UK NA to BS EN 1997-1 are higher because existing UK practice suggests the correlation factors in EN 1997-1 may lead to unsafe designs.

© The resulting characteristic resistance is significantly reduced compared with the EN 1997-1 calculation.

© In addition, the R4 resistance factor is higher than that given in Annex A of EN 1997-1, leading to a much-reduced design resistance in DA1-2.

© The partial factor on the total resistance must be used because no attempt has been made to separate the base and shaft components of resistance.

© A design to BS EN 1997-1 suggests DA1-2 is the most critical and that the requirements of the standard are not met.

® If we separate the base and shaft resistances, then a lower resistance factor may be applied to the shaft component (since ys < Yt), but not to the base (since Yb = Yt). Assuming a relatively small proportion of load taken by the base (15%), which is not untypical for floating piles, implies that the piles would be satisfactory to DA1-2. This emphasizes the importance of representing the pile behaviour as accurately as possible if unnecessary conservatism is to be avoided.

13.14.4 Design of continuous flight auger piles from cone tests

Example 13.4 demonstrates how continuous flight auger (CFA) piles may be designed from the results of field testing.

An investigation comprising both cable percussion boreholes and cone penetration tests (CPTs) was carried out at a site in Richmond, West London.19 A series of CPTs was carried out and Figure 13.19 shows a typical result from the site. The CPTs confirm the visual identification of the strata from the boreholes and provide data for direct pile design based on cone resistance. The sequence of strata is approximately 8m of Kempton Park Gravel overlying London Clay.

The design of a pile group comprising six, 400mm-diameter, 6m-long, continuous flight auger (CFA) piles is to be based on four CPT ground profiles. All three Design Approaches are considered. Design Approach 3 is not suitable for designing piles based on ground tests, as it requires partial factors to be applied to material properties that are not derived directly from the test.

Figure 13.19. Typical cone penetration test profile for site in Twickenham, through Kempton Park Gravel

The calculation for Design Approach 1 is also re-worked using the correlation and partial factors recommended in the UK National Annex to BS EN 1997-1.

Notes on Example 13.4

O Four cone penetration (CPT) tests have been used to derive average shaft and base resistance, using standard procedures to obtain derived values of shaft and base resistance.

© Partial factor sets to be used for permanent and variable actions are defined in Annex A to EN 1997-1.

© Correlation factors are defined in Annex A to EN 1997-1 for four ground profiles. Correlation factors are applied to the mean and minimum of the data set.

Example 13.4

Design of continuous flight auger piles from cone tests Verification of strength (limit state GEO)

Design situation

Consider the design of continuous flight auger (CFA) piles for a site in Twickenham, London. Ground conditions at the site comprise dense, becoming loose gravelly, SAND. Cone penetration tests have been perfomed at the site to a depth of 8m. (Data courtesy CL Associates.) The limiting average unit shaft resistance ps and limiting unit base resistance pb at each cone location are estimated to be:

(120kPa^ (2800kPa^

120kPa 100kPa v120kPa y pb =

3000kPa 2000kPa v3000kPa y

A group of N = 6 piles with diameter D = 400mm and length L = 6m are required to carry between them a permanent action FGk = 2100kN together with a variable action FQk = 750kN. The weight density of reinforced kN

concrete is Yck = 25-(as per EN 1991-1-1 Table A.1).

Design Approach 1

Actions and effects The self-weight of pile is WGk f 2 ^ n x D

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