"

6.8 Load and Resistance Factor Design of piles subjected to axial compressive loads according to AS2159

According to AS2159, the design capacity of piles subjected to axial compressive loads is determined as the product of the ultimate geotechnical strength in axial compression (collapse load Qf) and of the basic geotechnical strength reduction factor φgb. The term basic refers to designs which are not supported by pile loads tests-these are covered later in this Part. The product of the ultimate geotechnical strength in axial compression (collapse load) and the basic geotechnical strength reduction factor (i.e., the design bearing capacity) should be higher than the design action load, S* resulting from Ultimate Limit State (ULS) load combinations. It is reminded that S* is determined according to the provisions of the standard relevant to the specific structure e.g. AS1170.0 and AS1170.1 for structures other than bridges, and AS5100.2 for bridges.

(6.1) {\varphi _{gb}}{Q_f} \ge {S^ * }

The basic geotechnical strength reduction factor φgb is determined via a risk assessment procedure, based on Table 6.1.

Table 6.1. Weighting factors and individual risk ratings for risk factors (AS2159).
Risk factor Weighting factor (wi) Typical description of risk circumstances for individual risk rating (IRR)
1 (Very low risk) 3 (Moderate) 5 (Very high risk)
Site
Geological complexity of site 2 Horizontal strata, well-defined soil and rock characteristics Some variability over site, but without abrupt changes in stratigraphy Highly variable profile of karstic features or steeply dipping rock levels or faults present on site, or combinations of these
Extent of ground investigation 2 Extensive drilling investigation covering whole site to an adequate depth Some boreholes extending at least 5 pile diameters below the base of the proposed pile foundation Very limited investigation with few shallow boreholes
Amount and quality of geotechnical data 2 Detailed information on strength and compressibility of the main strata CPT probes over full depth of proposed piles or boreholes confirming rock as proposed founding level for piles Limited amount of simple in situ testing (e.g. SPT) or index tests only
Design
Experience with similar foundations in similar geological conditions 1 Extensive Limited None
Method of assessment of geotechnical parameters for design 2 Based on appropriate laboratory or in situ tests or relevant existing pile load test data Based on site-specific correlations or on conventional laboratory or in situ testing Based on non-site-specific correlations with (for example) SPT data
Design method adopted 1 Well-established and soundly based method or methods Simplified methods with well-established basis Simple empirical methods or sophisticated methods that are not well established
Method of utilising results of in situ test data and installation data 2 Design values based on minimum measured values on piles loaded to failure Design methods based on average values Design values based on maximum measured values on test piles loaded up only to working load, or indirect measurements used during installation, and not calibrated to static load tests
Installation
Level of construction control 2 Detailed with professional geotechnical supervision, construction processes that are well established and relatively straightforward Limited degree of professional geotechnical involvement in supervision, conventional construction procedures Very limited or no involvement by designer, construction processes that are not well established or complex
Level of performance monitoring of the supported structure during and after installation 0.5 Detailed measurements of movements and pile load Correlation of installed parameters with on-site static load tests carried out in accordance to AS2159 No monitoring

Note: The pile design shall include the risk circumstances for each individual risk category and consideration of all of the relevant site and construction factors.

More specifically: Each one of the risk factors of Table 6.1 is rated on a scale from 1 to 5 for the nature of the site, the available geotechnical information, pile design method and installation procedures, and an individual risk rating (IRR) is assigned to each one of the factors. The individual risk rating ranges from IRR = 1 for very low risk level to IRR = 3 for moderate risk level, up to IRR = 5 for very high risk level. Accordingly, the overall average risk rating (ARR) is determined as the weighted average of the product of all the risk weighting factors wi times the IRR of each factor:

(6.2) {\rm{ARR}} = \dfrac{{\sum {\left( {{w_i}{\rm{IR}}{{\rm{R}}_{\rm{i}}}} \right)} }}{{\sum {{w_i}} }}

An example application is provided in Table 6.2. For a given ARR value, the basic geotechnical strength reduction factor φgb is determined from Table 6.3, depending on the level of redundancy in the piling system. Systems with a high degree of redundancy would include large pile groups connected with stiff pile caps, pile raft foundations, and pile groups with more than 4 piles. Systems with low redundancy would include isolated heavily loaded piles, and piles set out at large spacing within a group.

Table 6.2. Estimation of the average risk rating ARR after AS2159.
Risk factor wi IRRi wiIRRi
Geological complexity of the site 2 3 6
Extent of ground investigation 2 2 4
Amount and quality of geotechnical data 2 4 8
Experience with similar foundations in similar conditions 1 1 1
Method of assessment of geotechnical parameters for design 2 3 6
Design method adopted 1 2 2
Method of utilising results of in situ test data and installation data 2 2 4
Level of construction control 2 2 4
Level of performance monitoring of the supported structure during and after construction 0.5 5 2.5
Σwi 14.5 ΣwiIRRi 37.5

{\rm{ARR}} = \dfrac{{\sum {\left( {{w_i}{\rm{IR}}{{\rm{R}}_{\rm{i}}}} \right)} }}{{\sum {{w_i}} }} = \dfrac{37.5}{14.5}=2.58

Table 6.3. Basic geotechnical strength reduction factor φgb for different average risk ratings (AS2159).
Range of average risk rating, ARR Overall risk category φgb for low redundancy systems φgb for high redundancy systems
ARR ≤ 1.5 Very low 0.67 0.76
1.5 < ARR ≤ 2.0 Very low to low 0.61 0.70
2.0 < ARR ≤ 2.5 Low 0.56 0.64
2.5 < ARR ≤ 3.0 Low to moderate 0.52 0.60
3.0 < ARR ≤ 3.5 Moderate 0.48 0.56
3.5 < ARR ≤ 4.0 Moderate to high 0.45 0.53
4.0 < ARR ≤ 4.5 High 0.42 0.50
4.5 < ARR Very high 0.40 0.47

 

License

Icon for the Creative Commons Attribution 4.0 International License

Fundamentals of foundation engineering and their applications Copyright © 2025 by University of Newcastle & G. Kouretzis is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Share This Book