Some Limit State Design Experiences in Australia
University of New south Wales, ADFA campus, Canberra, ACT 2600, Australia email@example.com
Abstract: Current limit state geotechnical codes in Australia are briefly discussed. Selected geotechnical projects that benefited from the use limited state design are briefly presented.
Limit state design in Australia has a short history. The first limit state geotechnical code was introduced by AUSTROAD, an umbrella organization representing the interests of all state road authorities, in early 90s. Geotechnical design codes in Australia are developed by Standards Australia and various state road authorities. The former is referred to by an AS number. As its name implies, Standards Australia has been developing codes intended for applications in Australia in general. As Australia is a spatially large country and the ground conditions and practices vary across Australia, geotechnical codes developed by Standards Australia tends to be flexible. The various state road authorities in Australia have also been developing their own codes, either individually or collectively via AUSTROAD. Currently limit state geotechnical codes in Australia are: AUSTROAD (1992), AS 2159 (1995), R57 (2002) and AS 4679 (2002). AUSTROAD (1992) is mainly for the design of piles and footings although it also contains brief recommendations on the design of simple retaining structures. AS 2150 (1995) is for piling. R57 is for the design of reinforced soil wall. It was developed by the Roads and Traffic authority, New South Wales, as a design specification. AS 4679 (2002) is for the design of “common” earth retaining structures, including reinforced soil walls. It has been adapted for use by other state road authority such as Queensland Main Road and referred to by a different reference.
partial material factors fm (i). The two factors are simply related by Φ m (i)=1/fm (i). In LRFD, R* is obtained by applying a single partial factor to Ru, the ultimate capacity/resistance calculated in the usual traditional way using non-factored parameters, that is R* = Φ RRu (3)
Both formats were adopted in the current limit state codes in Australia. In general, LRFD format is used for pile design whereas MFA format is used for the design of retaining walls. 2.2 Characteristic Values Different wordings are used in different codes in the definition of characteristic values. AUSTROAD (1992) defines characteristic values for soil and rock parameters as values that “should be selected such that, in the opinion of the designer, the probability of a more adverse value occurring in the field is not greater than 5%”. It also clearly points out that a number of factors, such as testing type, field geology, failure modes etc, need to be considered in the selection of characteristic values. The Australian piling code, AS 2150 (1995), defines characteristic value in such a way that it is only applicable to material strength as assessed by a standard test. Hence, one may argue that it is not relevant to geotechnical parameters. This does not present any problem as this code adopted LRFD for the determination of the design (factored) geotechnical capacity of a pile. The definition adopted by R57 is essentially identical to that of AUSTROAD (1992). However, as explained in Mak and Lo (1996), “the value” should be the spatial average over the potential failure domain, and is close to the population mean because the failure domain is large relative to the scale of fluctuation. Hence, the variability is mainly due to uncertainty is in the estimation of population mean from limited data. The design (factored) friction angle for select fill, is assessed directly (Simpson 1992). This is achieved in R57 by prescribing a material factor of unity to the friction angle at constant volume state. AS 4678 (2002) defines characteristic values as representative value. For soil properties, it is noted as a cautious...
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