2795
Characteristics Values in Rock Socket Design
Valeurs caractéristiques d'ancrage sur roche
Look B., Lacey D.
Sinclair Knight Merz Pty Ltd, Australia
ABSTRACT: The substructure of the Gateway Bridge comprises 1.5 metre diameter bored piers socketed into sedimentary rock.
Characterisation of the rock strength properties, through goodness-of-fit tests, showed the use of non-normal distributions produced
realistic characteristic strengths, while comparable predictions based on a Normal distribution showed unrealistically low values
existed below the 20
th
percentile reliability. Since limit state codes imply characteristic design strengths should be derived from
conservative (low) percentile values, erroneous characteristic strength values may be produced due to an assumption of a Normal
distribution. Two land based test piles fitted with Osterberg cells tested the sedimentary bedrock for shaft capacity at the bridge site,
and “Characteristic” rock strengths required by various rock socket design methods to replicate observed pile shaft capacity have been
back-calculated. This paper assumes that all considered design methods are equally “correct”, and compares the required design
values (the selection of which are often subjective) to their relative location within the applied strength profile distributions.
RÉSUMÉ : L'infrastructure du pont "Gateway Bridge" a des piliers forés de 1,5 mètre de diamètre ancrés dans de la roche
sédimentaire. La caractérisation des propriétés de résistance de la roche, faîte par le biais de test d'ajustement, a montré que
l'utilisation de loi non-normale a produit des caractéristiques de résistance plausibles alors que des prédictions comparables basées sur
une loi normale pouvaient devenir artificiellement basses pour un degré de probabilité au 20ième centile. Certaines normes d' états
limites suggèrent que les caractéristiques de résistance devraient être dérivées à partir de valeurs (au bas mot) de bas centile, des
valeurs de caractéristiques de résistance erronées pouvant être calculées en assumant une loi normale. Deux piliers tests installés sur
terre-sèche avec des cellules d'Osterberg ont été utilisés pour tester la capacité de la roche sédimentaire par rapport au pilier mais les
valeurs de résistance caractéristique de roche nécessitées par les diverses conceptions d'ancrage sur roche ont été rétro-calculées afin
de reproduire la capacité observée des piliers. Cet article assume que toutes les méthodes de conception considérées sont tout aussi
"correctes" et dérive les valeurs caractéristiques de conception requises (la sélection desquels est souvent subjective) afin de
reproduire les capacités observées des piliers durant des tests à grande échelle.
KEYWORDS: Rock socket, bored piles, characteristic design value, statistical distribution, Osterberg pile tests, sedimentary rocks
1 INTRODUCTION
The Gateway Upgrade Project (GUP) was the largest road and
bridge infrastructure project ever undertaken in Queensland,
Australia. The six lane bridge structure spans 1.6 km between
abutments with a main river span structure of 520 metres. This
paper focuses on rock socket design procedures applied to two
large-scale, land-based pile load tests conducted for this project.
The rock founding conditions varied across the bridge
footprint as summarised in Table 1. Characterisation of rock
strength properties included the derivation of site-specific
correlation of Point Load Index (I
s(50)
) data with Uniaxial
Compressive Strength (UCS) test results, and a statistical
analysis of resulting datasets.
The two test piles (TP1 and TP2) installed with Osterberg
Cells were constructed to investigate the rock socket behaviour
under high loads and identify any constructability issues prior to
construction of the two river piers. This paper considers various
accepted methods of pile rock socket design and compares their
applicability to the load tests completed at this site.
Rock socket design methods typically have similar
formulations for the estimation of side shear capacity, but
produce varying results due to their method of derivation, and
the available data or tested rock types used for formulation.
While the rock type may be a governing factor, this paper
assumes that all the methods produce “correct” pile designs, but
require varying “characteristic” design input values to produce
equivalent results. Reliability theory implies a moderately
conservative or cautious estimate should be used as the
characteristic design value, yet without a statistical basis the
selection of appropriate characteristic values remains subjective.
Table 1. Background Data
Location /
Pier
No. of Bored Piers
Key Geological Issue within
Rock Founding Layers
Land 5 –
Southern
10
(+ TP1)
Dipping Coal seam layer within
zone of influence of pier.
River 6
24
River 7
24
Random Shear zones with
varying length of piles
Land 8 –
Northern
10
(+ TP2)
Uncertain and inconsistent data
with possible weak zones
1.1
Background
Whilst driven piles were used extensively across the GUP site,
the river span of the Gateway Bridge is founded upon 1.5 metre
diameter bored piers socketed into sedimentary rock. Piers 5
and 8 are located on the riverbank while Piers 6 and 7 are
located within the river. River piers consisted of 24 piles that
extended to a depth of over 50m below the river level, and each
of the land based piers consisted of only 10 bored piers. Day et.
al. (2009) provides further GUP foundation and project details.
For the bridge foundation the key geological features were:
o
The basement rock consisted of Triassic aged material.
This includes layers of sandstone, siltstone, mudstone and
low grade coal formed about 220 to 180 million years ago.
This material does not have any significant folding, but is
known to have faulting as a consequence of crustal
tension in the Tertiary period.
o
Deposition of Quaternary Alluvium occurred in the recent
past. This site is located close to the mouth of the
