2925
Characterization of Sensitive Soft Soils for the Waterview Connection Project, New
Zealand
Caractérisation de sols mous sensibles pour le projet de raccordement Waterview en Nouvelle-
Zélande
Bobei D.C.
1
, Locks J.
2
Aurecon Group Pty Ltd
1
Level 2, 116 Military Road, Neutral Bay, Sydney, Australia
2
Level 4, 139 Carlton Gore Road, Newmarket, Auckland, New Zealand
ABSTRACT: The paper presents the results and interpretations of data collected during the procurement phase of SH16 motorway
upgrade. The strength and consolidation characteristics are investigated for a soil unit, labeled AH, which is identified to manifest a
response typical for a sensitive soft soil. The estimateof undrained shear strength based on empirical methods is found to have
limitations to predict the undrained shear strength of the sensitive AH soil. The one-dimensional compression response of the virgin
AH soil is proposed to estimate using a relationship between the liquidity index and the vertical effective stress. The predictive
capability of this relationship is demonstrated by numerical simulations of settlement monitored during the construction and post-
construction phase of the original SH16 motorway embankment.
RÉSUMÉ : Cet article présente les résultats et les interprétations des données recueillies lors de la phase de projet de mise à niveau de
l’autoroute SH16. La résistance et les caractéristiques de consolidation sont étudiées pour un type de sol, noté AH, qui est
représentatif d’un sol mou sensible. L'estimation de la résistance au cisaillement basée sur des méthodes empiriques est peu adaptée
pour prédire la résistance au cisaillement non drainée du sol sensible AH. La réponse en compression unidimensionnelle du sol vierge
AH est modélisée par une relation entre l'indice de liquidité et la contrainte effective verticale. La capacité prédictive de cette relation
est démontrée par des simulations numériques des tassements mesurés pendant la phase de construction et post-construction du
remblai original SH16 de l'autoroute.
KEYWORDS: clay, sensitive soil, critical state soil mechanics, SHANSEP, settlement.
1 INTRODUCTION
The Western Ring Route (WRR) is an ambitious project
initiated by New Zealand Transport Agency (NZTA) toprovide
a 48km alternative route for improvement of traffic flow around
the Auckland city center. One significant work component of
the WRR project is the upgrade of the State Highway 16
(SH16), where part of the route is passing through an estuarine
environment. This section of the motorway, referred to as the
Causeway, has experienced significant settlements over a period
of 60 years of service life. Today the traffic lanes are prone to
flooding during storm and king tide events.
2 PAPER OBJECTIVES
NZTA has commissioned Aurecon to undertake an in-depth
investigation into the soil conditions present along the
Causeway Section. The geotechnical investigation includes
over one hundred exploratory holes, in addition to numerous
older holes drilled during the planning phase for the original
Causeway in the late 1950s.
The field investigations were complemented by laboratory
testing for the purpose to provide details on strength and
compressibility of the Causeway estuarine soil materials.The
compressibility and behaviour of AH soils is investigated in the
critical state framework. The ability of SHANSEP formulation
to predict the undrained shear strength is investigated based on
the piezometer (CPTu) data.
Based on the non-linear one-dimensional compression
manifested by the sensitive AH soil, a framework of analysis is
proposed to predict the non-linear one-dimensional compression
of AH soil. An example of soil settlement analysis is carried
out to predict the magnitude and rate of settlement development
of the original SH16 motorway embankment during the
construction and post-construction stages.
3 GEOLOGICAL CONDITIONS
A detailed geological model was developed from the
information collected during five (5) stages of site
investigations. The ground profile consists of geological
conditions which adopt a layering code system as summarized
in Table 1.
The marine sediments (AH soil unit) belong to the Late
Pleistocene-Holocene age with a deposition environment
starting between 8,000 to 14,000 years ago, which still carries
on today.
The AH deposit generally consists of uniform normally
consolidated Silty Clays. The very soft strength characteristic
of the AH soils is in direct contrast to the typically
Table 1.Geological layer codes adopted for SH16 Causewayalignment.
Geologic
Age
Unit
Layer Code
Description
Late
Pleistocene
– Holocene
Marine
Sediments
AH
Marine clays and silts
– estuarine muds
ATcl
Clays and Silts
ATs
Sands and Silty Sands
ATo/ATp
Organic Clay/Peat
Pliocene -
Pleistocene
Tauranga
Group
ATv
Rhyolitic Silt and
Sand (volcaniclastic)
ER
Residual ECBF Soil
EW
Weathered ECBF
Sandstone – Soil and
Rock Fractions (20 <
N < 50)
Miocene
East Coast
Bay
Formation
(ECBF)
EU
Unweathered ECBF
Rock – Sandstone and
Siltstone (N > 50)
