50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
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Terrace Tunnel South Portal to the Basin Reserve,<br />
in Wellington City. The bypass is predominately at<br />
street level, but a section of the northbound lanes<br />
from Willis Street / Abel Smith Street intersection<br />
to Ghuznee Street involves construction of a<br />
depressed two-lane road. Opus International<br />
Consultants is Transit's consultants for this project<br />
from its inception to detailed design and currently<br />
construction management, and has developed<br />
innovative approaches to resolve the geotechnical<br />
challenges.<br />
The depressed section will pass under a new bridge<br />
at Vivian Street and will require extensive retaining<br />
structures. The innovative trench design<br />
incorporates a propped reinforced trough structure<br />
and 200 m long soil nail retaining walls between<br />
the Terrace Tunnel portal and Abel Smith Street.<br />
The up to 7 m high soil nail walls are supported by<br />
a total length of about 7000 m of soil nails. This<br />
will be the largest soil nail wall in New Zealand.<br />
Extensive geotechnical site investigations have<br />
been carried out to determine ground and<br />
groundwater conditions in the vicinity of the bypass<br />
route, particularly where the proposed road will be<br />
depressed below existing ground level, and adjacent<br />
to existing buildings. In the propped wall section,<br />
groundwater levels will be temporarily lowered<br />
during construction, then allowed to recharge. In<br />
the soil nail sections, groundwater levels will be<br />
permanently lowered. During construction a<br />
limited amount of excavation will be opened at any<br />
one time to minimise the amount of drawdown in<br />
groundwater levels. Artesian pressure relief wells<br />
are being drilled into bedrock to relieve<br />
groundwater pressures are underway. Building<br />
condition surveys, before during and after the<br />
construction are being undertaken.<br />
A programme was developed to monitor the<br />
groundwater, ground deformation, and subsidence.<br />
Instrumentation comprises of piezometers,<br />
inclinometers and settlement stations which were<br />
installed prior to commencement of excavation.<br />
Multiple piezometers were installed in many of the<br />
boreholes to record and monitor the groundwater<br />
levels in different aquifers. Over 90 vibrating wire<br />
and standpipe piezometer combinations were<br />
installed in 41 different boreholes throughout the<br />
Wellington Inner City Bypass area, measuring a<br />
number of different aquifers. Vibrating wire<br />
piezometers were used at locations important for<br />
construction monitoring, as they respond more<br />
quickly to groundwater level changes that may<br />
occur during construction, especially in the bedrock<br />
aquifer that will be drawn down during construction<br />
of the walls.<br />
The piezometers were installed in a number of site<br />
investigation stages. All 41 piezometer sets will be<br />
monitored regularly throughout the Wellington<br />
Inner City Bypass construction, a difficult<br />
undertaking in a dynamic urban environment, with<br />
traffic, vandalism, road maintenance, and urban<br />
development making continued monitoring<br />
challenging work.<br />
POSTER<br />
TRANSITION FROM S<strong>OF</strong>T-SEDIMENT TO<br />
WEAK ROCK DEFORMATION IN<br />
TURBIDITES <strong>OF</strong> THE MIOCENE<br />
WAITEMATA PIGGY-BACK BASIN,<br />
NORTHERN <strong>NEW</strong> ZEALAND<br />
K. B. Spörli &J.V.Rowland<br />
Department of Geology, University of Auckland,<br />
Private Bag 92019, Auckland.<br />
(kb.sporli*auckland.ac.nz)<br />
The well known, spectacular shore platform and<br />
cliff exposures of the Miocene Waitemata Group on<br />
eastern Whangaparaoa Peninsula 50 km north of<br />
Auckland provide a world-class example of weak<br />
rock deformation, the neglected domain between<br />
soft-sediment and hard rock deformation. As a new<br />
convergent plate boundary propagated southwards<br />
through northern New Zealand in the Late<br />
Oligocene, the Northland Allochthon was<br />
emplaced. The allochthon subsequently formed a<br />
mobile base for the Waitemata piggy-back basin,<br />
resulting in a complex interaction between tectonic<br />
and gravity-driven shallow deformation. Quartzpoor<br />
clastic sequences of the Waitemata Group at<br />
Whangaparaoa display a protracted sequence of<br />
deformations: D1, syn-sedimentary slumping at the<br />
top of the sedimentary sequence and including<br />
seismites; D2, large scale deeper-seated sliding and<br />
extensional low-angle shearing, associated with<br />
generation of boudinage and broken formation; D3,<br />
contractional shearing at low angle to bedding<br />
associated with thrust faults and inclined folds,<br />
indicating transport mostly to the SE; D4, thrusting<br />
and folding in the opposite direction; D5, further<br />
folding including sinistral shear; D6, steep faults.<br />
The deformation sequence suggests continuous or<br />
intermittent southeast-ward transport of units with<br />
increasing sedimentary and structural burial. Phase<br />
D3 boudinage and broken formation are localised<br />
and, with a few exceptions, occurred by brittle<br />
deformation. Subsequent folds show little variation<br />
in bedding thickness around their hinges. By phase<br />
D3, the rocks had therefore passed from the softsediment<br />
state to low levels of consolidation.<br />
However, with a compressive strength of ~5 MPa<br />
they are weak rocks even today. These<br />
deformations, although more intensive than<br />
elsewhere, are typical for the entire Waitemata<br />
basin. Such weak-rock deformation must be<br />
important in other sedimentary basins, especially in<br />
those associated with active convergent plate<br />
50 th <strong>Kaikoura</strong>05 -82- <strong>Kaikoura</strong> <strong>2005</strong>