2022-09-06 P&E Agenda - Item 5 - Attachment 1 - Six Mile Creek Bridge Replacement Offset Report - TMR

The existing Six Mile Creek bridge is a one-way, single span timber structure, approximately 55 m long. The
bridge has priority movement to southbound vehicles. Currently, insufficient site distance combined with a
high approach speed for the oncoming priority vehicles, make it difficult for vehicles to judge when to give
way, and when stopped, to decide when it is safe to proceed. There are observed occasions of northbound
vehicles failing to give way at the bridge.
The existing timber bridge at Six Mile Creek, currently provides immunity for only small flood events to an
~50% annual exceedance probability event. Due to the bridge being a timber structure, following over toping
events the road remains closed whilst the bridge is inspected and deemed safe for traffic. The purpose of the
proposed action is to improve the condition of the existing transport network through installation of a new
two-lane bridge with improved flood immunity. A concrete bridge is designed for overtopping and therefore,
will not need to remain closed following flood events for maintenance inspections.


1.3          Construction methodology
The proposed action comprises replacing the current single-lane timber bridge that crosses Six Mile Creek
with a new two-lane bridge and constructing new southern and northern road approaches (refer Figure 1.2
and Figure 1.3). The new bridge location is proposed to be positioned to the west of the existing bridge,
approximately 15 m downstream.
The approach roads have been built as close to the existing alignment as possible to minimise impact on the
environment. The new alignment will sit to the east of the existing road so it can be used as a side track
during construction eliminating the need for temporary works. The geometric design was complicated by two
reverse curves intersecting at the bridge location. As such, the bridge has a twist in it due to its
superelevation changing across the length of the structure. This complicated the structural design leading to
the thicker than usual bridge deck, which impacted the hydraulics and afflux up stream. To address this
issue the road had to be realigned vertically.
The construction methodology includes a working platform (piling pad) to be constructed next to the high
bank on the southern approach. The platform allows construction activities to be undertaken whilst avoiding
disruption to the creek flow (during non-flood conditions) and therefore maintaining fish passage (refer
Figure 1.4). The construction machinery will use the platform as a base to cast in place the piles adjacent to
the lower bank. Using this methodology results in the works not requiring a coffer dam of the creek to keep
the work area dry.
The bridge piles will be constructed using cast in place concrete piles with a steel lining. A hole is drilled into
the ground, the steel tube is inserted, and the concrete poured in. The steel is not removed once the
concrete is set. This reduces time by half working adjacent to the creek and it is also less disruptive than
driven piles.
All ancillary areas (e.g. construction compound) during construction will be located within existing cleared
areas adjacent to the project, no additional clearing will be allowed for site compounds or storage.
Following the construction of the new bridge, the existing timber bridge is proposed to be removed and
timber used as replacement parts for maintenance on other bridges in the network.




                             Project number 511784 File Six Mile Creek - Matters of National Environmental Significance Report 2021-12-03 Revision 1   2