Narrows Interchange Monument EarthCache
Hidden : 8/30/2013
Difficulty:
2 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

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Geocache Description:

The Listed Coordinates are for a monument marking the official opening of the Narrows Interchange on 30th November 1973 by John T. Tonkin, Premier of Western Australia.

The monument hosts a number of panels depicting the process involved in preparing the ground for this $22.5 million project.

The nearest practical parking is possibly Barrack Street to the east or from Esplanade Station; you can walk or cycle from both locations to the Listed Coordinates.

Lesson: To learn the importance of geology in major infrastructure projects and how different geological properties can be utilised as possible solutions to geological problems.

The project was undertaken over a number of phases:
     1957-59: Construction of Narrows Bridge and Kwinana Freeway.
     1964-69: Drain and Stabilise Area Reclaimed for Interchange.
     1969-73: Construct Bridges, Roads and Landscaping.


EarthCache Details:

The monument panels are numbered 1-5, although there are other ‘generic’ panels without numbers. These panels will guide you through the geological problems and solutions that were adopted to allow construction of one of the biggest projects of this type in the world at the time. The answers to the questions can be found on these numbered panels, which outline the various stages of development.

Panel 1 - Reclamation from Swan River.
To construct the interchange it was necessary to reclaim land from an old river bed filled with deep mud. This mud had to be compressed to minimise future settlement. These explanatory panels depict this sequence of work.

Question 1: Use the diagram on the panel to identify geological materials A, B and C in the image below.


Panel 2 - Construction of Sand Drains.
A hollow steel tube was driven to the bottom of the mud and filled with sand. The tube was then withdrawn, leaving a vertical sand drain through which water can escape from the mud. A total of 43,000 sand drains were installed (Gregory, 2009).

Question 2a: How many completed sand drains are depicted in the diagram on Panel 2?

Question 2b: Suggest why the sand is more suitable for this purpose?

Panel 3 - Compaction of Mud to Stabilised Road Foundations.
A sand load was placed and excess water removed from the mud by the pressure of the sand load. Subsequent reduction in water content permitted compaction of mud with a resultant gain in strength.

Question 3: What was the purpose of the sand for this phase?

Panel 4 - Sinking Of Caisson for Bridge Foundation.
A caisson is essentially a tube that allows excavation of a vertical shaft while retaining the walls. Firstly, a caisson segment was assembled on surface. Soil was excavated from inside of segment causing it to sink while further segments are cast at surface. The process continued until caisson reached the bedrock.

Question 4: What term (beginning with ‘M’) is used on Panel 4 to describe what the bucket is being used for?

Panel 5 - Bridge Building and Landscaping.
Free-standing piers founded on the bedrock were constructed inside the caisson to support the superstructure, which was cast in-situ. Lakes were excavated, roads formed and landscape treatment was completed.

Question 5: According to Panel 5, what does the caisson protect the bridge pier from?


The logging requirements are repeated here for convenience:
Q1)   Identify geological materials A, B and C.
Q2a)  How many completed sand drains are depicted in the diagram on Panel 2?
Q2b)  Suggest why the sand is more suitable for this purpose?
Q3)   What was the purpose of the sand for this phase?
Q4)   What term (beginning with ‘M’) is used on Panel 4 to describe what the bucket is being used for?
Q5)   According to Panel 5, what does the caisson protect the bridge pier from?

Sources:
Information panels on the monument.
GREGORY, J.A. 2009, 'Perth's Waterfront and Urban Planning 1954-93: the Narrows Scheme and the Perth City Foreshore Project', State of Australian Cities, Crawley, W.A., N/A, pp. 1-19.

All images © 'Geology Boy'.


Please use the following link to submit answers to this Earthcache: GeologyBoy.Earthcache@gmail.com
(This link pre-formats a subject line with the EarthCache reference so I can easily distinguish it from my other EarthCaches.)
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I hope you enjoy your visit to Narrows Interchange Monument!

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