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Collapsed Retaining Wall Reconstruction in Looe

 

A sequence of illustrative Photographs – Elm Tree Road

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Original wall profile, viewed from downhill position, providing support to Elm Tree Road. 3.5m high on average with base embankment about 1.5m over steep slope at about 450 down to a series of old stone walls, forming steep terraces to the rear of housing, overlooking Looe. The retaining wall dates from early 1950’s.

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Wall Collapse – Note plastic covering to protect against further collapse due to rainfall. The whole road area was subsiding downhill, endangering the stability of uphill properties together with utility services.

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The photograph above provides a section view of the collapsed wall (rotation failure about the foundations). The original concrete wall had some steel bar and rope as reinforcement with trench fill foundations and a backing of stone masonry. This profile shows the rotational failure about the foundations and allegedly the result of the removal of passive ground support and undermining of the wall foundations.

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A view down the location of the collapsed wall to show the topography and consequent high risk of works of remediaition. A design solution had to go hand in hand with a method of work and risk profiling that we value engineered in the design office.

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We decided to reduce levels within the “crater” of the collapsed zone by benching the slope profile to provide a safe working platform for installing a soil nail restrained embankment. This provided a strip of about 3m from which the collapse debris could be removed and where foundations involving low vibration “Odex” formed 220mm diameter piles could be installed (see below) to support a ground anchor tied support slab for pre-cast concrete retaining wall elements. Key features of this first stage action were to stabilise the uphill housing foundations, utilities and provide a narrow access for housing occupiers.

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Completion of piling.

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Steel reinforcement and other preparations are being made for the supporting ground slab and anchor blocks tied into and supported off the piling.

In all, some 30m’s of new retaining wall was required.

In order to ensure good line and to ensure the permanent works were provided on the land of the owner responsible for these remedial works, a steel wire “string line” was installed (see the lower part of the photograph). An old fashioned plumb bomb hung from the wire rope enabled a simple means of accurate placement of the shuttering for the concrete and back face of the pre-cast concrete retaining wall elements.

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Completed ground slab and ground anchor installation (under inspection and proof testing). Note the first stage of the pre-cast retaining wall installation in the foreground.

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A view of the pre-cast concrete retaining wall elements being erected onto the pre-formed concrete foundation. Note the temporary ground retaining treatment with sand filled bags to provide safe support for the crane.

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Where parts of the original wall did not collapse, but were only slightly distorted, a series of ground anchors were installed to reinstate the wall stability for the long term.

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The photographs above and below provide two views of the completed works.

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