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Construction, design and engineering
Structural Engineering and design planning

ProjectCollapsed Retaining Wall Reconstruction
DescriptionA sequence of illustrative Photographs – Elm Tree Road
ClientPrivate Client
LocationLooe, Cornwall
ValueLooe, Cornwall
ScopeFollowing the collapse of a major retaining wall in a residential area, PDP were responsible for the full lifecycle of remediation including assessment, design, planning through to construction and handover. Of key importance was collaborative working with stakeholders such as the Parish Council, media and residents. This was an innovative and challenging project given the difficult location and need for a solution which halted any further collapse or damage to local property.Tight control over capital expenditure was vitally important to ensure the most cost-effective solution for the client whilst minimising risk.

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The original wall profile is shown in Figure 1, viewed from a downhill position. It provided support to Elm Tree Road and was 3.5m high on average with a base embankment about 1.5m over a steep 450 slope, below which were a series of old stone walls, forming terraces to the rear of residential housing.

The retaining wall dated from the early 1950’s.

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Figure 2 highlights the emergency works PDP instructed following the collapse. – Note plastic covering to protect against further destabilisation due to rainfall. The whole road area was subsiding downhill, endangering the stability of uphill properties together with utility services.

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Figure 3 provides a section view of the collapsed wall. The original concrete wall had both steel bar and rope 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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Figure 4 shows a view below the site of the collpase. This details the topography and consequent high risk of works of remediaition to structures below the site. A design solution had to go hand in hand with a strategy which minimised risk whilst meeting the needs of local stakeholders. Given the highly visable nature of the collapse and other recent collpases in Looe, PDP were dealing on a regular basis with public questions and media attention.

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Figure 5 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 Figure 6) 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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Figure 6 Foundations, involving low vibration “Odex” formed 220mm diameter piles.

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Figure 7 shows a longitudinal perspective of the steel reinforcement and other preparations which were 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).

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Figure 8 shows the 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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Figure 9 illustrates 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.

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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Figure 10 Completed view of the retaining wall.

Gallery