B5109 Llansadwrn Embankment

The site consists of a 90m section of degraded masonry wall located on the embankment shoulder which supports the B5108 near Llansadwrn, Anglesey as it crosses over the Afon Cadnant.

Completed nailed edge beam and parapet wall. Completed nailed edge beam and parapet wall.
Llansadwrn Embankment Llansadwrn Embankment - embankment stabilisation.

A wall with an upstand of around 600mm is present on the carriageway, acting as a parapet in lieu of road restraint to vehicles.

The wall was in disrepair and the road showed tell tale signs of spreading and lateral cracking, particularly on the southbound lane and most pronounced as it crosses over the river. Intervention was required to prevent the loss of the edge of the road and maintain this vital north / south route between Beaumaris and Red Wharf Bay for the local community and tourists. The road serves a local bus route and is located on a stretch without alternatives routes and very few places to safely turn a vehicle around. The worksite is unsighted on the approach from either direction as it is located in a topographical dip, this meant careful consideration to the safe management of traffic was required. Temporary traffic lights were installed which were required to cover an area of around 1000m, to ensure traffic was held at a safe distance.

Griffiths reviewed the geology which showed fill over Alluvium consisting of clay, sand/gravel with peat layers over Glacial Till consisting of gravelling clay. The ground model is consistent with that associated with long term settlement and displacement of the soft foundation soils due to the weight of the embankment resulting in cracking and displacement of the road over time.

The project consisted of deconstructing of the masonry wall forming the edge of the carriageway, excavation into the embankment to form a new reinforced cast insitu foundation, soil nailed into the Glacial Till below to provide restraint. New carriageway drainage, surfacing and re-construction of the masonry together with regrading of the embankment were also completed.

The existing parapet was taken down carefully and set aside for re-use.

Drilling soil nails using grout flush. Drilling soil nails using grout flush.
Carriageway edge beam construction Carriageway edge beam construction complete with highway drainage.

Griffiths worked with our design partner Jubb to rationalise a simple but effective soil nailed edge beam design to provide carriageway restraint without the need to fully reconstruct the road and embankment.

The design was developed in a short timeframe in order to address the road movement before it started to adversely impact road users and to take advantage of completing construction in the drier summer / autumn months.

The design was verified by completing suitability tests on the soil nails early in the construction programme rather than completing a prolonged design programme, which is the traditional approach where time permits. The benefit of this approach was to curtail the development programme and get to site quicker to fix the issue.
Initially the client had expected the road to be fully closed whilst the repairs were undertaken, however Griffiths reviewed the construction activities and sequencing and were able to close one lane only for the duration of the works allowing passage of road traffic under lights.

Embankment excavation and marking out soil nail locations. Embankment excavation and marking out soil nail locations.

Fifty-seven 38mm diameter, 9m long soil nails were drilled at a 1.5m intervals through the embankment and Alluvial deposits into the Glacial Till below. The soil nails were drilled at 102mm diameter utilising Griffiths owned Ripamonti THV hydraulic excavator mounted drilling rig and 13T excavator.

A reinforced concrete edge beam was cast in 12m long bays to a 600mm high by 450mm wide dimension, within timber shuttering, designed and installed by Griffiths. The reconstruction included the installation of 3 gully pots and a 225mm carrier drain within the carriageway. The concrete was placed direct from the concrete wagon chute whilst traffic was held for 15-minute intervals.

The stone set aside from the deconstruction phases was sorted and re-used to construct the carriageway parapet. The reasoning was two-fold in that waste and carbon impacts were reduced but also importantly the visual appearance of the finished works would look identical from the perspective of the road user.

The project was self delivered by Griffiths with support from our Traffic Management subcontractor Quantum and surfacing contractor Hogan.

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