By Patricia Guerra-Escobar
The Pont Briwet Project in Penrhyndeudraeth, North Wales, U.K., consisted of the replacement of a historic timber trestle bridge with a new 446-foot (136-m) long concrete viaduct and the upgrade of the approach road to the north and south of the crossing (Figure 1). The site is set within the floodplain of the Afon Dwyryd (River Dwyryd), at the boundary of Snowdonia National Park, with tidal levels up to 15 feet (4.6 m). The existing 22-span wooden viaduct dated back to the 1860s and carried the single-track Cambrian Coast Railway and a single-lane road (Figure 2). It had served the area well for 160 years but was no longer suitable for modern transport requirements. The new viaduct provides a wider rail and a two-lane road, as well as links the local footway and cycleway networks on either side of the estuary. A key challenge of the project was building the viaduct within the tidal estuary while meeting the requirements on flood risk and water quality within an Area of Conservation of Natural Resources Wales.
The new viaduct comprises seven 66-foot (20-m) integral spans, formed from pre-stressed beams supported on concrete crossheads and large diameter concrete piles installed into the estuarine sands and gravels. For the causeway and ramps, it was necessary to find a solution to construct temporary embankments to gain access for the construction of the foundations of the viaduct within the river. The initial design considered permanent sheet-pile retaining walls; however, this solution would have required a large amount of work to take place within the tidal river and would have had a significant impact on the cost of the project and the environment.
As an alternative to sheet-pile walls, we proposed to construct reinforced earthwork embankments using uniaxial geogrids and a specific granular material for the embankments’ starter layers to allow the installation to take place directly in open water. The temporary causeway was required to extend 263 feet (80 m) into the tidal estuary from the southern shore and 66 feet (20 m) from the north (Figure 3).
The reinforced soil slopes with uniaxial geogrids were designed to increase the stability of the slopes so that heavy plant machinery could travel on the access roads, 3.3 feet (1 m) away from the edge of the embankment.
The height of the embankments ranged from 6.6 to 13.5 feet (2 to 4.1 m) with slope angles of 1V:1.5H and 1V:2H. The causeway working platforms were required to support large-tracked piling rigs and cranes, in addition to an 882-ton (800-tonne) mobile crane required to lift the bridge beams into place.
The top level of the temporary embankments was at 17.1 feet (5.2 m) AOD (above ordinance datum) and maximum water level was assumed at 13.9 feet (4.3 m) AOD. For the causeway, geogrids were used for the stability of the slopes and for the basal reinforcement of the working platform to support the pressures applied by the cranes and piling rigs.
The top level of the causeway was at 10.5 feet (3.2 m) AOD with seabed varying from 3.3 feet (1 m) AOD to -8.2 feet (-2.5 m) AOD and maximum water level assumed at 8.5 feet (2.6 m) AOD. During the temporary works, a woven geotextile (wraparound) was used to protect the slopes from erosion and to prevent the fines from being washed out with the constant changes of water level.
The main challenge for the design of the reinforced embankments was to keep the distance from slope edge for piling rigs and cranes to 3.3 feet (1 m), with a maximum slope angle of 1V:1.5H. The variation of the water levels due to the tidal activity added a further complication. For the design, it was considered that during the construction the causeway would be completely submerged on various occasions yet needed to be back in use immediately after the tidal events. In the design, we included some reinforced areas on the ramp approaches, above the predicted flood levels, to allow the tracked plant to remain safe during these events.
For each section of the causeway, a stability analysis of the reinforced embankment was carried out considering the different pressures of the rigs and the cranes, the maximum water level in each case, and the minimum distance to the edge of the embankment. Based on the results, the slope angle for each section and the minimum distance were determined.
The maximum height of the embankment was 13.5 feet (4.1 m) in section CH240 and the minimum height was 6.9 feet (2.1 m) in section CH380. For both sections, CH240 and CH380, the global safety factor was less than 1.00 for the scenario with a slope angle of 1H:1.5V and a distance of 3.3 feet (1 m) from the edge. So, the proposal for these sections was to increase the slope angle to 1H:2V and to maintain the distance of 3.3 feet (1 m) to the edge of the embankment. Figures 4a and 4b show the results of the stability analysis of the Section CH240 Embankment—Access Road and the CH380 Temporary Access Road—Causeway.
For the fill material it was very important to consider the use of local resources, to provide environmental and cost benefits to the project.
The starter layer was specified with a granular material Class 6B with no fines, and the embankment fill was designed with a locally modified Class 6N material (classifications according to the Specifications for Highways Works, Series 600).
Reinforced fill properties:
- Class 6B: Selected coarse granular material used for starter layer, with no fines (Particle sizes: 11.8 inches (300 mm) to 4.9 inches (125 mm)
- Class 6N: Selected well-graded granular material to use for embankment fill
- Friction angle Φ’=40˚
- Unit weight γ=129 pounds per cubic foot (20 kN/m3)
- Cohesion C’=0 psf (0 kPa)
Ground conditions in the estuary comprised deep loose sands over dense gravels incorporating cobbles overlying rock.
Foundation soil properties:
- MD Sand
- Friction angle Φ’=33˚
- Unit weight γ=122 pounds per cubic foot (19 kN/m3)
The reinforced embankments were designed based on the limit equilibrium analysis in accordance with BS8006-1:2010 for the internal stability and Eurocode 7 (BS EN 1997-1:2004) for the global stability. We used the method from Rowe and Soderman (1985) to estimate the stability of a reinforcement embankment for bearing capacity and the methods of reinforcement proposed by Giroud, Holtz and Bonaparte (1985), modified here in conjunction with the properties of the geogrids (Figure 5).
For the working platforms, we used the BRE470 guidance with a modified method based on bearing capacity and loading distribution to incorporate the biaxial geogrids for reinforcement.
The new bridge was constructed in two phases to keep the existing railway line in service during construction. The design of the causeway needed to consider effects on the existing and new bridge, and the final design was checked to meet Network Rail design requirements for working adjacent to a live rail line.
Reinforced embankment solution
The final design included the results for each section in the south and north areas of the causeway (Figures 6 and 7), considering all the scenarios described above. The results for the embankment section with the maximum height and the maximum slope angle are described below.
- Maximum height: 13.5 feet (4.1 m)
- Slope angle: 1V:2H
- Minimum distance to the edge: 3.3 feet (1 m)
- Basal reinforcement: 3 layers of biaxial geogrid of 2,056 pound-force/foot (30 kN/m), spacing 11.8 inches (300 mm), reinforcement length 65.6 feet (20 m)
- Slope reinforcement: 6 layers of uniaxial geogrid of 3,083 pound-force/foot (45 kN/m), spacing 19.7 inches (500 mm), reinforcement length 32.8 feet (10 m)
- Face protection: Woven geotextile wraparound each layer (“C” shape with 4.9 feet [1.5 m] at the bottom and top of each layer plus layer thickness)
During the construction period, the causeway was completely submerged by tidal activity on several occasions and back in use for construction activity within hours of the tidal event. To account for these events, we extended some areas of the embankments to include safe areas on the ramped approaches, above the anticipated flood levels, allowing space for the tracked plant.
The use of reinforced earthworks embankments with geogrids and granular material Class 6B and Class 6N allowed the construction of the causeway and access roads for the new bridge, keeping the existing timber viaduct in service during construction.
The main challenge for the design of the reinforced embankments was to keep the distance from slope edge for piling rigs and cranes to only 3.3 feet (1 m), with a maximum slope angle of 1V:1.5H while coping with the variation of the water levels due to tidal activity. Tidal levels varied by up to 15 feet (4.6 m) with very high spring tides experienced during the construction phase. In addition, two periods of major storms and flooding occurred during construction, and no damage occurred to the causeway or embankments.
The reinforcement of the embankments and access roads using geogrids also allowed the construction of working platforms and safe areas for the heavy plant to operate under all conditions, even the most extreme tidal conditions. One of the major benefits was that even though the platforms were submerged on a regular basis, very little, or almost no, remedial work was required after each tidal event to bring them back into operation. Without the reinforced working platform solution, the project deliverables would have been impossible to achieve within the spatial constraints imposed by the scheme land boundary and zone of influence of the operational railway.
A very detailed and careful construction plan was produced to integrate the permanent embankment construction with the temporary works and to ensure that the existing railway line could stay operational during construction. The project was successfully constructed in an efficient and cost-effective way, overcoming all the environmental and operational constraints of the scheme.
The project was constructed in two phases, with the completion date of the temporary earthworks in May 2015 and permanent earthworks in July 2015. The new viaduct was open to the public in August 2015.
Patricia Guerra-Escobar is a principal engineer for Geosynthetics Ltd. based in Hinckley, Leicestershire, U.K. All figures courtesy of the author.
BRE. (2004). “Working platforms for tracked plant: Good practice guide to the design, installation, maintenance and repair of ground-supported working platforms.” Watford, U.K.
British Standard, BS8006-1:2010. Code of practice for the strengthened/reinforced soils and other fills. British Standard Institution, London, U.K.
British Standard, BS EN 1997-1:2004. Eurocode 7. Geotechnical design—Part 1. British Standard Institution, London, U.K.
Bonaparte, R., Holtz, R., and Giroud, J. (1987) “Soil reinforcement design using geotextiles and geogrids,” in Geotextile testing and the design engineer, ed. J. Fluet, ASTM International, West Conshohocken, Pa.
Hewson Consulting Engineers. (2013). Pont Briwet Transportation Improvement Scheme. Pont Briwet Viaduct Temporary Causeway Form F003. H206/HCL/T/FM/103. Rev. A. July.
Highway Works MCHW. (2016). Specification for Highway Works Series 600—Earthworks, Vol. 1. U.K.
Holtz, R. D., Christopher, B. R., and Berg, R. R. (1997). Geosynthetic engineering. BiTech Publishers Ltd.
Koerner, R. (2005). Designing with geosynthetics, 5th ed., Pearson, U.S.
Pavco, Departamento de Ingeniería. (2009). “Manual de diseño con geosintéticos.” Octava Edición, Bogotá, Colombia.
Rowe and Soderman. (1985). “An approximate method for estimating the stability of geotextile reinforced embankments.” Canadian Geotechnical Journal 22.
TENAX SpA, (2003). “The design of reinforced soil walls using Tenax geogrids.” Vigano, Italy.
TENAX SpA, (2011). “Reinforced soil raft concept: Design guidance.” Vigano, Italy.
SIDEBAR: Project Highlights
The Pont Briwet Bridge Project
Client: Cyngor Gwynedd
Location: Penrhyndeudraeth, Gwynedd, North Wales, U.K.
General Contractor: Hochtief UK
Principal Designer: Hewson Consulting Engineers
Specialist designer: Geosynthetics Ltd.
Geosynthetics manufactures and products: Tenax: Uniaxial Geogrid TT and Biaxial Geogrid LBO HM: Tenax
Rhyno: Woven Geotextile