The use of geosynthetics for culvert foundation reinforcement

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1 The use of geosynthetics for culvert foundation reinforcement Advantage Report Volume 17 no. 4 March 2016 Mark Partington, R.P.F., M.Sc. Senior Researcher, Resource Roads Restricted to Members and Partners of FPInnovations fpinnovations.ca

2 FPInnovations is a not-for-profit worldleading R&D institute that specializes in the creation of scientific solutions in support of the Canadian forest sector s global competitiveness and responds to the priority needs of its industry members and government partners. It is ideally positioned to perform research, innovate, and deliver state-of-the-art solutions for every area of the sector s value chain, from forest operations to consumer and industrial products. FPInnovations staff numbers more than 525. Its R&D laboratories are located in Québec City, Montréal, and Vancouver, and it has technology transfer offices across Canada. For more information about FPInnovations, visit: Follow us on: SUMMARY As part of FPInnovations ongoing work on the challenges of constructing and managing resource roads across wetlands, a culvert foundation improvement study was launched in In that study, FPInnovations observed and documented the use of a geogrid and woven geotextile for improving culvert foundations on a resource road built in Ontario. After four years of remeasurements, the use of the geogrid and woven geotextile has not shown to improve the culvert foundations at this site. All the culverts settled the most in the first year after they were installed and have continued to settle but to a lesser degree. ACKNOWLEDGEMENTS This project was financially supported by the FPInnovations / Natural Resources Canada contribution agreement. The author would also like to thank EACOM Timber Corporation for its partnership in this project. CONTACT Mark Partington, R.P.F., M.Sc. Senior Researcher Resource Roads mark.partington@fpinnovations.ca 2016 FPInnovations. All rights reserved. Unauthorized copying or redistribution prohibited. Disclosure for Commercial Application: If you require assistance to implement these research findings, please contact FPInnovations at info@fpinnovations.ca.

3 1. INTRODUCTION Canada s northern forested landscapes feature numerous wetlands, such as fens, bogs, and swamps that present environmental and operational challenges for resource roads. The effects of these roads on the many ecological functions of wetlands are of increasing concern to Canada s resource industries, governments, northern communities, and conservation organizations. Among the difficulties in the planning and construction of resource roads which cross wetlands are the low bearing capacity of the subgrade material and the placement and installation of adequate drainage and water-crossing structures. However, both wetlands and resource roads should be able to function with careful planning, knowledge of the various wetlands and their ecological functions, and the development and use of best management practices. FPInnovations, in partnership with EACOM Timber Corporation, undertook a study to assess a method for improving culvert foundations (Partington 2014, 2015). At the study site, the poor bearing capacity of the deep organic soils is a particular challenge that required improved culvert foundation techniques. It was known that in other areas of Canada biaxial geogrids have been used in an attempt to reinforce the culvert foundation. The forest company and contractor decided that at this site, since the geogrid was going to be used as subgrade reinforcement in the road, it would also be installed directly under two of the culverts while the remaining culverts would be installed on the forest floor with no provision for an improved foundation (see Figure 1). On soft, compressive soils like this, the entire road embankment is expected to settle; the amount will depend on the fill thickness (i.e., total mass of the fill) and on the bearing capacity of the native soils. Geosynthetics are often used to help minimize differential settlement by bridging weaker sections along the road. It was believed that the geogrid could act as a type of hammock by supporting the culvert and preventing any significant settlement due to the dead load of the road fill material. In addition, a woven geotextile was placed under the geogrid, a practice that was standard for the operations at this site. Figure 1. High-density polyethylene (HDPE) pipe installed over a geogrid and a woven geotextile. FPInnovations Page 3

4 Biaxial geogrids have been used that way in a variety of forest roads across Canada to improve culvert foundations and to reduce settlement of a culvert on soils of low bearing capacity (Partington & Thiam, 2014). A modified version of this method has also been employed in Scotland, where geogrids and lightweight fill are used to support culverts built on peatland roads (Forestry Commission Scotland, 2010). Although the design and application described in this report were neither endorsed nor designed by FPInnovations or a geotextile supplier, they have provided an opportunity to document a field practice being employed on a resource road. The documenting and monitoring of the installation have led to implementation and design recommendations for the possible continued use or modification of this practice. 2. SITE DESCRIPTION The study site is located in Northern Ontario on the Spanish Forest Crown Land Management Unit and is part of a new primary forest road being built to cross the Wahnapitae River. The planned northern approach to the river required the road to cross 200 m of 6-m-deep organic soils; the 80 m closest to the river is untreed (see Table 1). Table 1. Site characteristics. Culvert Culvert foundation reinforcement Foundation soils Tree root mat present Fill height over culvert (m) 1 None Soft (3 m of organic soil) Yes Geogrid + woven geotextile Soft (3 m of organic soil) Yes None Very soft (6 m of organic soil) No (untreed) Geogrid + woven geotextile Very soft (6 m of organic soil) No (untreed) 0.90 Since surface water is routinely present at this location during snowmelt and periods of increased precipitation, drainage through the road had to be allowed for. It was decided that four 450 mmdiameter pipes of high-density polyethylene (HDPE) would be installed (see Figure 2). FPInnovations Page 4

5 0 Figure 2. An aerial view of the constructed road and culvert locations. In order to assess the ability of the geogrid to help reduce settlement of the culvert s foundation, geosynthetics were laid under two of the culverts. The remaining two culverts had the geosynthetics placed over them. The installation of the four pipes was completed in early December Shortly after that, road construction was stopped for the winter. Road construction began again in the fall of 2012 in preparation for the construction of the bridge. At that time additional fill was added to the road although, as of December 2015, the final road grade was not yet complete. The final preparation of the road approach to the bridge crossing is expected to be completed in 2016, at which point the final road grade, surface material, and any necessary rip-rap will be placed. 3. METHODOLOGY The initial survey of the culvert elevations took place in December 2011, immediately after construction, and four annual remeasurements were done in November 2012, 2013, and 2014 and in August The surveys used a Leica Rugby rotating laser level with six benchmarks established throughout the 200-m-long study area. 4. RESULTS The changes found in the average elevation after four years of culvert elevation remeasurements are presented in Figure 3. FPInnovations Page 5

6 Figure 3. Average settlement of the culvert ends, with and without geosynthetic foundation reinforcement. This graph illustrates the average change in elevation of the ends of the four culverts in relation to their original position after installation in November The greatest change in elevation for all of the culverts was in the year following construction (2012). The ends of the pipes sank from their original position by as little as 0.02 m (for culverts 1 and 2) and as much as 0.14 m (for culverts 3 and 4) in the four years following construction. This change in elevation was not unexpected given the low bearing capacity and the moisture conditions at this site. Pipes 1 and 2, located in the road section with shallower organic soils and a tree root mat, settled less than pipes 3 and 4. Pipes 3 and 4, located on deeper organic soils and in saturated soil, settled more. Culverts installed on soils with a low bearing capacity will often exhibit uplift at the ends. This results in a banana-shaped pipe since the section of the pipe that is under the road fill sinks under the dead load of the road material (see Figure 4). FPInnovations Page 6

7 Figure 4. Culvert position, relative to the 2011 east end elevation, four years after installation. Note: The center values for 2011 have been estimated. This graph presents the elevation of the ends and the centre for three of the culverts. The centre elevation was measured only in 2015 and not in the previous remeasurement periods. In 2015, four years after installation, the centres of the culverts were lower than both ends at two of the three locations. Culvert 2, with the reinforced foundation, did not show any additional settlement in the centre in relation to the ends and seemed to be settling uniformly. The geogrid may be providing the extra support needed to prevent additional settling along the centre of the pipe. 5. DISCUSSION All the culverts settled most in the first year after installation regardless of their location along the road. That was a result of the additional road fill and heavy traffic that occurred as part of the bridge construction at the site during the first year following the installation of the culverts. Since then, there has been no further traffic on the road and no additional road material has been added. In subsequent years, the settlement rate closely matched conservative estimates of linear elastic modelling for fill settlement on organic soils. FPInnovations Page 7

8 It is also believed that all of the culverts are settling at the same rate as the road itself. The geosynthetics may not be providing enough reinforcement for the culvert foundations on this site. That is evident from the measurement of the average settlement of the culvert ends, which shows similar results whether the geosynthetics were installed above or below the culverts. In order for the geosynthetics to provide reinforcement, differential settlement is needed to provide the required levels of strain so that the geosynthetics can resist the settlement of the road fill material. To achieve this, the geogrid must interlock with the overlying materials so that resistance can be provided against the sinking of the road fill material into the native organic soils. The coarse sand used as the road construction material at this site is too fine to interlock with the geogrid, and as a result the necessary strains and tension are not likely achieved. Further evidence of uniform road and culvert settlement is shown by the fact that the condition of the road surface has not changed at the location of the culverts. If a culvert were settling more slowly than the road, a hump would be created in the road. If a culvert were settling faster, there would be a dip in the road surface at the location of the culvert. The method used in this monitoring study to measure the culvert end elevations did not capture the greater degree of settling that is occurring at the middle of the culvert underneath the road base. The reference point was the end of the culvert nearest to the road embankment. That allowed for quick and efficient field measurements and did not impede road use or traffic. However, to obtain information about the settling of the culvert under the middle of the road, FPInnovations was able, in the fourth year after installation, to excavate the road to the centre line of the culvert in three of the four locations. The elevations of the culverts at the road centre line confirmed that the middle of the culverts had settled more than the culvert ends at two of the three culvert locations. The increased rate of settling of the middle section of the culvert is also evident from the uplift of the culvert ends that has taken place at this site. 6. IMPLEMENTATION The following recommendations are to be considered when installing cross-drain culverts on sites with poor bearing capacity and where culverts are expected to settle. Consider the use of geosynthetics, such as geogrids and geotextiles, as options for road reinforcement and culvert foundation improvement. Ensure that the installation of these materials follows the design engineer s, manufacturer s or supplier s instructions; that will allow the greatest benefit of these products to be achieved. Ensure that the fill material is matched with the geosynthetic product to achieve optimum performance. For example, geogrids are designed to work with a well-graded granular material (as opposed to sand) with a top particle diameter close to the opening size of the grid cells. On sites with poor bearing capacity, allow the road to settle for at least a season before installing cross-drain culverts. Since the majority of road settlement is expected in the first year after construction that delay in the installation of the cross-drain culverts can allow the culverts to be installed after that initial period of settlement is over. FPInnovations Page 8

9 Alternative cross-drain culvert installation techniques, such as cambering the culvert during the installation, can help to allow for the settling of the pipe after construction. That technique, where the centre of the culvert is installed at a higher elevation than the ends, allows the culvert to settle to a level grade after the entire road has settled. Keep the cross-drain culvert as short as possible so the fill of the road embankment or any riprap that may be present can weigh down the ends of the pipe. This will help to prevent excessive culvert uplifting. Consider using corrugated steel pipe for culverts where culvert end uplift is expected. The increased stiffness of the material can mitigate some of the deformation that may occur when the culvert settles. 7. REFERENCES Forestry Commission Scotland, Scottish Natural Heritage. (2010). Floating roads on peat. 82 p. Partington, M. (2014). Foundation improvement options: Geogrid for small culverts. Field Note. Pointe- Claire, QC: FPInnovations. 4 p. Partington, M. (2015). The use of geosynthetics for culvert foundation reinforcement: Three year study results. Technical Report 21. Pointe-Claire, QC: FPInnovations. 14 p. Partington, M., and Thiam, P.M. (2014). Culvert foundation improvement design concepts. Field Note. Pointe-Claire, QC: FPInnovations. 5 p. FPInnovations Page 9

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