Construction of Embankments over Culverts and Backfills for Buried Pipelines Using Geofoam Blocks

Culverts are crossings constructed through highway and railway embankments to accommodate pedestrian or vehicle passage or to convey surface water in the direction of natural drainage. Compared with buried pipelines, culverts are generally shorter in length and may be constructed from concrete/reinforced concrete (cast-in-place or precast), steel (single-piece or multi-plate), or polymer-based materials such as HDPE, PVC and PP. Culverts may be manufactured in various cross-sectional shapes, including box, circular, elliptical and arch configurations, and in a wide range of dimensions. Depending on project-specific conditions, culverts may be constructed on site or manufactured off site as precast/prefabricated units and subsequently assembled at the project site.

Twin-cell reinforced concrete box culvert serving as a bridge
Figure 1. A twin-cell reinforced concrete box culvert serving as a bridge (Source: http://www.hudsoncivil.com.au)

In addition to being constructed in various dimensions, culverts may also be covered by soil fills of different thicknesses depending on project-specific conditions. Where the soil cover above a culvert is relatively thin, or where no fill is placed above the culvert, vehicle axle loads constitute one of the most important design loads affecting the culvert section in addition to seismic loads (Figure 1). As the soil cover thickness increases, the effect of vehicle axle loads on the culvert decreases, while the effect of the soil fill load increases (Figure 2). Like all engineering structures, culverts are designed to safely resist dead and live loads in accordance with the design criteria specified in applicable standards. Where culverts are located beneath thick soil fills, the required structural section dimensions may become considerably larger in order to satisfy these design criteria. This naturally increases the initial project cost.

Twin-cell reinforced concrete box culvert beneath thick soil fill
Figure 2. A twin-cell reinforced concrete box culvert located beneath a relatively thick soil fill (Source: http://www.hudsoncivil.com.au)

More economical structural sections can be designed by constructing the fill above culverts using geofoam blocks, which can be up to 100 times lighter than conventional compacted soil fill. Where thick fills are to be constructed over existing culverts, a geofoam block solution can also help prevent potential structural damage (Figure 3).

Geofoam block fill constructed above a reinforced concrete box culvert
Figure 3. Application in which the fill above a culvert was constructed using geofoam blocks: By extending the geofoam block fill up to the highway pavement structure, the structural safety of the reinforced concrete box culvert was improved. (Alton, Iowa, USA) (Source: https://www.achfoam.com/)

When highway or railway embankments crossing soft clay deposits are widened, it is important to ensure not only the structural stability of existing culverts, but also adequate performance against foundation soil settlement and bearing capacity failure. If the additional widening embankment is constructed using conventional compacted soil fill, primary consolidation settlements developing in the foundation soil may cause structural damage to existing culverts. As discussed in Construction of Highway Embankments Using Geofoam Blocks and Construction of Highway Lane Widening Embankments Using Geofoam Blocks, ground improvement works can extend the construction period and, depending on the local ground profile, increase the overall project cost. Constructing the additional widening embankment using geofoam block lightweight fill can prevent differential settlements in the foundation soil and thereby reduce the risk of structural damage to existing culverts (Figure 4).

Geofoam block embankment constructed above an existing culvert at UTA FrontRunner
Figure 4. Application in which the fill above a culvert was constructed using geofoam blocks – UTA FrontRunner: As part of a railway embankment widening project, the additional embankment was constructed using geofoam blocks, preventing differential settlements that could otherwise have caused structural damage to the existing culvert. (Draper, Utah, USA) (Bartlett et al., 2015)

The field construction stages followed for geofoam block lightweight fill above culverts are the same as those described for geofoam block highway embankments in Construction of Highway Embankments Using Geofoam Blocks (Figures 5 and 6).

Geofoam block roadway embankment above a twin-cell reinforced concrete culvert
Figure 5. Geofoam block roadway embankment above a twin-cell reinforced concrete culvert
Geofoam block roadway embankment above twin pipe culverts
Figure 6. Geofoam block roadway embankment above twin pipe culverts
Use of Geofoam Blocks as Lightweight Fill over Buried Pipelines

Buried pipelines are essential engineering infrastructure used within urban areas for utilities such as potable water, wastewater, natural gas, electricity and communications, as well as for transporting valuable fluids such as oil and natural gas between cities and countries. Depending on their intended application, pipelines may be constructed from concrete/reinforced concrete, steel, or polymer-based materials such as HDPE, PVC and PP. Similar to culvert applications, geofoam block lightweight fill can provide advantages for buried pipelines in terms of initial project cost, performance and/or construction duration.

By designing newly constructed buried pipelines with geofoam block lightweight fill, dead loads can be minimized and the load distribution around the pipe can be made more uniform (Figure 7). This enables optimization of the pipe section and may result in more economical solutions or improved structural safety.

Geofoam lightweight fill used to reduce loads on a buried pipeline
Geofoam lightweight fill providing uniform load distribution around a buried pipeline
Figure 7. Geofoam block lightweight fill application used to reduce the load acting on a pipeline and provide a more uniform load distribution around the pipe (Source: http://geofoamintl.com/)

If the dead and/or live loading conditions above an existing buried pipeline are expected to change, relocating the pipeline or implementing other conventional protective measures may increase the total project cost and extend the construction period. Instead, geofoam block lightweight fill can provide both a more economical and a faster solution (Figure 8).

Road embankment constructed over an existing buried pipeline using geofoam blocks
Figure 8. Construction of a roadway embankment over an existing buried pipeline using geofoam block lightweight fill: Prior to construction of the embankment, excavation was carried out to remove a thickness of soil equivalent to the vertical surcharge generated by the geofoam blocks and pavement structure, resulting in a “net-zero” increase in load. This avoided conventional measures that would otherwise have increased the total project cost and extended the construction period. (Waxahachie, Texas, USA) (Source: http://www.thermafoam.com)

One of the developing applications of geofoam blocks is the protection of pipelines against ground movements caused by faulting or other mechanisms. For flexible pipes made of ductile steel or polymer-based materials, geofoam blocks can be positioned according to the direction of relative ground movement—for example, above the pipeline in the case of normal faulting or along the sides of the pipeline in the case of strike-slip faulting—to reduce bending stresses induced by permanent ground displacement (Bartlett et al., 2015) (Figures 9 and 10). In addition to flexible pipes, geofoam block lightweight fill solutions can also be designed to protect relatively rigid pipes, such as concrete or reinforced concrete pipes, against damage caused by ground movements (Bartlett et al., 2015).

Geofoam lightweight fill protecting a buried pipeline against normal-fault ground displacement
Figure 9. Geofoam block lightweight fill application designed to protect a buried pipeline against ground movement: To protect the pipeline against permanent vertical ground displacement caused by normal faulting, geofoam block lightweight fill was placed above the pipeline. (a) Placement of a 0.6 m diameter ductile steel pipe in the trench, (b) placement of geofoam blocks and a geomembrane cover above the pipeline, and (c) construction of a reinforced concrete load distribution platform above the geofoam blocks prior to construction of the roadway pavement. (Salt Lake City, Utah, USA) (Bartlett et al., 2015).
Geofoam lightweight fill protecting a buried pipeline against strike-slip fault displacement
Figure 10. Geofoam block lightweight fill application designed to protect a buried pipeline against ground movement: To protect the pipeline against permanent horizontal ground displacement caused by strike-slip faulting, geofoam block lightweight fill was placed on both sides of the pipeline. (a) Trench formed with specially cut geofoam blocks and the steel pipeline, and (b) backfilling of the geofoam block trench with pervious concrete. (San Francisco, California, USA) (Source: https://tunnelingonline.com)
Use of Geofoam Blocks as a Compressible Inclusion

Where the thickness of fill above a culvert or buried pipeline is very large relative to the dimensions of the buried structure, a full geofoam block lightweight fill solution may increase the initial project cost. In such cases, more economical solutions may be achieved by constructing the fill with conventional compacted soil and placing a geofoam block “compressible inclusion” above the buried structure. The soil stresses acting on culverts and pipelines buried beneath thick fills depend on differential settlement within the soil–structure system and the resulting soil arching mechanism. If the buried structure is rigid, negative soil arching develops in the soil above the pipe, causing the vertical stress acting on the buried structure to exceed the overburden stress, defined as the unit weight of the soil fill multiplied by the fill thickness. Under these conditions, culvert or pipe section dimensions may need to be considerably larger to satisfy the design criteria, naturally increasing the initial project cost. The geofoam block compressible inclusion concept can reduce the vertical stress acting on a buried rigid structure to values substantially lower than the overburden stress (Figure 11). Unlike geofoam block lightweight fill, a geofoam block compressible inclusion is designed to undergo sufficient deformation under design loads to mobilize a positive soil arching mechanism in the overlying soil fill.

Geofoam blocks used as a compressible inclusion above culverts or pipes
Figure 11. Use of geofoam blocks as a “compressible inclusion” above culverts or pipelines

The density and dimensions of a geofoam block compressible inclusion are determined on a project-specific basis considering the design loads. After compacted soil backfill has been placed up to the crown elevation of the culvert or pipe, a geofoam block compressible inclusion of the specified dimensions and density is installed above the structure (Figure 12). As the compacted soil fill is raised, the geofoam compressible inclusion undergoes greater compression than the soil fill adjacent to the buried structure. The resulting differential settlement causes positive soil arching to develop in the soil above the rigid buried structure, reducing the vertical stress acting on the structure below the corresponding overburden stress. Data obtained from instrumented project sites have shown that geofoam block compressible inclusions can reduce the vertical stress acting on buried culverts or pipes by more than 50% relative to the overburden stress (Sun et al., 2009; 2011; Vaslestad et al., 2011; 2019). Placement of geofoam block compressible inclusions against the walls of reinforced concrete box culverts can also reduce lateral earth stresses acting on the structure (Meguid et al., 2017). In addition to rigid buried structures, geofoam block compressible inclusions can reduce soil stresses acting on flexible buried structures and improve their structural response (Kang et al., 2007; Akınay and Kılıç, 2019; Kılıç and Akınay, 2019). More economical designs can therefore be achieved for culverts and pipelines beneath thick fills by accounting for the reduced soil stresses provided by geofoam block compressible inclusions.

Geofoam block compressible inclusion installed above a buried concrete pipe
Figure 12. Placement of a geofoam block “compressible inclusion” above a concrete pipe (Eidanger, Norway) (Bartlett et al., 2015)

REFERENCES

  • Bartlett SF, Lingwall BN, Vaslestad J (2015) Methods of protecting buried pipelines and culverts in transportation infrastructure using EPS geofoam. Geotextile Geomembrane 43(5): 450–461
  • Sun L, Hopkins TC, Beckham TL (2011) Long-term monitoring of culvert load reduction using an imperfect ditch backfilled with geofoam. Transp Res Rec 2212:56–64
  • Sun L, Hopkins TC, Beckham TL (2009) Reduction of stresses on buried rigid highway structures using the imperfect ditch method and expanded polystyrene (geofoam). Research report KTC-07-14/SPR-228-01-1F, Kentucky Transportation Center, University of Kentucky, Lexington Kentucky
  • Vaslestad J, Sayd MS (2019) Load reduction on buried rigid culverts, instrumented case histories and numerical modeling. In: Arellano D., Özer A., Bartlett S., Vaslestad J. (eds) Proceedings of 5th International Conference on Geofoam Blocks in Construction Applications (EPS2018), Kyrenia, May 9-11, 2018, 115-128
  • Vaslestad J, Sayd MS, Johansen TH, Wiman L (2011) Load reduction and arching on buried rigid culverts using EPS geofoam. Design method and instrumented field tests. Paper presented at the 4th international conference on geofoam blocks in construction applications, Lillestrøm, Norway, 6–8 June 2011
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