Modern highway construction and development sometimes may require construction of high embankments where the fill height above the top of buried rigid and flexible utilities may be greater than 10 m (32.8 ft). The high embankment fills may impose significant earth loads on the underground structures.

This bulletin will discuss the use of GeoSpec lightweight fill material as a compressible material above deeply buried utility lines. Compressive loads on buried utility lines have been shown to be dependent upon installation conditions.

Pipe installations are called trench installations when the pipe is located completely below the natural ground surface. Frictional forces between the sides of the trench and the backfill material help to support the weight of the soil overlaying the pipe. Pipe installations are called embankment installation when soil is placed in layers above the natural ground. The vertical earth pressure on a rigid pipe installed using embankment construction method is generally greater than the weight of the soil above the structure because of negative arching.

When high embankment fills are required over rigid culverts, the installation methods typically used are positive projection installation or induced trench installation. The induced trench installation method introduces a zone of compressible material such as EPS geofoam above the culvert.

The concept behind this method is to reduce the vertical earth pressure on the utility by inclusion of a lower density fill material (compressible layer), thus reducing the load over the utility, but also inducing positive arching. The compressible material above the utility line causes differential settling of the column of soil directly above the utility relative to the adjacent soil. The earth load from the column of soil directly above the culvert is partially supported by the shear forces developed on the soil interface with adjacent soil columns, thereby resulting in some of the load being redistributed to the adjacent fill.

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