Analysis of geocell reinforced soil covers over large span conduits.

Road, highway, and railway systems comprise the fundamental infrastructure of our modern societies - the transportation of commodities and services across Canada. Billions of dollars have been invested in these systems and the demands on them in terms of service life and capacity continues to increase. Concurrently, decreasing budgets for maintenance and renewal require that new and innovative techniques be employed to maintain bridges in serviceable condition or to replace them. The use of soil-steel bridges with geocell soil reinforced cover is an innovative short to medium bridge span replacement alternative. The use of these composite bridge structures show promise to be an economical replacement alternative for the aging and deteriorating short to medium span highway bridges.

A soil-steel bridge consists of corrugated structural steel plate conduits that are assembled on site in circular, elliptical or arch shapes and backfilled with granular soil. They are often used to support highway pavements and railway tracks due to their quick construction, durability, low maintenance and construction costs. Relatively large span conduits, up to 16.8 metres, have been constructed in recent years. Current deign codes (Ontario Highway Bridge Design Code [OHBDC], American Association of State and Highway Transportation Officials [AASHTO]) restrict the minimum cover soil thickness over large span conduits to about sixty percent the conduit diameter to prevent cover soil tension or shear failure. Thus, current design codes specify cover thickness above the crown that is greater than or equal to one metre for bridge spans greater than 6 m in diameter. Due to fixed project geometric constraints, (site grade) the use of large span soil-steel bridges is often precluded as a short or medium span bridge replacement alternative. Preliminary research, by Drs Knight and Bathurst at the Royal Military College of Canada (RMCC), indicates that thinner depths of soil cover are possible when one or more layers of geocell-soil reinforcement are introduced into the soil cover (Knight and Bathurst, 1997 and Bathurst and Knight, 1998).

Geocell is a generic term describing a class of geosynthetic products manufactured from thin strips of polymeric material (usually high density polyethylene) bonded or welded together to form a three-dimensional cellular network that can be filled with compacted soil (Figure 1). The effect of cellular confinement on the infill soil is to increase the stiffness and shear strength of the confined soil.

By reinforcing the soil cover with geocell it appears that current specified cover soil depths for large span flexible conduits may be significantly reduced without the promotion of cover soil tension or shear failure. Thus, this reinforcement technique has the potential to increase the range of application of large span soil-steel bridges as a short or medium span bridge replacement alternative by allowing soil-steel bridges to meet site geometric constraints.

A laboratory and numerical investigation program is in progress with two main themes. The first is the performance of one-tenth model tests to develop an understanding of the behaviour of soil-steel bridges with thin reinforced soil cover and to produce data sets for numerical simulators validation or calibration. The second is an investigation into the use of FLAC and FLAC3D for predicting the behaviour of soil-steel bridges with thin geocell reinforced-soil covers.

It is anticipated that this research will lead to the development of design charts for soil-steel bridges with geocell reinforced-soil cover, and the use of soil-steel bridges with thin geocell reinforced-soil covers as an economical replacement alternative for the aging and deteriorating short to medium span highway bridges.