The current design practice of geosynthetic reinforcements assume the backfill to be purely frictional irrespective of the soil type. However, the recent edition of AASHTO LRFD bridge design specifications [1] allows for the inclusion of cohesion in the design of geo-reinforced slopes. Nevertheless, practitioners need to be aware that unlike purely frictional backfills, cohesive soils are subject to the formation of tension cracks. These cracks make the slope less stable and therefore need to be properly accounted for in any geo-reinforcement design relying on soil cohesion.

Abd & Utili [2] derived a semi-analytical method for uniform c - phi slopes accounting for the presence of cracks that provides the amount of reinforcement needed as a function of soil cohesion, tensile strength, angle of shearing resistance and slope inclination employing the limit analysis upper bound method. The formulation was extended to the seismic case in Abd & Utili [3]. In [2 and 3] is shown that accounting for the presence of even a modest amount of cohesion may allow using locally available cohesive backfills to a greater extent and less overall reinforcement.

In this paper first a summary of the main tenets of the theoretical formulation to design the reinforcement is laid out, then several example case studies are presented to illustrate the application of the new theory to various cohesive backfills representative of the locally available soils in different geographical regions including those located in seismic areas. Calculations of the required reinforcement strength and embedment length are carried out for both uniform and linearly increasing reinforcement distributions. Also the economic savings that can be achieved by accounting for the presence of cohesion are estimated.

References:

1. AASHTO (2012). Bridge design specifications. American Association of State Highway and Transportation Officials. Washington DC., USA.
2. Abd A., Utili S. (2017). Design of geosynthetic - reinforced slopes in cohesive backfills. Geotextiles and Geomembranes, 45: 627-641.
3. Abd A., Utili S. (2017). Design of geosynthetic reinforced slopes in cohesive soils subject to seismic action. Procedia Engineering, 189: 898-907.