Embankments and tunnels

Abstract

Embankments can generally be constructed to great heights without concern for failure or excess settlement when founded on frictional materials or rock. Firm to stiff cohesive soils having undrained strengths in excess of 75 kPa provide sufficient bearing capacity for low embankments (H < 15 m) and embankment settlements will be small provided the preconsolidation pressure (p ^’_c ) is not exceeded over a significant depth in the soil profile. The exceptions to these general statements would be cases where adverse groundwater flow (artesian conditions) or ground ice (discontinuous permafrost subject to melting) could induce local failure or excessive settlements. Similarly, stiff materials have adequate strength to be self-supporting during tunnel-boring operations and sufficient stand-up time to remain stable until a tunnel liner is installed. The exceptions in the case of tunneling operations are loose or saturated frictional materials prone to raveling or piping into underground openings, and materials susceptible to squeezing or to rapid swelling and softening under stress release. These exceptions may require special construction techniques but the major problems in designing low embankments and tunnels arise when soft sediments are encountered. This chapter discusses the basic design and construction considerations related to embankments and tunnels in soft cohesive sediments.