12The majority of geoengineering applications occur in the unsaturated zone, which is in the near-surface 13 region forming the connection between meteorological phenomena above and saturated ground below. 14 The key characteristic of the unsaturated zone is that water is in tension or, put another way, pore water 15 pressure is negative. Moisture content, as well as most material properties, vary spatially and temporally 16 in the unsaturated zone and coupled processes are common. In geoengineering applications in the vadose 17 zone, unsaturated soils may be present during part or all of its design life. The question is how or when 18 to consider the unsaturated soils principles in an analysis or design. Although most geoengineering 19 applications have an unsaturated component, use of unsaturated soil mechanics in practice lingers 20 behind the prolific number of publications due uncertain benefit of accounting for unsaturated effects, 21 complexity, and conservativeness among other reasons. The focus of this colloquium is to continue 22 bridging the gap by illustrating unsaturated soils principles using application-driven examples in the 23 areas of capillarity as well as flow, strength, and deformation phenomena. As principles of unsaturated 24 soils become more understood and demand increases for incorporating climate change effects in design, 25 use of unsaturated soils principles in practice will continue to increase. 26