Disregard surface soil in the design of deep foundations, i.e., drilled shafts and driven piles. The disregarded depth is the amount of surface soil that is not included in the design of the foundation due to potential erosion from design flood or check flood, future excavation, seasonal soil moisture variation (shrinkage and swelling), lateral migration of waterways, and other factors. Amount of disregard may be different based on the design check being performed. For axial and compressive loading, disregard a minimum amount of 5 ft. over non-water crossings and 10 ft. over stream crossings. For abutments, disregard the portion of foundation passing through embankment fills. Note that the length of disregard may differ depending on the design check being performed.

When permanent casing is used for deep foundation installation, disregard side resistance with respect to axial design checks.

For projects where the existing ground line is at an elevation considerably higher than the proposed grade line (roadway is to be depressed) soil softening, swelling or heave must be accounted for in design of embankment slopes, roadways, retaining walls and foundation elements. Soils in these conditions respond to the removal of overburden (unloading). This response could have a dramatic impact on the design approach taken.

Additional considerations for disregard depth are required when encountering downdrag or scour. See Section 4 of this Chapter for downdrag. Information regarding disregarded depth and scour methodology at bridge foundations can be found in Section 6 and within the TxDOT Scour Analysis Guide.