UFC 3-220-01N
15 AUGUST 2005
structure.
8-3.2.3.4
Differential Settlement. Complex structures are more susceptible to differential
settlement because of the potential for large variations in loads carried by each component
foundation. In the multilevel stepped structure (Figure 8-3.3 (a)), the foundation supporting the
lower level offset component must also support the volume of backfill over that part of the
structure. Measures must be taken to ensure that the proper functioning of all elements is not
hampered by differential settlement. The increased cost of proper design and construction
where unusual or difficult construction procedures are required is insignificant when compared
with the cost of the structure. The cost of remedial measures to correct deficiencies caused by
improper design and construction usually will be greater than the initial cost required to prevent
the deficiencies.
8-3.2.3.5
Downdrag. In addition to conventional service loads, cut and cover subsurface
structures are susceptible to downdrag frictional forces between the structure and the backfill
that are caused by settlement of the backfill material adjacent to and around the structure.
Downdrag loads can be a significant proportion of the total vertical load acting on the structure
and must be considered in the structure settlement analysis. Structure-backfill friction forces
may also generate significant shear forces along the outer surface of structures with
backfill, roughness of the structure's surface, and magnitude of earth pressures acting against
the structure. Techniques for minimizing downdrag friction forces generally include methods
that reduce the structure surface roughness such as coating the structure's outer surface with
asphalt or sandwiching a layer of polyethylene sheeting between the structure's outer surface
and fiberboard (blackboard) panels. Backfill settlement and associated downdrag can also be
minimized by requiring higher backfill densities adjacent to the structure.
Figure 8-3.4 Excavation Subject to Bottom Heave
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