31 Jul 95
Figure 3. Finite element mesh for Bonneville tieback wall
(3) Excavate in front of the wall to the depth
alignment was 0.08 in. of heave. Figure 6 shows the
necessary for installing the uppermost tieback.
calculated lateral earth pressure distribution on the
wall at the end of construction in comparison to the
(4) Apply the tieback proof load.
lateral earth pressure distribution assumed during
earlier design studies. The design earth pressures are
(5) Reduce the tieback load to the lockoff load.
larger than the calculated earth pressure at the top of
the wall and smaller at the bottom of the wall. The
(6) Add the stiffness of the tieback to the mesh
undulations in the calculated pressure diagram result
and excavate to the level of the next tieback.
from the concentrated tieback reload forces that were
Steps 4 through 6 were repeated until the bottom of the
excavation was reached. These construction steps are
g. Parameter studies during design. Parameter
illustrated in Figure 4 for four levels of tieback.
studies were performed during design to investigate
the effects of reduction of the soil stiffness and the
f. Results. The results of the analyses included
consequences of failure of the top anchor at the end
values of wall deflections and moments, lateral earth
of construction. The SSI analysis of the wall was
pressures on the wall, ground surface movements
repeated using values of soil stiffness equal to one-half
behind the wall (including movement of the relocated
of those obtained form laboratory tests. This change
railroad line), and soil stresses in the ground behind
caused wall deflections to increase by about 65
percent, wall bending moments to increase by about 40
percent, and heave of the railroad line to increase by
Some of the results are shown in Figure 5. The
about 60 percent. The analysis of tierod failure
calculated wall deflections at the end of the excavation
resulted in a wall movement of about 1.45 in. toward
are away from the excavation and
the excavation, to a position 0.78 in. past the vertical.
toward the railroad line. This occurs because the large
This lateral movement of the wall was accompanied by
tierod reload forces pulled the wall toward the railroad
a 0.14-in. drop of the ground surface at the railroad
line. The maximum calculated deformation is a small
line location, to a level of 0.06 in. below the original
amount, 0.67 in. The calculated vertical movement of
the ground surface at the railroad track