Tieback Wedge Method. The Tieback Wedge Method was developed by Bell, et
al. (1975) as an extension of the trial wedge method from traditional soil mechanics
(Huntington, 1957), and has always been the appropriate design method for
geosynthetic-reinforced MSE walls. In an MSE wall with geosynthetic
reinforcements, the failure plane is assumed to develop along the Rankine rupture
surface defined by a straight line oriented at an angle of 45+φ/2 from the horizontal
and passing through the toe of the wall. Sufficient deformation is assumed to occur
for an active earth pressure condition to exist from top to bottom of wall. The
Rankine failure plane is not modified by inclusion of the extensible geosynthetic
reinforcements. Therefore, reinforcement strain actually allows the failure plane to
develop and the geosynthetic reinforcements, acting as tiebacks, restrain the active
wedge from failing. This contrasts sharply with the Coherent Gravity Method, where
the shape of the bilinear boundary between the active and resistive zones is based on
the location of maximum reinforcement tension, the failure plane does not actually
develop, the active wedge does not displace, and the inextensibility of the steel
reinforcements prevents structure deformation