City of Frisco Engineering Standards

Engineering Standards
including, sliding, overturning, bearing pressure, global stability, and structural capacity of the
wall itself. The design of the retaining wall shall meet the following minimum factors of safety:
a. Sliding: 1.5
b. Overturning: 2.0
c. Bearing Pressure: 3.0
d. Global Stability: 1.5
D. Avoid perching wall on slopes steeper than 8H:1V. When walls must be placed on slopes steeper
than 5H:1V, or the retaining wall height or the combined wall and slope heights exceed 8-ft., the
geotechnical engineer shall conduct a short-term and long-term global stability analysis using
applicable soil strength characteristics, geometry, and loading conditions (including load
surcharge, hydrostatic, etc.). The Design Engineer is responsible for the design of the wall
system, including its global stability.
E. A series of two or more walls built in tiers shall be considered a single wall in height for the
purposes of conforming to these Engineering Standards when the base of the upper tier is set back
from the base of the lower tier less than two times the height of the lower tier wall.
F. If TxDOT standard sheets pertaining to cast-in-place spread footing structural retaining walls are
utilized, the Design Engineer shall ensure that the actual wall geometry and loading conditions
are applicable to the standard wall design selected. The Design Engineer shall ensure that
interruptions to the wall stem or footing reinforcement by openings, utilities, geometric changes
or curved sections of the wall do not compromise the design and performance of the wall. No
TxDOT standard wall design shall be modified unless the Design Engineer designs, draws, and
seals the modified standard.
G. TxDOT standard sheets pertaining to cast-in-place spread footing walls provide a choice between
high pressure (HP) and low pressure (LP) footings. Selection by the Design Engineer is a function
of the loading, geometry, and allowable soil pressures. TxDOT standard sheets pertaining to
cast-in-place spread footing walls are developed based on the wall being drained, and the
design parameters for foundation and retained soils of a cohesion factor of zero, a friction
factor of 30 degrees, and a unit weight of 120 pounds/cubic foot. Give special consideration to
the site-specific geotechnical requirements and whether a TxDOT standard wall design is
applicable. Also give special consideration to walls subject to potential inundation.
9.06 Slope Stability Design Criteria
A. All slopes exceeding 8-ft. in height with a steepness of 4H:1V or greater, regardless of soil type,
cut, or fill, shall be evaluated for global stability for both the short-term and the long-term
conditions. Additionally, any known areas of existing fill, deleterious material, or soft soils which
have a height over 4 feet or slope angle greater than 6H:1V shall be evaluated for global stability
for both the short-term and the long-term conditions. Specific site conditions may require
evaluation for additional types of slope failure, such as bearing capacity, settlement, shear, and
undercutting. Calculations pertinent to the analysis shall be submitted with the construction
drawings when required by the City.
B. Use the following data to analyze global stability of a slope:
Section 9 – Structural Design Requirements May 2012 Page 9-8