City of Frisco Engineering Standards

Engineering Standards
2. Geotechnical engineering recommendations. The strength and depth of subsurface formations
determine the type of foundation chosen. In general, drilled shafts are well suited to areas
with competent soil and rock, and is the preferred foundation type subject to concurrence of
the geotechnical and structural engineers’ concurrence. Alternative foundation types require
approval in writing by the Director of Engineering Services.
3. Corrosive conditions. Salts, chlorides, and sulfates are detrimental to foundations. Where
these conditions exist, the Design Engineer shall take preventative measures. Use sulfateresistant
concrete as defined in these Engineering Standards as required.
B. The structural engineer shall use the geotechnical investigation recommendations as minimum
design criteria. If in the structural engineer’s judgment, the structural design needs to be based
upon more conservative geotechnical design criteria, the structural engineer shall provide the
more conservative design.
C. Disregard surface soil in the design of drilled shaft foundations. The disregarded depth is the
amount of surface soil that is not included in the design of the foundation due to potential erosion
from scour, future excavation, seasonal moisture variation (shrinkage and swelling), lateral
migration of waterways, and recommendations of the geotechnical investigation.
D. Drilled shaft capacity relies upon penetrating a specific stratum a specified depth. The plans shall
provide a note instructing the contractor and field personnel of the penetration requirement. The
plans shall identify the specific type of material to be penetrated and the minimum penetration
depth. The plan may allow for the drilled shaft to be shortened if the founding stratum is
encountered at a shallower depth, and it requires the shaft to be lengthened if the founding
stratum is not encountered at the expected elevation.
When the founding stratum is present at or near the surface, the structural engineer shall consider
the load-carrying capacity along with the stability of the superstructure on the foundation. For
these conditions, a minimum drilled shaft length shall be specified on the plans and the drilled
shaft will not be allowed to be shortened from plan length, but it may be lengthened if the
founding stratum is not encountered at the expected elevation.
E. Bridge foundations for new bridges over waterways require a scour analysis. A scour analysis
shall be conducted in accordance with the following:
1. TxDOT guidelines in “Evaluating Scour at Bridges” (HEC-18).
2. Abutment scour does not need to be calculated. However, abutments shall be protected
against potential scour through use of flexible revetment, where possible or hard armoring.
3. Design bridge foundations to withstand the scour depths for either the 100-year flood or
smaller flood if it will cause scour depths deeper than the 100-year flood.
4. Check the bridge foundations against the scour depth associated with the 500-year flood. This
flood event is considered an extreme event and the factor of safety on the bridge foundations
shall be greater than or equal to 1.
9.05 Retaining Wall Design
A. The Design Engineer is responsible for ensuring that the retaining wall selected for a given
location is appropriate. The retaining wall selection process shall consider the following:
Section 9 – Structural Design Requirements May 2012 Page 9-6