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The City has observed premature distress on many of the heavily traveled streets and
on streets built on clay subgrade soils with high plasticity indices (P.I. >20). A
map is provided in Appendix J that represents the general soil PI distribution within
the Austin area, but this does not relieve the designer's responsibility to provide
a geotechnical report and to design to the site specific soil condition. The minimum
pavement standards included in this TCM do not relieve the design engineer from the
responsibility of designing a cross section that is appropriate for the soil conditions
to meet the required design life of 20 years. A clay subgrade classification of CH
(Unified Soil Classification System) with a minimum Liquid Limit of 50; more than
50% passing a #200 sieve and a P.I. greater than 22 is considered highly expansive
requiring special pavement design considerations.
Pavement designs that are appropriate for soil conditions result in pavements that
are maintainable over the entire useful life of the pavement structure. This is achieved
with proper stabilization of subgrade soils that may necessitate using a combination
of modification techniques for the subgrade soils: such as, but not limited to removal
of objectionable soils, reinforcement strategies, and/or subgrade moisture control
features described at the end of this subsection.
The program described herein was adopted from the Texas Department of Transportation
(TxDOT) design system currently being utilized by the TXDOT and its local districts
and is modified for municipal applications. The TXDOT design system was adapted from
the American Association of State Highways and Transportation Officials (AASHTO),
Interim Design Method, with modifications for local conditions and needs.
Modifications to the TXDOT highway programs were undertaken to make the highway programs
for rural highway design more suitable for municipal conditions. These improvements
included the addition of: 1) curb and gutter costs, 2) subgrade excavation costs,
3) additional costs associated with future overlays including thickened edge, edge
milling and overlay tapering, 4) the effects of the distribution of heavy trucks on
city streets of different classification and 5) revising the traffic modeling.
It is important to note that this program may not produce appropriate critical stresses
in flexible pavements designed for relatively low Average Daily Traffic (ADT) values.
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