The purpose of this policy is to help determine and set minimum pavement design standards for streets in Hillsboro, Kansas.
The City’s current subdivision regulations specify required pavement widths. Widths vary depending on the size of right-of-way and the type of street (collector or local residential) in urban areas. The subdivision regulations also indicate roadway widths for suburban roads. These typically provide 24 feet of driving surface with a shoulder area and ditches to handle drainage. This policy will suggest pavement type and thickness for anticipated uses.
Pavement design must take into account the type of existing soils, the proposed paving material contemplated traffic loads and the type of base stabilization, if any.
In general, base stabilization, usually to a depth of 6” under the proposed pavement, adds sufficient extra support to reduce required pavement thickness. As such, on all urban streets 6” of base stabilization (usually lime, fly-ash or soil cement) is recommended. Rates of application and optimum moisture for the engineered rate of compaction is determined from laboratory analysis of representative samples of the soil. As soil conditions change, the precise method of stabilization may vary. During the construction phase, the compacted modified sub-base should be field tested to see that it has been constructed within the acceptable ranges of compaction and moisture. A rock base of AB3 may be added instead of the modified soil procedure outlined. The rock base to vary depending on the traffic load as determined by the Engineer.
Traffic consideration is required to determine the necessary thickness for a type of pavement, along with an anticipated life expectancy. Establishing the number of applications of equivalent 18-kip single axle loads for local and collector streets over the design life of the road. Independent studies have found that over a 40 year period this number was determined to be 117,000 and 290,000 passes respectively. Charts have been developed to determine pavement thickness required given the bearing capacity acquired through base stabilization and traffic loading over time.
Following in chart form are recommended pavement thickness for given pavement types and street types:
PAVEMENT PAVEMENT 6” OF STABILIZED
STREET
TYPE
TYPE THICKNESS SUBGRADE
Local Residential Asphaltic 6” Yes
Local Residential Concrete 5-1/2” Yes
Collector Street (includes Asphalt 7” Yes
commercial,
multi-family)
Collector Street Concrete 6-1/2” Yes
Industrial Industrials require special considerations to match design life with
traffic loading and pavement type.
Suburban streets generally have a greater right-of-way (usually 70’ or more) to allow for ditch drainage on each side of the road surface. Prior to any pavement improvements to this type of road, all cross-road culverts need to be checked for sufficiency in size as well as optimum location and correctness relative to positive drainage. Only when the drainage system is properly in place can the pavement improvement hope to last over time. Water must not be allowed to stand in ditches as its proximity to the pavement and base can weaken the street due to shrink and swell due to freeze and thaw cycles.
Two approaches are looked at for suburban streets. One is to incorporate the same standards as previously suggested for urban streets. The same requirements for a stabilized sub-base with 6” of asphalt for a local residential street could be equally employed, with the exception of curb and gutter sections. Concrete would not be economical in this type of construction.
A second approach is less expensive, but would require substantial maintenance on a more regular basis. This method involves placing 3 to 4” of an approved aggregate or rock base on a compacted earth base. The street should be properly crowned to shed water. Upon this rock base is placed a prime and double seal coat for a wearing surface. As indicated, this method is less expensive, but usually requires maintenance every three years.
These suggestions should be considered “minimums” and are subject to material and construction control. As pavement sections become thinner, material and construction control become more critical. This needs to be accomplished during construction as a street can fail in tough Kansas winters regardless of design excellence considering drainage, subgrade, traffic, etc. To this end, control by testing and inspection is encouraged to provide uniform and lasting streets with minimum maintenance.
See attached drawings for:
Rigid Pavement - Residential Streets
Flexible Pavement - Residential Streets
Rigid Pavement - Collector Residential Streets
Flexible Pavement - Collector Residential Streets
