2722-1

Design- County/City Contracts

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In the area of local government improvements under the County/City Contract Improvement Program, the requests are becoming greater; but with a reduced funding availability, the breach between supply and demand is widening. Good design practice and engineering judgment must be used in design of a county/city contract project and preparation of an estimate. While it is imperative that the design and construction be equal to that required to carry the anticipated traffic and loading, extra care must be taken to see that construction of facilities do not exceed that required. In utilizing engineering, consideration must be given to local conditions and nature and extent of road usage. Site conditions and effects of the overall construction on the surrounding environment must be considered. In line with minimizing the design to that absolutely necessary for the intended service, the following guidelines shall be used in preparing future estimates. Strict compliance with these guides is not intended as a substitute for use of good sound engineering judgment.

1. Overall Design
   1. The basic design will be in accordance with the Department's Low Volume/Local Road Guidelines as shown in the County/City Contract Board Policy.

   2. Grading should be set as closely as possible to accomplish a balance of earthwork. Grades may be established using minimum cover over drainage structures as check points and utilizing a balance of material from grading ditches in order to minimize use of borrow material. Maximizing allowable grades and minimizing elevation on flat grades will generally effect a savings in amount of earthwork. However, inundated area grades must be raised sufficiently to assure the integrity of the roadway.

   3. Base course materials should be set up utilizing local material to the fullest extent possible. Where economical, substandard materials may be utilized with the addition of a stabilizing agent or with a blend of other local materials or stone.

   4. Generally pavements for roads with daily traffic counts under 800 will be surface treatment. For roads with higher daily traffic counts, plant mix may be used, particularly in urban areas. Where justification exists, plant mix may be considered for roads with lower daily traffic counts if economically feasible.

2. Drainage Design Guides for Roads with Daily Traffic Less than 1000
   1. Storm Drain Pipe: Design so that a 10-year storm will not flow over an unpaved roadway or a 25-year storm will not flow over a paved road. Due to the greater flow at these structures, more importance should be placed on possible damage from a greater flood. If floods greater than listed above are to flow over the roadway, care should be taken to provide low points in the grade away from the structure.

   2. Box Culverts: Design so that a 10-year storm will not flow over an unpaved roadway or a 25-year storm will not flow over a paved road. Due to the greater flow at these structures, more importance should be placed on possible damage from a greater flood. If floods greater than listed above are to flow over the roadway, care should be taken to provide low points in the grade away from the structure.

   3. Bridge Structures: Bridges should be designed so that a 50-year flood will not reach the bottom beams. When desirable, this can be accomplished by allowing the water to flow over the roadway at low points away from the bridge.

Document History:

• added to Manual of Guidance: 08/06/75
• revised: 10/30/95
• added to TOPPS: 05/02/96
• revised: 05/04/04
• reviewed: 08/23/05
• thin plant mix overlays deleted from section A. 4
  Factors to be Considered deleted from section B: 04/13/07

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