05-13-1998 Council Agenda - Laserfiche WebLink

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PASER --- Evaluation Moisture and poor pavement drainage are significant factors in pave- ment deterioration. Some assessment of drainage conditions is highly recommended in addition to rating pavement surface condi- tions. While you should review drainage in detail at the project level, at this stage simply include an overview drainage evaluation at the same time as you evaluate surface condition. Consider pavement surface drainage as well as lateral drainage (ditches or storm sewers). It is important that the pavement can quickly shed water off the surface into the lateral ditches and that the ditches be adequately large and deep to drain the pavement and remove the surface water efficiently into adjacent waterways. Look at the roadway crown and check for low surface areas that per- mit ponding. Paved surfaces should have approximately a two per- cent cross slope or crown across the roadway. This will provide ap- proximately 2 -1/2 inches of fall on an 11 foot traffic lane. Shoulders should have a greater slope to improve surface drainage. The ability of a pavement to carry heavy traffic Toads depends not only on the pavement materials (asphalt surfacing and granular base), but also the strength of the underlying soils. Since most soils lose strength when they are very wet, it is important to provide drain- age to the top layer of the subgrade supporting the pavement struc- ture. in rural areas, this is done most economically by providing open ditches that allow soil moisture to drain laterally. As a rule of thumb, the bottom of the ditch ought to be at least one foot below the base course of the pavement in order to drain the soils. This means mini- mum ditch depth should be approximately two feet below the center of the pavement. Deeper ditches, of course, will be required in order to accommodate roadway culverts and maintain the flow line to adja- cent drainage channels or streams. You should also check culverts and storm drain systems. Storm drainage systems that are silted in, have a large accumulation of debris, or are in poor structural condition will also degrade pavement performance. Use the simple visual rating scale below to evaluate drainage. Rating Description Good Fair Poor Adequate drainage. No evidence of flooding, erosion, ponding or water damage. Ditches or storm sewer (curb and gutter) adequate size and good condition. Culverts clean and sound. Pavement has adequate crown and drains well. Ditches or storm sewers present but below standard. Shallow ditches or storm sewer causes occasional flooding. Pavement has some crown. Culverts in sound condition but some sediment present. Evidence of frequent flooding, ponding, erosion or drainage damage. No ditches, storm sewers or curb and gutter. No pavement crown— causes ponding. Culverts in poor structural condition or filled with sediment. Page 36 Assessing drainage conditions What to look for when assessing drainage