Figure 3-13.-Types of culverts.a deep V-bottom ditch, the side slope is 3:1, 2:1, or 1:1.CulvertsThe side slope for a trapezoidal ditch is 3:1,2:1, or 1:1.The flat bottom is generally 2 feet wide but can rangefrom 1 foot to 6 feet or more.Interceptor DitchesThe volume of water draining into a roadway ditchcan be decreased by the use of shallow ditches thatextend around the top of the cut and intercept the waterdraining from the original ground toward the roadway.An interceptor ditch shown in figure 3-12 is dug 2 or 3feet behind the backslope limits. Its size depends on theoriginal ground slope, runoff area, type of soil andvegetation, and other factors related to runoff volume.Diversion DitchesAs it leaves the cut, water from the roadway ditchescannot be allowed to pond in the ditches or against theroadway fill. Therefore, diversion ditches are dug tocarry the water away from the roadway to natural drains.These drains can be rivers, streams, gullies, sinkholes,natural depressions, or hollows.Sometimes it is necessary to have the water flowfrom one side of the road to the other or have the roadcross a small stream. You do this with cross drains. Theyare called culverts if they are 10 feet or less in width.Over 10 feet wide, they are called bridges. Culverts aremade of many materials, such as corrugated metal,reinforced concrete, concrete pipe, timber, logs, andeven open-ended oil drums. The type of materialselected is dependent upon various factors including, inpart, the type and life expectancy of the road.For permanent roads and highways with concrete orasphalt paving, the most durable of materials, such asreinforced concrete or concrete pipe, should be used.Concrete pipe is one of the strongest and most durablematerials used in making culverts. The shell thicknessand length depend on the pipe diameter. (The larger thediameter, the thicker the shell and longer the section.)Pipe diameters are nominal inside dimensions. Forsemipermanent and temporary roads, the designengineer may choose to use materials such as thoseshown in figure 3-13.3-13
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