• Home
  • Download PDF
  • Order CD-ROM
  • Order in Print
DRAINAGE - 14071_72
Figure  3-12.Drainage  systems.

Engineering Aid 2 - Intermediate Structural engineering guide book
Page Navigation
  56    57    58    59    60  61  62    63    64    65    66  
Figure 3-10.-Check dams. falls upon the surface, or traveled way, is drained by crowning the surface; that is, constructing the traveled way so that the middle is higher than the edges. The   traveled   way   in   curves   is   drained   by superelevating the surface; that is, constructing the traveled way so that the inside edge of the curve is lower than the outside edge. The  water  that  drains  from  the  surface  continues over the shoulders. The shoulders always have a slope greater than, or at least equal to, the surface slope. This slightly  increases  the  speed  of  the  draining  water  and therefore increases the rate of drainage. The water then flows from the shoulder down the side of the fall, if in a fill section of a roadway. If the section is in a cut, the water flows into a roadway ditch. Roadway ditches are not normally in a fill section. Roadway  Ditches The  functioning  of  a  roadway  ditch  is  the  most important factor in roadway drainage. If this ditch, which runs alongside the roadway, becomes obstructed or is inadequate for the volume of water, then the road- bed becomes flooded. Not only can this block traffic, but it can also wash away surface and shoulder material. There are several factors to consider in determining the size and type of roadway ditches, such as volume of water to be carried, the slope of the backslope, soil types, the “lay of the land,” and the maximum and minimum ditch  grades. The slopes of the surface, shoulders, and backslopes affect the volume. A steep slope increases the rate of runoff,  thereby  causing  a  greater  instantaneous  volume of water in the ditch. On the other hand, a lesser slope decreases  the  rate  of  runoff  but  exposes  more  surface area  on  the  backslope,  which  increases  the  amount  of runoff. The choice of slopes to be used is governed by other factors, however. The foremost factors are whether the additional   excavation   is   needed   in   the   roadway construction  and  the  type  of  soil.  A  lesser  slope  would be required if the cut is in sand instead of clay or rock. The usual cut slope, or backslope, is 1 1/2:1 (1 1/2 foot horizontal, 1 foot vertical). This slope maybe decreased for sandy soil or greatly increased for rock cuts. The usual ditch slope, from the shoulder to the bottom of the ditch, is 3:1. All these soil types have different amounts of runoff. The runoff from a sandy soil is small, but from a clay soil or solid rock it is large. An important design factor is the ditch grade itself. The  minimum  grade  is  0.5  percent,  and  the  desirable maximum grade is 4 percent. A grade greater than 4 percent would cause excessive erosion due to the greater velocity of the water. In this case, low dams of wood or stones, called check dams (fig. 3-10), are built across 3-11







Western Governors University

Privacy Statement
Press Release
Contact

© Copyright Integrated Publishing, Inc.. All Rights Reserved. Design by Strategico.