Installation Of Culvert Bridge.docx

INSTALLATION OF CULVERT BRIDGE There are two types of culverts; the corrugated metal or plastic tubes position in a crossing to allow water to pass through without damaging the roads. Culvert corrugation provides a greater strength-to-weight ratio than smooth pipe. This helps support the weight of the vehicles, while mimicking the roughness of a natural stream bottom, which slows the speed of flowing water. Culvert installation work To determine the right size of the culvert, you need to consider the following dynamics of; i. ii.

Stream flow and how it changes with season Peak flow during spring runoff and its tail off from late summer and early fall

The annual fluctuations can seriously erode exposed soil and cause inadequate crossing structures to fail, hence culverts should be sized at a minimum to handle the annual peak flow and installed in a manner that will protect the culvert’s strength over time. To determine the culverts correct size, filed measurement of the stream crossing should be taken when it’s at its lowest. Requirements for the field measurement. i. ii. iii. iv. v.

Tape measure Four stakes A hammer String Notepad

Step one: Steps for measurements. i. ii. iii. iv. v.

Check intended crossing i.e. the highest water mar, this is where any vegetation meets bare dirt and rock. Along the river bank, put stakes at the high water marks on each bank. Tie a string between the stakes Measure the distance between stakes Record it as the high water stream width (HWSW)

Step two: Measure or locate the lowest water mark. i.

This borders the flattest portion of the stream bottom. Establish the other two stakes at this level and measure the lowest water stream width (LWSW)

Step three: final measurement is the depth of the stream at high water. This is the distance between the stream bottom and the high water line string. If the stream bottom is highly irregular, ones need several measurements and average them. Using the simple formula to determine the stream’s cross-section area (A) A=(

𝐻𝑊𝑆𝑊+𝐿𝑊𝑆𝑊 2

) * h (depth)

A=(

5+2 2

) * 1 (depth)

A = 3.5 If you measure; HWSW = 5 feet, LWSW = 2 feet and 1 foot for the stream depth at high water, then the stream’s cross-section area would be 3.5 square feet. The culvert’s cross-section area should be at least 1.25 times the cross-sectional area of the stream, if the culvert is for a shallow ditch or seasonal stream. But, if the stream is subject to flooding, you should install a larger culvert in order to avoid damages due to water backing up and flowing over the roadway during flooding If flooding is likely, multiply the stream area by 1.75(for 25 years floods) or by 3 (for 50-100 year flood levels. In this case, the culvert cross-sectional area would be 3.5 * 1.25 = 4.375; 3.5*1.75 = 6.125; or 3.5 * 3 = 10.5 Determining the length prior to culvert installation. ii. iii.

Culvert length depends on how wide you want your roadway as well as the diameter of the culvert and the depth of dirt fill placed on top of the culvert to make the earthen bridge. Set culvert perpendicular (90 degrees) to the road’s centerline. Culverts not set in at 90 degrees will require a greater length.

To calculate length (l) of the culvert, add up the width of the roadbed (RB), the width of the two side slopes (SW), plus 1 foot extensions (E) from either end of the culvert; L = RB + SW + 2E The side width (SW) will depend upon the diameter of the culvert (CD, and depth of the road fill (RF) and the slope of the sides (S. SW = (CD + RF) x S x 2 For e.g. if you 10 feet wide roadbed and have a 24 inch diameter culvert, 1 foot of earthen fill on top of the culvert, 1

a standard 1.5:1 side slope (1 feet wide for every 1 foot in height) and 1 foot extension on either side, the 2

calculation is; SW = 2 foot culvert diameter + 1 foot fill = 3 feet * 1.5 slope = 4.5 * 2 = 9 L = 10 foot roadbed + 9 feet of side width + 2 feet of culvert extensions = 21 feet long. Equipment require for installation work. Backhoe or excavator and Soil compactor (mechanical, blunt-ended or long-handled compactor) i. ii. iii.

Excavate the streambed 6 inch deep to make space for the fill on which the culvert will be laid. As you excavate, follow the existing streambed gradient (slope), which typically runs from 2 -6 percent. Slope less than 2 percent can result in excess build of sediment and debris inside the culvert. If the culvert is set too high on either end, water can work its way underneath the tube, causing the bridge to wash out.

iv. v. vi. vii. viii. ix. x. xi.

If the culvert is set too deep, it can plug with debris After excavation, spread a 6 inch deep bed of rock free soil or sand over the bottom and compact it. When compacted, the fill will be about 4 inch deep, allowing the culvert to be set slightly below the stream grade so the water will drop at bit as it enters. Now, place and center the culvert. Be careful to minimize any abrasions to the pipe’s protective exterior coating. Next, fill around the culvert with 6 inch layer of dirt. Compact each layer before you add another and be sure the fill materials adjacent to the culvert is rock free. Continue adding fill over the culvert to a depth of at least 1 foot; this will protect the culvert form failure due to the weight of road traffic Finally, seed the bare fill dirt slopes with native grass and apply mulch. Also place flat stones or broken pieces of concrete in the streambed at each end of the culvert to protect the fill material from erosion. Or pour concrete protection i.e. known as armoring and will be useful when the water flow increases in the spring.