Chapter 6 in Highway Engineering book, Slides of Highway Engineering

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6 - 6 CHANNEL

1. Side slope with a ratio 2:1 or even flatter is most acceptable, except on rock or other hard materials where channels are lined.
2. The design of crown ditches, gutters, stream channels, and culverts flowing partially of fully, are based on the principles of flow in an open canal.

6 - 6 CHANNEL

6 - 7 CULVERT

1. Culvert is loaded vertically by the wheel load of vehicles; the earth fill covering the culvert; and the horizontal passive or active earth pressure.
2. The stress calculation for pipe culverts is based on the assumptions regarding the methods of under pipe support, soil support at its sides and loading distribution over it.
3. Corrugated metal pipes are more flexible that could tolerate greater deformations.

6 - 7 CULVERT

6 - 7 CULVERT

INSTALLATION OF CULVERTS

1. Culverts are installed in the original bed of stream with their slope and flow line conforming to the natural channel or canal
2. In mountainous or rolling terrain, departure form channel alignment, either upstream of downstream, may divert the current flow to one side of the channel.
3. Culverts on skewed channel are relatively longer and costly. But channel changes and erosion protections are provided with structures at one or both ends to reduce the skews.
4. Inverted siphons should be avoided whenever the water carries sediments of debris.
5. Stagnant water trapped inside the culverts sag is highly objectionable that must be avoided.
6. Most culverts start upstream with headwalls and terminate downstream with end wall.

6 - 7 CULVERT

6 - 7 CULVERT

CONDUITS Conduits are laid on bedding or footing that conforms to the following standard specifications:

1. Conduit bedding is classified into Class A, B and C.
2. In laying culvert pipes, the requirement of Class C bedding is applied if no bedding class is specified.
3. Class A bedding shall consist of a continuous concrete cradle confirming with the plan detail.
4. Class B bedding is extended to a depth of not less than 30% of the vertical outside diameter of the conduit.
5. The minimum thickness of the bedding materials beneath the pipe is 10 centimeters.
6. The layer of bedding material is arranged and shaped to fit in the conduit for at least 15% of its total height.
7. When a bell and spigot type is used, the recesses in the bottom trench are shaped to accommodate the bell.
8. Class C bedding should be extended to s depth not less than 10% of its total height.
9. For flexible pipes, the bed is roughly shaped and a bedding blanket of sand is specified.

6 - 7 CULVERT

JOINING THE CONDUIT PIPES Rigid conduit pipes are the bell and spigot type, tongue and groove, or other types that may be specified.

1. In joining conduit pipes, the ends are fully entered into the inner surface evenly flushed. Joints are connected by: a. Portland cement mortar or grout b. Rubber gaskets c. Oakum and mortar or joint compound d. Plastic sealing compound e. A combination of these types or any other as may be specified.

6 - 7 CULVERT

2. Joint mortar for concrete pipes consists of 1 part by volume of Portland cement and 2 arts of approved sand with water necessary to obtain a refined consistency.
3. Mortar joint is made with excess mortar to form a continuous bead around the outside joint of the pipe and smoothly finished on the side.
4. Rubber gaskets are installed to form a flexible watertight seal.
5. Flexible conduits are firmly joined by coupling bands.

6 - 7 CULVERT

6. For that portion above the trench on each side of the conduit pipe, the width of backfill is equal to twice the diameter of the conduit or 3.50 meters whichever is smaller.
7. After being bedded and backfilled, all conduits are protected by cover fill 1.00 meter high, before any heavy equipment is permitted to cross during the construction of the roadway.

6 - 8 DESIGN OF UNDERDRAIN

Underdrain is categorized under item 501 of DPWH Standard Specifications, which provides that: “This item shall consist of constructing underdrain, using pipes and granular filler material underdrain pipe outlets and blind drains using granular material in accordance with the specifications and in reasonably close conformity with the lines and grades shown on the plans or as established by the engineer.”

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