Culvert

1. What is Culvert?

Culvert is similar to a bridge in that it is a structure that allows pedestrians and vehicles to cross obstacles, such as roads and railways. However, the terms bridge and culvert are used differently. Factors such as type of obstacle, usage, surrounding terrain, constructability, and span length are used as criteria to distinguish between culverts and bridges.

In accordance with the National Bridge Inspection Standards (NBIS) in the Hydraulic design of Highway Culverts of the Federal Highway Administration (FHWA), span length of structures less than 20ft (6.1m) are defined as culverts. Although this criterion is not a standard that distinguishes the two structures, it is commonly used. A conceptual diagram of the FHWA standards and the geographical differences in which the culvert and bridge are installed are shown in the figure below.

2. Types of Culverts

Culverts are used mainly for crossing streams or for drainages. Culverts are also used to provide grade separated crossing pedestrian and bicycle traffic on roadways or railways. In this page, structures installed for crossing streams or drainages are classified as buried structures, and structures installed for pedestrian passage are classified as grade separated structures.

A. Buried structure

Buried structures are generally used for conveying water. They are used to cross streams in locations where roads or railroads pass, or for underground drainage. Since these structures are not large, they are classified as culverts rather than bridges, and structures that meet the required capacity are selected from the standard design table without detailed design.

The shape of a bridge structure should be chosen according to its application and situation. In most cases, pipe culverts or box culverts are used, but depending on the situation, precast concrete arch culverts, three-sided culverts (bottomless culvert, open bottom culvert), or long-span corrugated steel culverts are also used.

Since these types of structures transmit vertical loads through the structure’s capacity and soil arching action, which is one of the soil-structure interaction, there is a significant difference in the way load is transferred depending on the shape of the structure. Furthermore, the barrel (size, shape, material) and inlet configuration (pipe, end section, headwalls, wingwalls, bevels, and tapers) must be designed according to FHWA’s ‘Hydraulic design of Highway Culverts’ in order to provide adequate hydraulic capacity.

An example of a structural analysis model of a box culvert, and the loads being applied is shown in the figure below.

B. Grade-separated Structure

Culverts are also installed on routes through which roadways, railways, or major drainage-ways pass giving passage to pedestrian traffic or cyclist, and depending on the region, culverts also provide passage to equestrian, cattle or camels. Structures used for these purposes are classified into overpass and underpass depending on the location of the passage. Bridges are also classified into overbridges and underbridges, which have similar roles but differ in the size of the structure. Culverts can be used when they are more economical than bridges, considering both constructability and hydraulic and structural requirements. Safety and aesthetics are also important factors when considering culverts.

Structures made for traffic management are generally made of three-sided culverts and have longer span lengths than structures for drainage systems. These structures are classified as long-span culverts in FHWA’s ‘Hydraulic design of Highway Culverts’, and the span length is proposed in the range of 20ft to 40ft (7m to 14m). However, recently, culverts having longer span length have been manufactured due to the development of new manufacturing methods and materials.

Three-sided culverts can be classified into two types: flat top and arch top. A structural analysis model and a conceptual diagram of the applied loads is shown in the figure below.

3. Difference between Culvert and Bridge

Culverts and buried culverts used for drainage systems and waterways are clearly distinguished from bridges. However, they may not be easy to distinguish if they serve a similar role to that of bridges. Culverts and bridges have many differences, including the design method, construction method, maintenance method, etc., but some of the most important differences are as follows:

A. Type of Obstacle

The span length, which is a common criterion for distinguishing culverts and bridges, defines what kind of obstacles the structure should cross. Bridges are structures made to cross valleys, rivers, and even seas, so their span length is inevitably long. However, culverts are structures that are installed in narrow areas, so their span length is short. When choosing between culverts or bridges, it is not possible to choose only considering the type of obstacle, but the economical, constructability, and surrounding environments must all be considered as well.

B. Structural

 Culverts are either completely covered or bordered on both sides of the embankment material. Therefore, not only the vehicle loads, but also the weight of the soil covered on the culvert or the lateral soil pressure should also be considered in the design. The soil covering or surrounding the culvert plays an important structural role. For example, soil stability directly affects the culvert’s structural performance, while lateral soil pressure enhances the culvert’s ability to support vertical loads.

The components of culverts are simple compared to the components of bridges, which are composed of various components including superstructures (deck, girder, bearing) and substructures (pierhead, pier, pilecap, pile, abutment, etc.). Culverts can consist of only pipes or concrete boxes that can become passages like tunnels, and if necessary, wing walls or foundations are added.

C. Maintenance

Culverts, like bridges, require maintenance according to standards, but there are differences in the details. In the case of culverts installed for drainage purposes, management is necessary because the waterway may be blocked by debris and sediment. Furthermore, culverts must be managed to prevent leakage that may occur in scours and joints that occur in inlets and outlets to prevent loss of bearing capacity or undermining.