Culvert Bridge Rehabilitation

Excite Development---Culvert Bridge Rehabilitation

I. Members

Name NetID 

Lee Gilbert lsg14

Ellen Huang ech19

Tracy Huang*

tyh3 

Cesar Perez cap42 

Sophie Vos  shv2 

*contact personWant to join?! Then talk to one of us or Maria Gibbs (maria.gibbs@duke.edu)!

II. Introduction:

Duke University Excite Development and Bridges to Prosperity are partnering in pursuing a pedestrian bridge project in El Salvador that will provide access to schools, markets, and emergency medical care for three rural farming communities in San Jose Villanueva. Excite Development is a student organization whose mission is to support community-driven development programs worldwide through the design and implementation of sustainable engineering projects, while fostering responsible leadership. Duke students are currently designing a bridge to connect the 36 families of El Guayabo, El Porvenir and Tula to San Francisco and the adjacent Panamerican highway. During the rainy season, which lasts from June to November, the San Antonio River will rise up to 2 meters, completely submerging the existing culvert vehicular bridge for up to four days at a time. When the crossing is flooded, the communities are left isolated. Children cannot cross the river to reach the school they attend on the other side and their parents are unable to sell their produce or purchase food and supplies. The plan is to design and implement a 40-meter suspended pedestrian bridge that will provide a safe crossing year round, as well as rehabilitating an existing culvert vehicular bridge. Currently, the culvert bridge stands, but is in danger of deteriorating beyond repair. With each flood, the ends of the bridge that meet the ground weaken as water passes over the deck and through the holes in the stone and mortar structure. Furthermore, leaf, twig, and branch build-up at the culverts prevents efficient water flow, and could lead to increased pressure on the bridge. Furthermore, the deck of the bridge is simply poured concrete; it has no steel rebar or any other type of reinforcement. These factors make rehabilitation of this bridge a high priority, especially if school attendance may increase to 90%.

III. Problem Statement:

The current vehicular bridge spanning the river is useable but in order for it to remain passable over a long time span it will require a series of repairs. The culvert bridge currently has two noticeable failure points. During heavy rainfall there is a large amount of debris that flows down the river and clog the culverts. This increases amount of stress put on the bridge and leads to greater erosion. This erosion mainly occurs on the upstream face of the bridge where it can already be seen that the mortar and rock in the wall are degrading. The other source of damages occurs during the flooding of the river. The water level can reach up to a meter or more above surface of the vehicular bridge. This not only means the bridge is receiving the equivalent forces of a dam but the abutments to the bridge are inundated and are susceptible to erosion. The current deck of the bridge is also not reinforced and thus very weak in the tension induced be the water flow. Both the issue of the face of the bridge eroding and the deck itself have the ability to lead to a failure of the bridge rendering it impassible even during the dry season.

IV. Research Avenues:

With the main focus of the El Salvador project being on the construction of the pedestrian bridge there are constraints on resources and time available to fixing the existing culvert bridge. Due to these constraints it would not be feasible to construct an alternate rout for vehicles to take across the river leaving the option of securing the current culvert bridge.In order to find information on bridge rehabilitation members of our group contacted civil engineering professors as well as other engineers who have had experience with bridge construction or design. Our main source of information has come from Dwayne Lee who is a retired army corps engineer. He has previously worked with Maria Gibbs on the construction of a bridge in Bolivia and was able to give some ideas on why the bridge may be failing and possible ways to slow or halt the damages.

V. Progress made:

The bridge rehabilitation sub-project is still primarily in the research stage and it will remain this way until the start of the spring semester. At which time the sub-project will be largely taken up by the CE 185 class. The current focus of the group is on data collection as well as the summery and presentation of the collected information. It is important that the CE185 class has the majority if not all of the information that they will need to design bridge repairs, as valuable time would be lost if they were missing information on the bridge.This being said there are currently three main reparations that are being looked into. Firstly, there is the issue of the eroding upstream face of the bridge. In order to minimizing the focusing of water pressure there is the possibility of covering the upstream face of the bridge in mortar. This would allow a greater distribution of water pressure and not allow the rocks lining the face of the bridge to erode. Secondly, riprap could be added to the upstream abutments of the bridge. The riprap would be formed out of wooden cribs containing stones, which would absorb the impact of the river and dissipate the force currently felt by the bridge abutments. Lastly, covering the deck in a reinforced concrete slab would decrease the amount of surface fracturing and erosion of the driving surface. The reinforcement would allow the concrete to resist tension and not fracture as the pictures show of the existing bridge. Ideally all three of these methods of restoration and strengthening would be implemented to secure the existence of the bridge.

a) Mortar

Water is a powerful force that can quickly erode a bridge if built incorrectly. The current culvert bridge is slowly beginning to fall apart due to the erosion of the upstream face of the bridge. The mortar that holds the stones in place is slowly crumbling and allowing the stones supporting the bridge to fall out (figure 1). This erosion will continue to occur as water flow has pressure concentrations at sharp corners. The best way to slow this cycle is to re apply mortar and ensure that the water is not able to concentrate its forces or enter into the bridge.In order to reapply the mortar with the most effective results it will be necessary to strip the current bridge as much as possible. In order to ensure the soundest and most force resistant seal all of the weakening mortar needs to be broken out. By using a piece of rebar it is possible to both take out and put in new mortar a fair distance into the bridge. A mixture proposed by Dwayne is one part cement and four parts sand. The water content should be on the higher end to allow the mortar to penetrate the furthest into the bridge. The curing time for the mortar is around 14 days at which time it can begin to provide resistance. The whole upstream side of the bridge should be covered in this method leaving the face looking like figure (2).

Figure 1: existing culvert bridge with loosely held stones

Figure 2: rock wall held soundly together by mortar

b) Abutment Protection

Protecting the bridge abutments from the direct force of the river has many benefits worth working for. The river will concentrate its forces on the weak and jagged parts of the abutment wall as well as the intersection of the abutment and the ground it sits on. These impacts lead to the weakening and erosion of the abutment and potential the undercutting of the abutment, which leads to transverse cracks in the bridge and even the washing away of the bridge. Our team has looked into two main options to halt this process by lessening the force of the water on the abutments. Historically the creation of a rock riprap apron both absorbs the forces of the river and will prevent the scouring of the riverbed under the abutment. Another prevention method is the creation of a secondary bridge abutment that would take the main impact of the water flow.Rock riprap has been used for centuries to alter the natural effects of erosion and consequently there is a large amount of information on the subject. The primary force lessened by riprap is the development of a vortex system that grows over time and scours the toe of the bridge abutment (figure 1). By placing an apron of riprap around the abutments the water is not able to form into a vortex leading to a greatly dissipated force (figure 2). In order to determine where to place the rip rap various formulas have been created based on the size and weight of the rocks being used and the velocity of the water flow. While this apron does a good job of stopping the build up of a vortex it does little to weaken the force on the sides of the abutments.The addition of a second abutment on the upstream side of the bridge will absorb the brunt impact of the river. In most circumstances abutments are created in order to halt the erosion of the ground around the bridge but this would not be possible for the culvert bridge in question. Another difference to common abutments is that at the location in question it would not be possible to drive pilings into the ground or create large concrete structures. One possible solution is to create two wooden cribs filled with riprap on the upriver abutments (figure 3). The reason for creating a crib is that it uses the most readily available material as well as the fact that it would be self-stabilizing against the force of the water.The crib could be constructed in much of the same fashion as a pier or dock base, with interlocking wooden beams. The connected nature of the logs as

well as the weight of the rocks will anchor the structure in place avoiding the issue of using pilings. The crib also will not have to be impervious to water as it will dissipate enough energy from the water flow by the uneven nature of the ballast rocks and the upstream log faces.

Figure 1: bridge near collapse after one of the piers has been undercut by the river

Figure 2: bridge piers surrounded by riprap

Figure 3: log crib filled with stones

c) Reinforced slab

The road slab is one of the most important parts of the bridge. By holding the road surface a solid piece the water will flow over the road and not create cracks and erode the sides of the bridge. When cracks form in the slab water is able to get under the road slab and it can create voids large enough to damage the road slab. The existing road slab is not reinforced and there are already pieces separating from the main body of the slab (figure 1). A non reinforced concrete slab is able to with stand large forces in compression however they are much weaker in tension. By constructing a reinforced concrete slab over the top of the current road the bridge will be able to take much greater forces without forming cracks.In order to place a reinforced slab over the existing road it must be ensured that there will not be failures in the old road after it is covered. To do so any existing cracks will need to be filled in with sand and compressed as much as possible. If the sand is not compressed or cracks are not filled in then the new slab will be susceptible to fracture at the same location. An initial concept for the reinforced deck is a four inch thick slab with quarter inch rebar placed evenly through out (figure 2).

Figure 1: current culvert bridge with cracked road surface

Figure 2: rebar aligned and ready for pouring of concrete

Areas to investigate further:

In the time before the CE185 class the group will look into other sources of information on bridge restoration. With the suggestions provided by Dwayne, the team will be able to ask more specific questions and gain more information in response. It will also now be possible to look into current bridges to see if any have similar structures to aid in there stability. Lastly, the data that has been acquired will be organized and compared to the information given by Dwayne in order to tell if any more information needs to be found out about the bridge before designs of reparations are begun.