Tag: Bridge

Jacks: Not Just a Kid’s Game?

Photo of A-jacks laid out on ground

Streambed scour is defined as fluctuation in the vertical position of a streambed, or the depth of the stream, as material is eroded and/or degrades. Some degree of streambed fluctuation is a natural process within the types of gravel-bedded rivers that we see; however, scour can also occur as a result of a change in the natural streambed conditions. Hoyle Tanner is currently assisting the New Hampshire Department of Transportation (NHDOT) with providing scour stabilization measures using an innovative system: A-jacks.

The History of Stream Crossing Design – If Only we Knew Then What we Know Now!

When the initial network of roads and highways was developed in New Hampshire, there was a different thought process towards designing infrastructure such as bridges, culverts or pipes, that crosses rivers and streams than there is today. Currently we would examine a stream from all angles to determine how to best approach designing a stream crossing that will not change the stream’s natural flow, depth or substrate (riverbed material). Stream crossing designs in the 1970s did not prioritize this stream information, and, as a result, in some situations the crossing structure has changed the stream’s parameters such as width, depth and flow.

The Results: Scour Pools & Stream Channel Changes

The most common example of this is where a stream crossing is too small to meet the stream’s bankfull width, or the width the stream needs when it is at a maximum flow and creates a pinch-point in the stream. Think of a water hose: When you pinch it you can create more pressure as the water comes out. In those situations, as water is forced through the smaller opening, water flows increase in speed and energy, and the water exiting the crossing can erode, or scour, the streambed, banks, or both. This can often result in a small area immediately downstream of the crossing that is deeper than the stream is upstream of the bridge or culvert – this is called a scour pool. If the amount of scour comes close enough to the culvert, pipe or bridge foundations, it can erode the ground under the crossing and risk destabilizing the crossing, including the road on top of the crossing.

When faced with these situations, stabilizing the stream bed and banks while protecting the culvert/pipe and road from being affected are interconnected goals.

A-Jacks: New Technology to Address an Old Problem

NHDOT routinely surveys stream crossings to determine if they are stable or if work should be done to either prevent scour from occurring or resolve scouring that is currently happening and may impact the crossing structure. In Woodstock, the stream crossing of Interstate-93 over Eastman Brook that was installed in 1972 is composed of a twin cell (or 2-sided) concrete box culvert; each side is 18’ wide. Original installation included riprap (stone) at the inlet and outlet of the culvert to prevent scouring. This riprap has washed away at the downstream outlet, and despite repairs of adding riprap on several occasions, the stream continues to scour downstream. Over time, this scour will jeopardize the stability of I-93, which is not an acceptable situation. Eastman Brook carries water that flows out of the White Mountains that can seasonally flow fast enough to carry even the largest riprap boulders downstream, particularly in spring, due to snowmelt combined with heavy rainstorms. Is there a different solution?

Hoyle Tanner’s experienced bridge design engineers proposed the use of A-Jacks in this location. They consist of two concrete T-shaped pieces joined perpendicularly at the middle, forming six legs. A-Jacks are designed to interlock into a slightly flexible, highly permeable matrix that will remain in the streambed. The highlight of this design is the ability of the A-Jacks system to spread out the energy that comes from water flowing quickly out of the culvert, allowing for increased resistance to the erosive forces of flowing water. Because they lock together in place, they can flex yet effectively stay put where they are installed.

The patented, two component design allows economical transport and on-site assembly. Just as you would picture a pile of jacks when you dump them onto the floor to play the game, A-Jacks interconnect and are assembled by sliding one half into another to form a complete unit. Rows of A-Jacks are assembled to interlock in horizontal as well as vertical directions. A-Jacks can be installed either randomly or in a uniform pattern. NHDOT has previously installed A-Jacks in four locations across the state and was open to the idea of using this alternative for the Woodstock scour stabilization project.

A-Jacks were installed downstream of the stream crossing in Woodstock this summer for approximately 87 feet. As shown in the photos, the streambed was excavated to  a depth to allow for installing a double row of 48” A-Jacks that raised the streambed elevation to meet the bottom lip of the outlet of the culvert and tied into the natural grade of the stream downstream of the crossing; this will allow for improved aquatic organisms and fish passage through the crossing by preventing the situation shown in the before photo, which is called a perched outlet. Thus, the design will accommodate the highest stream flows and will keep water running through the culvert during the low flows of summer so the stream doesn’t disconnect, and wildlife and fish can pass freely. Can’t see them? Clean washed gravel and stone was placed on top of the A-Jacks to fill the small voids (or spaces) between the individual units, resulting in simulation of a natural streambed.

If you look closely at this photo, you can see the tips of some of the A-Jacks sticking out of the streambed. This was intended to produce the natural variation in depth that a natural cobble-gravel-bolder streambed would have. 

Hoyle Tanner’s Environmental Coordination team effectively coordinated between NHDOT, the bridge designers, the NH Department of Environmental Services (NHDES), the US Army Corps of Engineers (USACE) and the NH Fish and Game Department (NHF&G) to obtain agreement from each respective permitting agency that the A-Jacks, despite technically being viewed as “fill” in the streambed, were necessary in this location and would result in the best overall result that met the goals of the agencies involved.

By using innovative design techniques, our team was able to effectively stabilize an important piece of infrastructure, prevent future scouring of the stream and damage to the stream crossing, and re-create a natural streambed with improved functionality for fish and wildlife. Just a day’s work for our talented bridge designers and environmental coordinators. Let us know if you have a tricky scour issue that you would like us to take on!

The New Wave: How 3D Bridge Modeling is Shaping the Future

3D image model of bridge

OpenBridge Designer features a complete package of software that allows both 3D geometric modeling analysis and the design of steel, reinforced and prestressed concrete bridges within the same program. The benefit of using this software is its interoperability that allows us to move from modeling to analysis/design back into modeling to make geometric adjustments, then back to design and eventually into the development of CADD drawings. This software allows us to make real time adjustments and have graphics that keep up with the calculations instead of using multiple programs for different elements and stages of design.

We’re only just beginning to use this software for 3D modeling, but the program’s applications and innovation have already begun to shape the future of our industry.

Why We Use It

It seems that the future of our industry is going to be digital (with deliverables consisting of electronic data and information plan generation), and the use of paper plans may eventually disappear one day. An analogy: On the highway side, there’s OpenRoads Designer and in theory, they’re going to be able to take the 3D OpenRoads electronic files and provide them to the contractor, who will be able to use that electronic version of the roadway geometry without needing a paper plan set to build the road (since the data can be transmitted to the construction equipment).

OpenBridge is still in the infancy, and this type of capability will likely be sometime in the future for bridge construction. It is easier to build roads using 3D models instead of paper plan sets, but maybe not for bridge designs yet.

A big benefit of 3D modeling and this program is that it enables us to utilize a single software package for the major components a typical bridge consists of and – perhaps more importantly – help us to find conflicts for those components or other elements. One thing I like about this program is the ability to use “clash detection” to figure out if you have a conflict. Let’s say we did our geometry, and we modeled everything and designed our girder, but then we needed to find the minimum vertical clearance over this corridor – the program would report that clearance, and we’d be able to say whether or not we achieved the required minimum clearance. We can make adjustments from there if we have violated the design vertical clearance parameter.

How to Use It

We can start with a blank canvas when we begin using the program, but it is better to have a general idea of the design and layout of the structure (location, alignment, single span, multi-span), and to have a general idea of what type of structure it’s going to be. The nice thing about the software is that you can run it with the full 3D modeling through analysis and design, or you can start off and just do what we call a “standalone” model. Bridge Information Modeling (BIM) workflow is the full 3D geometric modeling and analysis/design tool through plan drawing development. With standalone models, in which we only use the software analytical and design tools, we can develop different structural models, to evaluate various bridge types and layouts and then work back to develop a BIM workflow to finalize the geometry and design. We want to have preliminary design done and structure type selected so that the BIM workflow has a starting point, and we know where we will end up. That’s not to say you won’t make design changes when you start BIM, but you want to be close.

We have used the OpenBridge Designer components when they were individual software packages and as standalone models now that they are all in single software package, but we are in the infancy of using the BIM workflow.

Who it Helps & Where it’s Headed in the Future

It helps designers with the process, and it helps the design work flow. When using the software, I have the capability to design the superstructure and substructure within the same software package I can go through the program and design a girder and then could add another girder, remove a girder or change the girder size without building an entire new model. The workflow is the same. Then, the substructure design could be updated with any of the girder changes I made, again without creating a new model. The models are updated as you work through your design and the BIM process.

The way we do it right now, we use multiple software programs for different design components, but I can do all that they can do with OpenBridge Designer. In theory, this will make our designs more efficient. There’s also less room for error if I don’t have to take data out of a 2D model for design loads, for instance, and then input them into several different software packages. I can just use this one software package to generate the design loads and complete my designs.

I believe this type of software is the future, but we need to be well-versed in this program before using it heavily. We recognize what this software and others like it have to offer us as designers and the future of 3D design, and are excited to learn how to design bridges with this new set of tools.

Want to learn more about OpenBridge Designer or how our bridge team can help your community? Contact me!