Preliminary Engineering Analysis and Design
The goal of PE design is to develop an engineering solution that meets the transportation need while avoiding and minimizing environmental impacts.
A road’s physical location on the Earth’s surface, as defined by its latitude and longitude data, is called a road’s horizontal alignment. A road’s horizontal alignment determines where the road is in reference to other landmarks.
A road’s horizontal alignment has the most impact and influence on environments and communities.
For a new highway, the horizontal alignment determines where the highway will be, which neighborhood it will go to, and which part of the city it will serve.
For a road-widening project, the horizontal alignment identifies where the additional needed right of way will be acquired. The horizontal alignment totally controls decisions as related to acquiring the needed right of way from left side, right side, or both sides of the existing road.
Figure 4.12a illustrates a 6015 feet segment of the horizontal alignment (from STA 167+95 to STA 1207+80) representing alternatives 8, 14, and 15 for the proposed new Florida Gulf Coast Parkway. The legend panel at the bottom left corner indicates that a white-lines represent property boundaries, dashed grey-lines represent proposed right of way line, and light grey shaded areas are wetland. The alignment shows the bridge has two tangent segments, and these two tangent segments are connected by a simple circular curve starting from STA 211+ 42.08 (PC) to STA 230 + 62.09 (PT). Tlie curve has a total length of 1920.01 feet.
The proposed highway will need additional right of way from the west side of the existing road as indicated by the dashed grey proposed right of way (ROW) boundary. No other land is needed on the east side of the existing roadway (the existing white property boundary coincides with the proposed grey ROW line).
The alignment also crosses a narrow piece of wetland between STA 220 and STA 224. Wetland impact evaluation needs to be performed.
Horizontal alignment provides boundary information for:
■ Right of way need and relocation assessment
■ Wetland encroachment assessment
■ Wildlife and habitat area encroachment assessment
■ Historical and archaeological site encroachment assessment
■ Floodplain encroachment assessment
■ Highway noise impact assessment
■ Access need assessment
■ Neighborhood cohesion analysis
How a road negotiates hills and valleys along its horizontal alignment is called vertical alignment. Vertical alignment determines how flat or steep a road is and how high or how low a roadway is as compared with its abutting lands.
Figure 4.12a Illustration of a horizontal alignment. (Courtesy of Florida Department of Transportation, www.fdot.gov.)
Figure 4.12b shows a vertical alignment for the proposed State Road 64 Intercoastal Waterway bridge. The horizontal grid labeled with the station information characterizes the linear distance. The vertical grid offers elevation information. The scales used are different.
Grey-lines represent the proposed bridge profile. The actual bridge begins at STA 119+22.99 and ends at STA 153+74.64, a distance of 3451.64 feet. The 3451.64 feet bridge has a 784 feet long vertical curve with an up/down 4% grade. While the top grey-line of the bridge profile shows the actual pavement surface elevation, the bottom grey-line indicates the superstructure’s bottom elevation. The distance between the bottom and top grey-lines is the superstructures thickness, which is 8 feet as indicated. Identifying the bottom grey-line is critical as it affects the vertical clearance for water bore traffic (e.g., ships). Here the vertical clearance is 65' from mean high water elevation per U.S. Coast Guard permitting specification.
At the bridge’s left end, an 850 feet vertical curve is used to connect the bridge to the touchdown roadway. On the bridge’s right end, a 1,400 feet long vertical curve is used to transition the bridge down to the touchdown road. The vertical alignment extends all the way to other crossing roadways on both ends. This is very critical as impacts from the proposed bridge to other roadways and connecting streets must be fully understood.
The proposed vertical alignment also shows the existing profile (dashed light grey). The existing profile is much lower, with a channel width of 131.21 feet.
Also, water depth information, as depicted by the mean high-water elevation (black line) and the ground line (dashed black line), are also provided.
Vertical alignment offers data and information on:
■ Earth fill and cut quantity estimation
■ How to tie down a roadway with abutting roadways and driveways
■ Guardrail and other roadside feature needs assessment
■ Fore-sloping/back-sloping and construction limit determination
■ Potential highway noise abatement wall location and height evaluation
■ Bridge vs. causeway in dealing with wetland and wildlife crossing
■ Underpasses for potential wildlife crossing analysis
■ Various vertical clearance needs analysis
A typical section developed during PE offers not only an engineering solution but also a great public involvement tool due to its ease of understanding. A typical section offers a clear illustration on:
■ The right of way (width) needed to accommodate a proposed roadway.
■ What a proposed roadway looks like - an arrangement of traveled lanes, bicycle lanes, sidewalks, and other elements.
Figure 4.12b Illustration of a vertical alignment. (Courtesy of Florida Department of Transportation, www.fdot.gov.)
■ Roadway components to accommodate different constraints (e.g., guardrail with a steep fore slope vs. a wider road with a flatter fore slope).
Design alternatives are the derivatives of three key elements - horizontal alignment, vertical alignment, and typical sections. The objective of developing alternatives is to minimize environmental impact and control project costs while fulfilling transportation needs and gaining public support.
Environmental Impact Evaluation
Design engineers begin a design with a wide range of known constraint factors such as (a) 100-year floodplain locations, (b) known wetland areas, (c) known hazardous material locations (e.g., landfill), (d) known wildlife and habitat areas, (e) locations of businesses, (f) residential buildings and homes, (g) known archaeological sites and historic buildings or districts, and (h) both present and future land-use plans.
Once design alternatives are devised, other professionals, such as the right of way agents, relocation specialists, wetland scientists, noise specialists, historians, and archeologists conduct environmental assessments for each alternative. Based on a comprehensive evaluation, design engineers further compare alternatives, modify alternatives, devise new alternatives, and select a final preferred alternative.