Category:143 Practical Design
From Engineering Policy Guide
|Osage County Route D|
|St. Louis County I-64|
|Barry County Route 37|
|Greene County Route 160|
|Safe and Sound Bridges, Pettis County|
Funding for highways is rarely adequate to cover all the needs of the system, and a recessed or depressed economy only complicates the issue. In 2004, MoDOT recognized such an economy on the horizon. Realizing it had no control over its future funding stream, the department began to focus on the most efficient way to spend its money, however small the amount.
This change in philosophy marked the birth of the Practical Design concept. Under the Practical Design model, a project is structured solely on the basis of its purposes and need, no more, and no less.
For example, if a bridge over a creek can no longer convey traffic safely, the purpose and need is to provide for that crossing. In the past, however, design standards would dictate that the new structure be wider, higher and longer than the existing. There was also a tendency to upgrade other highway aspects in the general vicinity just because they were there.
|While practical design has given MoDOT designers increased flexibility, it is strictly governed by three ground rules:|
|Safety. Every project designed with practicality in mind, must get safer. There is no room for compromise where safety is concerned.|
|Communication. There is collaboration in developing every practical solution. Communication among designers, stakeholders and administration officials is open, tolerant and frequent.|
|Quality. The practical solution must function properly and cannot leave a legacy of maintenance challenges. With Practical Design, quality remains at the forefront.|
Practical Design pointed out that, in many cases, the existing bridge had functioned flawlessly for decades. As such, it suddenly began to seem ridiculous to needlessly inflate the new structure when its only deficiency was its poor structural condition.
Practical Design defines the scope by focusing on achieving the project purpose and need while considering the surroundings of each project. It encourages sensitivity to where the project is located, whether it is an interstate or a letter route and allows the surrounding context to help determine the design criteria. Practical Design's goal is to produce the best value for the least cost. Life cycle costs must be considered so the burden is not shifted to maintenance.
MoDOT identified the small percentage of its standards that accounted for the vast majority of its expenditures. Those standards were studied and rewritten with greater flexibility, allowing for more innovation on the part of designers. Even with the greater flexibility, standards in the practical design system function as a starting point for project decisions, rather than a destination. The real project decisions are based entirely on the purpose and need of the facility as well as its context.
Practical Design provides good solutions across the entire system, as opposed to perfect solutions in isolated locations. In this manner, the entire highway system improves and even a modest budget can be stretched to provide quality transportation for Missouri’s taxpayers.
143.2 Practical Design Concepts to Consider
After several successful years of implementing practical design concepts during both the design and construction of MoDOT projects, several areas of opportunity have been identified. Although not a complete list, the following concepts are typical items that should be considered during design.
143.2.1 Pavements and Shoulders
For typical roadway project, materials generally make up a significant portion of the total project costs. On projects that are not let using alternate or optional bid, using the lowest grade, thickness and width of material usually will provide the least expensive alternative. This must be tempered with project expectations, the scope of work and the purpose and need, but designers should always be looking at the cost effectiveness of the proposed work.
In any project that replaces areas or segments of pavement, consideration should be given to reuse of existing materials. This includes the possible rehabilitation of existing pavement in place. There may be benefit to keeping small areas such as turn lanes, ramp segments and shoulders adjacent to new work and adjusting the condition or grade of these locations with overlays or other additional structure. A significant number of Value Engineering Change Proposals are associated with reuse of existing pavement or shoulders, by some type of rehabilitation, and elimination of complicated staging, traffic control or construction processes.
Shoulders of existing facilities are often in good shape and do not require the same additional structure that may be warranted on the mainline pavement. In instances where the shoulder cross slope and the algebraic difference of slope from the pavement to the shoulder will allow, the overlay on the shoulder can be tapered. Even a modest reduction of the outside overlay thickness on a shoulder can reduce material costs enough to be warranted. Additionally, this taper can help reduce the generated edge drop off and the impacts to roadside hardware.
Considerable expense is often generated by the need for retaining walls. In depth consideration should be given to the variables associated with these decisions. Adjusting slopes and right of way lines might prevent the need for MSE walls that require lifetime maintenance and upkeep. In these instances, consideration must be given to the total project costs including construction, right of way, utilities and long term maintenance. A cost estimate for the possible scenarios should be considered.
On even small projects, significant excess or borrow can be very expensive. Every effort should be made to adjust the profile grade such that the project generates a slight excess. It is typically easier to negotiate disposal of a small amount of excess material than the complications associated with a material deficiency. In instances where borrow material cannot be avoided, consideration should be given to all available resources in the project area. Flexibility for the contractor general equates to improved pricing and assists our partners to minimize project costs. Where MoDOT owns right of way in the project area, consideration should be given to the possibility of borrow material from these available sites. When the material that is available in the area varies greatly, guidance on any necessary construction requirements for each specific material could be provided in the contract documents to assist contractors in preparing their bid based upon the most cost effective material for their means and methods.
143.2.3 Drainage, Culverts and Pipe
Culvert pipe liner is often used as a low cost alternative to replacing the culvert. This is a cost effective strategy, however, allowing the contractor flexibility to replace the culvert in lieu of the liner can result in project cost reduction. When contract pipe liner installation is proposed, consideration should be given to allowing optional replacement in the bidding documents if the project location, traffic impacts and staging can facilitate the replacement option.
Paved ditch liner is often used to protect up channel flow areas from eroding during storm and drainage events. However, consideration should be given to alternative materials including rip rap, shot rock, or a variety of fabricated materials designed to replace paved ditch which serve the same function without some of the common drawbacks to traditional paved ditch. The paved ditch location in respect to traffic and the clear zone must be considered, along with the estimated costs.
143.2.4 Roadside Hardware
Typically, pavement rehabilitation strategies are developed based upon the condition of the pavement, the desired project outcomes and the constructability given a specific roadway. An additional consideration for shoulders or for pavement locations where shoulders are narrow or non-existing, is the impact to slopes, such as median slopes in approach to guard cable or the impact to existing roadside hardware. With the minimum guardrail height of 27 3/4 inches, guardrail replacement can become a significant project cost fairly quickly if not considered during the development of the pavement and shoulder rehabilitation strategy. As already discussed, tapering the shoulder thickness is one possible solution to assist in this area. Additionally, consideration should be given to coldmilling, either both the mainline and shoulder, or just one or the other, to provide solutions that help reduce the impact to slopes and roadside hardware. Pavement and shoulder cross slope requirements should be met, but opportunities still may exist to use the coldmilling in reducing overall impacts. Coldmilling costs are often offset or negated by the material salvage value if the coldmilling is approximately 1” or greater.
143.2.5 Temporary Traffic Control
Traffic control costs can add up quickly and, although necessary to construct a project, are often one time costs that add expense without much value to the project. Anything that can be done to reduce the cost of temporary traffic control should be considered, however, the underlying expectation of motorist and worker safety should not be forgotten. Standards demonstrate the typical application for traffic control, however, engineering judgment is necessary to use the correct strategies to ensure both motorist and worker safety when establishing the project’s traffic control requirements. Consideration should be given to traffic volumes, alternative routes, existing conditions and a variety of additional variables that make this element of our work complicated.