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Section 5: Geometric Schematic

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Overview

This section discusses the process of refining alignments and geometrics, performing analyses on geometrics, and preparing preliminary plans and layouts. Most of the data collected and calculated is shown on a “geometric” schematic - a schematic having computed alignments. Work related to schematic development includes performing hydraulic studies, determining right of way needs, and identifying utility conflicts.

After developing preliminary schematics and performing associated preliminary design, the project cost estimate and, sometimes the project scope are updated to reflect changes.

Requirements for presentation of information in schematics are included in the Roadway Design Manual, Chapter 1, Section 3.

Developing the Geometric Schematic includes the following tasks. All tasks should be performed concurrently as much as possible. However, the following sequence of work tasks will likely be necessary:

2500. Develop preferred geometric alignment

2505. Perform preliminary geotechnical surveys

2510. Prepare pavement design report

2520. Refine typical sections

2530. Consider impacts on historic structures

2540. Perform detailed Level of Service analysis

2550. Determine guide signing and operational controls

2560. Submit requests for new access points to interstate highways

2570. Perform preliminary planning for weigh stations

2580. Perform preliminary planning for bridges

2590. Establish preliminary retaining and/or noise wall locations

2610. Perform hydrologic study

2620. Perform preliminary hydraulic analysis/design

2630. Determine right of way and access needs

2640. Identify existing utilities on geometric schematic

2650. Identify potential utility conflicts

2660. Establish preliminary illumination locations

2670. Conduct constructability review

2460. Update cost estimates

2690. Update project scope

1400. Review scope, cost, and staff requirements of project development

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2500: Develop preferred geometric alignment

Description. A preferred geometric alignment is developed after preliminary schematics are developed (see 2350: Evaluate geometric alternatives) and a preferred alternative selected. Related disciplines should be consulted in reviewing alternatives and establishing the preferred alignment.

Changes to the preferred alignment after a public hearing may require TxDOT to repeat the review and approval process for the environmental document and hold an additional hearing. However, minor adjustments to alignments may be made by the designer, if feasible and if the change does not compromise project design criteria. Substantial changes to the approved schematic will require submission of several copies of the revised schematic to the Design Division/FHWA.

A checklist of items to show on a schematic is available in the TxDOT Roadway Design Manual, Chapter 1, Section 3.

A list of sub-tasks involved in developing a schematic is available in this manual under 2350: Evaluate geometric alternatives.

Pertinent Project Types. New location, added capacity, or controlled access projects or projects requiring an Environmental Impact Statement.

Helpful Suggestions.

  • Avoid changes to ROW limits and locations of ramp gores shown on the schematic after Design Division and FHWA approval.
  • Consider underground features, such as utilities and septic tank systems, when making alignment adjustments.
  • Begin identifying and securing approval of design exceptions/waivers (see 2870: Submit design exceptions/waivers for approval).
  • Consider conducting a value engineering study (see 2700: Conduct a Value Engineering (VE) study).

Responsible Party. Project manager

Resource Material. TxDOT Roadway Design Manual

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2505: Perform preliminary geotechnical surveys

Description. Preliminary geotechnical surveys are performed to help guide early project layout and design, and to determine 1) feasibility of the preliminary design and 2) limitations on construction staging. Preliminary geotechnical testing serves as a preview to determine the following factors:

  • whether an additional final geotechnical survey (see 5460: Perform final geotechnical surveys) is needed.
  • preliminary geotechnical surveys can vary from simple, visual inspections to various forms of subsurface exploration, depending on information needed.

For large structures, preliminary geotechnical surveys form the basis for more rigorous testing. On major projects, a small number of preliminary borings should be obtained to aid in preliminary project layout.

Geotechnical surveys for pavement design should be done at this time and may include roadway pavement testing such as Falling Weight Deflectometer (FWD) testing.

Pertinent Project Types. All projects except preventive maintenance and restoration projects.

Responsible Party. Project manager

Sub-tasks.

  • Determine needs for more data and field testing after evaluating existing geotechnical information.
  • Forward the information to the district pavement engineer, structural engineer, and landscape architect for their use.

Helpful Suggestions.

  • The Field Operations Section of the Bridge Division is available for geotechnical engineering support in areas of foundations, retaining walls, embankment stability including slope stability and settlement, and stabilization of soft soils under roadway and embankment.
  • The Pavement Section of the Construction Division is responsible for technical support for pavement design such as falling weight deflectometer, ground penetrating radar, dynamic cone penetrometer, and seismic pavement testing.
  • The district laboratory and Materials Section of the Construction Division are responsible for assisting with soils exploration and testing.
  • A right of entry (see 2150: Obtain right of entry) or other written evidence of permission must be obtained before entering private property.
  • Results may influence project design (e.g., high water table, hazardous materials).

Critical Sequencing.

  • Perform hazardous materials investigations (see 3280: Perform hazardous materials survey assessment and investigation) before performing this task to avoid spreading contamination.
  • Preliminary geotechnical surveys should be performed before developing a pavement design (see 2510: Prepare pavement design report) or establishing preliminary retaining and/or noise wall locations (see 2590 Establish preliminary retaining and/or noise: wall locations).
  • Locate existing utilities before conducting subsurface exploration.

Resource Material.

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2510: Prepare pavement design report

Description. The pavement structure represents one of the single most costly items on the typical highway improvement project. One important reason for doing a preliminary pavement design is to facilitate and enhance the accuracy of the cost estimate.

The preliminary pavement design is performed by a design engineer and requires the evaluation of various data to determine a proper pavement structure, including materials properties, environmental conditions, and cost. Traffic data (see 1430: Obtain Traffic Data) and geotechnical data (see 2505: Perform preliminary geotechnical surveys) should be reviewed. There are benefits and costs to be considered in selecting rigid pavement or flexible pavement types. The designer's job is to assess all facts and factors (including life-cycle costs and user-delay costs) and maximize the benefits.

The designer must determine the purpose of the new pavement structure - such as improving structural capacity or improving functional characteristics such as ride quality or skid resistance. For overlays and rehabilitation projects, it is essential to perform pavement evaluations to identify the types of distress (such as base failures, asphalt stripping, pumping, etc), determine the causes for distresses, and suggest a rehabilitation strategy that will address the problem. For new pavements it is important to perform a full geotechnical investigation to determine whether there is a need for stabilization of the subgrade material in accordance with the stabilization guidelines developed by the Construction Division.

If significant changes occur during project development in project scope, pavement type, traffic data, etc., the report may need to be revised and resubmitted.

Pertinent Project Types. All projects on the interstate highway system, and all projects on other facilities except preventive maintenance.

Responsible Party. Roadway design engineer or district pavement engineer

Sub-tasks.

  • Collect necessary data, such as the following:
    • “as-built” construction plans; field evaluation data
    • Existing and projected traffic data (see 1430: Obtain Traffic Data.) (request early in project development)
    • Geotechnical investigation
    • current pavement construction cost information
    • drainage data
    • historical performance information
    • Pavement evaluation
    • available funding.
  • Contact district maintenance personnel for additional information on roadway pavement history.
  • Check the Pavement Management Information System (PMIS) for historical data on pavement conditions. PMIS is available on the TxDOT mainframe computer system.
  • Perform a condition survey - a site visit to assess pavement surface distress.
  • Determine the requirements of the new pavement - what loads must it carry and how long before the first overlay is required.
  • Determine the type of pavement to be built.
  • Determine the different materials to be used for the different layers of the pavement structure.
  • Determine the Performance Grade (PG) to be used for the Hot Mix asphalt layers.
  • Determine the pay schedule to be used for evaluating the ride quality.
  • Perform a life-cycle cost analysis as part of using automated design software (e.g., FPS19).
  • Prepare the pavement design and develop a Pavement Design Report. (See the Pavement Design Guide, Chapter 2, Section 8, Pavement Design Report).
  • Send the design to the district pavement engineer for review and approval. The district pavement engineer is responsible for ensuring compliance with FHWA and TxDOT design policies and procedures. If a district does not have a certified pavement engineer, then pavement design approval reverts to the Materials and Pavement Section of the Construction Division.
  • The district pavement engineer sends the approved copy of the pavement design report to the Pavement Section of the Construction Division for review and analysis. Revise the project cost estimate since pavement is a major portion of the project cost. (See 2460, Update Cost Estimates).

Helpful Suggestions.

  • Contact Transportation Planning and Programming Division for both existing and projected traffic data (see 1430: Obtain Traffic Data).
  • The Materials and Pavement Section of the Construction Division is available for assistance as needed. Also, refer to the Pavement Design Guide.
  • Contact the Materials and Pavement Section, through the district pavement engineer, for information on the FWD testing, the dynamic cone penetrometer test, the use of ground penetrating radar, and for a pavement design report format.
  • The district laboratory can run the triaxial test, Atterberg limits, and calculate potential vertical rise as part of the geotechnical investigation.
  • For large, complex projects that take years to design, the pavement design may need to be re-evaluated during detailed design. It is possible for traffic counts and patterns to change enough to require pavement design change.
  • Soil types, weather conditions and material availability vary widely among districts. Consult with the district pavement engineer and construction engineers before starting a design.
  • Pavement design must consider potential effects of groundwater on pavement performance along with other factors such as proposed roadway grades.

Critical Sequencing.

  • Because one of the purposes of the pavement design is to estimate the cost of pavement related items, the pavement design should be performed before updating the cost estimate.
  • This task should occur after collecting traffic and geotechnical data (see 2505: Perform preliminary geotechnical surveys) (including roadway pavement testing) and after setting preliminary grades.

Resource Material.

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2520: Refine typical sections

Description. As the project progresses, a preferred design alternative is selected, and revisions are made to the design. Typical sections need to be updated to show those changes. Information on (See 2360: Develop typical sections) their requirements is available in this manual. Design criteria for features to be shown on the typical sections can be found in TxDOT's Roadway Design Manual. If preliminary pavement design information (see 2510: Prepare pavement design report) is available at this time, it may be added to the typical section.

Review proposed typical sections with local public agencies responsible for cost participation in the project

Pertinent Project Types. All projects.

Responsible Party. Roadway design engineer

Critical Sequencing. Revisions to the schematic are determined as the schematic is refined, hydraulic studies are performed, and a detailed Level of Service analysis is done.

Resource Material. TxDOT Roadway Design Manual, Chapter 2

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2530: Consider impacts on historic structures

Description. A mitigation plan (see the Environmental Manual) may be needed for projects affecting buildings, structures (including bridges), sites, etc., which are included, or eligible for inclusion in the National Register. If a plan is needed, the project manager should begin considering how mitigation plans will be incorporated into the project.

Pertinent Project Types. All projects except preventive maintenance and restoration projects.

Responsible Party. Project manager

Helpful Suggestions.

  • Coordinate all development with the district environmental coordinator.
  • Request that the landscape architect review the mitigation plan so that landscape and aesthetics are considered.
  • The Environmental Affairs Division will coordinate with the State Historic Preservation Office, which will review the PS&E package for all projects involving historic buildings, structures, or landscapes.

Authority Requirements.

  • National Historic Preservation Act of 1966
  • Department of Transportation Act of 1966
  • National Environmental Policy Act of 1969
  • Historical and Archeological Data Preservation Act of 1974
  • Intermodal Surface Transportation Efficiency Act of 1991.

Resource Material.

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2540: Perform detailed Level of Service analysis

Description. Earlier in the process, a preliminary Level of Services (LOS) analysis of project alternatives (see 2310: Perform preliminary Level of Service (LOS) analysis) should have been performed to aid in selecting a preferred alternative.

At this stage, a detailed LOS analysis may be needed to compare different geometrics to refine the design proposal. For example, the analysis can be used to refine geometrics such as shoulder and lane widths, profile grades, and frequency of access points.

Results of the detailed LOS analysis are incorporated into establishing preferred alignments (see 2500: Develop preferred geometric alignment) and (see 2520: Refine typical sections).

A LOS analysis is usually performed for the current year using current traffic and geometric conditions, and for the future “design” year using traffic projections and proposed geometric designs. The LOS analysis may show whether a proposed design will meet future transportation needs.

Pertinent Project Types. New construction and reconstruction projects on multi-lane highways and freeways, and projects with ramp modifications.

Responsible Party. Roadway design engineer

Sub-tasks.

  • Review traffic data obtained (see 1430: Obtain Traffic Data) on present and projected traffic volumes and other data such as percent trucks, design hourly volume, and the directional distribution.
  • Obtain data on current and proposed roadway features (e.g., number of lanes, offset to obstructions, lane widths).

Helpful Suggestions. Several computer software programs are available to run the analysis.

Critical Sequencing. Perform as part of refining the preferred alignment and geometrics.

Resource Material.

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2550: Determine guide signing and operational controls

Description. Traffic control guide signs guide vehicle operators along streets and highways. Guide signs show route designations, destinations, directions, distances, services, points of interest, and other geographical, recreational, or cultural information.

There are different types of guide signs used on freeways or expressways. These include advance signs for interchange exits, supplemental signs, exit direction signs, diagrammatic signs, and miscellaneous signs.

Intelligent Transportation Systems (ITS) are an effort to improve safety and maximize operational efficiency of the transportation system by enabling TxDOT to detect incidents more quickly; provide information immediately to the public on incident location, its severity, effect on traffic flow and expected duration; and, change traffic controls to accommodate traffic flow changes due to the incident.

Pertinent Project Types. Mainly freeway/expressway and arterial highway projects; however, there are some rural applications.

Responsible Party. Roadway design engineer or traffic engineer

Sub-tasks.

  • Obtain traffic data (see 1430: Obtain Traffic Data) and accident/crash data (see 2190: Obtain traffic accident data).
  • Determine guide signs needed to increase roadway efficiency and safety.
  • Consider including ITS strategies in the project.

Helpful Suggestions. Design guide signs to be legible to drivers approaching them and to allow adequate time for the driver to respond.

Critical Sequencing. Obtain traffic information (see 1430: Obtain Traffic Data) before performing this task.

Resource Material.

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2560: Submit requests for new or revised access points to interstate highways

Description. Regardless of funding or oversight, new or revised (e.g., ramp reversal or relocation) access points on interstate highways must be approved by the Federal Highway Administration (FHWA). The FHWA must ensure there is either no or only minimal adverse effect on the operation of the interstate facility.

A request for access modifications must be documented in an “Interchange Access Justification Report (IAJR).” The IAJR should provide information to support a request for the approval of new or revised points of access on completed sections of the interstate system. A checklist of information to include in this report is being added to the Roadway Design Manual, Chapter 1. A Level of Service analysis (see 2540: Perform detailed Level of Service analysis) will be required. Submission of the schematic layout is also generally required.

Pertinent Project Types. Projects involving new mainlane access points or revisions to existing access points on the interstate system.

Responsible Party. Project manager

Helpful Suggestions.

  • Consider including a line diagram showing LOS results and traffic volumes.
  • Informal coordination with FHWA during schematic preparation may streamline the FHWA review process later.
  • Coordination and approval of the IAJR can take time since it must be approved by FHWA Headquarters staff in Washington, DC.

Critical Sequencing. This task should be done after preparing a schematic.

Authority Requirements. 23 USC §111

Resource Material. TxDOT Roadway Design Manual

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2570: Perform preliminary planning for weigh stations

Description. The need for new weigh/inspection stations and the determination of their location must be coordinated between the TxDOT administration and the Department of Public Safety (DPS). The DPS provides permanent scales, if needed, and also provides manpower to operate weigh/inspection stations. Construction of new weigh stations must be authorized by Texas Transportation Commission minute order. Projects that sever existing weigh stations should include construction of a replacement station as part of the project.

The need for a “weigh-in-motion” detector/station and its location is determined by the Transportation Planning and Programming Division of TxDOT. These stations are used to gather vehicle information - not for law enforcement.

Pertinent Project Types. Individual weigh/inspection station projects or highway improvement projects including weigh stations.

Responsible Party. Project manager

Sub-tasks.

  • Obtain Commission authorization for any new weigh/inspection station construction.
  • Contact the DPS to obtain details on weigh/inspection station design needs, consistent with Commission direction.
  • Develop preliminary plans for the weigh/inspection station.

Helpful Suggestions. Clarify funding issues in question before beginning planning or design.

Critical Sequencing. If part of a larger project, perform this task when developing other preliminary plans.

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2580: Perform preliminary planning for bridges

Description. The location of proposed bridges must be determined early in project development. Preliminary planning (see the Bridge Project Development Manual) includes estimating bridge limits, span lengths, and bent locations. Bridge layouts are created from this information and forwarded to the Bridge Division for approval. Accommodations for future bridge widening should be considered at this time.

If a railroad crossing is involved, the Project Development Section of the Bridge Division processes the layout through the Railroad Section of the Traffic Operations Division.

Culverts with a total span greater than or equal to 20 ft (6.1m) are considered bridge-class culverts and their design must follow span bridge guidelines.

Pertinent Project Types. Bridge projects or other projects that include bridge class structures.

Responsible Party. Roadway design engineer

Sub-tasks.

  • Identify applicable Federal Emergency Management Agency (FEMA), U.S. Corps of Engineer and U.S. Coast Guard constraints. (See 2340: Identify requirements for crossing navigable waters).
  • Identify type, size, and locations of existing bridges.
  • Determine vertical and horizontal clearance requirements.
  • For bridge widening, determine load rating, and obtain condition survey (see Bridge Project Development Manual).
  • Consider widening the existing structure versus replacement.
  • Determine type, size, and location of proposed bridges.
  • Review proposed bridge cross sections with local public agency when grade separation structures involve facilities not owned or maintained by TxDOT.
  • Initiate geotechnical surveys (see 2505: Perform preliminary geotechnical surveys) for foundation design.

Hel