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An Integrated Approach to Sustainable Roadside Design and Restoration

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3. Design Strategies and Tools

• 3.2 Geotech
  • 3.2.1 Introduction
  • 3.2.2 Key Requirements
  • 3.2.3 Relationship with Other Disciplines
  • 3.2.4 Trade-offs and Considerations
  • 3.2.5 Recommended Approaches
  • 3.2.6 Geotech Checklist

3.2 Geotech

Geotechnical Design Considerations:

• Slope stabilization and maintenance
• Drainage
• Structure foundations
• Earth pressure data

3.2.1 Introduction

Geotech, or Geotechnical Engineering, is concerned with the behavior of earth materials. In the roadside environment, it is largely a function of the extent to which the design of the roadway impacts the surrounding topography. This must be accomplished both longitudinally along the roadway alignment and laterally from the roadway to the edge of the right-of way. Where the road has a minimal impact on the surrounding terrain, such as a roadway through relatively flat countryside, the engineering of the roadside can be minimal and confined to design issues related to safety, landscaping, and drainage. Where the design of the roadway impacts the surrounding terrain to a greater extent, such as extensive cuts and fills, more intensive design issues are encountered and need to be addressed.

Long-term stability of the slopes is of paramount importance for the safety of the traveling public. Design issues may include slope stabilization and maintenance, drainage, and foundations for structural retaining features, as well as supplying soil pressure data to structural engineers for the design of the walls. Other geotechnical elements of roadside design include roadside structures that might require foundation elements, such as roadside signage or noise-barrier structures.

3.2.2 Key Requirements

The geotechnical discipline is generally seen as having a support role to other design disciplines. Geotechnical professionals have a responsibility to provide geotechnical recommendations to project managers or other designated members of an interdisciplinary team.¹ Sustainability issues can be addressed at any stage of the design or construction process; however, it is generally best to introduce them as early as possible in order to be fully integrated into the design process. Early participation in site investigations can assist in developing sources for materials either disturbed or filled for the purpose of roadside construction.² Specifically, early investigations should:

• Identify the potential reuse of materials within the project limits
• Focus on areas where elevation changes may necessitate slope stabilization
• Identify where soil cut or fill may require retaining walls or rock slopes/cuts

Additional geotechnical support may be needed for the structural discipline in the form of input to calculations and designs for retaining wall structures. Other structures needing geotechnical input for foundations along the roadside include signs, noise abatement structures, and large culverts.³

3.2.3 Relationship with Other Disciplines

Geotechnical engineering is intimately connected to the geology, geomorphology and the hydrology of a specific site. In this, the built environment must fit into the surrounding environment in such a way that the built environment transitions smoothly to the surrounding landscape. Developing sustainable solutions requires consideration of:

• The cost involved in grading the roadside transition zone (including drainage considerations)
• What, if any, materials are needed to stabilize this transition and how these materials can be chosen for their sustainability properties
• How the cost and level of effort to maintain the roadside can be minimized within the parameters of safety, environment, and aesthetics

Slope designs are incorporated to create a roadside space that is safe, economical, and aesthetically pleasing. Other disciplines to consult with include primarily hydraulic design and structural engineering.

HYDRAULIC DESIGN: The focus is on drainage, both on the macro-scale of the surrounding terrain and on the meso-scale of the drainage from the roadway itself. Erosion control within the drainage channels is another area where geotechnical and hydrological disciplines must work together.

STRUCTURAL ENGINEERING: Coordination is required to design the structural elements of any retaining wall features needed to stabilize slopes or to manage slides or rock-falls. The selected methods for slope stabilization have to take into account the type of vegetation that the landscape is intended to support.

Other structures may be required in the roadside environment, either for public information, safety, or other purposes. These could include signage, guardrails, animal control, and noise abatement structures. All of these require geotechnical input with regard to their foundation elements.

If soil is amended, the depths of soil treatment and the corresponding effect on slope stability need to be considered. If placed too close to a wall or foundation, plant root systems may eventually cause damage to a structure.

3.2.4 Trade-offs and Considerations

The following are trade-offs to consider when integrating sustainable solutions into geotechnical design. Each solution will vary in consideration of the unique context of each project.

• Natural slopes versus retaining structures: Natural slopes require less input and impact to the landscape but elevation changes may not allow for natural slopes. Retaining structures can help reinforce stable slopes. The drawbacks to retaining structures are the costs, visual impact to the roadside environment, and the need for maintenance over time (depending on material).
• Sculpted rock cuts versus straight cuts: This is largely a matter of aesthetics. However, sculpted rock cuts, while more expensive, can be designed such that rockfall mitigation efforts over time can be minimized.
• Local materials versus imported materials: In general, local materials are preferred. However, some specifications for materials can be restrictive, prohibiting the use of local materials. In some cases, the specifications for materials can be relaxed to allow for the use of local materials without significant effect on the final product.
• Consideration of total lifecycle costs versus straight construction cost: Many design elements that reduce maintenance costs can be incorporated into a design package for little to no extra cost if they are considered and implemented early in the design process. In other cases, additional costs added to the construction budget can result in significantly lower maintenance costs over the life expectancy of a product.

3.2.5 Recommended Approaches

The intent of geotechnical professionals is to leave in place a landform that is safe, stable, and cost-effective. Where slopes must be managed, the structures for this purpose can be designed with the intent of using local products and materials. Other design features should include consideration of the aesthetic quality of the structure, making it blend into the landscape. For this to be sustainable, the following approaches are recommended:

• Use locally available materials, where appropriate (Figure 3-14).
• Consider use of a gabion⁴ structure, where appropriate, as a retention structure with locally extracted rock material as a filler, both for aesthetic and sustainability purposes. Gabion structures can also be used as noise mitigation facilities.
• Consider maintenance of the right-of-way environment in the roadside design. This can be achieved by minimizing the area where mowing is required or where irrigation is needed to support desired vegetation.

View larger version of Figure 3-14

Figure 3-14: Rock wall material (Manitou Springs, Colorado)

 

3.2.6 Geotech Checklist

General

• Ensure that slope angle can be supported by the roadside material.
• Design for mitigation and prevention of future rock fall events.
• Ensure that there is adequate support for the structures. Where possible, minimize the area of the ground to be disturbed by excavation for the foundation.
• Design for minimal maintenance requirements.

Vegetation

• Ensure that vegetation, if used to anchor soil, is designed for the long term and will minimize encroachment.
• Ensure that little to no irrigation is needed after the initial establishment of the vegetation.
• Consider vegetation as a tool that can bolster the palette of geotechnical strategies. Rockwork and vegetation root structures can significantly increase slope stability when designed together.

Hydrology

• Verify that the design of the wall system allows for adequate drainage.

Footnotes

1 PDDM Section 6.1.2

2 PDDM Section 6.3.1

3 PDDM Section 6.3.1.2.3

4 A gabion wall is a retaining wall made of rectangular containers (baskets) fabricated of thick galvanized wire, which are filled with stone and stacked on one another, usually in tiers that step back with the slope rather than vertically.

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