Volume Reduction of Highway Runoff in Urban Areas: Final Report and NCHRP Report 802 Appendices C through F (2015) / Chapter Skim
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Appendix E Geotechnical Considerations in the Incorporation of Stormwater Infiltration Features in Urban Highway Design (White Paper #3) 1 Introduction ....................................................................................................................................
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1 Introduction Design of infiltration systems includes a feasibility evaluation to determine areas where infiltration might be suited, as well as site-specific evaluations to quantify potential environmental and geotechnical impacts. Addition of water to the subsurface can result in minor to significant impacts on existing conditions and infrastructure within the vicinity of the infiltration feature, ranging from reduced infiltration to settlement or slope failures.
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5. Basins and localized depressions – design incorporating basins or localized depressions, ranging in size from relatively small (with footprint less than 200 square feet)
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2.1.2 Consideration of Total versus Incremental Risk In standard roadways designs, here are various pathways for water to enter the road base and shoulder material in the absence of intentional infiltration (e.g., seepage through cracks and joints in the pavement, lateral drainage)
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stormwater infiltration. The designer should be able to provide answers to the following questions, in addition to those above: • What is the potential impact on the groundwater table from stormwater infiltration?
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Additional resources, such as design and as-built information regarding existing structures should be obtained, as appropriate. When representative conditions and input parameters are compiled, final design analyses and calculations can be performed to evaluate the geotechnical impacts.
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subsurface utilities and/or underground utilities may pose geotechnical hazards in themselves when infiltrated water is introduced. 3.1.2 Planning Level Feasibility Screening Recommendations At the planning phase, the designer should identify underground utilities (including abandoned, existing and proposed)
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3.1.3 Design Considerations and Potential Mitigation Measures During the design stage, the impacts of stormwater infiltration on utilities should be further evaluated. Design drawings and as-built records of utilities should be obtained and reviewed, if available.
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The designer should understand the typical subsurface conditions in the slope areas: • How deep is the groundwater table? • Are there existing seeps or springs in the slope?
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potential infiltration of the factor of safety, the designer working with the geotechnical engineer must determine what factor of safety would be considered acceptable. The acceptable factor of safety will be dependent on the duration of the impacts of the stormwater infiltration and the whether some slope displacement would be considered acceptable.
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The results of the detailed stability analyses should be evaluated with respect to allowable risks (see discussion in 3.2.2) and regulatory guidelines.
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pathways, this can increase the potential for slope instabilities by creating weak spots in the slope face. However, this factor is inherent in all drainage design, regardless of whether stormwater infiltration is proposed or not.
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moisture can dissolve or soften the cementation or structure of the soil, causing rapid and possibly extensive settlement. Planning Level Feasibility Screening Recommendations Impacts from soil collapse may include damage to hardscapes, utilities and foundations as well as changes in site drainage patterns that may lead to additional impacts.
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3.3.3 Expansive Soils Overview The designer must consider the presence of potentially expansive soils in and around structures and improvements when considering infiltration in design. Expansive soils are soils that experience volume changes with changes in moisture content.
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the project area for the presence of clays soils with significant clay fractions. The NRCS Web Soil Survey (http://websoilsurvey.sc.egov.usda.gov)
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3.3.4 Frost Heave/Thaw Overview Upward displacement of soil resulting from formation of ice in the subsurface, called frost heave, has the potential to cause significant damage to pavements, utilities, lightly loaded structures and the proposed VRA. In addition, the cyclic nature of frost heave/thaw cycles has the potential to substantially deteriorate roadway and subgrade layers leading to eventual damage and/or failure of roadways.
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hazards are identified, the designer should evaluate potential impacts and mitigations as early in the design phase as possible, as effects from frost heave can be significant. Design Considerations and Potential Mitigation Measures If potential for frost heave is identified, detailed subsurface information and foundation design of all potentially impacted features should be obtained to evaluate which feature components are located within the frost zone and whether mitigation measures can be sufficiently incorporated.
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computed using some standard soil parameters. The reader is cautioned that there is considerable variation in the properties of fine-grained soil which correspondingly leads to large variations in settlement.
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example, subsurface pumping of water through a silt layer could result in significant and progressive fines migration, development of subsurface pipes/fissures and possible settlement. Fine-grained cohesionless soils such as silts and very fine sands, and dispersive soils are considered the most susceptible to piping.
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Planning Level Feasibility Screening Recommendations During the feasibility evaluation of potential stormwater infiltration sites, the designer should evaluate if the potential sites are located within liquefaction susceptible zones. As discussed above, this can be evaluated be determining if the three general criteria for liquefaction are present.
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3.4 Retaining Walls and Foundations 3.4.1 Overview Retaining walls, including basement walls and bridge abutments, are common features within or in close proximity to urban roadways. These structures are designed to withstand the forces of the earth they are retaining and other surface loading conditions such as nearby structures.
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cracking of structural elements and overall decrease in the structural integrity and/or serviceability of the structure. Guidance for simplified calculation of foundation bearing capacity and retaining walls are provided in textbooks such as Bowles (2001)
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additional drainage features. The designer should determine if the VRA has the potential to impact moisture conditions within the pavement section: • Are existing roadways in the area exhibiting signs of moisture-related damage?
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paper #4 (Appendix F) provides further discussion of permeable pavement design, including design to meet both structural and hydrologic design requirements.
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Additional drainage features may provide another method to reduce the potential for infiltrated water to impact sensitive features. For example, a subsurface drain could be installed upgradient of a retaining wall or slope that would intercept subsurface moisture and direct it away from the sensitive feature.
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4.3 Design of Features to Incorporate Infiltration By either retroactively or proactively designing structures or other features to accommodate increased infiltration, undesirable geotechnical impacts can be minimized. Retroactive approaches would apply to existing structures, slopes, utilities, retaining walls, and other features that were previously constructed without consideration of stormwater infiltration and would be potentially influenced by the addition of stormwater infiltration.
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The feasibility of these approaches is expected to vary greatly on a site-by-site basis, and should be evaluated using "what if" scenarios based on site-specific information. Not all mitigation measures may be physically or economically feasible.
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of potential opportunities and constraints for specific categories of VRAs related to geotechnical considerations. The guidance in these tables is intended to provide a brief summary and synthesis of the information presented in Section 3 and 4 and is not intended to replace the need for sound engineering judgment based on project-specific data.
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Table 1. Summary of Geotechnical Considerations Geotechnical Hazard Category Example Indicators of Elevated Risk1 Example Design Implications1 Utility Considerations • Presence of utilities in ROW • Historic infrastructure • Underground vaults below groundwater table • Permeable backfill in trenches • For existing and proposed utilities: Allow adequate setbacks or otherwise control area of impact and/or limit flow within utility corridors with cut off walls, • For proposed utilities: Design utilities to allow for infiltration, if needed Slope Stability • Presence of slopes greater than 20 percent (5V:1H)
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Geotechnical Hazard Category Example Indicators of Elevated Risk1 Example Design Implications1 Dispersive Soils and Piping • Moderate to high subsurface gradients • Dispersive soils or fine-grained cohesionless soils • Design filtration systems to reduce subsurface erosion • Reduce subsurface gradients • Lime treatment of dispersive soils Liquefaction • Saturated, loose to medium dense sandy and silty soils, and • Rapid cyclical loadings (earthquakes) • Design to eliminate one or more of the three key risk factors • Design to balance consequence of failure versus probability of earthquake Retaining Walls and Foundations • Presence of retaining walls or foundations within influence area • Finer grained soils with bearing strength sensitive to moisture content • Potential for significant increase in weight of surface layer or elevation of groundwater • Avoid infiltration near retaining walls and foundations • Provide features, such as cutoff walls or drains, to control lateral migration, if needed • For proposed features, may be possible to allow for infiltration in design assumptions; incidental infiltration may already be assumed in standard calculations Pavement Impacts • Moisture sensitive base, subbase, or subgrade materials • Insufficient drainage systems to limit water contact with pavement system • Poorly draining base or subbase • Increased pavement section thickness to accommodate reduced subgrade modulus • Design of free draining base, subbase layers • Increased maintenance requirements • Inclusion of filter in pavement design to limit fines migration 1 – Examples provided to identify typical indicators of risk and possible design implications.
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Table 2. Summary of Potential Opportunities and Constraints for Specific Categories of VRAs Category of VRA Characteristic Properties Example Opportunities and Constraints related to Geotechnical Issues1 Opportunities Constraints Direct infiltration into roadway subgrade • Broad footprint; may only receive direct rainfall or equivalent • Road subgrade has important structural considerations, particularly for flexible pavement design • Broad footprint may allow infiltration in relatively dense soils • Standard roadway designs typically account for wetting of subgrade • Rigid pavement design (i.e., concrete)
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Category of VRA Characteristic Properties Example Opportunities and Constraints related to Geotechnical Issues1 Opportunities Constraints Channels, trenches, and other linear depressions offset parallel to roadway • Tends to be located 10 or more feet from travel lanes • Typically effective water storage depth is between 6 inches and 36 inches • Tributary area ratio may be low • May be fully or partially infiltrated • Channels with positive grade are common drainage features; have relatively limited increase in risk • Due to horizontal separation, features have less potential to damage roadway • Some settlement may be tolerable • Greater potential for impacts out of ROW due to proximity to ROW line. • Greater potential for mounding due to concentration of infiltrating footprint.
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6 Recommended Contents of Geotechnical Infiltration Feasibility and Design Reports A "Geotechnical Infiltration Feasibility and Design Report" or equivalent is recommended to summarize the geotechnical feasibility of stormwater infiltration and associated design parameters. The exact contents of this report may vary as a function of project type, site conditions and associated conditions of the concern, regulatory context, and agency preference.
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• Recommended pavement design parameters, such as the modulus of resilience of subgrade soils in presence of infiltration systems Various other elements may be appropriate to include in the report, at the discretion of the responsible geotechnical engineer. 7 References ARA, Inc., ERES Consultants Division.
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Los Angeles County Department of Public Works (LACDPW)
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