Category HIGHWAY ENGINEERING HANDBOOK

Once a proper selection has been made of feasible wall types that satisfy the necessary constraints, design consists of determining the earth pressure against the back of the wall and then proportioning the wall so that it will be structurally sufficient to satisfy a number of traditional checks. These checks include stability against sliding and overturning, and foundation bearing pressure limits. Clearly, satisfying the traditional checks would be of no value if the entire structure were to move because of some condition not related to any of these three checks. Therefore, it is also important that the designer be assured that the wall is globally stable—i. e., that no deep-seated slide or slip surface exists.

An important and essential part of the design of retaining walls consists of deter­mining the earth pressure on the back of the wall. The earliest theory of earth pressure traces back to Charles-Augustin de Coulomb, who published his work in 1773. Coulomb’s theory presented a method by which a designer could determine the pressure that dry, granular, cohesionless material would exert upon the back of a wall constructed to restrain the material. His work was based on the theory that failure is characterized by a wedge-shaped mass of the supported sand material that slides down along a sloping plane such as is shown in Fig. 8.6.

The Coulomb theory assumes a hydrostatic distribution of pressure such that the resultant forces R (reaction needed to hold wedge in equilibrium) and P (summation of normal pressure times area) act at the lower third point of the planes upon which they act, planes ab and ac, respectively. The force R acts at an angle of friction of soil on concrete, ordinarily 25°, while P acts at an angle of friction of soil on soil, generally assumed to be 34°. This latter angle will vary significantly from 34° to 40° or more. Because of the different angles of friction, the theory produces an error in the result;

however, the error is generally accepted as negligible. In essence, if it is assumed that no friction exists between the earth and the wall, the pressure determined from the Coulomb theory is the same as that determined from the Rankine theory. Thus, because of its simplicity, the tendency is to use the Rankine theory. See Art. 8.2.3 for an example of active pressure calculation.

It is evident that the theory as expressed in Fig. 8.6 does not suggest a particular plane of failure. Thus, the pressure determination of the Coulomb theory is traditionally left to graphical methods, in particular those first developed by J.-V. Poncelet, and later by a German engineer, Culmann. These constructions, which allow for the complete determination of lateral pressure acting on the wall (i. e., magnitude, direction, and point of application), are not further discussed herein. However, several failure planes are usually assumed, pressure from each assumption is graphically determined, and an envelope line of pressure is developed from these pressure points from which the max­imum pressure can be determined. The methods are laborious but straightforward and may again gain in popularity with the increasing use of computers.

At approximately the same time as Culmann’s construction was developed, a Scottish physicist, W. J. M. Rankine, presented his theory in a work called On the Stability of Loose Earth, a theory that remains in active use. Rankine assumed a mass of loose earth of infinite extent, and a planar top surface subjected to its nonweight. The theory assumed granular backfill material without cohesion, but was adapted in 1915 by a British engineer to allow for cohesion.

Evaluation factors that can be used on selected conceptual wall designs include the following:

• Constructibility

• Maintenance

• Schedule

• Aesthetics (appearance)

• Environment

• Durability or proven experience

• Available standard designs

• Cost

The sum of all weight factors should be 100 points. To simplify the selection process, minor factor(s) may be removed from the rating matrix. This is readily achieved by assigning the same score for minor factors on all the selected feasible wall types.

8.1.1 Notes on Using the Worksheets (Figs. 8.2, 8.3, and 8.4)

1. Factors that can be evaluated in percentage of wall height

A. Base dimension of spread footing

B. Embedded depth of wall element into firm ground

2. Factors that can be described as “large (high),” “medium (average),” or “small (low)”

A. Quantitative measurement

(1) Amount of excavation behind wall

(2) Required working space during construction

(3) Quantity of backfill material

(4) Effort of compaction and control

(5) Length of construction time

(6) Cost of maintenance

(7) Cost of increasing durability

(8) Labor usage

(9) Lateral movement of retained soil

B. Sensitive measurement

(1) Bearing capacity

(2) Differential settlement

3. Factors that can be appraised with “yes,” “no,” or “question” (insufficient information)

A. Front face battering

B. Trapezoidal wall back

C. Using marginal backfill material

D. Unstable slope

E. High water table or seepage

F. Facing as load-carrying element

G. Active (minimal) lateral earth pressure condition

H. Construction-dependent loads

I. Project scale

J. Noise or water pollution

K. Available standard designs

L. Facing cost

M. Durability

4. Factors that can be approximated from recorded height

A. Maximum wall height

B. Economical wall height

A classification system is an essential part of the description and selection of different earth retaining wall types. Figure 8.1 indicates the many types of walls that are possible.

Earth pressure walls can be classified logically into three categories according to their basic mechanisms of retention, or into three categories based on their source of support. The retention mechanisms include internally stabilized, externally stabilized, and hybrid systems. The sources of support are described as gravity, semigravity, and nongravity.

An externally stabilized system uses a physical structure to hold the retained soil. The stabilizing forces of this system are mobilized either through the weight of a mor – phostable structure or through the restraints provided by the embedment of the wall into the soil, if needed, plus the tieback forces of anchorages.

An internally stabilized system involves reinforced soils to retain fills and sustain loads, adding reinforcement either to the selected fills as earth walls or to the Retained Earth directly to form a more coherent stable slope. These reinforcements can either be layered reinforcements installed during the bottom-to-top construction of selected backfill material, or be driven piles or drilled caissons built into the retained soil. All this reinforcement must be oriented properly and must extend beyond the potential failure plane of the earth mass.

A hybrid or mixed system is one that combines elements of both externally and internally stabilized systems.

Regarding sources of support, gravity walls derive their capacity through the dead weight of the wall itself or through an integrated mass that can be either externally or internally stabilized. They can further be classified into four types. The first is an inter­nally stabilized system: earth walls with either facing covered cuts in situ doweled with uniformly spaced top-to-bottom constructed nails or selected fills reinforced with tensile reinforcements, which can be either metal (inextensible) reinforcements or geotextile (extensible) reinforcements. The second type is an externally stabilized system, either modular precast concrete walls or prefabricated metal bin walls. Third is an externally stabilized system—generic walls such as masonry, stone, dumped-rock, and gabion walls. The fourth type is an externally stabilized cast-in-place mass concrete wall or low-cost cement-treated soil wall system with anchored precast concrete facings.

Semigravity walls derive their capacity through the combination of dead weight and structural resistance. Semigravity walls designed with different shapes can be further classified into two groups: first is the conventional cast-in-place cantilever concrete wall, and second is a prefabricated system wall with cast-in-place base and many

kinds of innovative precast or posttensioned stems. Semigravity walls are, in general, externally stabilized systems. They can be constructed either on spread footings or on deep foundations, such as caissons or piles, as foundation conditions may demand.

Nongravity walls derive their capacity through lateral resistance, either by embed­ment of vertical wall elements into firm ground, by anchorages provided by tiebacks, by dowel actions provided by piles, or by caissons drilled into a stabilized zone. They can be classified into, first, an externally stabilized system with embedded cantilever wall elements, sheet piles, drilled shafts, or slurries; second, similar embedded walls utilizing multiple anchorage tieback systems; and third, internally stabilized systems such as creeping slopes externally covered with multianchored facings and internally doweled with pile or caisson inclusions.

Wall selection is an iterative process that involves cycles of preliminary design and cost estimation. The first and most important step is to define the design problem with design objectives and constraints. The objective of almost all design problems is least cost, although there will be many cases, particularly in urban areas, where objectives will include aesthetic and environmental considerations as well. Costs such as those for materials and construction are much easier to quantify than are aesthetic and envi­ronmental costs. In the latter instances, it is sometimes difficult to verify which one of the feasible solutions is the best. In order to find solutions that are at least feasible, constraints such as serviceability requirements (wall horizontal movement, vertical differential settlement, etc.) and spatial limitations (rights-of-way, underground ease­ments, etc.) should be defined as comprehensively as possible. Designs (wall types) that meet the prescribed constraints are all feasible solutions. A ranking of these feasible solutions (wall types) is required. The ranking should include spatial behavior and economic factors as discussed later in this article. Ideally, the wall with the highest rank should be adopted for detailed design; the rest can be used as design alternatives or discarded if the selected wall is confidently lowest cost, or is the only wall that satisfies all the established design requirements.

At the beginning of the selection process, rough sketches labeled with wall types should be adequate to screen out unfeasible types. As the selection process proceeds, a conceptual design with preliminary dimensions should be generated. Factors affecting the selection of an earth retaining structure may be grouped into three categories: spatial constraints; behavioral constraints; and environmental, aesthetic, and economic con­siderations. Factors to be considered for each of these categories are listed below.

1. Spatial constraints

A. Functions of wall

(1) Provide room for roadway at front of wall

(2) Retain roadway at back or top of wall

(3) Provide for grade separation, landscaping, or noise control

(4) Provide for ramp or underpass wall support

(5) Provide for temporary shoring of an excavation

(6) Ensure stability of steep side slope

(7) Flood control

(8) Serve as bridge abutment

(9) Other

B. Space limitations and site accessibility

(1) Right-of-way boundaries

(2) Geological boundaries

(3) Access of material and equipment

(4) Temporary storage of material and equipment

(5) Maintaining existing traffic lanes or widening

(6) Temporary and permanent easement

(7) Other

C. Proposed finished profile (using combinations of different wall types along the wall alignment may be the optimal solution)

(1) Limit of radius of wall horizontal alignment

(2) Cut or fill with respect to original slope

(3) Minimal site disturbance:

(a) Anchored wall with minimal cut

(b) Stepped-back wall on terrace profile

(c) Superimposed or stacked low walls

(d) Mechanically stabilized earth (MSE) wall with truncated base or trapezoidal reinforced zone

D. Check available space versus required dimensions

(1) Working space in front of wall (shoring, formwork, etc.)

(2) Wall base dimension

(3) Wall embedment depth

(4) Excavation behind wall

(5) Underground easement

(6) Wall front face battering

(7) Superimposed walls or trapezoidal profile of wall back

2. Behavioral constraints

A. Earth pressure estimation (magnitude and location)

(1) The magnitude of the earth pressure exerted on a wall is dependent on the amount of movement that the wall undergoes.

(2) The vertical component of earth pressure is a function of the coefficient of friction and/or relative displacement (settling) between wall (stem, facing, and Reinforced Earth mass) and retained fill.

(3) Compaction of confined soil may result in developing of earth pressure greater than active or at-rest condition.

(4) For complex or compound walls such as bridge abutments, battered-faced walls, superimposed walls, and walls with trapezoidal backs, a global limit equilibrium analysis is required.

(5) For embedded cantilever walls, profiles of lateral pressures acting on both sides of a wall are affected by the location of the center of wall rotation (pivot point), which is construction-dependent.

(6) For multianchored embedded cantilever walls using a minimum penetra­tion depth where there is no static pivot point, the soil pressure profile is anchorage design-dependent and should be developed with the recognition of beam-on-elastic foundation principles.

(7) At the ultimate limit state, the location of the horizontal earth pressure resultant moves up from 0.33 to 0.40 of the wall height.

B. Groundwater table

(1) Reduce hydrostatic pressure if possible by an appropriate drainage system.

(2) Introduce special precautions to reduce corrosion.

(3) Prevent soil saturation; an appropriate groundwater drainage system is required except when the water table level must be maintained to prevent settlement of adjacent structures.

C. Foundation pressure estimation

(1) Uniform average pressure by Meyerhof effective width method for mechanically stabilized earth wall systems

(2) Maximum toe pressure by flexural formula method for unreinforced or reinforced concrete-type walls

D. Allowable bearing capacity estimation

(1) Allowable bearing capacity is limited by and related to preset settlement or differential settlement criteria.

(2) Earth walls integrated with wider flexible bases are allowed higher bearing capacity and tolerate more settlement than rigid walls on spread footings.

E. Allowable differential settlement

(1) Settlement is a time-dependent behavior.

(2) Top-of-wall settlement is a sum of settlement from wall and from subsoil strata.

(3) Allowable settlement should be evaluated by considering tolerable move­ment of superstructure and wall precast facings.

(4) Simple-span bridges tolerate more angular distortion between adjacent footings than continuous-span bridges.

(5) Tolerable (vertical and horizontal) movement of a wall facing is a function of panel joint width and pattern of connection.

F. Earth pressure on wall facing

(1) The rigidity and slope of a wall facing affect the development of lateral pressure and displacement at facing.

(2) The earth pressure is reduced with a decrease in facing stiffness, while the facing deformation is only slightly increased for a decrease in stiffness.

G. Settlement and bearing capacity improvement techniques

(1) Surcharge (two-phase construction) to hasten anticipated settlement

(2) Drainage (wick drain) to hasten anticipated settlement in fine-grain silt and clay substructure materials

(3) Excavation and compaction of a portion of weak foundation material

(4) Addition of reinforcement to subsoil

(5) Use of lightweight fill material to minimize loads beyond existing precom­pression of foundation materials

H. Methods of reducing settlement on reinforced mass

(1) Increasing compaction of fill material

(2) Using more reinforcements (length, area, and spacings of reinforcements)

(3) Cement treatment of fills

(4) Reducing clay content of fill

(5) Using high-density in situ micronails

I. Earth pressure applied at facing

(1) High: facing with posttensioned anchors

(2) Medium-high: mechanically stabilized earth wall with full-height panels

(3) Medium: rigid concrete facing with inextensible reinforcements

(4) Medium-low: concrete panel facing with extensible reinforcements

(5) Low: concrete panel facing with nailed soil

J. Wall base width

(1) Wall types, foundation types

(2) Allowable bearing capacity of spread footing

(3) No tension allowed at heel of spread footing

(4) Internal and external stability of wall

(5) Reinforcement length to control lateral movement of Reinforced Earth wall

(6) Hybrid walls to reduce wall base width

K. Toe penetration depth of embedded cantilever wall

(1) Water cutoff consideration

(2) Heave in front of wall

(3) Bearing capacity

(4) Stability of passive toe kickout

(5) Slope of ground in front of wall

L. Wall sensitivity to differential settlement

(1) High: cast-in-place concrete retaining walls

(2) Medium: earth walls with inextensible reinforcements, geogrid walls with facings, precast modular walls

(3) Medium-low: geofabric walls without facing

(4) Low: gabion walls, crib walls, embedded cantilever walls, multianchored cantilever walls

M. Potential settlement of retained mass

(1) High: embedded cantilever walls

(2) High-medium: some concrete modular walls, geofabric walls

(3) Medium: cast-in-place concrete retaining wall, concrete modular walls, geogrid walls

(4) Medium-low: earth walls with inextensible reinforcements

(5) Low: multianchored embedded cantilever walls

N. Relative construction time

(1) Long: cast-in-place concrete walls

(2) Medium: earth walls with reinforcements

(3) Short: embedded cantilever walls, multianchored embedded cantilever walls, precast modular walls

O. Wall design life

(1) Structural integrity

(2) Color and appearance

P. Load-carrying capacity and settlement of deep foundation

(1) Maximum frictional resistance along the pile shaft will be fully mobilized when the relative displacement between the soil and the pile is about *4 in irrespective of pile size and length.

(2) Maximum point resistance will not be mobilized until the pile tip has gone through a movement of 10 to 25 percent of the pile width (or diameter). The lower limit applies to driven piles, and the upper limit is for bored piles.

(3) The ultimate load-carrying capacity is the sum of pile point and total fric­tional resistance.

(4) Pile-to-cap compatibility should be considered, especially with battered piles and semirigid pile-cap connection.

(5) For the estimation of group efficiency in vertical and horizontal displace­ment, calculation of pile group, pile diameter, spacing, soil type, and total number of piles should be considered.

Q. Fill material properties

(1) The lower the soil friction angle, the higher the internal earth pressure restrained by the wall.

(2) The lower the soil friction angle, the lower the apparent friction coeffi­cient for frictional reinforcing systems.

(3) The higher the plasticity of the backfill, the greater the possibility of creep deformation, especially when the backfill is wet.

(4) The greater the percentage of fines in the backfill, the poorer the drainage and more severe the potential problem from high water pressure.

(5) The more fine-grained and plastic the fill, the more potential there is for corrosion of metallic reinforcement.

R. Fill retention versus cut retention

(1) Fill retention (bottom-to-top construction)

(a) Earth walls (extensible and inextensible tensile reinforcements)

(b) All semigravity walls

(c) Modular walls, generic walls, and mass concrete walls

(2) Cut retention (top-to-bottom construction)

(a) Earth walls, soil nails

(b) All nongravity walls

3. Environmental, aesthetic, and economic considerations

A. Environmental constraints

(1) Ecological impacts on wetlands

(2) Effect of corrosive environment on structural durability

(3) Water pollution, sediment, or contaminated material

(4) Noise or vibration control policy

(5) Stream encroachment

(6) Fish and wildlife habitat or migration routes

(7) Unstable slope

(8) Other

B. Aesthetic constraints

(1) Urban versus rural

(2) Design policy of scenic routes

(3) Acoustic or aesthetic properties of wall facing

(4) Antigraffiti wall facing

(5) Avoiding valley effect of long or high wall

(6) Other

C. Economic considerations

(1) Construction schedule

(2) Availability of fill material

(3) Supply of laborers

(4) Heavy equipment requirements

(5) Formwork, temporary shoring

(6) Dewatering requirements

(7) Available standard designs

(8) Temporary versus permanent wall and future widening

(9) Cost of drainage system

(10) Design and installation of wall attachments

(11) Negotiated bidding and design/build on nonstandard projects

(12) Maintenance cost, readjustment, and remodeling

(13) Uncertainty of site and wall loads

(14) Complexity of project

(15) Height differences in finished or base grades

(16) Number of wall turning points

(17) Scale of project

(18) Length or height of wall—quality control of fill material

(19) Posttensioning, grouting, trenching, slurry

(20) Pile driving, caisson drilling

(21) Precasting, transportation, and inspection

(22) Quantity of excavation

(23) Quantity of backfill material

(24) Experience and equipment of local contractor

(25) Proprietary product and quality assurance

(26) Other

The logical consequence of considering these factors is to reduce the number of fea­sible wall types. The first stage of the decision process eliminates obviously inappropriate walls through spatial and behavior constraints before considering economic factors. The behavior constraints involve the properties of the earth the wall must retain and the ground it rests on. A detailed geological investigation and soil property report is needed in the second stage of the decision process. At this stage, conceptual designs with dimensioned wall sections and subsoil strata are required. In the third stage, behavior constraints and economic constraints should be repeatedly or simultaneously considered.

After identification of the feasible set of wall types (a subset of the available walls), work proceeds on the more refined or detailed preliminary designs. Then a rating of these feasible designs should be made.

To consider the various factors during the selection process, use the worksheets shown in Figs. 8.2, 8.3, and 8.4, along with the properly defined design problems (objectives and constraints) and cost requirements (Fig. 8.5). These sheets form a part of the documentation in support of the final selection(s).

After the worksheets are completed, a list of selected wall types with conceptual designs should be generated. A rating matrix can then be developed for a qualitative eval­uation of the selected alternatives. On the basis of each evaluation factor, a qualitative rating between 1 and 5 can be given each alternative. The qualitative ratings are usually multiplied by weight factors reflecting the importance of the factors; usually, cost – and durability-related factors are given higher weights than the rest. The alternative(s) with the highest score is (are) then selected for final design and detailed cost estimation.

The intent of this procedure is to identify equally satisfactory alternative wall types. The plans or specifications will provide the opportunity for the contractor to select from the acceptable alternatives, should the designer make the decision to permit alternative walls. The specifications will outline the acceptable alternatives with dimensioned conceptual designs and indicate the requirements for the contractor to submit final site-specific details. These submitted (design/build) shop drawings should clearly establish that the design criteria are satisfied. They may include aesthetic features, bearing capacity and stability requirements, design computations for the alternative site-specific selection signed and sealed by a licensed professional engineer, and other data as may be necessary to document compliance with project needs.

A. J. Siccardi, PE.

Formerly, Staff Bridge Engineer
Colorado Department of Transportation
Denver, Colorado

S. C. (Trever) Wang, Ph. D., P. E.

Senior Engineer

Colorado Department of Transportation
Denver, Colorado

Retaining walls are an important element in highway construction. They are most fre­quently constructed in the highway environment to retain a mass of earth. They are also used to enable the highway designer to establish grade lines for roadways at dif­fering elevations when such roadways are in close proximity to one another and are to be constructed within limited rights-of-way, as is generally the case in densely populated urban locations.

Until 1972, when the first Reinforced Earth wall in the United States was built in California, retaining walls utilized in highway construction were usually plain gravity or reinforced concrete walls. Now, the use of mechanically stabilized earth (MSE) walls has become widespread in construction throughout the United States. Because early walls included metal strap reinforcement as the primary mechanism for stabilizing the soil, corrosion of the reinforcement and lack of long-term durability were a major impediment to immediate acceptance. Currently, the utilization of metal reinforcing requires the addition of sacrificial galvanizing materials selected to ensure the design life of the structure.

More recent earth reinforcement systems utilize geosynthetic materials, which are deemed inert to attack by deicing salts used on the highways. Salts are a primary inducer of corrosion in metal reinforcement. Long-term creep characteristics of geosynthetic reinforcements, however, must be carefully considered. There are also increasingly more specialty-type walls, such as the soil nail type for both temporary and permanent wall locations, especially for slope stabilization where slope materials are appropriate for nailing. Each of these wall types is discussed briefly in this chapter.

The material in this chapter is drawn from many sources, including personal experi­ence, but primarily from the following sources: (1) Section 5, “Retaining Walls,” American Association of State Highway and Transportation Officials (AASHTO), Standard Specifications for Highway Bridges, 17th ed., 2002; and (2) Subsection 5 of the

Colorado Bridge Design Manual, “Earth Retaining Wall Design Requirements,” Colorado Department of Transportation. A list of references is given at the end of the chapter.

Retaining walls can also be designed by the Load and Resistance Factor Design (LRFD) method as given by AASHTO in LRFD Bridge Design Specifications. This is a method of proportioning structural elements by applying factors to both the loads (load factors) and the nominal strengths (resistance factors). The specified factors are based on the mathematical theory of reliability and a statistical knowledge of load and material characteristics. The load factors are multipliers (typically greater than 1.0) that take account of the variability of different types of loads, such as earth loads and live loads. Resistance factors (typically 1.0 or lower) account for inaccuracies in theory and variation of properties. Although AASHTO’s goal is to use LRFD for all new construction, the traditional methods are currently the choice of most retaining wall designers and, hence, are the focus of this chapter.

The following criteria are necessary to ensure satisfactory impact performance of

luminaire supports.

• Use only designs that have been approved as crashworthy by the FHWA.

• The FHWA has established upper limits on the support mass and height of luminaire supports. These limits are applicable even when the breakaway characteristics have proven acceptable by crash testing. The maximum acceptable support weight (mass) is 1000 lb (454 kg), and the maximum luminaire support height is 60 ft (18.3 m). These values are increased from the limits of 600 lb (272 kg) and 50 ft (15.2 m) cited a few years ago. Any further increases in these limits should be based on full-scale crash testing and an investigation of vehicle characteristics beyond those recommended in NCHRP Report 350 [13, 19].

• Breakaway devices are designed to operate by being subjected to horizontal forces (device placed in shear). The devices are designed for this to occur when impacted at a typical bumper height of about 20 in (510 mm). Locating luminaire supports where they will be impacted at a different height will result in forces directed parallel to the support and thereby loading the devices in tension and compression. This results in improper operation of the breakaway device and possibly severe impacts and injuries to vehicle occupants. Superelevation, slope rounding, offset side slopes, curves, curbs, vehicle departure angle, and speed can all influence the striking height of a typical bumper. Negative side slopes should be limited to 1:6 between the roadway and the luminaire to help ensure that errant vehicles strike the support at an acceptable height [13].

• Use a wiring system that allows all circuit components to be shielded from impact, preferably underground, and that ensures that all electrical energy potentially available at the pole foundation surface is limited by the current-limiting fuses. Conductors pro­tected only by a circuit breaker should be not be accessible in the pole base.

• The major cost of a luminaire assembly is the pole, foundation, and breakaway devices. Select luminaires for performance and for a design flexibility that allows more selection of pole locations to produce a lighting system with fewer potential hazards.

• As a general rule, a pole will fall in line with the path of an impacting vehicle. The mast arm usually rotates so that it is pointing away from the roadway when resting on the ground. Consideration, however, must be given to the fact that falling poles may endanger pedestrians and may pose a danger to other motorists.

• A maximum 4-in (100-mm) stub height must be maintained to prevent vehicle snagging. Quick-disconnect electrical circuitry should also be used to facilitate the breakaway mechanism, to reduce the hazard of electrical shock from exposed wiring after impact, and to ease repairs.

• Foundations should be properly sized for surrounding soil conditions. Foundations that move through the soil upon impact place the breakaway mechanisms in bending rather than shear, resulting in improper actuation.

• Curbs, regardless of their shape or height, will elevate an impacting vehicle. The rise in height begins approximately 18 in (460 mm) from the curb and can extend as far as 10 ft (3050 mm). When possible, therefore, luminaire supports should be placed 10 ft (3050 mm) from the curb. If this is not possible, then they should be located closer than 3 ft (610 mm) from the curb. Luminaire poles placed between 3 and 10 ft (610 and 3050 mm) behind curbs increase the chances of improper break­away operation.

• If a luminaire support is placed behind a barrier, it may not be necessary to provide a breakaway feature. In general, if the support is within the design deflection dis­tance of the barrier, then either the barrier should be stiffened or a breakaway pole support should be used.

• Some agencies place luminaire assemblies on top of concrete median barriers. High-angle impacts, or impacts by large trucks or buses, can cause a luminaire mounted on top of a barrier to be struck. Breakaway design is not recommended for this type of installation because of the risk that a downed pole might pose to oppos­ing traffic.

• If a luminaire support is to be placed on top of a concrete barrier, then the barrier must be adapted to fit the pole base. Concrete safety-shape types are typically designed with an approximately 6-in-wide (150-mm) top surface. Since luminaire bases are typically 8 to 12 in (200 to 305 mm) in width, it is necessary to either widen the barrier top to 12 in (305 mm), or flare the barrier in the area of the luminaire.

• Design alternatives should be investigated with the goal of reducing the number of luminaires used along a section of roadway. Higher mounting heights can signifi­cantly reduce the total number of supports needed. Tower or high mast lighting can be used to effectively illuminate major interchanges. This method reduces the num­ber of poles and locates the supports much farther from the roadway.

• It should be noted that some agencies are experiencing problems with the failure of aluminum T-bases due to environmental loads. It is believed that this kind of failure, such as shown in Fig. 7.79, is initially caused by minor impacts from mowing units and other maintenance equipment. This causes small cracks at the bottom flange of the T-base that grow under environmental loads. The result is eventual separation of the bottom flange from the T-base as in Fig. 7.80.

Maintenance activities usually fall into one of two categories: demand or routine. Demand-responsive maintenance is response to random occurrences such as luminaire failures—i. e., lamps, fuses, ignitors, ballasts—or pole knockdowns. Routine mainte­nance is scheduled activities such as group lamp replacement or luminaire cleaning that are intended to produce a certain level of performance of the lighting system and eliminate some of the demand maintenance [18].

7.30.2 Maintenance Guidelines

A comprehensive discussion of roadway lighting maintenance is presented in “Design Guide for Roadway Lighting Maintenance,” IESNA DG-4. In addition to the factors affecting maintenance, this guide includes information for establishing a maintenance management system that will be helpful to agencies attempting to upgrade maintenance activities.

Maintenance must be considered from the earliest design stages of a lighting project. Top-quality materials should be specified and then arranged or located to protect the components from the potential hazards of the environment, whether these be rain, mois­ture, ultraviolet degradation, or threat of vehicular impact. After a system is installed and tested for operation and for component integrity, proper maintenance procedures can produce continued high performance of the roadway lighting system. If the lighting system is not properly maintained, the responsible authorities may expose themselves to potential liability—plus increased costs if expendable items are not replaced as they reach the end of their service life, because they can cause other components to fail.

7.30.1 Maintenance Operations

There are many reasons to routinely maintain a lighting system. The first reason is that only through good maintenance can the system continue to perform as designed. No matter how much knowledge and skill goes into the design, and how much care is put into the installation inspections and final system testing, the system will not provide the performance expected of it if regular maintenance is not performed. In addition to the legal liabilities of a substandard lighting system, the condition of the system reflects the civic concern of the responsible agency. A lighting system containing faults such as burned out lamps, dirty luminaires, or knocked-down poles reflects a poor attitude that is very noticeable. Another factor is that the electrical energy costs are more or less constant even though the light on the roadways may be significantly reduced, so the economic efficiency is decreased.

Before any lighting system is accepted as complete, or preferably before the electricity is turned on, several tests should be conducted to ensure the quality of the components:

Insulation tests. The contractor should measure the conductor insulation resistance to ground of each lighting circuit using a 500-V megohm-range type instrument. A record should be made of each phase conductor’s resistance to ground. The circuits should measure a minimum of 250,000 Q resistance to ground before the power is turned on. The test should be arranged to test splices and all components of the circuit. Ground resistance test. Using an instrument designed for the purpose, the con­tractor should measure the resistance of each ground rod. A written record of the value should be signed and given to the inspector. Any ground rod with a resis­tance of 25 Q or less is acceptable. Additional ground rods, up to a maximum of three at each location, should be installed to reach the 25 Q.

High mast lowering test. Each high mast lighting assembly should be tested by completely raising and lowering the luminaire ring once. Further testing of the latching operation for top latch devices is necessary. Each luminaire ring should be unlatched, lowered a minimum of 6 ft (2 m), then raised and relatched a total of five times to demonstrate its acceptability.

Photocontroller test. The control circuit of the lighting system should be demon­strated to show it operates properly in both manual and automatic modes. The

Voltage tests. The supply voltage at the lighting control center should be measured and recorded. With the luminaires energized and at full brightness, the voltage at the last luminaire of the circuit should be measured to ensure no more than a 10 percent voltage drop is present.

The conventional method of supplying electric energy to the high mast ring, with the luminaires at the top of the pole, uses a normal twist-lock plug and cap set. Some of these have a neoprene cover to provide some degree of waterproofing, but oftentimes this cover is either poorly installed or missing entirely. The result has been premature failure of the plug and cap set due to moisture. In some cases, the connectors will short-circuit between phases, and in others a large hole is burned through the connector body where the electricity arcs to a grounded structure. This latter case is particularly dangerous to the maintenance technician who attempts to reset a tripped circuit breaker while standing directly in front of the power cord connector. A method to avoid this has been developed by MG2/Duraline by utilizing a submersible cable connector that is impervious to moisture [14].

7.28.2 Foundation Installation

A sometimes overlooked hazard on the roadside is a poorly installed foundation. Any foundation, regardless of the design, can be a hazard unless care is taken to ensure that neither any portion of the foundation nor the predicted breakline of the breakaway device is above the 4-in (100-mm) line. Strict adherence to details, such as discussed in Art. 7.26, will produce a safe foundation even on a steep slope.

7.28.1 Conduit on Bridges and Median Barriers

The current design standards for bridge rail and median barrier rail are very similar. Consequently, the method for installing conduit for one is applicable to the other. In the past, the most common method for installing bridge conduit was to attach it to the underside of the bridge deck. This required both the installer and the maintenance crews to work outside the bridge rail and underneath the slab by constructing special scaffolding or using expensive trucks with articulating booms. A method has recently been developed that places the conduit and junction boxes within the bridge rail. The conduit may be galvanized rigid, PVC, or high-density polyethylene (HDPE). HDPE is available on large reels in lengths up to 60 ft (1500 m). The junction box is a curb box (Fig. 7.76) and may be a standard galvanized cast iron, or a box with reinforced fiberglass sides and a polymer concrete ring and cover as manufactured by CDR [17]. The CDR box has the advantage of not requiring the cover to be grounded for safety reasons. Details of this wiring method with junction box and luminaire foundation are shown in Fig. 7.77.

This same method is also used for installing conduits for median-mounted lighting systems. In some areas, whenever a median of a divided highway is closed and a median barrier installed, an empty HDPE conduit is installed inside the barrier for future use. The advantage of using the HDPE in this case is that there are no joints required other than at bridge ends. No special skill is required to install the HDPE, so the concrete barrier construction is not delayed. When the conduit is for a future light­ing system, no foundations are installed at the time the rail is constructed. A section of the barrier can be removed at a later time when the lighting system is installed to allow the conduit ends to be connected to the junction box placed at each luminaire location. Details of this foundation are shown in Fig. 7.78. Luminaire poles can be

easily installed on existing barriers without the encased conduit by using a modifica­tion that places the conduit in the roadway shoulder below grade and connected to the barrier rail-mounted junction box.