Highway and Railroad Engineering PDF

PUP Civil Engineering Department #WeLearnAsOne Module 1 | Highway Engineering Learning Objectives Upon successful completion of this course, the student should be able to: Understand the functions of a highway and railway engi...

PUP Civil Engineering Department #WeLearnAsOne Module 1 | Highway Engineering Learning Objectives Upon successful completion of this course, the student should be able to: Understand the functions of a highway and railway engineering; Understand the geometric design controls and criteria; Understand the elements of horizontal alignment, including being able to design and set out circular curve elements and circular and transition curves; Understand the elements of vertical alignment, including being able to design and set out vertical curves; Being able to coordinate of horizontal and vertical curves; Understand the cross-section elements; Course Material 1. BASIC HIGHWAY DESIGN DATA 1.1. Field Survey Investigation - Involves determination of the physical location, alignment, gradients, sight distances, cross sections, and other design elements of highways. Highway Location - It consists mainly of reconnaissance, topographic surveys, establishment of horizontal and vertical controls, centerline staking, centerline profile and cross-sectional leveling, bridge site survey, parcellary survey, and other surveys related to highway engineering. The survey shall be under the direct supervision of a Locating Engineer. Reconnaissance - Reconnaissance is carried out in order to plan the best possible horizontal and vertical alignments. Rock cuts, agricultural farms, steep side slopes, slides and other controls are identified. Preliminary Survey - In the preliminary survey the topography of the strip or strips flagged is obtained and from which a topographic map will be prepared to be utilized as the basic framework for projection of the line in the office. Office Projection - This is a trial and error process to obtaining the best line, in consideration of constraints such as alignment, grades, sight distances and compensation. Final Location Survey - Final location survey is done to transfer the office projection of the best line to the actual site in the field. 1.2. Field Investigation Proposed Sites for Stream Crossings - Important for hydrologic and hydraulic considerations Road Alignment - Can produce a major impact on the environment, the fabric of community, and highway users. Existing Utility Services - Records obtained from utility service providers should be verified in the field 1.3. Soil Investigation - Obtaining design data and analyze in detail the soil problems in order to decide the most suitable investigations, method and equipment to be used. 3  PUP Civil Engineering Department #WeLearnAsOne Subsurface Investigation - Subsurface investigation includes investigation of the area below the subgrade level. Subgrade Investigation (CBR value) - Subgrade investigation involves detailed investigation of the soil surface on which the pavement is constructed. Widening of Existing Pavements - Any pavement widening should have a design depth of at least as thick as that of the existing pavement. Sampling and Testing The following tests should be made on the subgrade samples obtained from test pits and boreholes: o Mechanical Analysis – AASHTO T 88 or 27 o Specific Gravity – AASHTO T 100 or 84 or 85 o Atterberg Limits – AASHTO T 89 or 90 o Moisture-Density Relationship – AASHTO T 180 or 99 o CBR% – AASHTO T 193 o Natural Moisture Content Classification of soils would be made in accordance with AASHTO M 145, and all dry samples should be prepared in accordance with AASHTO T 87. 1.4. Existing Pavement Evaluation - Whilst test pits and borings can give all the subgrade data, only a pavement inspection combined with some background history of the pavement can guide the Pavement Engineer in his evaluation on the remaining life of the pavement and the original quality of its construction. Concrete Roads - Rough surface, poor joints and scaled surfaces would indicate poor or weak concrete Potholes, cracking and pumping may indicate localized areas of poor concrete or inadequate subgrade compaction and drainage. Asphalt Roads - Potholes indicate generally inadequate pavement strength for the traffic Longitudinal and transverse cracking and depressions generally indicate subgrade or sub-base failure 1.5. Drainage and Recommendation - To maintain all parts of the highway (i.e. surface, subsurface, slope and structure) in an excellent drainage condition, prevent traffic congestion and slip accidents caused by flooding water on road surface 1.6. Design Controls - Topography, land use, traffic and vehicle data form the major controls for highway design as these have pronounced effect on highway location, geometrics and determination of the type of highway. Anticipated Traffic Volume - Traffic Loading (Cumulative) Character of Traffic – Weight, Dimension, Mobility Design Speed - Maximum safe speed that can be maintained Highway Capacity – Geometric Features and Axle Load Classification of Highway – Function and System Accident Information - Conduct of analysis to determine safety enhancement 1.7. Requirements for Speedy Plan Preparation 1.7.1. Plans - The final horizontal alignment shall be plotted on a scale of 1:1000 m, and the following items shall be shown on the plan: Plans shall show the centerline of the project road, the width of the roadway and shoulders and the right-of-way. 4  PUP Civil Engineering Department #WeLearnAsOne Azimuth, distance, elements of curve, coordinates, superelevation and widening of every curve, and design speed shall be specified. Each sheet shall have a north arrow indicator and lines representing the coordinates. Contours shall be plotted at 1.00 m intervals, however if contour lines are too close together an interval of 5.00 m may be used. The minimum extent of contour line should be within the Road Right-of-Way. Elevation of benchmarks with accurate descriptions, reference points and controlling points with azimuth and distance shall be shown. Information and data shall be provided regarding existing roads, intersections and railways, existing rivers and waterways, existing houses and structures, public utilities, land classifications and others. All existing and proposed structures, such as bridges, box culverts, pipes and other drainage, slope protection structures, traffic signs, road markings, safety barrier and lighting columns shall be indicated – which may involve a number of plans for clarity. Include typical roadway section and existing Road Right of Way limit. 1.7.2. Profile - Longitudinal profile of existing ground and finished grade lines shall be plotted on a scale of 1:1000 m horizontal and 1:100 m vertical. For mountainous areas a scale of 1:200 for vertical may be used. The following items shall be shown on the longitudinal profiles: Elements of every vertical parabolic curve. The percent grades indicated by a plus (+) for ascending and minus (-) for descending. The finished grade and existing ground elevations for every full station. The station number in kilometer including invert elevations and a description of all existing and proposed structures, such as bridges, box culverts and pipes. The maximum flood elevation in flooded areas and ordinary and highest water elevations of river, creek and canals. Side ditch profile indicating the gradient, invert elevations and outfall. Superelevation and widening diagrams. 1.7.3. Detailed Cross Section - Cross sections at every 20 m full station, at intermediate breaks on the ground and at bridge approaches and drainage structures shall be plotted on a scale of 1:100 m horizontal and vertical. The following shall be indicated on the cross-section drawings: Existing ground profile and template roadway section. The general manner of treating slopes in cut and fills, including warping and rounding. The manner of superelevating and widening in curves. Coordinates of the existing ground and template roadway section. Finished grade and natural ground elevations of roadway centerline. Area of cut and fill, and quantities of all other involved items of work. Drainage structures including side ditches. Slope protection. 1.8. Geotechnical Drawings - The geotechnical data in these drawings shall include the complete soil survey data for the project, the approved sources of borrow, aggregate, sub-base, aggregate base, concrete aggregates and asphalt aggregates. 5  PUP Civil Engineering Department #WeLearnAsOne 2. GEOMETRIC DESIGN - Embraces the grade line, alignment and the width of the several component parts including intersections and roadside facilities. 2.1. Basic considerations in the geometric design of a new highway or redesign of an old highway Environment - minimum effect to the environment Safety - provided with necessary roadside treatment, and road safety control devices Construction Methodology - simple as possible from the standpoint of the builder Maintenance - least/reasonable cost Motorists’ Convenience - Suitable to traffic volume; safe for driving and ensure confidence for motorists Minimum hazard - Consistent and must avoid surprise changes in alignment, grade line, and sight distance Aesthetics - Pleasing to the user and to those who live along it 2.2. Alignment Choice and Terrain Adaptations The approach and methodology in respect of the practical work with alignment choice and terrain adaptation can be divided into three steps: Inventory of constraints and opportunities; Route planning; and, Detailed design of the alignment. 2.2.1. Step 1: Inventory - It is important to analyze the terrain to understand its constraints and possibilities. Constraints could be areas not allowed to be used or only to be used as constraints, e.g. existing or planned buildings, rivers, roads, geotechnical difficult areas etc. Possibilities to create the desired right scale and rhythm are the talent of the designer to combine the technical requirements with the freedom given by the terrain area. It is important in the initial phase to get good knowledge and a visual concept about the terrain. The terrain should be “walked”. Good maps, terrestrial and aerial photos are essential, but it should be stressed again that the solution of the road design is found in the terrain. 2.2.2. Step 2: Route Planning - There are two alternative methods for identifying the route: the tangent method and the arcs method. Tangent Method - The straight strategy gives the road line iteratively by defining straights and then to combine these by arcs. An advantage could be effective use of sight distances. 6  PUP Civil Engineering Department #WeLearnAsOne Arc Method The other method starts by selecting arcs with radii that fit in well with the scale and form of the landscape. These arcs are then linked together with transition curves, i.e. clothoids of larger arcs. It is important in this strategy to avoid dilemma sight distances and to create sufficient overtaking sight distances. This method is more likely to result in a route which is well adapted to the terrain. The road location and alignment procedure should start on the map by sketching suitable alignment alternatives. Impressions and notations from the landscape and its characteristic forms and properties should be used to create alternatives anchored in the landscape. The straight line is normally not in harmony with the landscape. The terrain adaptation must, as already stressed, be combined with the partly contradictive requirement to create sufficient overtaking sight distances and to avoid dilemma sight distances. Left-hand bends, even with large radii, will rarely have sufficient sight distance for safe overtaking. The designer should mentally visualize the three-dimensional form. This could be supported by using simple profile sketches to analyze the phasing between horizontal and vertical alignment. Intersections, interchanges etc. should be considered already in this stage. 2.2.3. Detailed Design - Having found a route in harmony with the terrain using the sketch technique above horizontal geometric elements can be calculated and a first profile produced. Technical requirements should also be checked such as: minimum horizontal and vertical elements; combined elements for visual guidance; stopping, dilemma and overtaking sight distances; and, speed profiles. Always check what percentage of the road will have overtaking sight distance. This will have a major impact on safety and level-of-service at medium high traffic flows. The coordination between vertical and horizontal alignment should be checked using perspective images. It is important to learn how to select points for perspectives and how to interpret perspective images. 7  PUP Civil Engineering Department #WeLearnAsOne 2.3. Sight Distance is the length of roadway visible to a driver. Ability to see ahead is of the utmost importance in the safe and efficient operation of the highway. Designers are encouraged to calculate and report the percentage of road length where the sight distance is adequate for safe overtaking as a useful design safety indicator. 2.3.1. Stopping Sight Distance - Available distance on a roadway to enable a vehicle traveling at the design speed to stop before reaching a stationary object. (break reaction plus braking distance). Formula: SSD = 0.278tv + v2/254(f+G). 2.3.2. Passing Sight Distance - Distance required for a driver to see a sufficient object to complete the passing maneuver without cutting off the passed vehicle in advance of meeting an opposing vehicle appearing during maneuver. Formula: PSD = 0.278t1 [V-m + a(t1)/2] + 0.278Vt2 + (var. between 30 and 75m) + (2(0.278Vt2)/3. 8  PUP Civil Engineering Department #WeLearnAsOne 2.3.3. Decision Sight Distance - Distance required for a driver to initiate and complete safely and efficiently the maneuver of an unexpected or otherwise difficult-to- perceive information source or hazard. Formula: D = 0.278tv + v2/254(f+G). 2.3.4. Intersection Sight Distance - The corner sight distance available for a vehicle approaching an intersection to see oncoming vehicles approaching from crossing legs (the left and right). 2.3.5. Criteria for Measuring Sight Distance Vertical Control for Stopping - A height of object of 0.15 m is assumed. Horizontal Control for Stopping - Height of eye of 1.15 m to an object on the road surface of height 0.15 m. Control for Passing - Height of eye of 1.15 m and a height of object of 1.40 m. 9  PUP Civil Engineering Department #WeLearnAsOne 2.4. Horizontal Alignment - Horizontal curves are, in effect, transitions between two tangents. These deflection changes are necessary in virtually all roadway alignments to avoid impacts on a variety of field conditions (e.g. right of way, natural features, man- made features). The following discusses the several types of horizontal curves that may be used to achieve the necessary roadway deflection: 2.4.1. Simple Curves - Simple curves are continuous arcs of constant radius that achieve the necessary roadway deflection without an entering or exiting taper. It is the most used. The radius of the circle determines the “sharpness” or “flatness” of the curve. 2.4.2. Compound Curves - Compound curves are a series of two or more simple curves with deflections in the same direction. Compound curves are generally used on the roadway mainline to meet field conditions (e.g. to avoid obstructions that cannot be relocated) where a simple curve is not an option. 10  PUP Civil Engineering Department #WeLearnAsOne 2.4.3. Reverse Curves - A reverse curve consists of two simple curves joined but curving in opposite directions. For safety reasons, the designer should not use this curve unless necessary. 2.4.4. Spiral Curves - Spiral curves provide an entering transition into a simple curve with a variable rate of curvature along its layout. 11  PUP Civil Engineering Department #WeLearnAsOne 2.5. Design Elements of Horizontal Curves 2.5.1. Design Speed - Defined as the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern. It is the basis that will literally put shape to the different elements of the highway. 2.5.2. Superelevation - The tilting of roadway to help offset centripetal forces developed as the vehicle goes around a curve. Along with friction they are what keeps a vehicle from going off the road. Three methods of attaining Superelevation: rotation about centerline, inside edge and outside edge. 12  PUP Civil Engineering Department #WeLearnAsOne 2.6. Transition Length - The superelevation transition length is the distance required to transition the roadway from a normal crown section to the full design superelevation rate. The superelevation transition length is the sum of the tangent runout distance (Lt) and superelevation runoff length (Lr). Superelevation runoff and tangent runout lengths are calculated as follows: (Source: AASHTO Geometric Design of Highways and Streets 2011) 13  PUP Civil Engineering Department #WeLearnAsOne Example Superelevation Problem: A. What is the minimum radius of curvature allowable for a roadway with a 100 km/h design speed, assuming that the maximum allowable superelevation rate and the pavement coefficient of friction are 0.80 and 0.12, respectively? B. What is the actual maximum superelevation rate allowable under AASHTO recommended standards for a 100 km/h design speed, if the maximum value of ƒ and minimum curve radius allowed by AASHTO for this speed are 0.12 and 490m respectively? Round the answer down to the nearest whole percent. C. For a two (2) lane road (3.35m per lane), with a normal corss-slope of 1.50%, design speed of 100 km/h, and maximum superelevation rate of 4.0%. Find superelevation runoff length and tangent runout length? Solutions: 14  PUP Civil Engineering Department #WeLearnAsOne 2.7. Curve Widening On modern roadways and streets with 3.65 m travel lanes and high-type alignment, the need for widening generally is not required in spite of high speeds, but for some conditions of speed, curvature and width it may be appropriate to widen the travelled way. The amount of widening of the travelled way on a horizontal curve is the difference between the width needed on the curve and the width used on a tangent. 𝒘 = 𝑾𝒄 − 𝑾𝒏 where: w = widening of travelled way on curve, m Wc = width of travelled way on curve, m Wn = width of travelled way on tangent, m The travelled way width needed on a curve (Wc) has several components related to operation on curves, including: the track width of each vehicle meeting or passing (U), the lateral clearance for each vehicle (C); width of front overhang of the vehicle occupying the inner lane or lanes (FA) and a width allowance for the difficulty of driving on curves (Z). These components are illustrated in Figure below. 𝑾𝒄 = 𝑵(𝑼 + 𝑪) + (𝑵 − 𝟏)𝑭𝑨 + 𝒁 𝑼 = 𝒖 + 𝑹 − √𝑹𝟐 − ∑ 𝑳𝒊𝟐 𝑭𝑨 = √𝑹𝟐 + 𝑨(𝟐𝑳 + 𝑨) − 𝑹 𝑽 𝒁 = 𝟎. 𝟏( ) √𝑹 15  PUP Civil Engineering Department #WeLearnAsOne Widening components on open roadway curves The lateral clearance, C, provides clearance between the edge of the travelled way and nearest wheel path and for clearances between vehicles passing or meeting. For design, assume C = 0.9 m. 16  PUP Civil Engineering Department #WeLearnAsOne 2.7.1. Application and Design Procedure for widening Widening should transition gradually on the approaches to the curve to provide a reasonably smooth alignment of the edge of the travelled way and to fit the paths of vehicles entering or leaving the curve. The principal points of concern in the design of curve widening, which apply to both ends of roadway curves, are presented below: Location of Widening. On simple curves, only apply the widening on the inside edge of the travelled way. On curves designed with spirals, widening may be applied on the inside edge or divided equally on either side of the centerline. In the latter method, extension of the outer-edge tangent avoids a slight reverse curve on the outer edge. Transition. Transition the curve widening gradually over a length sufficient to make the whole travelled way fully usable. Although a long transition is desirable for traffic operation, it may result in narrow pavement slivers that are difficult and expensive to construct. Aesthetics. From the standpoints of usefulness and appearance, the edge of the travelled way through the widening transition should be a smooth curve. Do not use a tangent transition edge. Distribution. On roadway alignment without spirals, smooth and fitting alignment results from attaining widening with one-half to two-thirds of the transition length along the tangent and the balance along the curve. This is consistent with a common method for attaining superelevation. The inside edge of the travelled way may be designed as a modified spiral, with control points determined by either the width/length ratio of a triangular wedge, by calculated values based on a parabolic or cubic curve, or by a larger radius (compound) curve. Otherwise, it may be aligned by eye in the field. 17  PUP Civil Engineering Department #WeLearnAsOne Given: Widening on Curves For a two (2) lane highway (3.60 m per lane) with a design speed of 40 km/h, and radius of 50 m. Problem: Find widening along the curve using Single-Unit Truck (SU-9) design vehicle type. Note: Refer to tables above for the values of the following: A = front overhang of inner-lane vehicle, m L = wheelbase of a single unit or tractor, m u = track width on tangent, m Li = wheelbase of design vehicle between consecutive axles (or sets of tandem axles) and articulation points, m Solution: Step 1: Using Equation 3.9, compute for the extra width allowance, m 𝑉 𝑍 = 0.1 ( ) √𝑅 Step 2: Using Equation 3.8, compute for the width of front overhang of inner-lane vehicle, m 𝐹𝐴 = √𝑅2 + 𝐴(2𝐿 + 𝐴) − 𝑅 Step 3: Using Equation 3.7, compute for the track width of design vehicle (out-to-out tires) on curves, m 𝑈 = 𝑢 + 𝑅 − √𝑅2 − ∑ 𝐿2 𝑖 Step 4: Using Equation 3.6, compute for the track width of design vehicle (out-to-out tires) on curves, m 𝑊𝑐 = 𝑁(𝑈 + 𝐶) + (𝑁 − 1)𝐹𝐴 + 𝑍 Step 5: Using Equation 3.5, compute for widening of travelled way on curve, m 𝑤 = 𝑊𝑐 − 𝑊𝑛 18  PUP Civil Engineering Department #WeLearnAsOne 2.7.2. Principal Points of Concerns in Widening on Curves On simple curves, widening should be applied on the inside edge only. On curve design with spiral, widening may be placed on the inside or divided equally between the inside and outside curve. Curve widening should be attained gradually over a length sufficient to make the whole of the traveled way fully usable. Recommended minimum width of widening is 0.60m. 2.8. General Controls for Horizontal Alignment The design of horizontal alignment involves, to a large extent, complying with specific limiting criteria. These include minimum radius, superelevation rates and sight distance around curves. In addition, the designer should adhere to certain design principles and controls that will determine the overall safety of the facility and will enhance the aesthetic appearance of the roadway. These design principles include: Consistency. The alignment should be consistent. Sharp curves at the ends of long tangents and sudden changes from gentle to sharply curving alignment should be avoided. Directional. The alignment should be as directional as possible consistent with physical and economic constraints. On divided roadways, a flowing line that conforms generally to the natural contours is preferable to one with long tangents that slash through the terrain. Directional alignment will be achieved by using the smallest practical central angles. Use of minimum radii. Avoid the use of minimum radii, if practical, especially in level terrain. High fils. Avoid sharp curves on long, high fills. Under these conditions, it is difficult for drivers to perceive the extent of horizontal curvature. Compound curves. Do not use compound curves on the roadway mainline. Alignment reversals. Avoid abrupt reversals in alignment (reverse curves). Desirably, provide a sufficient tangent distance between the curves to ensure proper superelevation transitions for both curves and to allow time for the motorist to perceive the next decision point. Typically, 2 seconds of travel time. Broken-back curvature. Avoid where possible. This arrangement is not aesthetically pleasing, violates driver expectancy and creates undesirable superelevation development requirements. Coordination with natural/man-made features. The horizontal alignment should be properly coordinated with the natural topography, available right-of-way, utilities, roadside development and natural/man-made drainage patterns. Environmental impacts. Horizontal alignment should be properly coordinated with environmental impacts. Intersections. Horizontal alignment through intersections may present special problems (e.g., intersection sight distance, superelevation development crossover crowns). See “Intersections” for the design of intersections. Coordination with vertical alignment. general design principles for the coordination between horizontal and vertical alignment. Bridges. Horizontal alignment must be coordinated with the location of bridges. The need for curvature and superelevation development should be evaluated for each bridge location. 19  PUP Civil Engineering Department #WeLearnAsOne 2.9. Vertical Alignment - Parabolic vertical curve has been used to design the profile of highways. It has properties that make it easy for laying out the alignment of a roadway in the field. 2.10. Nature of Terrain - To characterize variations in topography, topography is generally separate into three classifications according to terrain: Level. In level terrain, sight distances, as governed by both horizontal and vertical restrictions are generally long or can be made so without construction difficulty or major expense. Rolling. In rolling terrain, natural slopes consistently rise above and fall below the road or street grade, and occasional steep slopes offer some restrictions to normal horizontal and vertical roadway alignment. Mountainous. In mountainous terrain, longitudinal and transverse changes in the elevation of the ground with respect to the road or street are abrupt, and benching and side hill excavation are frequently needed to obtain acceptable horizontal and vertical alignment. 2.10. Gradient “For economy of vehicle operation, grades should be as flat as possible.” In areas subject to inundation, grades should be established 0.50m above water level. 20  PUP Civil Engineering Department #WeLearnAsOne Grades of bridges should allow 1.50m freeboard above the maximum flood water elevation. Maximum grade widely used is 6.0%. On through cut sections, grades should at least be 0.50% to provide longitudinal drainage. 2.10.1. Maximum Grades - “Rural and Urban Freeways”, “Rural Roads”, and “Urban Streets” present criteria for maximum grades based on functional classification, urban/rural location, type of terrain and design speed. Wherever practical, use grades flatter than the maximum. Only use the maximum grades where it is absolutely necessary. 2.10.2. Minimum Grade Uncurbed roadways. It is acceptable to provide a minimum longitudinal gradient of approximately 0.0%, but only if the roadway will not be curbed in the future. Ensure that the pavement has adequate cross slopes and that the flow lines of the outside ditches have adequate drainage. Curbed streets. The median edge or centerline profile on roadways and streets with curb and gutter desirably should have a minimum longitudinal gradient of 0.5%. Where the adjacent development or flatter terrain precludes the use of a profile with a 0.5% grade, provide a minimum longitudinal gradient of at least 0.3%. 2.11. Critical Length of Grade - Critical Length of Grade - The critical length of grade is the maximum length of a specific upgrade on which a truck can operate without an unreasonable reduction in speed. The roadway gradient, in combination with the length of the grade will determine the truck speed reduction on upgrades. 21  PUP Civil Engineering Department #WeLearnAsOne The following will apply to the critical length of grade: 1. Design vehicle. Figure above presents the critical length of grade for a 120 kg/kW truck. 2. Criteria. Figure above provides the critical lengths of grade for a given percent grade and acceptable truck speed reduction. Although this figure is based on an initial truck speed of 110 km/h, it applies to the design or posted speed. For design purposes, use the 15 km/h speed reduction curve in determining the critical length of grade if exceeded. 3. Momentum grades. Where an upgrade is preceded by a downgrade, trucks will often increase their speed to ascend the upgrade. A speed increase of 10 km/h on moderate downgrades (3% to 5%) and 15 km/h on steeper downgrades (6% to 8%) of sufficient length are reasonable adjustments to the initial speed. This assumption allows the use of a higher speed reduction curve. However, the designer should also consider that these speed increases may not always be attainable. If traffic volumes are sufficiently high, a truck may be behind another vehicle when descending the momentum grade thereby restricting the increase in speed. Therefore, only consider these increases in speed if the roadway has a Level of Service equal to C or better. 4. Measurement. Vertical curves are part of the length of grade. Figure above illustrates how to measure the length of grade to determine the critical length of grade. 5. Application. If the critical length of grade is exceeded, flatten the grade, if practical, or evaluate the need for a truck-climbing lane. Typically, only two-lane roadways have operational problems that require truck-climbing lanes. 6. Roadway types. The critical-length-of-grade criterion applies equally to two-lane (single carriageway) or multilane roadways (dual carriageway), and applies equally to urban and rural facilities 22  PUP Civil Engineering Department #WeLearnAsOne Notes: 1. For vertical curves where the two tangent grades are in the same direction (both upgrades or both downgrades), 50% of the curve length will be part of the length of grade. 2. For vertical curves where the two tangent grades are in opposite directions (one grade up and one grade down), 25% of the curve length will be part of the length of grade. 23  PUP Civil Engineering Department #WeLearnAsOne Figure 3.2: Measurement for length of grade 24  PUP Civil Engineering Department #WeLearnAsOne Example Nos. 1 and 2 illustrate the use of figure above to determine the critical length of grade. Example No. 3, indicate the successive gradients and lengths of grade on the roadway segment. ******** Example No. 1 Given: Level Approach G = +4% L = 450 m (length of grade) Rural Arterial Problem: Determine if the critical length of grade is exceeded. Solution: G1 yields a truck speed reduction of approximately 10 km/h. G2 yields a speed reduction of approximately 2 km/h. The total of 12 km/h is less than the maximum 15 km/h speed reduction. Therefore, the critical length of grade is not exceeded. Example No. 2 Given: Level Approach G1 = +4.5% L1 = 200 m G2 = +2% L2 = 150 m Rural Arterial with a significant number of heavy trucks Problem: Determine if the critical length of grade is exceeded for the combination of grades G1 and G2. Solution: G1 yields a truck speed reduction of approximately 10 km/h. G2 yields a speed reduction of approximately 2 km/h. The total of 12 km/h is less than the maximum 15 km/h speed reduction. Therefore, the critical length of grade is not exceeded. 25  PUP Civil Engineering Department #WeLearnAsOne Example No. 3 Given: Example illustrates the vertical alignment on a low-volume, two-lane rural collector roadway with no large trucks. Problem: Determine if the critical length of grade is exceeded for G2 or for the combination upgrade G3 and G4. 26  PUP Civil Engineering Department #WeLearnAsOne Solution: Use the following Steps: Step 1: Determine the length of grade using the criteria in Figure 3.2. For this example, the following calculations are used: L Step 2: Determine the critical length of grade for the roadway in both directions. Use figure above to determine the critical length of grade For trucks travelling left to right, enter into Figure 5.1 the value for G3 (3.5%) and L3 = 320 m. The speed reduction is approximately 13 km/h. For G4 (2%) and L4 = 270 m, the speed reduction is approximately 5 km/h. The total speed reduction on the combination upgrade G3 and G4 is 18 km/h. This exceeds the maximum 15 km/h speed reduction. However, on low-volume roads, one can assume a 10 km/h increase in truck speed for the 3% “momentum” grade (G2), which precedes G3. Therefore, a speed reduction may be as high as 25 km/h before concluding that the combination grade exceeds the critical length of grade. Assuming the benefits of the momentum grade, this leads to the conclusion that the critical length of grade is not exceeded. Next determine the critical length of grade for trucks travelling in the opposite direction. On Figure 3.1, enter in 3% for G2 and L2 = 320 m. The speed reduction is 10 km/h. Because the speed reduction is less than 15 km/h, the critical length of grade for this direction is not exceeded 2.12. Truck Climbing Lane - It is desirable to provide a truck-climbing lane as an added lane for the upgrade direction of a roadway where the grade, traffic volumes and heavy- vehicle volumes combine to degrade traffic operations from those on the approach to grade. See Figure 3.5. Truck climbing lane 27  PUP Civil Engineering Department #WeLearnAsOne 2.13.1. Location Guidelines - A truck-climbing lane may be necessary to allow a specific upgrade to operate at an acceptable level of service. The following criteria will apply: 1. Two-lane roadways (single carriageway). On a two-lane, two-way single carriageway, provide a truck-climbing lane if the following conditions are satisfied: The upgrade traffic flow is in excess of 200 veh/h; and The heavy-vehicle volume (i.e. Trucks, buses and recreational vehicles) exceeds 20 veh/h during the design hour; and One of the following conditions exists: o The critical length of grade is exceeded for the 15 km/h speed reduction curve (see Figure 3.1), or o The level of service (LOS) on the upgrade is E or F, or o There is a reduction of two or more LOS when moving from the approach segment to the upgrade; and The construction costs and the construction impacts (e.g. Environmental, right- of-way) are considered reasonable. 2. Multilane roadways (dual carriageway). Provide a truck-climbing lane on a dual carriageway if the following conditions are satisfied: The directional service volume for LOS D is exceeded on the upgrade; and The directional service volume exceeds 1000 veh/h/lane; and One of the following conditions exists: o The critical length of grade is exceeded for the 15 km/h speed reduction curve (see Figure 3.1), or o The LOS on the upgrade is E or F, or o There is a reduction of one or more LOS when moving from the approach segment to the upgrade; and 2.13. Vertical Curves - Vertical curves are curves that provide transitions between two sloped roadways, allowing a vehicle to negotiate the elevation rate change at a gradual rate rather than a sharp cut. The design of the curve is dependent on the intended design speed for the roadway, as well as other factors including drainage, slope, acceptable rate of change, and friction. These curves are parabolic and are assigned stationing based on a horizontal axis. 2.13.1. Crest Vertical Curves - Wherever practical, longer stopping sight distances should be used, particularly at decision points. The basic equations for length of a crest vertical curve in terms of algebraic difference in grade and sight distance follow: 28  PUP Civil Engineering Department #WeLearnAsOne When the height of eye and the height of object are 1.08 m and 0.60 m respectively, as used for stopping sight distance, the above equations become: Where S is greater than L, minimum lengths of vertical curves in meters are expressed as 0.6 times the design speed in kph. Design values of crest vertical curves for passing sight distance differ from those for stopping sight distance because of the different sight distance and object height criteria. Using 1.08 m for the height of object results in the following specific formulas: 2.13.2. Sag Vertical Curves - Sag vertical curves are in the shape of a parabola. Typically, they are designed to allow the vehicular headlights to illuminate the roadway surface (i.e. the height of object = 0.0 m) for a given distance “S.” The light beam from the headlights is assumed to have a 1 degree upward divergence from the longitudinal axis of the vehicle. These assumptions yield the following basic equations for determining the minimum length of sag vertical curves: 29  PUP Civil Engineering Department #WeLearnAsOne 2.13.3. Underpass - Check sag vertical curves through underpasses to ensure that the underpass structure does not obstruct the driver’s visibility. Use the Equation 3.8 to check sag vertical curves through underpasses where the sight distance is less than the length of curve (S L). 𝐴𝑆2 𝐿= 800(𝐶 − 1.5) Use to check sag vertical curves through underpasses where the sight distance is greater than the length of curve (S>L). SL 30  PUP Civil Engineering Department #WeLearnAsOne 2.14. Cross-Sectional Elements - These comprise the types of surfaces, the width of pavement and the shoulders, the cross slopes, medians, sidewalks, and drainage channels and side slopes. 2.14.1. Pavement - For the purpose of defining the width of pavement, the pavement is regarded as the running surface, excluding shoulders, regardless of the width of the pavement courses which support the running surface. 2.14.2. Surface Type - The selection of surface type is determined based on the traffic volume and composition, soil characteristics, climate, performance of pavements in the area, availability of materials, energy conservation, initial cost, and the overall annual maintenance and service-life cost. Types of surfaces broadly are referred to as high, intermediate and low inconsideration of the effect on geometric design. A 2.14.3. Cross Slope – The purpose of the cross slope is to drain the pavement on tangents and on curve and to provide superelevation on horizontal curves. 2.14.4. Lane Widths - The width of pavement is determined by the lane width, which depends on the width and size of vehicles, speed of travel, the annual average daily traffic and the width of shoulders. Width of pavement is determined by the lane width. Desirable lane width is 3.65m which allows large vehicles to pass without either vehicle having to move sideways towards the edge of pavement. Lane width as low as 2.75m may be used on grounds of economy. 2.14.5. Shoulders - Shoulder width is measured from the edge of the pavement to edge of usable formation and excludes any berm, rounding or extra widths required to accommodate guide posts, guard fencing, etc. Shoulders must be sufficiently stable to support occasional vehicle loads in all kinds of weather. Shoulders on fill preferably should be wider than in cuts although the present practice is to make them equal. Regardless of the width, shoulders should be continuous. Although, it is desirable that shoulder be wide enough for a vehicle to be driven completely off the travel way, narrower shoulders are better than none at all. Shoulder width of 0.60m may be considered on difficult terrain and on low- volume highway. DPWH Department Order No. 40 Series of 2012 “Guidelines on Shoulder Paving Along National Roads”. Item 3: Paving of shoulders shall be considered along road sections under any of the following conditions: AADT > 1250 vehicles Roadway with frequent turning movements High embankment (provided with guardrail requirements) 31  PUP Civil Engineering Department #WeLearnAsOne e > 7% Those designed with curb/gutter/ lined canal Where pedestrians are normally concentrated with steep and long gradient exceeding 6% and 100.0 meters respectively 2.15. Combination of Horizontal & Vertical Alignments Curvature and grades should be in proper balance. Tangent alignment or flat curvature at the expense of steep or long grades and excessive curvature with flat grades both represent poor design. Vertical curvature superimposed on horizontal curvature, or vice versa, generally results in a more pleasing facility, but such combinations should be analyzed for their effect on traffic. Sharp horizontal curvature should not be introduced at or near the top of a pronounced crest vertical curve. Such an arrangement can be avoided if the horizontal curvature leads the vertical curvature, and by using design values well above the appropriate minimum values for the design speed. Only flat horizontal curvature should be introduced near the bottom of a steep grade approaching or near the low point of a pronounced sag vertical curve. On 2-lane highways and streets, the need for safe passing sections at frequent intervals and for an appreciable percentage of the highway length often supersedes the general desirability for coordination of horizontal and vertical alignment. Both horizontal curvature and profile should be made as flat as feasible at highway intersections. 32  PUP Civil Engineering Department #WeLearnAsOne 33  PUP Civil Engineering Department #WeLearnAsOne 34  PUP Civil Engineering Department #WeLearnAsOne 2.16. Road Safety 2.16.1. Road Signs - Road signs contain instructions that the road user is required to obey. These provide warning of hazards that may not be self-evident and information about directions, destinations and points of interests. Also, since signs are essential part of the road network system, the information provided should be concise, consistent and most importantly, these should be installed at conspicuous or designated spots along the roadway. Moreover, prior approval of the DPWH Secretary or the Head of the Office concerned for the installation of the same should be sought No road signs shall bear any advertising or commercial message, or any other messages that are not essential to traffic control. Placement of unauthorized signs within the road right-of-way or close to the roadway is not allowed. The display of unofficial and non-essential sign is likewise not permitted. 2.16.2. Classifications Road signs are classified as follows: 2.16.3. Regulatory Signs (Type R) - Regulatory signs indicate the application of legal or statutory requirements, i.e. obligation to give way at intersections, speed limits, prohibition of movements at intersections and control of parking of vehicles. The notable exceptions to this are STOP (octagonal), GIVE WAY (triangular) and some manually operated banners used in road works. It is important that these shall be removed promptly if the legal requirements become inconsistent with the present conditions. 35  PUP Civil Engineering Department #WeLearnAsOne 2.16.4. Warning Signs (Type W) - Warning signs notify road users of the condition on or adjacent to the road that may be unexpected or hazardous. Should not be used if, under normal conditions, the driver can see the potential hazard ahead. 2.16.5. Guide or Informative Signs - These inform and guide the road users of directions, distances, locations of services and points of interests. 2.16.6. Instructional Signs (Type S) - These are used at locations where ordinary guide or regulatory signs do not achieve the desired results. Also, these guide motorists of the direction or follow a course of action. 2.16.7. Hazard Markers (Type HM) - These are used to emphasize a marked change in the direction of travel and the presence of an obstruction. 36  PUP Civil Engineering Department #WeLearnAsOne 2.17. Standard Application Identical conditions should always be treated with the same type of signs so that road users can readily anticipate the course of action required. Basic requirements for road signs are as follows: Fulfill a need Command attention Convey a clear, simple message Command respect Give adequate time for proper response 2.18. Design Standardization of shape, color, dimensions, inscriptions and illumination or reflectivity is important so that various classes of signs can be easily recognized. The following design principles are considered: The driver should not be distracted by a road sign from his driving. Road signs should be understood by the driver traveling at any given speed and must have sufficient time to take appropriate response safely. 2.19. Weighbridge Station National roads and bridges are damaged by freight trucks and trailers whose gross weights exceed the allowable limits. In line with this, DPWH has installed weighbridge stations at strategic locations along national roads. 37