CVE140 - CHAPTER 3- ROAD CROSS SECTION.docx

**ROAD CROSS SECTION AND ITS IMPORTANCE** In highway engineering, a Road Cross Section is a detailed representation of the highway\'s vertical slice at a specific point. It shows the arrangement and dimensions of various components that make up the roadway and its surrounding areas. This cross sect...

**ROAD CROSS SECTION AND ITS IMPORTANCE** In highway engineering, a Road Cross Section is a detailed representation of the highway\'s vertical slice at a specific point. It shows the arrangement and dimensions of various components that make up the roadway and its surrounding areas. This cross section is crucial for understanding the construction and functional aspects of the highway. **Fig. 1: TYPICAL ROAD CROSS SECTION** **ROAD PAVEMENT LAYERS** 1. **Surface/Wearing Course** The top layers of pavement which is in direct contact with the wheel of the vehicle. Usually constructed of material in which bitumen is used as binder materials. **2. Base course** It is the layer immediately under the wearing surface (Applies whether the wearing surface is bituminous or cement concrete and or more inch thick or is but a thin bituminous layer). As base course lies close under the pavement surface it is subjected to severe loading. The material in a base course must be of extremely high quality and its construction must be done carefully. **3. Sub Base:** It is a layer of granular material provided above subgrade generally natural gravel. It is usually not provided to a subgrade of good quality. It is also called granular subbase. **4. Sub Grade:** Consists of the naturally occurring material on which the road is built, or the imported fill material used to create an embankment on which the road pavement is constructed. Subgrades are also considered layers in the pavement design, with their thickness assumed to be infinite and their material characteristics assumed to be unchanged or unmodified. Prepared subgrade is typically the top 12 inches of subgrade.  **Fig. 2: PAVEMENT LAYERS** **COMPONENTS OF ROAD CROSS SECTIONS** 1. **Carriageway or Traffic Lanes** - The part of the road intended for to travel on and that, depending on its size, may be made up of one or more lanes. - This width allows large vehicles to pass or overtake without a vehicle having to move sideways or towards the outer edge of a lane. - The lane width and the road surface condition have a substantial influence on safety and comfort of users on the roadway. So, wide lanes are generally safer and more comfortable to drive. - The table below shows the width of the different types of carriageways. --------------------------------- ---------------- **Class of Road** **Width** Single Lane 3.75 m Two Lanes, without raised curbs 7.0 m Two Lanes, with raised curbs 7.5 m Intermediate carriageway 5.5 m Multi- Lane Pavement 3.5 m per lane --------------------------------- ---------------- **TABLE 1: WIDTH OF CARRIAGEWAY** **Fig. 3: Single Lane Cross Section** 2. **Median** - A road on which traffic in one direction of travel is separated from that in the opposite direction is called a divided highway and the dividing strip in the middle of the roadway is known as median. - The vital functions of the median are: I. To reduce the chances of head on collision between the vehicles moving in opposite directions. II. To reduce the glaring effect of the head light of the vehicle moving in opposite direction in another lane. III. At intersections, these provide a refuge for the cross-traffic. **Fig. 3: Double Lane Cross Section with Median** - Two types of Medians I. **Raised medians** are elevated sections of roadway designed to separate opposing traffic lanes, improving safety by preventing head-on collisions and enhancing aesthetics with landscaping. They are commonly used on high-traffic roads where physical separation is important. II. **Depressed medians** are sunken below the road surface and primarily serve practical functions such as managing water runoff and providing space for utilities. They are often used in urban settings where drainage and utility access are crucial. III. **Flushed Medians** are painted diagonal lines down the center of the roads. It can also be used as a space or to provide room for vehicles waiting to turn left or right. It is commonly used on urban setting. It can be used on free-ways but with a median barrier. - **Median Safety Barrier** is a structure placed within the median to physically prevent vehicles from crossing into oncoming traffic. These are commonly made of concrete, steel, or wire rope. It is designed to absorb or redirect the impact of a vehicle, preventing crossover accidents. - Dimensions of Median and Median Safety Barrier - - - - - - 3. **Road Margins** - These are the areas that are not used as a regular roadway. There are many types of Road Margins which are: I. **Shoulders** - These are the portions provided along the carriageway to provide lateral support to the pavement and for giving working space for stopped vehicles. It should be strong enough to bear the weight of a fully loaded truck even in wet conditions. The color of the shoulder should preferably be different from that of the pavement so as to be distinct. It should be rougher than the traffic lanes so that users do not use it as regular traffic lanes. a. **Structural Function:** Lateral support to pavement. b. **Traffic Function:** Recovery area for any vehicle out of control, Emergency stop, Pedestrian/bike use, Clearance to lateral obstructions. - **Types of Shoulders** I. II. III. - **Shoulder Width** - - II. **Parking Lane** - These are generally provided on urban roads to allow kerb parking Parallel parking is generally provided for safe moving of vehicle. However, angle parking is also provided. - **Types of Parking** I. II. - **Parking Lane Dimensions** - - III. **Lay Bye** - These are provided near public conveniences with guide maps to enable drivers to stop clear off the carriageway. Lay bye normally has width of 3.0 m and at least 30 m length. IV. **Cycle Track** - These are provided in urban areas where volume of cycle traffic on the road is very high. Typically for one-way travel in the same direction as adjacent traffic. Exceptions include **contra-flow lanes or two-directional** lanes in certain designs. - Dimensions of Cycle Track - - - - - - - - Types of Bicycle Lanes I. II. III. V. **Drive Way** - These are used to connect the highways with commercial establishments like fuel stations, service-stations etc. It should be away from the intersections and have a large radius, but width should be minimized to reduce the length of cross walks. VI. **Footpath** - These are provided in urban roads having heavy vehicle as well as pedestrian traffic, to provide protection to pedestrians. - Minimum Dimension - - - VII. **Guard Rail** - These are provided at the edge of the shoulder when road is constructed on fill to prevent vehicles from running off the embankment Guard stones are installed at suitable distances to provide better night visibility on the curves under head lights of vehicles - Types and Designs - VIII. **Lay Bye / Lay-By** is a designated area on the side of the road where vehicles can pull over temporarily. Allows vehicles to stop without obstructing the main flow of traffic. - Dimensions of Lay Bye - - 4. **Kerb/ Curb** - A vertical or sloping member along the edge of a pavement provided for supporting raised footpaths or central median is called Kerb. It is desirable to provide kerbs on urban roads. - Based on their function kerbs are mainly divided into three groups: I. **Low or Mountable type Kerb:** Height of this type of kerb is about 10 cm with a slope or batter to help vehicles climb the kerb easily II. **Semi-Barrier type Kerb:** Height of this type of kerb is about 15 cm with a batter of 1: 1 on the top 7.5 cm. It is provided where pedestrian traffic is high. III. **Barrier type Kerb:** Height of kerb stone is about 20 cm with a batter of 1:4. - **Functions of kerb are** I. To facilitate and control drainage. II. To strengthen and protect the pavement edge. III. To delineate the pavement edge. IV. To present a more finished appearance. V. To assist in the orderly development of the roadside. 5. **Camber or Cross Slope** - Transverse slope provided to the road to drain off rainwater from road surface is known as camber. It is measured in 1 in \'n\' or n% (e.g. 1 in 50 or 2%). Main advantage of providing camber is quick drying of pavement which in turn increases safety and subgrade protection by drainage. - Requirement of camber of a pavement depends upon: I. Type of pavement surface II. The amount of rainfall - Camber may be provided in the shape of parabolic, elliptic or straight-line shape as shown in Figure 3.11. For cement concrete pavement straight line camber is provided while parabolic and elliptic shape are preferred by fast moving vehicles because they require frequent crossing of crown line during over taking operation. - The camber of shoulder should be atleast 0.5% steeper than the cross slope of adjoining pavement, subjected to a minimum of 3.0% and a maximum value of 5.0%. - Disadvantages of excessive camber are: I. Transverse tilt of vehicle and discomfort II. Formation of cross ruts or depression III. Center line tendency of vehicles in the movement +-----------------------------------+-----------------------------------+ | **Surface Type** | **Camber** | +-----------------------------------+-----------------------------------+ | High type bituminous surfacing or | 1.7 to 2.0% | | cement concrete | | | | (1 in 60 to 1 in 50) | +-----------------------------------+-----------------------------------+ | Thin bituminous surfacing | 2.0 to 2.5% | | | | | | (1 in 50 to 1 in 40) | +-----------------------------------+-----------------------------------+ | Water bound Macadam | 2,5 to 3.0% | | | | | | (1 in 40 to 1 in 33) | +-----------------------------------+-----------------------------------+ | Earth | 3.0 to 4.0% | | | | | | (1 in 33 to 1 in 25) | +-----------------------------------+-----------------------------------+ **Table 2: Camber for different road surfaces** 6. **Drainage** - It refers to the system and methods used to manage and direct surface and subsurface water away from the roadway and its surrounding areas. - Effective drainage is essential for maintaining the safety, longevity, and structural integrity of roads by preventing water-related issues such as flooding, erosion, and damage to the road surface and subgrade. - There are two types of Drainage which are: I. **Table Drains** - These are located on the outside verges and shoulders in cutting or alongside shallow raised carriageways in flat country. It\'s a fairly basic form of drainage which is generally unsealed and essentially it provides a place for water to drain to and pool away from the road and pavement surface. II. **Catch Drains** - These are installed in locations where we might be cutting, and we have the potential for water to drain down an embarkment onto the road surface. These would be located above the road surface and help to protect the batters of the roadway. 7. **Right of Way** - The right of way is the total land acquired for the construction of the highway. The width of this acquired land is known as land width and it depends on the importance of the road and its future development. The right of way should be adequate to accommodate all the elements that make up the cross-section of the highway. **Table 3: Right of way width for different classes of Roads in (m)** **PAVEMENT SURFACE CHARACTERISTICS** ***References:*** Department of Public Works and Highways, Philippines, Highway Design Volume 4, 2015 Madeeasy, Highway Engineering, Made Easy Productions, 2022