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Center line alignment influences haul cost, construction cost, and environmental cost e.
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During the reconnaissance phase and pre-construction survey the preliminary center line has been established on the ground. During that phase basic decisions regarding horizontal and vertical alignment have already been made and their effects on sfaety, construction, and environmental costs. The road design is the phase where those “field” decisions are refined, finalized and documented.
The preferred method for locating low volume sesign discussed in Section 2. The main difference between this and conventional road design methods is that with the former method, the laying out and designing of the centerline offset is done in the field by the road locator while substantial manul offsets are often required with the latter method Figure Adjustments in horizontal alignment can help reduce the potential for generating roadway sediment.
The objective in manipulating horizontal alignment is to strive to minimize roadway cuts and fills and to avoid unstable areas. When unstable or steep slopes must be traversed, adjustments in vertical alignment can minimize impacts and produce a stable road by reducing cuts and fills. The route can also be positioned on more stable ground such as ridgetops or benches. Short, steep pitches used to reach stable terrain must be matched with a surface treatment that will withstand excessive wear and reduce the potential for surface erosion.
On level ground, adequate drainage must be provided to prevent ponding and reduce subgrade saturation. This can be accomplished by establishing satety minimum grade of 2 percent and by rolling the grade. Achieving the required objectives for alignment requires that a slightly more thoughtful preliminary survey be completed than would be done for a more conventionally designed road.
There are two commonly accepted approaches for this type of survey: Figure 26 illustrates design adjustments that can be made in the field using the non-geometric design concept discussed earlier. Equipment needed for ether method may include a staff compass, two Abney levels or clinometers, fiberglass engineer’s tape 30 or 50 ma range rod, engineering field tables, notebook, maps, photos, crayons, stakes, flagging, and pencils. The gradeline or contour method establishes the location of the P-line by connecting two control points with a grade line.
A crew equipped with levels or clinometers traverses this line with tangents that follow, as closely as possible, the contours of the ground. Each section is noted and staked for mass balance calculations. Centerline stakes should be set at even 25 – and 50 – meter stations when practicable and intermediate stakes set at significant breaks in topography and at other points, such as breaks where excavation goes from cut to fill, locations of culverts, or significant obstructions.
On gentle topography with slopes less than 30 percent and grade is not a controlling factor, the centerline method may be used. Controlling tangents are connected by curves established on desgin ground.
The terrain must be gentle enough so that by rolling grades along the horizontal alignment, the vertical alignment will meet minimum requirements.
In general, this method may be less practical than the gradeline method for most forested areas. When sideslopes exceed mnual – 55 percent or when unstable slope conditions are present, it may be necessary to consider full bench construction shown in Roar Excavated material in this case must be end hauled to a safe location. Normally, the goal of the road engineer is to balance earthwork so that the volume of desibn equals the volume of cut plus any gain from bulking less any loss from shrinkage Figure Erosion rates are directly proportional to the total exposed area in cuts and fills.
Road cuts and fills tend to increase with smooth, horizontal and vertical alignment. Conversely, short vertical and horizontal tangents tend to reduce cuts and fills. Erosion rates can be expected to be lower in the latter case.
Prior to the design phase it should be clearly stated which alignment, horizontal or vertical, takes precedence. For example, if the tag line has been located at or near safeety permissible maximum grade, the vertical safwty will govern. Truck speeds in this case are governed by grade and not curvature. Therefore, horizontal alignment of the center line can follow the topography very closely in order to minimize earthwork. Self balancing sections would be achieved by shifting the template horizontally.
Roadway safety will be in jeopardy and the road shoulders will be impacted by off-tracking wheels if vehicle geometry and necessary curve widening are not considered properly. Continually eroding shoulders will become sedimentation source areas and will eventually weaken the road.
The main principle of off-tracking and hence curve widening, centers on the principle that all vehicle axles rotate about manua, common center.
Minimum curve radius is vehicle dependent and is a function of maximum cramp angle and wheelbase length see Figure These dimensions were used to develop Figures 35 to 38 for calculating curve widening in relation to curve radius and central angle.
Several different solutions to determine curve widening requirements are in use. Most mathematical solutions and their simplified versions give the maximum curve widening required.
Curve widening is a function of vehicle dimensions, curve radius, and curve length central angle. A graphical solution to the problem is provided in Figures 31 to This solution can be used for single trucks, truck-trailer combinations and vehicle overhang situations. This solution provides the maximum curve widening for a given curve radius. Steps 2 and 3.
Graphical solution for curve widening. The graphical solution for a stinger type log truck is shown in Figure Here, an arc with the bunk length L2 plus L3 is drawn with the center at C. Graphical solution for off-tracking of a stinger-type log truck. Simple, empirical curve widening formulae have been proposed by numerous authors and government agencies. A common method used in North America is: The above equations are adapted for the typical truck dimensions used in the United States and Canada.
Curve widening recommendations in Europe are given by Kuonen and Dietz et al. The approximation methods mentioned above are usually not satisfactory under difficult or critical terrain conditions. The following charts provide off-tracking for four common vehicle configurations–a single or two-axle truck, a truck-trailer combination, a stinger-type log-truck and a tractor-trailer lowboy combination.
The charts are valid for the specified vehicle dimensions and are based on the following equation Cain and Langdon, The maximum off-tracking for a given vehicle, radius and deflection angle occurs when the vehicle leaves the curve.
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Since vehicles travel both directions, the required curve widening, which consists of off-tracking OT plus safety margin 0. One half of the required curve widening should be added to the inside and one half to the outside of the curve Figure Figures 35 through 38 provide vehicle off-tracking for a given vehicle, radius, and deflection or central angle.
To this value, 0. Transition or taper length from tangent to curve vary from 9 to 18 m depending on curve radius. Recommended length of transition before and after a curve are as follows Cain and Langdon, Standard road width is 3.
Design vehicle is a stinger-type log-truck with dimensions as shown in Figure Depending on conditions, a safety margin of 0. Depending on the ballast depth, some additional shoulder width may be available for driver’s error. Taper length would be 15 m. Dowh widening guide for a two or three axle truck as a function of radius and deflection angle. The truck dimensions are as shown. Curve widening guide for a truck-trailer combination as a function of radius and deflection angle.
The dimensions are as shown. Curve widening guide for a log-truck as a function of radius and deflection angle. The tractor-trailer dimensions are as shown. Vertical alignment is often the limiting factor in road design for most forest roads. Frequently grades or tag lines are run at or near the maximum permissible grade.
Depending on road surface type, a typical logging truck can negotiate different grades. Table 16 lists maximum grades a log truck can start from. It should be noted that today’s loaded trucks are traction limited and not power limited. Once in motion they can typically negotiate steeper grades. Vertical curves or grade changes, like horizontal curves, require proper consideration to minimize earthwork, cost, and erosion dpwg.
Proper evaluation requires an analysis of vertical curve requirements based on traffic characteristics flow and safetyvehicle geometry, dseign algebraic difference of intersecting grades. Vertical curves provide the transition between an incoming grade and an outgoing grade. For convenience in design, a parabolic curve Figures 39 and 40 is used because the grade change is proportional to the horizontal distance.
The grade change is the difference between incoming grade and outgoing grade. The shorter the vertical curve can be kept, the smaller the earthwork required. Maximum grades log trucks can start on from rest Cain, They may be impractical because of construction and maintenance problems and may cause vehicles that travel in the downhill direction to lose control.
Factor for wet clutches, hydraulic torque converters, freeshaft turbines, or hydrostatic transmissions would desiign. Stopping Sight distance S: On crest curves, S is a function of desgn design speed of the road and driver’s comfort. Kuonen aafety an equation for minimal vertical curve length based on stopping distance:. Determine the minimum vertical curve length for a crest curve that satisfies the safe stopping sight distance. Vehicle clearance, axle spacing, front and rear overhang, freedom of vertical movement at articulation points are all factors to be considered in vertical curve design.
Passage through a sag curve requires careful evaluation of the dimensions as illustrated in Figure