Earthwork, Cross Sections, Mass Haul Diagram

Earthwork, Cross Sections, Mass Haul Diagram QUANTITY ESTIMATION FOR HIGHWAYS; by J. D. Innes It is generally necessary to consider the quantities of material to be excavated or generally used in the highway excavation process. This information can be useful in the development of a preliminary cost estimate for the work to be undertaken. It can also be used as part of the design process in the selection of the final cross-section and alignment characteristics of the roadway. Quantity estimates are often part of the information given to the contractors bidding to complete the work. In past years quantity estimation was a slow and tedious task which involved extensive and repetitive calculation. In more recent years, computer software has been developed whereby these calculations can be done automatically. This has improved the ability of the designers to consider various design alternatives. Computers have enabled the design process to improve. This is shown in Figure 1. The designer will generally use the information in a feedback process of improving the design. The contractor will gain an appreciation for the scale of the work to be undertaken, the equipment required, and so on. Material Quantity Estimates v___________________________v Road Designer ________________ Contractor Cross-Section and Templates The first step in determining the amount of earthwork that is required on highway project is to establish the physical characteristics of the facility. This is done by setting the outline or "cross-section template" for the highway. The template will determine the road width, the depth of the ditches, the slopes, the back slopes, guide rail locations, shoulder widths, etc. The template will show the different treatments for excavation (cut) areas and embankment (fill) areas. Both cut and fill sections are illustrated in Figure 2. Figure 2. - Original Grade Line & Template Section Fill Section Cut Section Cut/Fill or Transition Section A template section is overlaid on cross-sections of the existing grade along the proposed alignment. Typical cross-sections are shown in Figure 3. The elevation of the road surface in the template is set by the predetermined alignment profile of the highway, These cross sections allow the engineer to determine the area of cut and fill in the cross-section. The volume of cut and fill can then be calculated between successive cross-sections.` Determining Quantities The next step in estimating earthwork requirements is to calculate the amount of cut and fill. Quantities can be measured in terms of their weights, although, they are usually measured in terms of their volume. The process by which this is accomplished, whether done manually or by computer, is quite standard. The process is illustrated in Figure 4, wherein a short section of road is drawn schematically. Here a fill section as a short section of highway is shown. design cross-section (template) of the road is constant. The cross-section the fill at three points is shown - 1, 2, and 3 . The areas of the fill cross- sections are also shown - A1, A2, and A3 . These areas would be measured in square feet or square yards in FPS units or in

square metres in SI units. The distance between the fill sections is distance "1". The volume of material in this fill can be calculated using the 'Average-End-Area' method as represented by the formula: V = l x (A1 + A2)/2 where: V is the volume of material between stations 1 and 2 with fill areas A1 and A2. This is a simple, traditional, and accepted method. Figure 4. - Road Section Areas Material Swell and Shrinkage Materials used in embankments or fills that have been excavated will swell or shrink. It is important to determine how this swelling or shrinking will affect the earthwork volumes. The effect of these actions must be reflected in earthwork calculations. SHRINKAGE - When earth is excavated and hauled for use as fill, the freshly excavated material generally increases in volume due to the presence of air voids in the uncompacted material. However, when this material is compacted into place in a fill, its volume its volume is normally less than its original condition before it was excavated). This difference is defined as 'shrinkage'. The amount of shrinkage will depend on the type of material used. The amount of shrinkage will also depend on the depth of the fill. A shrinkage of 10 to 15 percent is typically found for shallow fills. Shrinkage of 40 to 50 percent for some types of material is possible. These high shrinkage values generally are used to allow for loss of material in the hauling process and loss of material at the toe of a slope. In peat or muck areas, shrinkage should not include settlement of fills due to consolidation. SWELL - when rock is excavated and placed in an embankment, the material will occupy a large volume due to the air voids that are introduced into the material. This increase is called 'swell'. Swell can amount to 30 percent or more. Swell is only taken into account when a rock fill is being considered. Swell is not considered for small amounts of loose rock or boulders placed in an embankment. Mass Diagrams are a useful method to graphically represent the amount of material that will be cut and used for fill on any earthwork job - particularly highway or railway projects. It shows the location of the mass balance points, the direction of haul, and the amount of earth to be taken to or from any location along the alignment. Mass diagrams are also extremely useful in determining the most economical distribution of material. A simplified mass diagram is shown in Figure 5. Definitions Related To Mass Diagrams 1. Excavation (E) - refers to any excavated material. There are two main categories for excavated material - ordinary material (O.M.) and rock. Both O.M. and rock are generally paid for by cubic metre in terms of excavation cost ($/m3). Rock is generally handled as a separate cost because it is substantially more expensive to excavate. 2. Free haul (F) - when material is excavated it will be moved over a certain distance free of charge. This distance is the 'free haul' and is normally specified by the contractor. 3. Overhaul (H) - is defined as and distance over which the excavated material must be hauled less the free haul distance. The rate for overhaul is normally specified by the contractor. This rate is normally given in $/m3/unit distance.

4. Borrow (B) - refers to the fill material that must be brought to the proposed highway site from outside the highway cross section. Borrow does not include the material that is excavated on site for use as fill , The borrow cost is normally given in $/m3, and this rate normally includes cost of excavating borrow. There may be surcharge for borrow if excavated from someone' s property (B>=E). 5. Economic Overhaul Limit (LE0H) - a distance beyond which it is uneconomic to overhaul. Therefore: E+(H+LOEH)=E+B

6. Limit of Economic Overhaul (LEH) - is the distance beyond which it is uneconomic to overhaul plus the free haul distance. i.e. LEH = F + LEOH Note : There may be all kinds of reasons why this economic formula may not be used. (E.g.. if you want to use all excavated material for fill for environmental reasons.

7. Waste - is the excavated material that can not be used for fill on the project site . Normally, an engineer will try to roughly balance the amount of cut and fill required on a project when at the design stage so that the amount of fill that must be hauled in or the amount of waste that must be hauled away are not excessive. Waste can also include excavated materials that are unsuitable for use as a fill because they have unacceptable engineering properties (such as peat, clays, etc.). Sometimes it is more economical to waste material and use borrow material from a borrow pit within the free haul distance . This occurs where it is necessary to haul excavated material long distances to use as fill. The cost of earthwork is the sum of the cost of the earthwork excavation the borrow, and the overhaul. All of the above information can be represented on a quantity evaluation sheet as shown in Table 1.` Links: General List of Transportation Topics Intro. to Transport, a Physical and Quantitative approach TRANSPO Introduction start page Tools page Home Index of MYNET files End to date, ams 990824