QUANTITY ESTIMATION & MASS-HAUL DIAGRAMS
•The highway design process often requires the designer to consider a number of constraints before arriving at a balanced design. In most cases, the balanced design is only achieved after several iterations have been completed. Political, social, environmental and economical forces will interplay when a project is in the design stage.
•The effectiveness of a design alignment is measured using a number of methods.
•One measurement tool which assists the designer to estimate the economic implications of a project is known as cost estimation.
•Estimates can be calculated using various levels of detail.
•A simple model may have global estimates for road construction (e.g. $1.2 million / km of two-lane-two-way highway, 110 km/h design speed), and other components of the project ($5 million per fully directional interchange).This model may be most appropriate during the preliminary design process.
•As a project progresses, a more detailed estimate is normally required.
Sample of Items to Quantify
•Right of way acquisition
•In-situ material excavation
•Erosion Control material
•Maintenance of traffic during construction
•One of the costliest items in the previous list is the excavation (and placement) of existing material.Cut material, if of adequate quality, is used to fill those locations where the existing elevation of the ground is below the design grade.
•The estimation of the quantity of material which must be excavated (cut), and the quantity of material required to raise the elevation of the existing ground (fill), is very useful in the development of a cost estimate for a project.
•The final cross-section and alignment characteristics of the roadway are often adjusted to bring the two quantities to unity.Stockpiling extra material or importing additional material are additional costs which can be avoided if the cut and fill quantities are in balance.
•Most of the other quantities listed previously are tied directly to the cut an fill estimates.The compilation of their estimates and combination with unit costs, enables the designer to arrive at an overall cost for the project.
•When construction projects are publicly tendered, quantity estimates are often part of the information provided to contractors who bid to complete the work.Quantity estimation by computer has provided marginal improvements to the accuracy of estimates
•However, its greater benefit has been to give designers the ability to consider multiple design alternatives in more detail which, in turn, enables them to arrive at better design.
The cross-section of the fill at three points, 1, 2, and 3,is shown.The areas of the fill cross-sections, A1, A2, and A3, are also shown.The “cross sectional” area of the amount of fill would be measured at standard points,or “station”, along the alignment.The distance between sections is normally constant.The volume of fill material between Section 1 and Section 2 can be calculated using the “Average-End-Area” method as represented by the formula:
V = ((A1 + A2)/2)*L
V = volume of material between Stations 1 and 2 with fill end-areas A1 and A2
L = segment length (in meters)
V = volume (m3), and A1 and A2 are end areas (m2)
Shrink and Swell
Excavated in-situ material used in embankments or fills may swell or shrink.It is important to determine the properties of a material in order to evaluate how these properties will affect the earthwork volume estimates.
•When earth is excavated and hauled for use as compacted fill, its volume once compacted 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.
•A shrinkage factor, or compaction factor, of 10 to 15 percent is common.Dense in-situ materials may be somewhat lower.
•Shrinkage of 40 to 50 percent for some types of material is possible.These high shrinkage values also account for other factors, such as loss of material in the hauling process and loss of material at the toe of a slope.Shrinkage should not include settlement of fills due to consolidation.
•When rock is excavated and placed in a fill, the material will occupy a larger volume due to the air voids that are introduced into the material.
•This increase is called ‘swell’ or ‘bulking’.
•Swell can be as high as 40 percent.
•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
•A Mass-haul diagram is a useful method to graphically represent the amount of material that will be cut and used for fill on any earthwork job, particularly for highway or railway projects.
•It illustrates 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.
•Excavation (E) Excavation refers to any material removed from its original location.Excavation is commonly split into two categories -ordinary material (OM) and rock.Both OM and rock are generally calculated by volume (cubic yard or meter) and costedin this manner; that is, in $/yd3or $/m3.These two quantities are normally handled as a separate cost items because rock is substantially more expensive to excavate.
•Occasionally, the estimator may not classify the material separately and attribute a cost to the total volume of material.This might be the case where geotechnical information is not complete enough to accurately assess the depth of bedrock, or if rock in the area can be easily excavated without blasting.
•Free haul (F) When material is excavated, construction contractors will normally move it over an established distance free of charge.This distance is the “free haul”, and is normally agreed upon between the client and builder.
•Overhaul (O) Overhaul is defined as the distance over which the excavated material must be hauled, less the free haul distance.The cost for overhaul is normally specified by the contractor.This rate is normally given in $/yd3/unit-distance or $/m3/unit-distance .
•Borrow (B) Borrow refers to the fill material that must be brought to the proposed highway site from outside the highway corridor. Borrow does not include the material that is excavated on site for use as fill . The borrow cost is normally given $/yd3or $/m3, and this rate normally includes the cost of excavating and transporting borrow.
•Limit of Economic Overhaul (L) The limit of economic overhaul is a distance beyond which it is not economic to overhaul.At this point, the cost to excavate and overhaul on-site material is equal to the cost of excavating and delivering borrowed material.
•Waste (W) Waste is the excavated material that cannot be used for fill on the project site.Normally, the designer will try to roughly balance the amount of cut and fill required on a project at the design stage, so that the amount of fill that must be transported in (borrow) 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.This occurs where it is necessary to haul excavated material long distances to use as fill.
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