Estimating the cost of any work items includes estimating the cost of labor, equipment, and material. The analysis of a given job requires an in-depth examination of the plans and specifications given with the bid documents, an evaluation of the soil investigation report, etc. Also, it is important to carry out a visit to the proposed site before estimating.
For earthwork estimates, the soil report and relevant geotechnical information about the soil and other subsurface conditions are usually given with the bid documents. However, the estimator or the quantity surveyor can use other sources which may be useful in developing an accurate estimate.
Swell and compaction factors
It is important to know the physical properties of soil for accurate estimation involving the cost of excavating and hauling earth. It is because of the change of volume such as swelling of soil during the construction operations.
During an earthwork operation, the soil is excavated from its natural state. Then, it is placed in a hauling unit and transported to the disposal area, where it is distributed and compacted. For example, 01 m3 of soil that is excavated from the ground may occupy 1.25 m3 after it is loosened and placed in the hauling unit. After the soil is compacted in place it may occupy 0.9 m3.
The soil to be excavated, in its undisturbed condition, is called bank measure. Similarly, after being compacted in place, the volume of the filled embankment or the pit is called the bank measure. Also, any additional requirements such as excavation supports should be added to the cost estimation of an excavation operation.
Calculating truck requirements
In excavation operations, the estimator or the quantity surveyor has to decide the optimum number of trucks required to transport excavated materials.
A simple formula can be used for these calculations based on the idea that it is desirable to have sufficient trucking capacity to ensure that the excavation equipment can operate continuously without having idling time waiting for trucks.
For example, 03 trucks will be required if it takes 10 minutes to load a truck and 20 minutes for that truck to unload and return for another load. Then, when the first truck is away, the two other trucks can be loaded.
Therefore, the number of trucks required can be calculated as;
The number of trucks obtained from Eq. 1 should always be rounded up, no matter how small the decimal. It is better to have more rather than fewer trucks so that the excavator is always kept occupied.
When estimating the material required for any work item, it is necessary to add a portion as the wastage of material used.
The quantities of material taken off are the unadjusted net amounts calculated from the drawings. Allowance for waste and spillage of this material can be made by increasing the take-off quantities or by increasing the price by a percentage factor considered reasonable.
The values of waste factors usually lie between 01 and 10 percent for different materials.
Example – Calculate the equipment and labor prices per m3 to excavate 3,000 m3 of a trench using 0.75 m3 backhoe costing LKR 6,700/day (day = 8 hrs), plus LKR 40,000 for transportation and set up charges. The expected output is 60 m3/hr with an operator and 0.5 labor at wages of LKR 400/hr and LKR 300/hr, respectively.
Example – Calculate the price of obtaining gravel from a pit located 16km from the work site, where the unit price is LKR 25/m3, using a loader with a rate of 50m3/hr (bank measure) and 12m3 trucks to transport the gravel. The hire charges for the loader and trucks are LKR 4,500/day and LKR 3,000/day respectively. The labor crew consists of one equipment operator at LKR 400/hr, two laborers at LKR 300/hr, and a truck driver at LKR 350/hr. The average speed of the dump is 20km/hr. The swell factor for gravel is 12%. Offloading time for trucks are 5 minutes.
First calculating the required number of trucks to have a balanced crew;
Secondly calculating the gravel supply price;
Example – It is required to determine the unit price for mass concrete given the following information:
Mass concrete quantity = 1,080 m3.
01 m3 of mass concrete comprises 250 kg cement; 0.55 m3 of sand (fine aggregate) and 1.02 m3 of 37.5 mm metal (coarse aggregates).
The prices of the materials are LKR 22,000/M.ton; LKR 7,500/m3 and LKR 3,500/m3 for cement, sand, and 37.5 mm metal respectively. Assume 10% wastage for all materials. The details of the crew used are shown in the table below. Assume overheads and markup 20%.
|Crew item||Number employed||Production rate||Unit rates|
|Concrete pump||1||30 m3/hr||LKR 1,500/hr|
|Concrete mixer||3||9 m3/hr/machine||LKR 1,000/hr/machine|