Different earth moving equipment have optimum output and cycle times which can be used as a basis for estimating anticipated productivity taking into account the tasks involved, task efficiency of the machine, operating efficiency and in case of excavators, the type of soil. Data for the factors to be taken into consideration can be obtained from timed observations, feedback information from people who have done similar work or are experienced or published tables contained in Manufacturer literature or reliable text books. For information sake, see Table below for approximate productivity rate per day for some labour-based tasks in road construction works. Most of these tasks are done with mechanical equipment. See Practical guide for preparation of Bill of Engineering Measurement and Evaluation (BEME) for building and roads.
Example
If a net excavation of 1500 m3 is required and assuming we are working with 1 m3 capacity bucket engaged in normal trench excavation in a clayey soil and discharging directly into an attendant haulage vehicle, determine the output and cycle time of the equipment?
Solution
A. Output of excavation equipment
Optimum output = 60 bucket loads per hour
Task efficiency factor = 0.8
Operator efficiency factor = 75% (typical value)
Anticipated output = 60 x 0.8 x 0.75 = 36 bucket loads per hour
An allowance should be made for the bulking or swell of the solid material due to the introduction of air or voids during the excavation process.
Net output allowing for 30% swell = 36 – (36 x 0.3) = say 25 buckets
B. Time to complete task
Net total excavation from bill of quantities =1500 m3
Time required to complete the task = 1500/25 = 60 hours or assuming an 8-hour day plus ½ hour maintenance time in days = 60/7.5 = say, 8 days.
C. Number of haulage vehicle required
Haulage vehicles required = 1 + (round trip of vehicle)/ (loading time of vehicle)
If the round trip is 30 minutes and loading time is 10 minutes,
Number of haulage vehicles required = 1 + (30/10) = 4
This gives a vehicle waiting overlap ensuring excavator is fully utilized which is economically viable.
