EXCAVATION AND BACKFILL WORKS

Before a new structure is built earthworks in the form of excavation is needed to be able to lay its foundations from the ground.

Here's a sample calculator to get the volume of soil to be unearthed with sample estimate on productivity rate on how much it may cost corresponding to the volume to be excavated.

An estimate of excavation will help us determine if there's a need of securing backfills in terms of soil or aggregates if unearthed soil is not sufficient to cover the backfill later.

Earthworks is essential when it comes to preparation of Bill of Materials and Cost estimate in our projects to be submitted with the complete working drawings when processing building permits.

Formula used is simply the width x length x height to get its volume.

Sample Computation:
➤ Footing size is 1.50m x 1.50m
➤ depth of footing: 1.50m + 0.10 for gravel bed (total of 1.60m)
➤ thickness of footing: 0.25m

➤ size of column: 0.25m x 0.35m 
➤ no. of footings=12 sets 

1. Compute for excavation and backfill

excavation    
     1.50m x 1.50m x 1.60m x 12 = ✔43.2cu.m

backfill

column volume: 

    0.25m x 0.35m x 1.25m = ✔ 0.109 cu.m say ✔0.10cu.m

footing volume: 

    0.25m x 1.50m x 1.50m = ✔ 0.5625 cu.m say ✔0.56cu.m

from total excavated earlier deduct the following:

● volume of footing = 0.25 x 1.5 x 1.5 x 12 sets

● volume of column = 0.25 x 0.35 x (1.5 - 0.25) x 12 sets

∴ 43.2cu.m - {[(1.50m x 1.50m x 0.25m)+0.10] x 12} = ✔35.25cu.m

Backfill have the same process but this time we deduct the volume of poured concrete (thickness of footing + volume of column) and what remains will be the volume for filling.

2. Excavation with added formworks

Some engineers would include additional allowance of 0.25m just around the size of footing being excavated (perimeter) or its four sides.

To compare from above sample computation

excavation    

    where footing: 1.5m + 2(0.25) = 2m length on one side with the same width

then add the two remaining sides to get the total volume.

or

    [(1.5m x 1.5m x 1.6m) + 4(0.25 x 1.5 x 1.50)] x 12 = ✔ 70.20cu.m

backfill (less footing volume & column)
     70.2cu.m - {[(1.5m x 1.50m x 0.25m)]+0.10 x 12} = ✔ 62.14cu.m

3. Excavation with added working space

If the footing is too deep and needs some space for proper adjustments of form-works and compacting during pouring of concrete then you may add on both sides/all sides at least 24" or 600mm for ease of movement.

excavation

4. Excavation Costs

Will vary on location, type of  equipment and rental costs, minimum daily rates or for some construction productivity rates. 

For the sake of sample computation if done by hand on structural excavation, we will be using the 6MH/cu.m

productivity_ratio_table

Given:

➤ 12 sets of Footing excavation

➤ 1.50m 1.50m x 1.60m (excavation)

➤ say we need 2 Laborers to do the excavation

a. Volume:

1.50m x 1.50m x 1.60m x 12 = ✔ 43.2cu.m

b. Total MH (manhours needed)

volume x MH/cu.m = needed MH

∴ 43.2cu.m x 6MH/cu.m = ✔ 259.20MH

c. Number of days to finish

computed MH/total MH per day of laborers = days

Say both Laborers A & B will work 8hours a day then you have a total of 16MH/day

∴ 259.20MH / 16MH/day = ✔ 16.20days say 17days

d. Labor Cost

Again will vary on location on minimum hourly rate to daily rate.

Let's just assume of $50 hourly rate per one laborer

Labor A= $50 x 8hours ($400/day)

Labor B= $50 x 8hours ($400/day)

            total: $800/day

∴ 17days x $800 = ✔ $13,600

You may follow the same process to compute for the labor cost for structural backfill but this time using the 5MH/cu.m or simply via ratio and proportion from computed excavation cost.

Backfill cost = 5($13,600)/6

∴ BFcost = ✔ $11,300

But as suggested by other engineers, it would also be more realistic to get actual data from your Foreman and investigate the actual MH(man hour) for each designated workers and try to breakdown the scope of work and number of days of completion.

If you have a project with a scope of pouring of concrete slab (suspended) then you may try to watch the detail explanation in breaking down the cost here.

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