Estimation of Earth workexcavation – Types, WithNumerical ExamplesEstimation of Earth work excavation in civil engineering isthe process of calculations of expected quantity required,amount of work to be done, the number of worker and equipmentrequired and total expenditure cost to complete any project.

Different types of quantities are required to be estimatedaccordingly to their types of work. For example, the amount ofexcavation, cutting and backfilling is required to becalculated in earthwork, the amount of concrete andreinforcement in RCC work and so on.

The earthwork excavation is taken in cubic meters, so thedetail of length, breadth and height of the work that is to beexcavated is the first thing we required in estimation.

Excavation shall include throwing of excavated earth materialat least 1m to clear the edge of the excavation. Theexcavation includes setting out of work, profile, siteclearance, dewatering and removal of soil.

However, if the structure is to be dismantled beforeexcavation, they are not included within it and separateestimation and rate analysis are required.

The cubic rate estimate method is generally preferred forearthwork (cost estimate) in which the amount of estimate ismeasured in cubic meters and are directly multiplied by cubicrate to determine the cost.

Different types of earthwork excavation are performed forbuilding construction, road construction, pile and pierconstruction, irrigation and drainage purpose.

Different methods are used todetermine the estimate ofquantities in the buildingconstruction work. They are;

A) Central line methodIn this method, the total centerline length is calculated inthe first order. The centre to centre length is denoted by (l)and breadth by (b). Thus, the total length of the centre line

is given,

Total length = 2(l+b)

Quantity of earthwork excavation = T. L. * w * h (h is heightof trench and w is width of trench)

As shown in Figure, it is clear that the inside intersectedcorner repeats twice when area through length and breadth istaken. And the outside corner is completely neglected. Thus,the intersected corner compensates the area od outside cornerwhich is exactly equal giving the actual area.

Special consideration is needed when more no of cross wall isincluded within.

Total length = 2l+nb- no of junction *w/2

B) Crossing methodThis method is also similar to center line method, but thecalculation of length is different here. The total outsideperimeter is taken instead of perimeter of center line. Thus,here each corner repeats during calculations and thus they arededucted.

Total length = 2(L+B) – 4w

And total quantity of earthwork excavation = T. L. * w * h (his height of trench and w is width of trench)

Special consideration for more number of cross wall is givenby;

Total length = 2L+ n *B- no of junction including corner *w

C) Long wall and short wall methodIn this method the long wall and short wall are first of allidentified. Then their respective length is calculated. Andthe separate quantity of both long wall and short wall iscalculated and sum up at last.

Long wall length (lw) = c/c length + w/2+ w/2

Short wall length (sw) = c/c length – w/2 – w/2

So,

Total long wall quantity of excavation = nl * lw*w*h (nl is no.of long walls)

Total short wall quantity of excavation = ns*sw *w*h (ns is no.of short walls)

D) Bay methodThis method is generally use for the slab and raft foundation.The plane area of panel is calculated and then by multiplyingwith depth quantity is determine. And multiplying by each

number of panel the total quantity is estimated

And, the different types of method used to calculate theearthwork excavation in road construction (in plain area),irrigation canal and drainage are:

E) Mid section area methodHere the area of mid cross section is taken for certainportion of length of road to be excavated (also forembankment). And then are multiplied with length to get volumeof earthwork.

Quantity of excavation = Area of mid section * length of thesection considered

And,

Area of mid section = area of rectangular portion + area oftriangular portion (reference to figure shown)

= B* d + ½*s*d² + ½*s*d²

= Bd + sd²

So, quantity of excavation = (Bd + sd²) * L

Where;

d is height (depth) at mid section

B is formation width

s is side slope

F) Mean sectional area methodIn this method instead of depth of mid section, the mean areaof the two ends of the certain length portion of road to beexcavated are calculated to determine the estimate. So,

Quantity of excavation = Mean of the area * length of sectionconsidered

Area of the both cross section (front and end face) arecalculated same as above. So,

Area of front face (A1) = B*d1 + s*d1²

Area of end face (A2) = B*d2 + s*d2²

Mean area (Am) =(A1+A2) /2

So,

Quantity of earthwork excavation = Am* L

G) Prismodial formula methodThis method is the more accurate and precise approach tocalculations. Here, the Mean depth of the section iscalculated to determine the area of the midsection of theconsiderable length of road excavation along with calculationsof the area of the front end and back end of it.

And, thus by Prismodial formula quantity of earth excavationis calculated.

Quantity od earthwork excavation = L/6 (A1+A2+4*Am);

A1 = B*d1 + s*d1²

A2 = B*d2 + s*d2²

Am = B*dm+ s*dm²; dm = (d1+d2) /2

The method to be used depends upon their suitability alongwith work type and the user.

Some simple example of each for more illustration are givenbelow.

1) Estimation of Earth workexcavation calculations forbuilding

Sept 1:First of all, the necessary drawing with proper and detaildimensioning should be made ready for estimation.

The specification also must be read properly before anyestimate. If possible, trench plan drawing can be preparedseparately for easy estimation. The trench plans are requiredespecially when the formworks are to be placed beneath theground for footing purpose.

Thus, the extra width of 20-30 cm is added to the width of theactual plan structure (wall or footing foundation) for such

trench.

Step 2:Determine the most bottom width of trench required to beconstructed and according to it, the full depth is to beexcavated.

(Note: But when the height of excavation is too deep thetrench are constructed in step wise such that width increaseupwards or in slope (increasing width upward) to maintainstability.

Step 3:determine the depth of excavation needed. Differentstructures are located at different depth so it must havenoted carefully. For example, the depth of footing is higherthan the depth of the foundation beam.

Step 4:Determine the volume of excavation needed using any odtechniques of estimate the quantity.

Footing: excavation for footing is determined. If the footingsare of the same dimensions overall building, then at the firstvolume required doe one footing is calculated and thenmultiplied by its numbers.

Foundation beam: Any one of the techniques above explain canbe used. For footing, the long wall and short wall method arecomparatively easier so it is select here for calculating.

2) Estimation of Earth workexcavation calculations for simpleplain road constructionGiven data:

Formation width (B) = 12m

Side slope for cutting (s:1) = 1.5: 1

Formation road gradient is(downward) 1 in 200. (So, change oflevel per chainage should be 30/200 =0.15)

Solution:

Step 1:Find out R.L of formation level. This is done by deducting0.15 from top R.L of 0 chainages (108.6) per each chainage.

Step 2:Find out depth of cutting or banking required.

Depth of earthwork = R.L of F.L – R.L. of G.L

If the depth value is in -ve sign then cutting(excavation) isindicated and if it is +ve then filling (banking) isindicated. (Note, here we only deal with excavation)

Step 3:Determine the chainage at which the process of excavation andfilling gets change by linear interpolation techniques.

(X-20)/(0-0.5) = (150-120)/(-0.6-0.5)

On calculation value of x is 133.63 which can be taken asapprox. 134 for easier calculation.

Step 4:Calculate the quantity of earthwork to be excavated using anyone of the methods as explained above (Here, we use themidsectional method)

Calculation is as shown in table: (note only excavation isshown so -ve sign is neglected)

I hope this article on “Estimation of Earth work excavation”remains helpful for you.

Happy Learning – Civil Concept

Contributed by,

Civil Engineer – Rajan Shrestha

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