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How Does Alpha Value Affect The Earthwork Computation?

Introduction #

Just the other day we received an interesting query from one of our users, Ms. Izzah from WKL.

In the project file, it has three survey point groups;

  1. OGL 1 – represents the original terrain
  2. OGL 2 – represents the original terrain 
  3. ABL – represents the as-built level

So, what is the difference between these OGL 1 and OGL 2? #

Based on the explanation given, these two survey point groups represent the original terrain which means both survey point groups have the same elevation. Upon further checking from the given project file and drawing file, we do notice that the elevation is exactly similar. However, there is a slight difference between the x and y coordinates for every point between these two survey point groups as depicted in the image below. Hence, one is expecting a nearly identical As-Built earthwork computation between OGL 1 and ABL (Case 1) with OGL 2 and ABL (Case 2).

Upon comparing the results of Case 1 and Case 2 using the old version of MES software, EW3D, she was met with an astounding 83.94% difference in the earthwork computation. How could two cases with supposedly identical elevation for OGL groups display such contrasting results?

OGL 1 and ABL
(Case 1)

OGL 2 and ABL
(Case 2)
Earthwork computation using DTM Method in EW3D

Checking Earthwork Computation in MiTS 2 #

Since the Earthwork module in MiTS 2 is an upgraded version of EW3D, we are planning to compare the earthwork computation between EW3D and MiTS 2. We requested Ms Izzah to share her EW3D project file together with her drawing file to further check on this issue. 

Without changing any settings or values, we ran the EW3D project file directly in MiTS 2, expecting the results to align closely. the values we obtained were quite disparate from the EW3D results!

OGL 1 and ABL
(Case 1)

OGL 2 and ABL
(Case 2)

To check further, we use the MCIntegrator feature in MiTS 2,which allows us to integrate our project file with Civil 3D.

Checking Earthwork Computation in Civil 3D #

Civil 3D was proven to give quite a close result with MiTS 2. But this time, we also find that the Civil 3D yielded different values when compared to both MiTS 2 and EW3D.

OGL 1 and ABL
(Case 1)

OGL 2 and ABL
(Case 2)

New Feature in MiTS 2 #

In MiTS 2, there are two new features which the settings of these features will greatly affect the earthwork computation. The two features are;

  1. Use all Survey Points: Yes/No
  2. Alpha value settings

Use All Survey Points Settings #

First, we changed the Use all Survey Points to Yes for both cases. You can find this setting by going to;

Options > Project Parameters > Earthworks > Analysis > Use All Survey Points: Yes

Alpha Value Settings #

Next, we change the alpha settings for every survey point group. What is the alpha value we should use? This alpha value provides granular control for the users to include and exclude the ground levels, depending on the user’s engineering judgement on how dense the surveyor points are necessary in order for accurate cut fill calculation. Hence, there is no right or wrong alpha value settings. You can see the image below to see the comparison on the triangulation of our survey points when we use minimum and maximum alpha value. When we use minimum alpha value, the triangulation will only include the area where there are survey points. Whereas when we use maximum alpha value, it will cover a larger area.

Minimum alpha value settings

Maximum alpha value settings

Since we are using Civil 3D as our benchmark, we will set the alpha value in MiTS until we get the same number of triangles as per Civil 3D. Observe images below which highlighted the number of triangles used in Civil 3D and MiTS 2. 

Civil 3DMiTS 2

Output from Civil 3D, MiTS 2 and EW3D #

After making sure that we are comparing apple-to-apple between Civil 3D and MiTS 2, we proceed to calculate the cut/fill volume using the DTM Method. However, in EW3D, we have a limitation where the alpha value could not be adjusted. Hence, we will just use the results as it is. From the earthwork computation we got, the output we have from Civil 3D and MiTS 2 are nearly identical with only less than 0.4% discrepancy.

Civil 3DMiTS 2EW3D
OGL 1 vs ABL
Cut volume1836452.45 m³1829313.36 m³1182482.03 m³
Fill volume2403430.84 m³2394955.70 m³1151167.10 m³
Net volume– 566978.38 m³-565642.34 m³31314.93 m³
OGL 2 vs ABL
Cut volume1804151.93 m³1797422.89 m³200052.91 m³
Fill volume2492520.69 m³2484382.56 m³123438.71 m³
Net volume– 688368.76 m³-686959.67 m³76614.21 m³

When we are sure that our earthwork computation in MiTS 2 has no issue, we can now check further on our issue for Case 1 and Case 2. Table below summarise the percentage discrepancy between Case 1 and Case 2 using three different softwares. 

Discrepancy between two casesCivil 3DMiTS 2EW3D
Cut volume1.77%1.76%142.12%
Fill volume3.64%3.67%161.26%
Net volume19.34%19.37%83.94%

From these results, we can see that when we use EW3D, the percentage discrepancy between both cases has a really huge discrepancy that is unacceptable. These discrepancies have been greatly reduced in MiTS 2 and Civil 3D.

Why is there still a significant discrepancy in net volume between both cases? #

The 19% discrepancy between both cases happened because we are comparing the result of cut and fill (net volume). When subtracting two large numbers, even a small difference between them can lead to significant variation in the final outcome. If we are comparing only cut volume for both cases, it has only 1.77% difference, whereas there is about 3.64% difference for fill volume. This percentage difference is expected as both survey point groups are not 100% similar. They have different numbers of points (OGL 1 = 8335 points and OGL 2 = 6070 points) and every point does not have the exact same location (x and y coordinates) as per shown in the first image of this article. Hence, these percentage differences between two cases are expected and acceptable.

Conclusion #

In conclusion, the study on alpha value’s impact on earthwork computation revealed significant insights. Initial comparisons using EW3D showed an unacceptable 83.94% discrepancy in net volume between two cases. However, further investigation using the upgraded MiTS 2 software, alongside Civil 3D as a benchmark, and adjusting the alpha value, significantly reduced the net volume discrepancy to around 19%. This 19% can actually be greatly reduced to a smaller discrepancy by observing only cut volume (1.7%) and fill volume (3.6%) for both cases. The new alpha value feature in MiTS 2 provided better control over ground levels, enhancing cut and fill calculations. 

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