All about civil construction knowledge- PARAM VISIONS

How to calculate the RL of a temporary benchmark on-site?/ How to transfer TBM in surveying?/Transferring & finding RL of TBM on construction site.

 Let us now go through the procedure of shifting & finding the RL of a temporary benchmark on the construction site.




Full form:

  BS 👉 Backsight

  FS 👉 Foresight

  RL 👉 Reduced level

  TBM 👉 Temporary benchmark

  HI 👉 Height of instrument


Procedure:

While reading the procedure, observe the above-given drawing (for each step) to understand the concept.

1. We have to set the instrument at station A. The location of the instrument station should be such that the staff held over TBM-1 & TBM-2 should be visible.

2. Note down the BS reading over the staff held over the TBM-1 & FS over the staff positioned over the TBM-2.

3. Shift the instrument position from station A to station B. After leveling & setting the instrument, take BS reading over TBM-2 & FS reading over the TBM-3. 

In this process, the shaft held over the TBM-2 should not be lifted or moved before observing FS & BS readings.

4. If the RL of TBM-1 is not known, consider it as100. 

Now, 

HI at station A = [RL of TBM-1 + BS from station A]

RL of TBM-2 =  [HI at station A - FS from station A]

Similarly,

HI at station B = [RL of TBM-2 + BS from station B]

RL of TBM-3 =  [HI at station B - FS from station B]

In this way, you can transfer the TBM to the desired locations on the construction site.


Let us consider an example to understand the procedure.

Eg:

The following readings were taken with a leveling instrument to transfer the TBM on the site.

0.695, 1.250, 1.675, & 0.470. The instrument is shifted after 2nd reading. The RL of the 1st reading taken over TBM-1 is 100. Find the RL of TBM-2 & TBM-3.


Calculation:




As you can observe in the above drawing, the instrument is shifted after the 2nd reading.

So,  1st, 3rd readings     👉 BS   👉 0.695 & 1.675

       2nd & 4th readings 👉  FS  👉 1.250 & 0.470

Please remember: 👉 While surveying, the 1st reading becomes BS & the last reading becomes FS. 

HI at station A 

  = [RL of TBM-1 + BS from station A]

  = [100 + 0.695]

   = 100.695


1. RL of TBM-2 

=  [HI at station A - FS from station A]

= [ 100.695 - 1.250]

= 99.445


Similarly,

HI at station B 

= [RL of TBM-2 + BS from station B]

= [99.445 + 1.675]

= 101.12


2. RL of TBM-3 

=  [HI at station B - FS from station B]

= [101.12 - 0.470]

= 100.65

To understand A to Z of surveying, click here.

Thank you for going through these calculation steps. Have a good day 😄.

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