Week 9

A commonly used method for setting out drainage is the profile board method. This involves a surveying setting a series of pegs with profile boards hammered into them at specific heights above the ground. A stringline can then be attached to the profile boards. The operator can then use this stringline and a traveller to gauge the depth of the excavation by ensuring the two heights match. The below image shows an existing manhole and three new manholes set for construction. In the field, mark out the positioning of all four and set up the profile boards for the operator to use a stringline and traveller with.

Equipment Required

Process

1) Before heading into the field, complete the “invert level calculations table” by calculating the invert level for the three new manholes using the gradients and lengths provided.

2) Transfer the existing invert level and new invert level (calculated in step 1) into the “traveller and profile board calculations table”.

3) Hammer one of the long pegs into the ground and choose the direction of the drainage run.

4) Set out the other 3 pegs at the distances with the distances above (totalling 84m). Ensure that you eye in the pegs to ensure you get a straight line.

5) Set up the level at approximately the midpoint and slightly off centre to the line.

6) Take the following readings using the level and stave:

a. Backsight to the first peg (MHEX),

b. Intermediate sights to the three additional pegs (Mh1,Mh2,MH3),

c. Foresight to the first peg (MhEx).

7) Calculate the reduced levels for each of the three additional manholes and place them in the “traveller and profile boards calculations table”.

8) Calculate the depth for each of the manholes to the invert level by using the calculation below and put the results in the “traveller and profile board calculations table”.

depth = Reduced Level - Invert Level

9) Decide on a traveller length. The length of traveller must be decided, this should be long enough so that you have some length above ground level, but not too long so that the user cannot hammer in the profile board at the desired height.

Note: The idea for a traveller is so that when an operator has dug the hole to the correct depth, the traveller board and the profile board line up, this can be to one side, as illustrated below, where the bottom of the profile board and top of the traveller align.

10) Calculate the height above ground using the equation below.

11) Hammer in the profile boards for each manhole at the correct height above the ground.

12) Check all the measurements from the ground to the profile boards edge are correct and eye the final work to check it looks ok.

Helpful tips

· On larger runs, it may be necessary to use multiple traveller lengths. When this is done, the manhole where the traveller changes should have two profile boards attached and marked up appropriately to avoid confusion.

· It is often useful to mark up which part of the profile board is at the correct height, e.g. drawing an arrow to the bottom of the profile board and top of the traveller.

· Standing at either end of the drain run, profile boards should be face/back on to you. Do not fit the profile boards to the side of pegs.

Results Tables

Practical Video Guide