how to understand ground levels

[jdumaurier]

Hi all,

Desperately needed help on understanding laser level and datum? Could someone please help me understand finish floor level.

I’m currently on a job at the moment where the finished floor level is 99.831 and I have to build a retaining wall which has a finish floor level of 99.600, this is made up with .300mm of grasscrete and then a .850mm deep concrete slap, how do I set up the laser and story stick?

Best Jonathan

[Stock]

Do you have a tbm?

What is your hard core level? do you have a dig level...........

[jdumaurier]

hi stock,

to be honest, I’m a finishing carpenter so ground works are not my forte... I’m currently doing a renovation, which has a FFL of 99.839, I will be building an extension on to the renovation which will have the same FFL

The 99.839 is in the architects drawing, now people have been telling me to put TBM on a wall at say a meter, but still confuses me..

[Stock]

OK so what you need to do is transfer the finished floor level outside and this will become your TBM.

According to your levels that will be 99839.

What you will do in future is set up your tripod and level and zero it, then every thing else will be below this.

So for argument sake the reading on the staff is 2350 then if you are digging down 230mm for top of grasscrete you move the receiver up the staff until it reads 2580.
Now the retaining wall is made up of 300mm grasscrete + 850 mm of a slab.
So 300+850= 1150mm
For this set up the tripod and level take a reading on the TBM. Then add 1150mm to this reading from above we take it our reading is the same 2350mm so reset the receiver to 3500mm this should be the level for the top of the hardcore or dig level if it poured on the ground.

[Stock]

Hmm made a hash of this so..............

[Vinpetrol]

Hi JD

Your quite right asking as I wouldn't expect the joiners I work with to know how best to work this out.
Stock may well of already sorted you out with this but if not I would recommend that you make your own datum and call it zero. This will keep it more simple.
Start with a given point that you know the level off . It may be a datum on site somewhere permanent . In your paperwork you will have a measurement for this and a measurement for your FFL (finished floor level) work out the difference between the two.
Call the datum zero.
if the difference between the two was say for example+ 254 mm
you then know your floor level is datum + 254 mm

if your wall level is 150 mm below FFL
you then know it is datum +254 mm minus 150mm

if you have 300mm of concrete below wall level
you then know it is datum +254 minus 150 minus 300.

if your using a rotating laser , set up the rotating laser, put your staff on your datum and set the receiver at the height of the laser. If you the wanted FFL you would mark the point on your staff that the receiver was set at and then measure 254 mm down the staff and re set the receiver at this height. The receiver is now set at FFL.
If you wanted a level set at 300 mm below FFL you then just measure up the staff 300 mm from the receiver and then move the receiver to this point it is now set at 300 mm below FFL

i hope this helps

vin