by Dirtman » Mon Jan 22, 2007 3:29 pm
Answering both of your questions here.
Is the specified slope of the pipe 0.6% or 0.12%? By what you stated in your first post, I’ll assume it’s 0.6% as 0.12% is quite flat.
Be VERY careful when dealing with percentages and decimal equivalents! They can be easily confused. 0.6% divided by 100 equals a decimal equivalent of 0.006, which is per length unit, whether feet or meters.
I will tell you from experience that if you have a grade rod that’s in feet, convert metric units to feet and use feet for all calculations. By the same token, if it’s a metric grade rod, use meters or centimeters. Trying to convert from system to system when you’re under the gun can be a disaster!
I strongly suggest you set a TBM (temporary bench mark) on something solid before you start any pipe installation. Unless the existing joint is anchored on some type of foundation, it is movable. Even if it is anchored, when you install the first joint, your elevation control is now gone!
Assuming your grade rod is in feet and tenths. Convert the invert elevation from meters to feet. 10m / 0.3048 = 32.81 feet.
To calculate rise per each pipe joint: Each pipe is 13' long (measured from the inside of the bell end to the end of the spigot end). The finished slope of the pipe will be 0.012 per foot (1.2%), so, 0.012' * 13' = 0.156' rise per joint.
The rise of each 13' joint of pipe is cumulative from the original invert elevation of 32.81 feet. At the end of the first joint, the new invert will be 32.97' (32.81 + 0.156), then 33.12' (32.97 + 0.156), etc..
Although you can calculate each joint, it’s easier to use total feet of new pipe. Multiply 0.012 by the total length of the new installed pipe. If three joints are installed, that would be 39', so 39 * 0.012 = 0.468'. Add this to 32.81 and you have a new invert elevation of 33.28, located 39' from the beginning invert elevation.
The formula here would be PL*0.012+32.81 = New Invert Elevation, where PL = pipe length, 0.012 = specified slope and 32.81 = the beginning invert elevation.
If you have a direct elevation rod (called a “Linker rod”), at the known pipe invert, set the linker rod to 2.81 feet. As the elevation is actually 32.81 feet, remember to add the 30' you don’t see! Now you can either set the rod AT the new invert elevation you want and read the existing elevation directly or set the rod tape TO the elevation you want and raise or lower the pipe.
If you have a standard level rod (Philadelphia, San Francisco, etc), where zero always begins at the bottom of the rod, first determine the height of the instrument (HI).
How do you determine the HI? Set up and level the instrument (laser, transit, etc.). Place the rod on a known elevation, in this case, the existing pipe invert. Look through the instrument (or raise the laser detector until it emits a continuous tone) and record the rod reading. Add to this the known elevation. If the rod reading is 8.22', add 32.81 (the invert elevation) to obtain the height of the instrument, which would be 41.03 feet. Use this HI for all elevation calculations.
To determine the elevation of any subsequent point, subtract the rod reading at that point from the HI. This is the elevation of that point.
To determine a rod reading for ‘x’ elevation, subtract the elevation you need from the HI. A good example would be the invert elevation at the end of the first joint, which is 32.97. The HI of 41.03 - 32.97 = 8.06, the rod reading you will have when the pipe is on grade.
It’s late and I’m tired, so my answer could be more confusing than it should be but if you have any questions on what can be a confusing subject for a novice grade checker, feel free to ask.
George