Right on Mr. Ward. Almost always, as soon as the blame game starts, everyone (in management anyway) starts ducking for cover and throwing stones at the other players, never even considering the absurdity of the expectation behind the charges.
Resection is essentially part of a traverse, lacking a beginning and a closing angle from existing control. If it needs to be as tight or nearly as tight as the control from which it is established, I prefer to begin set up on and backsighting existing control, traverse through the new point with distances observed forward and back (averaged this compensates for C&R and any collimation error which may exist in the inst for better vertical), and then close out occupying and foresighting existing control.
Resection always seems to add more error than actually traversing through. Even with your instrument in recent adjustment and your instruments C&R correction turned on, vertical will usually be your weakest component in a resection.
Coming off good control, you can get good results, just not quite as good as the contol you're working from and not quite as good as traversing through.
This a 1.5 Billion Dollar project... between TXDOT and the various contractors there will be plenty of micro-managing going on. Plus I'm sure the specifications were there before anybody bid on this project and the contractor(s) were awarded this huge project. As a tax-payer I want this project done to the specifications, as a Land Surveyor and a pretty good construction guy I can understand their position. If you can't achieve the stated specifications it is a lot better to negotiate a good position before you start moving dirt and placing concrete instead of the chaos that will inevitably occur.
This really depends on the geometry of each of the individual setups (the four random points) as well as the geometry of the known points.
A perfect application for the simulation feature in some least squares programs. You could put in the geometry, and compare the resulting error ellipses for each solution.
RB-
I don't do roadwork but have done XYZ pre-stressed cast concrete lay out with simple differential levelling and plumb bob & chain.
With a 400' equilateral (sort of) triangle as a base, 't'would seem fairly straightforward without the 'gizmo' approach.
But I may be revealing luddite tendencies.
YOS
Derek
Oh we are well in TxDoTs spec. This is internally, trying to meet our yield projections.
Are you doing an instrument measure up for these 3 point resections or is the HI=0.00 ?
The way to get some real vertical precision out of the free stationing is to go with HI=0.00 and let the instrument "occupy" a one-time, never to be occupied ever again position.
In this way I have done half-mile trig level runs with free stationing and hit the benchmarks within your hundredth, not just an absolute difference but 2 sigma in the least squares.
Seems to me that the wheel has been reinvented many times over already. 0.01' just isn't practical.
Specifications Specifications Specifications
Started typing up a response thinking you were speaking of Asphalt Concrete. Now I see you were talking about PCC. It more than likely has nothing to do with the surveyor.
This is why PCC should be paid for by plan quantity, not load slips based upon weight.
How was the PCC paid for? This is very important. The contractor may have bid the job knowing they were only getting paid plan quantity. They usually bid a little higher, knowing they might have an over-run that will not be paid by the agency.
However they do have to pay the concrete batch plant and the truckers.....First they will try to get a change order from the agency, once they are shot down there, they will approach the batch plant, then after getting shot down, they will approach the surveyor...?
I would be willing to lay a bet down that the PCC is paid for by the cubic yard. In this case, the contractor has to determine which item has the most profit....PCC or base grade? Then hold grade either high or low within the grade tolerance to adjust quantities. If they were to run .04' high on base grade, and build the PCC to the plan profile, they could cut the PCC quantities significantly. In the past doing something like this was impossible due to that pesky .04', however with the GPS/Laser controlled milling machines and graders, I think a smart contractor could take advantage of loose specifications on a large quantity project.
Another issue I would look at.....is the PCC item designated as a final pay item in the specifications? These are all things that the contractor takes into consideration when bidding a project. Many times they still cry foul, even though it was already taken into account in the bid price. JRL
Your only recourse is to core some of the completed areas. I suspect there will be cores that are 1/8th over (if not more), but there will also be an equal or greater number of cores than will go the other direction (less than required concrete depth).
This ususally shuts them up and puts the focus back where it belongs....calibration of the scales back at the batch plant.
Roadhand, you could actually do the academic math to predict the differences. That dx, dy,dz math has long gone from my daily use.
You have been in the field long enough to know what is really obtainable.
The atmospherics and geology can play havoc as concrete has expansion like steel.
Instruments could all be different tolerances and may or may not be in good adjustment.
Time to take all the white collar bean counters out into the field to get their shoes dusty and then take them to your pub for a beer to tell them that every one is working to their best.
The million dollar eighths savings are likely bean counter bonuses.....
RADU
Are they talking about the thickness tolerance being .01 or the finish grade being .01?
Typically, after a job is paved, it will be sample cored and the thicknesses measured. If it is shy of the thickness, there may be a penalty involved. Do they have machine control pavers? In my concrete experience, the subgrade is cut by machine control, every thing else is laid down according to the plan thickness plus compaction factor. By the time the stone and permeable asphalt is down, it's a guessing game. When you set stringline for the concrete paver, you may end up with concrete that is too thick or too thin. The concrete crew may end up adjusting your stringline. You'll be lucky if you get 1 inch tolerance. Good luck with the .01 - that's a fantasy. P.S. Don't forget to allow for final ride-ability milling.
Amen! 1/8" is an unachievable tolerance for normal construction work.
roadhand
best of luck with that, and please keep us up to date on how this shakes out
