Interesting thinks determined from an old bench loop

The topic of a couple of recent posts here have dwelt on the stability of benchmarks, particularly those in trees and utility poles.?ÿ This brought to mind a project I worked on in the early '90s when I was with the highway department.?ÿ I'd like to share the insight I gleaned from that project.

Back in about 1939, and in conjunction with a SH11 & US64 road project, state surveyors performed a leveling loop between Cherokee and Alva, OK.?ÿ It began at an existing BM established near the tracks at Cherokee, OK in 1928 by the US Geological Survey and ran almost 20 miles west to Alva, OK. along the highway route.?ÿ The lay of the land is flat and agricultural, even to this day.?ÿ Not surprisingly the crews utilized just about anything they could when establishing TBMs along the route, including "spikes in power poles".?ÿ Which BTW is just about all there was along the route at the time.?ÿ Drainage structures were yet to be constructed on this late '30s project.

We had the original level notes.?ÿ Performed by several different crews over a period of time these notes were structured and eloquent.?ÿ Their process included FS-BS / BS-FS at each turn with notes recording which rod was used on which TBM.?ÿ Although not exclusively, a good portion of the loop was performed in 3-wire fashion.?ÿ I don't remember the exact raw closure when they made it back to the original BM, but it was impressive; somewhere in the >0.05'.?ÿ A lot of time and effort went into this.?ÿ The entire loop was "adjusted" in a fashion I don't care for, but that's just personal preference.

Due to the ravages of time and a number of much shorter projects along the route conflicting elevations were starting to be discovered.?ÿ After the 1940 construction of the route the elevations were transferred to more permanent Xs on concrete headwalls and the like.?ÿ Notes indicated the subsequent projects recovered or used few, if any, of the "original" TBMs from 1939.?ÿ Incidental work over the years indicated there could be as much as 0.25' of "slop" floating around out there.?ÿ On a cold winter day I took several crews and our new digital levels up there to re-establish some 'good' vertical control.

We actually recovered the original bronze tablet from which this all started although the RR had been abandoned.?ÿ It appeared to be in good shape.?ÿ We also recovered three or four "spikes" in poles that could have possibly been from the original loop.?ÿ They at least fell in the proper place according to station & offset.?ÿ The only surviving TBMs that could be determined to be from the original loop were several chiseled Xs on the concrete bases of windmills and water wells that were well away from the roadbed and R/W.?ÿ There also was one RR spike in a large elm on some property in the little hamlet of Ingersoll that was from the original loop.?ÿ The trunk had apparently been blazed when the BM was established.?ÿ Several subsequent projects had utilized this for vertical control and I can only imagine that periodic "maintenance" was the only reason this BM was recovered as the bark had crept and nearly swallowed the spike.

We completed the new loop and had a raw closure of somewhere in the neighborhood of 25 mm.?ÿ Given the number of turns and the conditions I thought this was pretty amazing considering it was one of the first times I had used digital levels.?ÿ On a side note; the poles, to this day, are on the south side of the road bed.?ÿ It was winter with low sun and the level was always looking into the sun at the rod on the north (shaded) side of the pole.?ÿ The level didn't want to cooperate under those conditions.?ÿ We rectified this with the use of an old bathroom mirror we kept in the truck which was usually used to illuminate inverts of manholes.?ÿ We stationed a "mirror man" with the rod man to reflect the low sun onto the shaded rod.?ÿ It helped a lot.

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So basically what we discovered was that while the original loop was some really quality work, the TBMs set along the way fell prey to weather and time.?ÿ We had two spikes in poles that were probably from the original loop.?ÿ Both of them indicated elevations that were lower than when established.?ÿ We also had a number of nails in poles that had been set during a 1957-1970 period.?ÿ All of those also indicated elevations lower than when they had been established, some in the neighborhood of 0.25'.?ÿ Take away from this project:?ÿ Nails or spikes in utility poles probably have a tendency to settle and fall victim to gravity.?ÿ Not to mention the fact they can get damaged and replaced regularly.

Sadly, all of the Xs on the drainage structures set after the 1940 construction had been destroyed over the years. The structures had been either lengthened or enlarged in about 1970.?ÿ Even though the Xs were a good idea at the time, they were compromised by the practice of providing a wider 'clear roadway' distance and drainage capacity criteria.

The tree actually fared fairly well.?ÿ I don't remember exactly, but we determined its elevation to be >0.10' from when it had been originally recorded.?ÿ It was difficult to know for sure since the head was all but covered up.

And the 'winner' of the benchmarks that seemed to stand the test of time were the Xs that had been chiseled on the concrete bases of the windmills and water wells, far away from the roadbed.?ÿ It was difficult to be "exact" due to the closure error of the new loop and old, but a couple of them were within a few hundredths of their reported elevation from 1939.

What I determined was:?ÿ Avoid setting spikes in poles.?ÿ Trees are probably OK, but they have a tendency to "swallow" things.?ÿ Stay away from structures too close to the roadbed and set BMs in or on things that are inherently stable and away from incidental construction.

Thanks for listening.?ÿ 😉