Leveling work cannot be more accurate than the stability and integrity of each of the benchmarks which are used in the leveling circuit. The development of high accuracy in leveling instruments must be matched by a parallel development of carefully designed and carefully installed benchmarks if the desired accuracy is to be attained. This is especially important if the measurement of the long term stability of a structure or a machine is required.
A "suitable" or "appropriate" bench mark for highly accurate leveling is surprisingly difficult to attain. The desired properties of such a mark are quite simple to describe. The H5 is capable of accuracy of about +/-3 microns in 30 meters. The benchmarks used should be at least that good. Marks which are that stable are suitable for measuring the earth tide effect which has of magnitude of about +/- 2 x 10 (-7). If a network of six marks are spaced around a circle of 100 meter radius we are asking that the relative elevations of the marks not change by more than 10 microns due to any cause, long term or short term (about 1/10 part per million).
Much effort has been expended by many people on this probiem, with varying degrees of success . The difficulty lies in the fact that when looking at our world, there are many things which occur in the range of a few parts per million which must be considered.
As an example, consider establishing a set of benchmarks in a level field. The soil under foot has a thermal expansion of several parts per million per degree. It also expands and contracts with changes in water content - some soils far more than others. Diurnal and weekly temperature changes extend a meter or two below the surface. Annual changes of a degree of two can be detected at eight to ten meters below the surface. Except in unusual circumstances, the soil and the strata across a site will be neither uniform nor level so that these near-surface thermal changes will not be uniform. The simple rule then is, a benchmark attachment point should be deep - and isolated from the overlying soil. So that they act as a unit, they should all be the same depth below the natural surface. How deep is a matter of cost and difficulty, but the first rule is that eight to 10 meters is probably as deep as makes any sense in most cases.
Between 1964 and 1974 I obtained excellent results with sets of monuments in widely varying soils with hole depths of about eight meters. The marks, used with a tiltmeter, easily supported earth tide measurements and long term crustal deformation studies.
One additional advantage of having the attachment point be deep is that the local loading effect of a person or a truck does not reach it. This loading effect is especially serious when trying to work on the surface of ordinary reinforced concrete floors or sidewalks. One must carefully check each proposed mark in such circumstances because on the same floor, one spot may be stable to better than a micron, and a spot ten meters away may deflect fifty microns when a person stands on it.
These short term motions are almost always accompanied by large and unpredictable motions due to loading, rainfall, seasonal changes, etc. - so a concrete foor can seldom be relied upon for any accurate work. In general, a concrete floor is never well tied to the underlying earth. Usually a polyethylene sheet is present which guarantees that no real connection exists between the bottom of the concrete and the fairly loose layer of dirt or sand beneath the floor.
One can cut a hole in the floor and install a column or pier, usually of concrete, which provides the appearance of massive stability, but what usually results is a heavy, unstable, unreliable benchmark. There are three reasons for this. First, the column is not usually deep enough. Second, it is rarely isolated from the surrounding soil with a casing. Third, concrete is probably the worst possible material to use for a mark.
Concrete, with a bit of steel, is frequently used. There is enough steel to control cracking, but the finished unit really acts like, shrinks like, and swells like concrete without steel. Concrete, from the time it is poured, shrinks a few tens of parts per million per month for a few months and then settles down to shrink a few tens of parts per million per year for the rest of its life. Superimposed on this shrinkage (which is caused by a slow, continuing crystallization of the cement in the structure( is a length change due to changing moisture content (annual rainfall, etc.) which can easily amount to a few tens of parts per million per year
A concrete pillar three meters long can be expected to shorten 50 to 100 microns per year, and to cyclically change by 10 to 100 microns per year, even if its temperature is held absolutely constant for its entire life. The same concrete has a thermal expansion coefficient of ten or so parts per million per degree, so a three meter column will grow 30 microns with each degree of increase in temperature. The thermal changes are usually easy to sort out with insulation and a thermometer. The moisture related and long term changes are nearly impossible to isolate, and they are truly impossible to predict.
These marks were used very suceessfully in measurements of the earth tide effect at all sites . Diurnal effeets due to the monuments were below two microns in every installation.
For each mark
4" thin wall steel casing, 20'
2" galvanized pipe, 21'
Coupling, tapped plug for 2" pipe
3/4" Invar riser, each end tapped 1/2-13
1/2-13 invar studs 1-1/2" long to join riser rods
100 lb. Del Monte fine quartz sand
25 to 50 1/2" dia. x 24" steel pins, pointed on one end
To Install
Drill rig for 6" hole
Vacuum cleaner and 25' hose
1/2" dia x 24 ft mild steel drive rod
1/2" ID Electrical conduit and couplings (threaded
conduit) drive guide, 30 ft
Clamp to hold casing off bottom of hole
20 ft. tripod or guyed pole to hoist drive rod
Pulley and hoist rope - 1/4" nylon
Large pipe wrench and extension handle
Skilsaw and metal cutting blades for casing and 2" pipe
Flashlights
Misc hand tools
Gloves
This mark was developed to support a tiltmeter measurement program in the field in the 1960s. The goals were to have a design which would always result in a good stable mark, with a minimum of heavy equipment. The marks were relatively easy to install, easy to test, and they proved to be stable for years. So far as I am aware, there is no better mark design anywhere in the world today which can be applied to practically any soil condition. In addition, it is not expensive. We routinely installed three marks in one easy day with two or three men.
If I were requested to establish a set of stable benchmarks, I would use this design. The only time we ever had any trouble was at the Pearblossom site when we slowly realized that we might be driving pins for the rest of our lives. Even there, the process eventually yielded excellent marks.
At three of the sites, the marks were below the water table (we worked in wet holes) . Three sites were well above the water table. At three other sites the water table annually rose above and dropped below the attachment points.
Even at the site where the water table rose and fell, the marks stayed solid. None were located near an active, pumping, water well so we did not observe the temporary subsidence and tilting due to water pumping which is normal in such circumstances. One instrument on the edge of an oil field did routinely measure the tilting due to oil extraction related subsidence of the center of the field.
The integrity and stability of these marks over time has led me to believe that they would serve equally well as horizontal control points. The 2" pipe would need to be replaced by something stiffer - like perhaps a 6" pipe which would be some 27 times stiffer laterally.
I would probably start with a 6" pipe and a 10" thin wall casing in a 12" hole 15 to 20 feet deep. I would use 3/4" diameter pointed pins about four feet long (and a 3/4" steel drive rod) to keep the scale of things fairly constant. A suitable test for the assembly would be to use a long handled wrench and to apply about 1500 foot pounds of torque. A central invar rod would provide a vertical mark and a welded cap with a hole for the invar and a separate tooling ball could be used for the horizontal mark.