Enlightenme:
I think you are now headed in the right direction. Map these pipe trenches out on a facilities plot plan, so that the plant maintenance people and their subcontractors have some idea when they are working in restricted ground loading area. Then you do some special design and engineering at that time, depending upon the equipment to be used, loads to be hauled or lifted, and other pertinent circumstances at the moment. You should probably be able to develop some general rules and loadings for them for everyday maintenance work, but then must also set some limits where extra attention must be brought into play before work starts. This is an ongoing development process as you and your client gather more info. on existing plant conditions. I would want to study the reports from any earlier pipe failures, and their causes.
Regarding the 6" & 12" pavement and its bearing capacity, you need to enlist the help of a Pavement and GeoTech Engineer. This will be a function of the type of pavement, its base and sub-base, and the compaction of the soils below these. This is a double edged sword at pipe trenches; where proper pipe installation involves the pipe bedding, and compaction of the soil around and over the pipe; on the one hand this properly supports the pipe to take max. loads, but it also provides proper support so the pavement can perform in distributing wheel loads. This might involve some testing to get a handle on existing conditions; that GeoTech Eng. needs something to hang his hat on too, in giving you his best advice.
You should probably not be extrapolating or interpolating from graphs and charts, you said API explicitly says this, and that was the point of my first paragraph, in the earlier post. There are methods for doing this pipe loading design, there is literature out there, but it’s been a long time since I’ve done this kind of design problem. You can do a reasonable engineering design for a specific condition, and it may involve added timber mats to distribute loads during some work, for example. At locations which are always getting heavy loading, the fill around pipes could actually be a lean concrete/fly ash mix which supports the pipe better than soils. It is much more difficult to do this problem in reverse, assuming some perfect conditions and arriving at a max. allowable loading, which may never occur, because you are guessing at the variables. Otherwise, all you can say is what the API graphs and their footnotes allow you to say; and that all the pipe installation, backfilling and paving must have been done to industry standards and specs. The pipe manuf’ers. have good literature on their particular pipe. Otherwise, any imperfections can render the whole process meaningless. You shouldn’t be doing this work by yourself if you don’t have the proper experience, and you must learn how to do this engineering problem to render the proper service to your client. How are the allowable loading charts/graphs developed given all the variables, then you can use the charts with confidence, or extend them, or know when another calc. method is required because some variable has exceeded some limit.