A large construction firm has requested a quote for building monitoring. They are asking for a vertical accuracy of .005' (1/16? is what they specified). The situation is this:
Contractor-A built a row of three industrial building that I will call Buildings 1, 2, & 3 respectively. Each one is about 100'x300' and probably eight stories tall (I am visiting the site today to confirm height). The buildings are spaced 92' apart. Building-2 is subsiding and Contractor-Z, my potential client, has been hired to drive sheet piles on each side of Building-2, within the 92' spaces. The original surveyors are still monitoring and will continue to do so. I would simply be Contractor-B's CYA surveyor to help prove that the process of driving sheet piles does not increase the subsidence. I only need to be onsite when they are pile driving.
Mostly because of this forum, I convinced my employer to purchase Star*Net 9. I've been waiting for a an opportunity like this to run the preanalysis routine. However, our most experienced surveyor mentioned to me that he did a similar project with an optical level and a micrometer.
I will update this post on the specific site conditions later today.?ÿ I know that it is flat, but I'm not sure if I can get much more than 200' from any monitoring points.
Question: What tools and procedure would you use to verify that a building did or did not move .005' vertically?
If you had something monolithic within 1000?? I would think a digital level would be the best tool for the job.
Question: What tools and procedure would you use to verify that a building did or did not move .005' vertically?
Digital level.?ÿ Make sure your benchmarks are well outside the zone of influence.?ÿ?ÿ
Have you ever worked to such tolerances? It's a specialty field.
What confidence level is 0.005? Convert 0.005 at 95% to 0.0025 ft or 0.75 mm std error or rms and it's scary small. And 95% means you will be wrong 1 time in 20.
The first concern is finding or establishing reference elevations that won't move by a measurable fraction of 0.005 ft.?ÿ That takes a good monument.?ÿ ?ÿYou can hope the other crew has some in place, because any new one could settle a bit in the short term.?ÿ
Do you need to consider temperature changes in different materials? If there is much difference between your references and monitoring heights, a big change in weather could require correction.
I hope John Hamilton comments, because I seem to remember his dam monitoring values having a noise level not much better than that, although that might have included some seasonal variations.
Stainless steel rod to refusal with greased sleeve based on local conditions. Double run digital level using NGS procedures by Curt Smith.
The NGS site has all the software and instructions you need. It's worth a visit...
We have a number of regular jobs similar to this. Use three offsite datum points, so if there is any datum movement you can work out which one is the problem.
As Bill93 highlights there are lots of potential problems. Use invar staves (ideally 2, which avoids settlement problems at the instrument).
Keep all the sights as equal as possible - this means intermediate sights as well, even though you need more set ups.
If possible make all the change points on site permanent, use the same sequence each time.
If possible do the work at the same time of the day - just in case the whole area has diurnal movement. (We have one site about 1km. across where the diurnal movement can be up to 2 or 3mm., but it happens within a couple of hours of sunrise/sunset.
Digital level, heck yes. But not just any digital level. You are going to need a high grade model, and probably an invar rod.?ÿIMO, the 1/16" spec is not realistic for the application. A 1/8" spec would be sufficient and a lot easier to reliably achieve.?ÿ Nobody is going to believe you until movement passes 1/4" anyway.
Sure, work of this caliber was done before there were digital levels. Why would you want to?
Thanks for the input. I was too optimistic in thinking that I would get to walk the site today. I am at the mercy of three site managers but I was able to schedule a mandatory safety training session.
I'm going to review the NGS procedures and speak with my local Trimble representative, Duncan-Parnell, about digital level options.
Regardless of the quality of the existing control, I will plan on driving stainless to refusal. I'll use their control as an additional check.?ÿ
I've never done a monitoring project to such a small tolerance. I am going to provide them with quotes for 1/16",1/8",and 1/4" and see what happens.?ÿ
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Just remember that monitoring projects need to start way before the site work begins.?ÿ From my deformation monitoring experience you are going to want to set physical measurement points on the subject buildings. ?ÿI would suggest that you set stainless steel leveling studs.?ÿ You can find a plethora of monitoring specific stuff online, goecke dot de is a good source.?ÿ From my experience, most of it comes from Europe so you will need some lag time for getting it here.?ÿ Once you have set everything up, you should monitor the site for a period of prior any work being done on site.?ÿ Just measuring it once is a bad idea.?ÿ As others have said, you are going to want a 1st order level, digital preferred, and invar rods.?ÿ Just remember, this is not cheap work and your liability is high.
Good luck
If you are using trimble get the one piece invar rod only. Screw it up exactly the same way every time and you'll be golden.?
Yes, as stated above - run your procedure several times before any site activity to assure you re getting the right results - then keep to the same procedure - Balance every turn - setup in the same locations if you can - for one of my jobs, I ran up and down on the same points - double double run and checked the differences
Revisiting.....
A invar levelling rod is going to cost around $2k. A digital level worthy of an invar rod is going to be $8k. If one is not going to be doing this kind of work on a regular basis that's a cost that will be tough to recoup.?ÿ A 1/8" spec could be achieved with a common rod and level setup and a little extra care.?ÿ If the client is going to insist on the 1/16" spec, which?ÿ I think probably exceeds the needs of the job, I'd have to be very thoughtful about whether I was willing to participate.?ÿ
Probably the spec writer is thinking of reading the rod (or his tape measure) to the half-hundreth (1/16") and mistaking that for +/-0.005' accuracy in elevations. Quoting of the spec in fractions of inches is a clue to the sophistication of the writer.?ÿ ?ÿ
Stainless steel rod to refusal with greased sleeve based on local conditions.
A minimum of three that will reliably last the duration of the project plus the duration of any ensuing litigation.
I fancy an N3. No misgivings with that and hard copy hand written field notes make it easy for review by others if need be. Peg test very often ?ÿ
Bslanced sights, really good rods, Invar not a big issue working in close quarters, but ?ÿtrue rod; not any old construction Philly rod.?ÿ
Many years ago we purchased an N3 and NA2, with two pairs of 3m. invar staves, secondhand from the engineer's department of a major city, when they re-organised and upgraded.
Over the last 25 years the investment has paid back more than a hundredfold. Digital is nice, but for precise monitoring you don't actually need it. What you do need is care, taking your time, thinking everything through, working to be as repeatable as possible every time. If you stop for coffee at 10:00 am. then EVERY time you do the work stop for coffee at 10:00 am. It becomes a routine; then you don't make mistakes - if you do then since everytime is the same pattern it is easy to find out where things went wrong. More often, things haven't gone wrong but something external has happened. You can then isolate that and investigate what has happened to affect the results.
Remember, when you are monitoring you need to record to an order of magnitude greater than the amount which is significant - ie if a movement of 0.1 inch is significant to the structure being monitored then you should be recording to to 0.01 inch. That way there is no doubt about the relevance of the results you are producing.
