Monitoring - Establishing Control
Long time reader first time poster. Any help will be greatly appreciated.
I am being asked more and more to undertake structural movement monitoring for domestic basement construction. I was wondering if anyone can shine some light on their recommended processes for establishing and maintaining a reliable control network.
I have tried a number of techniques and each has brought differing results. I've found that working to sub-millimetre accuracy doesn't allow for the use of nails as my permanent/primary control due to the inaccuracies in holding a mini prism. I will always set-up with a resection and never over a station for this reason. Generally my set-up results for a resection using targets will be 0.5mm or less in E, N & H, and usually 1" or less in orientation.
However, whilst retro targets are ideal, I am concerned that they do not provide the longevity of a permanent control network. I know monitoring works are only ever temporary (at least in my applications), but I am concerned that if by chance all of my targets were to be picked off, it'd be game over. I've tried to use reflectorless points (paint marks on buildings for example) but centring over these is not accurate enough for my liking due to the lack of cross hair/defined point. It is common for me to (cheekily I know) stick retros on peoples property without their permission - usually low down on garden walls and in discreet places. I will also use items such as the base of bollards and lamp posts and actually it is extremely rare that people remove these/notice them.
I guess my question is: do you guys utilise both primary and secondary control networks for small scale domestic monitoring surveys? Or do you entirely rely upon targets in reliable positions and on the basis of good geometry and consistency?
Really appreciate your time.
Each site has its own challenges. Length of time for the monitoring has a big influence on the quantity and quality of control that I set. When I perform monitoring on mountain slopes, I will be sure to place reflecterless targets well away from the AOI.
Establishing realistic monitoring tolerances is the first and most important step. It's up to you to explain that a tolerance requirement of “Any movement” is unrealistic.
Using a high quality level and Invar rod will give better and faster results if elevation is your main concern.
Using two different colors of reflectorless targets, both cut in 1.5” squares, I put a 1/8” hole in the center of one and then stick it on the other which creates a color contrast that I can site. Sometimes I stick them on aluminum flat stock that I then drill and fasten to walls or other solid structures.
Like others here, I take advantage of Star*Net and use it to calculate the amount of redundant reading required to meet my minimum tolerance. A tightly run control network works, but I prefer resecting to reflectorless targets now that I've experimented with both.
Sub-millimeter isn't really practical, or required in most cases. I mean, it is going to be super expensive to achieve and typically structures will move that much, and more, in the normal course of a day. Architects writing specs may state that they want it that close without know WTF they are asking for. I do monitoring regularly. I tell my clients that until the numbers show things moving at least 0.02', and staying there, they haven't moved at all.
do you swear by undertaking observations in both faces to take an average?
Yes, also the best way to check your instrument and determine somethings not OK.
I once monitored a bridge that got hit by the 2009 storm in Georgia with a two-lane highway. I began in May 2012 and ended in February 2013 with 7 observations over 10 points that I drilled and glued into the bridge. I used a Leica DNA10 Digital Level and the most movement I saw over that time period was 0.026 feet. It was a steel and concrete structure and that is just the most extreme movement that I noticed over all the points.