Inclinometer Installation

[geozap]

I am working on a project in which we will we be installing an inclinometer and monitoring slope movement over time. Wondering if anyone had a standard procedure for the correct installation of an inclinomter in a borehole. I have monitroed them after installation but never had an opportunity to supervise the installation. Any information would be helpful.

 

[GeoPaveTraffic]

Any moneky can install an inclinometer today, however, there is alot to installing one correctly. I'm responding at a bad time, so I'm likely to omitt several important issues, but this is what you get after a day of deck deconstruction and several glasses of wine...

1. Drill the hole as straight as possible.

2. Put about 20 feet of casing together with a bottom cap. If the inclinometer is over about 60 feet you might want to pop rivit the bottom cap in place then seal it with tape.

3. Place the 20 feet of casing in the hole, fill with water to help the buoyancy. It is a good idea to put a chain wrench on the casing to prevent loosing the casing down the hole.

4. Add casing and continue until you have at least 3 feet of stick up above the ground.

5. While placing the casing keep one set of groves perpendicular to the slope. Do not twist the casing more than you absolutly have to. Otherwise you have a twisted casing and that is not good.

6. Place some drill rods in the casing to add weight, then grout the casing in place. The grout should have approximately the same stiffness as the soil.

7. Keep the casing in place during grouting, but don't just add weight to the top, if you do you will have "wet noodle" shape to the casing. Again not good.

8. Let the grout set, then take your readings.

9. An inclinometer can also be installed to function as an observation well. Just drill holes in the casing and use sand instead of grout for backfill. I only do this for relatively shallow installations.

Hope that helps.

 

[soiledup]

Yeah, all good advice. I have read that some companies manufacturing inclo's are suggesting that a sand backfill is better at mimicking the consistency of the natural ground than faffing around with grout consistencies.

Also you can buy 'snap on' casings that click together very neatly without the use of rivets. Should water be an issue in your boreholes you can get some extreme uplift pressures acting on the inclo if it is sealed up watertight. It can be advised to fill the inclo with water so that it 'sinks', rather than bobbing out of the hole post grouting!

It really sounds alot harder than it actually is.

 

[dgillette]

Happy Monday, to the extent possible.

Regarding #9:

At least a few years ago, Sinco folks recommended against sand as backfill. Sand can make it difficult to identify the precise location of a shear. Unless you can somehow compact the sand in place, it is likely to be softer/weaker than the surrounding material, which I believe is much worse than having it stiffer. (Not even sure it's a problem to have the backfill STIFFER than the surrounding material, unless, in adition to being stiff, it is so strong that the soil yields and flows around it instead of the grout cracking and shearing.) We have some sand installations, and we periodically dust off the data from 1981 to argue about - Was the slide in the slopewash, or was it within the shaly bedrock? We'll never know for sure.

DRG

 

[Mickney]

I always use grout when backfilling. Also (a no-brainer), orient the axes (groves) of the casing in the direction you desire; typically perpendicular and parallel to the crest of the slope. Also, I sometimes incorporate centralizers to ensure equal grout on all sides of the inclinometer casing.

 

[soiledup]

Yeah centralizers is a good comment.

I've had many arguments about grout consistency. One consulting engineer was writing a dissertation on the topic and produced a spec for the grout for the driller to mix. The grout was supposed to be a similar consistency to the surrounding ground. When the mixing was complete we had a grout a bit thicker than say tar. The engineer was delighted with this mix. Perfect. Very similar to the ground conditions (stiff clays). However, he was somewhat bemused when we had to ask how we were supposed to tremme it down the borehole as it was so thick we ha to shovell it out of the mixer! in the end we watered it done to a regular consistency grout and pumped it down. What I'm saying is that there has been a lot of work on this topic with no particular conclusions. I've found both methods to give good results provided you follow the inclinometers manufacturers guidelines. So do whatever makes your life a little bit easier.