What casue the installation failure of inclinometer casing?

[tuzhuanjia]

We are installing some inclinometers deeper than 100m. We already had two expensive failures and the reasons for me are not clear and hard to accept. I was not at the site at that moment.

The 297ft inclinometer casing was installed inside a 7" steel casing. After grouted to the top, the driller started pulling out the 7" casing and it got stuck. The driller rotated the steel casing but the inclinometer casing got broken at 96ft at the junction. It was about 4 hours from the start of grouting to the failure of the inclinometer casing. the grout mxing was 35 gallon water + 25lb betonite + 94lb cement, a typical mixing for stiff soils.

One reason was given that turning the steel casing induced torsional force to the inclinometer casing through the grout.

Fluid does not transfer shear or torsional force. After about 4 hrs, can the grout transfer the torsional force to the inclinometer casing from steel casing due to cememntation of the grout? Or in fact the failure was casued by other reasons?

A test on the grout in the lab will be condcuted to check the strength growth over time. Meanwhile, would anyone give more suggestions, ideas and explanation?

 

[BigH]

4 hours, from the mix you have seems like initial set could occur. Did you use a retarder in the grout mix? Could you? Secondly, why grout all the way to the top - for a hole this deep, you might have tried a tremie - pulling the casing us as you grout through a hose to the bottom and pull the hose up too - standard tremie techniques. What does your cement mill certificates say for the initial and final set of the cement?

 

[worldofmud]

Not hugely experienced with 'deep' installations however, casing/bore verticality seems to jump to mind-was the entire hole drilled in one string of casing or was there potential for movement of the 7 inch casing within a wider hole? Was the casing to the base of the hole?

Just a thought-hope it works out, the mind boggles at a 100m inclinometer...

 

[tuzhuanjia]

Thanks, BigH! We did not use retarder and we should use it.

Thanks, worldofmud! You have a good sense of the installation. Yes we had two casings: 10" and 7". the 10 casing was removed before 7" casing.

We did measure the grout strenght increase over time for other projects. It was unconfined compressive strength (pocket penetrometer) but not shear strength (tor-van). Did anybody do tor-van test on the grout?