Bentonite Cement Water ratio for Inclinometer in soft to firm clay 4

[Myoho]

I have specified an inclinometer to be installed in an embankment reservoir for monitoring lateral movement.

The borehole for the inclinometer shows soft to firm clay (say 45kPa).

I have asked the contractor to grout up the inclinometer with a bentonite / cement ratio of 2.5:1, which will be placed using a grouting pan.

The contractor used the following mix 2.5kg bentonite to 1kg of cement to 80 litres of water.

My personal feeling is that this mix will not give me a shear strength of 45kPa infact Id be surprised if it sets at all.

Ive been looking for design data for grout mixs but its proving difficult to find. Any advice would be appreciated.

 

[doc11]

Your right it wont set. Any more than 10% bentonite by weight is too much. Check with Slope Indicator They reccommend a couple of different mixes one with bentonite, the other with lime. One of them should work or at least give you a good starting point.

 

[Panars]

Slope Indicator gives two different grout mixes on their website, one for hard soils and one for soft soils. The grout mix ratios by weight for cement:water:bentonite are
1:2.5:0.3 and 1:6.6:0.4. You can find the page at

http://slopeindicator.com/support/inclinometers/technote-groutmix-inclinometers.html

 

[Myoho]

Many Thanks doc11 and Panars. Just a question about Fig1 on the paper, which shows water cement ratio against Unconfined compressive strength. Im after shear strength so I have just halved the value of the Unconfined compressive strength. Would you chaps agree with that?

My other concern is that the mix only uses a bentonite cement ratio of about 0.4 : 1.

I would be interesested in mixes using less water and more bentonite to try and match the deformation modulus of the clay.

 

[Panars]

For all practical purposes, the shear strength is half the unconfined compressive strength.

 

[msh69]

Is there any reason not to prefer a casing?

 

[doc11]

Those mmixes are pretty thick, most standard geotech drill rigs with a moyno pump probably wouldn't be able to pump anthing with less water without bringing in a grout pump.

Something we do alot is drill as small of a hole as possible (4"in dia. for 2 3/4") casing and use the grout valve that Slope Indicator sells. With the smaller annulus the grout mix becomes less critical.

Could you please clarify your last question, are you refering to not installing an inclinomenter an using something like a TDR cable instead?

 

[msh69]

Slope Indicator notes using the inclinometer casing as well as grout mixes on their website. I just wondered it was a matter of choice (technical, economical, etc.)

 

[doc11]

A little of both, no one grout mix will work in every situation, I have even used pea gravel and bentonite chips in certain situations. Throw regulators into the mix and it can get even more interesting.