Retrieving Tremie in Wet Piles while Keeping it under 3m of Concrete 7

[T2ioTD]

While pouring concrete in wet (full of bentonite) piles, how does the operators of the tremie know that there are around 3 m of concrete above it? They would see nothing, as the hole is filled with bentonite and concrete...

 

[MTNClimber]

Drop a weighted tape down the hole.

 

[BridgeSmith]

They should know the volume rate of concrete they're pumping in, and roughly the cross-sectional area of the hole. From that, they can calculate the rate the concrete should be rising in the hole. I believe the concrete pumpers generally have pressure gauges that show the back pressure in the line, which can also be used to estimate the depth of the concrete above the tremie outlet.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[T2ioTD]

MTNClimber: Can a man holding a rope sence the difference between bentonite and concrete with high slump?

 

[MTNClimber]

I guess it could depend on the mixes. The few deep shafts (40m+) that I've seen use designed bentonite mixes, yes I was able to tell the difference. But that's just anecdotal. I agree with Mr. Smith though. If the hole has good geometry (drilled in clay or temporary casing), you could figure out the theoretical volume.

 

[SlideRuleEra]

An experienced crane operator will develop a "feel" for friction between the tremie pipe and fluid concrete. This is based on how the crane reacts as the tremie is lifted. He/she won't be able to tell the depth is 3 meters but they will be able to tell if the tremie is embedded too deep or too shallow in fluid concrete. Technique long predates use of concrete pumps for similar applications such as placing cofferdam seals.

http://www.SlideRuleEra.net

 

[BUGGAR]

On BART in SF, a weighted tape was used in 7-inch slump concrete. They experimented with different shaped weights but wound up with just a 3-inch shackle and a very experienced tape man. A good tape man is the key.

 

[aeoliantexan]

The unit weight of concrete should be about twice that of the bentonite slurry. A milk jug filled with water and about half filled with sand should sink through the bentonite but float on the concrete. A concrete test cylinder with an eyebolt probably will do the same.