sand collape

[kkr90]

hi,
I am kartik,currently I am working in a bridge construction (piling)whenever we are doing the piling,we kind of saving 2 or 3 m3 than actual I don't know whats the problem ,somebody saying it is because of collapsing and i am not sure about that.our pile dia is 1.2m and we are using 1 piece of 8mm and 7 pieces of 6 mm,help me to figure this out ,and also i want the details of bucket dimensions

 

[Ron]

I get the gist of your question but you need to provide more info.

Is this an augered pile? What method is being used for concrete placement? What is the depth? What is the geotechnical profile?

 

[kkr90]

thanks for ur reply sir,our's is auger boring,and the depth is 41m below the cut off level,we are placing concrete by tremie method,and the geo-report is as follows
0-1.5m=sandy clay with gravels
1.5-6m=stiff brown sandy clay
6-10.5m=stiff greyish sandy clay
10.5-18m=dense blackish silty sand
18-24m=hard sandy clay
24-32m=yellow brown silty clay with gravels
32-33m=greyish boulders
33-42m=hard brownish silty clay

 

[Ron]

I see two layers that could give you a problem but in different ways. Sloughing could occur in the silty sand layer; however, it is noted as dense.
The other layer is the boulder layer...this could result in sloughing or voids. Sloughing would decrease your concrete volume and voids would increase the volume.

Keep in mind that placement by tremie gives you little control over the documentation of actual volume. If you placed the concrete through the auger as the auger was extracted, you could control both sloughing and volumetric measurement of the concrete used through knowing the pump displacement and counting the pump strokes.

 

[kkr90]

sir thanks for ur valuable reply ..if the sloughing is occurs in range of
32 m why it cannot be removed at the time of flushing,and because of caving the amount of concrete should increase but we kind of saving concrete ...we are using bentonite solution for flushing.the worst part is that we are going to do RPLT test on that pile group

 

[Ron]

The lateral pressure exerted by the bentonite slurry might not be sufficient to hold the material in place. For this reason, I would recommend using an augered concrete pile approach for your application. (See info on AugerCast piles or similar augered concrete piles

 

[BigHarvey]

The type of soil you are drilling through can't explaine your problems if drilled under bentonite slurry. Under bentonite, you should be drilling with a bucket and not an auger. Check the water table level. In order to have a stable excavation you need at leat 1.5m of head againt the water table. No artesian conditions ?

 

[kkr90]

In our site our water table is at a depth of 2.5 m,one time during placing of concrete the bentonite solution oozes out from the near by soil,is this a condition of presence of artesian ,and also we are using auger for boring and bucket for removing.and the client was not interested in augered concreting,suggest some measures to overcome this

 

[Ron]

kkr90...who is the engineer? You or the owner? When things go wrong he'll hold you responsible. If you're responsible you make the calls.

 

[BigHarvey]

Are you producing concreting reports with a curve showing real volume of concrete and theoretical one vs depth. This would give uou a hint where your underbreak comes from. Check diameter of tools, they should have the nominal diameter ie 1200mm and this should be maintained (change teeth, recharge welding, etc). Do you use a temporary steel casing on the first meters to prevent collapse in the top part ?. Are the characteristics of your bentonite slurry checked and within normal values (pH, density, filtration, viscosity, sand content, etc...) ?
You should not have problems with the soil and water table configuration you are describing.