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secant pile sinkholes

secant pile sinkholes

(OP)
We are drilling a secant pile wall for a cofferdam excavation. We had a sinkhole along the side of the guide wall. We are pulling the casing as we tremie the concrete. THe material varies from sands to clay. What can cause that- a drilling technique or a poorly constructed guide wall?

RE: secant pile sinkholes

Check the head pressure of the concrete against your casing extraction...you probably had a section "neck down" as you were pulling the casing, thus allowing subsidence.  

RE: secant pile sinkholes

(OP)
Thanks Ron- I will check with the super if he thinks the pressure got low.  I have heard some suggestion that the material under the guide wall could be too loose.  We had a pile of fill over this location for years that was excavated.  Then the trench was 3' deep before placing the guide wall.  Could loose material be the problem- I assumed the overburden from the fill would have compacted at least the first layers.  

RE: secant pile sinkholes

Loose fill is less likely to be the issue.  In that case, you would have more grout being pumped, so your grout volume per hole would show as being high.

RE: secant pile sinkholes

Loose material might flow into the pile as Ron describes. As he noted though, sufficient concrete head would counteract that.

Have the concrete volumes been checked and compared to the "design volume" of the pier?  If the material is entering the pier, then the grout volume would be too low.

Where is the end of the tremie in relation to the end of the casing and the concrete surface? Is ther sufficient slump to prevent bridging in the casing?

RE: secant pile sinkholes

(OP)
The volume going into the column exceeds the theoretical volume.  We are using about 30% more concrete than expected.  

I am confused about "bridging".  Could it be a problem with the mix design?

THank you for your help.  

RE: secant pile sinkholes

By bridging, I mean that the if the concrete did not have a high enough slump, it can catch on the sides of the casing, and raise up with the casing.  This will results in less head, and could cause some necking down as Ron described.

RE: secant pile sinkholes

You should check both the initial slump ( it should be around 20 cm or 8 inches ) and it should not decrease too much with time ( at least the concreting time ). If the high slump is obtained with water, it should decrease very quickly ( could drop to 13 cm in half an hour ). In this case use a plasticizer in sufficient quantity in order to have a slump value of 16 cm when you finish to concrete your pile.

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