Problems with off-shore CPT 1

[Hanks864]

Dear all,

I am looking for help to solve the problems I am having in the execution of off-shore CPT tests. The peoblem is that the soil is very hard/dense, which has led me to stop when it is needed and then perform a drillout and continue the test to the requested depth given by the client.

With the drillout the problem comes to the fact the the CPT rods are unconfined, I fabricated and use a series of centralizers that help me align the CPT rods, but I am experiencing extreme bending and I have broke many CPT tubes and lost a piezocone.

I wonder if you know any technique that would be suitable for my case?

Thanks very much,

Hanks

 

[GeoPaveTraffic]

The solution is to use an exploration technic other than CPT. Try drilling and sampling a real boring. A CPT is apparently not the right exploration tool for the conditions.

Mike Lambert

 

[Hanks864]

GeoPaveTraffic thanks for your response.

I am also performing borings with sample extraction and the performance is extremely good compared to the CPTs. Unfortunately I cannot replace the cPT test with a borehole, I have to do the job I was contracted to. What I need is to find a solution for the confinement of the rods.

Thanks...

 

[GeoPaveTraffic]

Your contract should have included a CPT refusal criteria. If it didn't, you have a serious problem.

You might be able to install a heavy drill casing with an ID slightly larger than the OD of your CPT rods through the drillout zone.

Mike Lambert

 

[coneboy]

Hank,
When your cone meets refusal and you conduct a drill out are you using casing?
Are you just drilling down with a drill out tool? You will have many problems with cone rods breaking if you are pushing the cone through drilled out soil that is has been disturbed.

On past barge CPT investigations we have periodically encountered obstructions or very dense soil layers that had to be drilled out and cased off. When the soils are drilled out we have always used mud rotary casing equipped with a shoe on the lead bit.


Yes, we have also encountered rod bendage and cone breakings on barge investigations in the past. In the contract documents I always state that "any equipment lost or damaged will be charged at replacement cost". If the client will not accept this condition then build into your proposal the cost for new equipment for each job.

We also use a string of specialized casing that is inserted inside the drill casing. This specialized casing must be the perfect size to take up the annular space and prevent rod bendage. The casing must have the perfect diameter to accept the electronic cone and take up the annular space. The specialized casing must inserted down the full lenght of the drill casing. Of course, if the head on the drill can not remain stable and straight during hard pushing operations then you will have an even more difficult time reaching the required depth.

Another item is barge movement. Is your barge staying in a relatively fixed and stable position?

I always have to add a little extra to my bid prices for barge CPT work as there are some risks and challenges in this type of work.
Coneboy

 

[Hanks864]

GeoPave, I am currently using a thick casing as you described with relative success. I have now the problem that at deeper depths this casing is too heavy it cannot be lifted with the machine. I am thinking to use a crane if the time comes when I cannot take it out with the machine alone.

Coneboy,

Yes, I use a casing when I do drillout. I am using HW casing at the very beginning of the perforation and then we continue only with HQ casing. Yes, my barge is relatively stable, it is a jack-up type platform, the only movement I get is sometimes when my boat hits the jackup. The way the contract was negotiated didn't include payment of broken equipment, I cannot do anything there.

But now I would like to change the focus on a CPT refusal criteria, the only thing I have right now is a 75 MPa tip resistance as a stop criteria, I think this is very crude and excesive, but I have had a hard time trying to change this with the client. Do you know any acceptable stop criteria for CPT test out there I could propose to them? I have been advised to read the ISSMGE 2001 recommendations but I haven't found them, if you know about it or could provide me a link for download I will appreciate it.

Thanks,

Hanks.