PVD / Obstruction clearance

[Canadaian]

Hi


As all you may know it is possible to encounter near surface obstruction during installation of Prefabricated Vertical Drains PVD ... I understand that pre augering ( or other method may be chosen by the Contractor) is used to clear the obstruction for a certain depth to facilitate the PVD installation.

For this project PVD is to be used for the first time in the project area and to mitigate risk and unjustified high bidding rate, the client proposed that obstruction clearance shall be a separated item from the PVD installation. The question: is there any criteria or method to be followed to make sure that the Contractor do not abuse the item (do preaugering unnecessarily) I am thinking about power criteria. I know that in one project there was a load cell installed in some way to the PVD reel and there was a limit to define refusal, but it seems to me so dedicated and" sophisticated". Is the PVD rig equipped with some device that can tell us the operator had used the maximum machine power, so we can switch to obstruction clearance technique?

Thanks in Advance.

kiki

 

[BigHarvey]

You must make a didtinction between obstructions ( blocks, etc ) and the need for pre-drilling due mechanical properties of the soil.
If you have an obstruction then whatever the installation method or power of the rig , it won't go through.
If you use static installation rigs, you should be able to go through cohesionless layers showing qc ( CPT ) of up to 12 MPa. If you use a vibro method you can go up to qc 25 MPa. Otherwise you will need pre augering. The values I gave are for cohesionless soils because if you have such values in clay you certainly do not need drains ( you can generally overcome qc 6 MPa with a static machine )
Given the production observed in PVD installation ( which are of about 5000 m per shift) it is true that you can't look for the need of pre drilling for each drain but consider areas where you do pre drilling or not. You might feel this as abusive from the contractor but it's not.

 

[ntschwanz]

On a project in ND, we installed PVD's through embankment fill, that contained cobbles and boulders in a matrix of lean clay, into an underlying CH deposit. We used a separate bid item for obstruction clearance of pre-augering in the fill material. We knew this was a problem from borings and based our quantity takeoffs from the fill thicknesses. Hitting obstructions with the hydraulically driven mandrel lifted the front of the excavator. In those cases the contractor had to pre-auger a second time. We used a split bid item for PVD and obstruction clearance.

 

[Canadaian]

BigH and ntschwanz

Thanks for your response.
BigH , the concern is surface and shallow obstruction not deep obstrcution. what is the approximate equivalent SPT "N" to the CPT "qc" youy did mentioned.

K

 

[BigHarvey]

See Robertson, Campanella & Wightman ( 1982 ) qc/N variation with grain size :

For D 50 = 0.005 mm qc/N is about 2 with qc in bars ( 1 bar = 0.1 MPa )
For D 50 = 0.1 mm qc/ N is about 4

typical refusal values for static machines is around N= 18 in sandy soils ( depends also on thickness of layer )

 

[BigH]

Canadaian - don't confuse BigHarvey with BigH! We're both great, bonbiff and loveable guys, though!! I've been involved with the installation of PVD through soft clays into stiffer clays some time back and, given our set-up, we found that an N value of between 7 and 8 in the clay was just about as far as we could go. I'm sorry in that I don't have any "pressure" data on how much the push capacity of the system was.

 

[Canadaian]

Thanks BigH for the response and clarification... I thought you are the same person.

Thanks again BigHarvey.