PVD / Obstruction clearance
PVD / Obstruction clearance
(OP)
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
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





RE: PVD / Obstruction clearance
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.
RE: PVD / Obstruction clearance
RE: PVD / Obstruction clearance
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
RE: PVD / Obstruction clearance
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 )
RE: PVD / Obstruction clearance
RE: PVD / Obstruction clearance
Thanks again BigHarvey.