Horizontal Directional Drilling - Bentonite 1

[SJcivil]

I am designing a pipe for an HDD application, the driller is using a Bentonite mixture for his drilling mud. When I do the ring deflection calculation for the pipe what can I assume for supporting soil for the pipe.

The Directional Driller has told me that the mud gets very hard after a 24 hour period, assuming that it fills the void between the outside pipe wall and the in-situ soil. can I assume that the pipe will be supported the same as what the in situ material is. The in situ material is a competent soil with an assumed 100 percent compaction.

I should note the pipe is HDPE Dr 11 which relies quite significantly on soil support for its strength.

Thanks

 

[civilperson]

No native soil that I have encountered has compaction of 100%. The range is from 58% to 93% with the median at 82%,(Standard Proctor). Glacial tills may give a higher in place density but I never have used it in a fill requiring a proctor.

 

[SJcivil]

Sorry, I did not mean 100% I was just reiterating that it is in situ soil. That is not really relevant to the question I had though.

 

[rconner]

You are asking a basic question that I think may be hard to answer in absolute terms based on the information supplied; however, I understand “bentonite” is basically/generically a "high(ly) plastic clay" and with some swell characteristics (see while not the same applications e.g.

http://cat.inist.fr/?aModele=afficheN&cpsidt=15219641

and

http://scitation.aip.org/getabs/ser...00119000002000242000001&idtype=cvips&gifs=yes

etc.).
While I believe some mixes of e.g. granular soils and bentonite can indeed be made quite firm (an example is mechanically compacted and sometimes cooked mixes of bentonite and sand etc. used in metal mold/casting), perhaps referring to it (at least in often some saturated conditions common to deep HDD?) as "very hard" could arguably thus be a little bit of a stretch in some situations. I believe there can also be at least temporary voids that occur in some circumstances around HDD-installed pipes.
I have however heard myself from some ostensibly quite experienced proponents of this business that the drilling fluid and “soil cutting” mixes around pipes do over a period of time basically take on (at least eventually) "characteristics" of the surrounding soil. Like you however I await more learned responses.

 

[SJcivil]

Thanks rconnor, have you ever had experience with drillers mixing grout or cement in with the drilling mud or pressure grouting outside the pipe after it has been installed. I would think using cement or grout with the drilling mud would be risky in terms of timing in getting the pipe pulled back through before the grout hardened or even getting it cleaned out of the drilling equipment.

 

[escrowe]

A bentonite/grout mix can be adjusted appropriately for the situation. Unless you intend to provide a mud
specification, I would suggest you present the contractor with performance requirements and get out of the way.

Really don't like HDPE...

Here's an example spec for HDPE HDD from my old stomping grounds.

http://www.wssc.dst.md.us/dsg-PERMI...ecs_2005/02446_Horiz_Directional_Drilling.pdf

And from my new region:

http://www.cityftmyers.com/Departme...ommand/Core_Download/Default.aspx?EntryId=368

 

[rconner]

I do not have such experience but understand "pressure grouting" of various setting cement mixes is done quite often and by various means (through exterior grout tubes or thru-wall ports) at least AFTER e.g. carrier pipes are pushed or pulled up inside larger casing pipes. While I suspect the purpose of this is to firmly fix the carrier in the casing for whatever reasons by those designers, I have not necessarily always understood this practice in all cases (so to speak), as it would appear to negate some original purposes of casings to begin with [that as I once read explained in the ASCE MOP #89, “Pipeline Crossings” includes the ability to withdraw the pipe at any time in service life if that were ever desired and for whatever reason etc.]
However, while I have not heard of this sort of thing being done actually during pulling installation of HDD pipelines installed directly without casings into a soil mass (perhaps for the worries you describe), I would not be surprised if this had also been done somewhere. However, I think it is well-known some “stuff happens” in some directional drills that can interrupt progress (or for that matter in basically any kind of construction involving folks and equipment etc.!) For instance I heard of a case last week in the middle of a pull of quite long pull of large diameter ductile iron pipe where the contractor ran out of drilling mud part way through the installation. He sent a worker to pick up another pallet of mud mix, and in so doing the worker (trying to “do good”, as most of us are wont to do) picked up two pallets. On the way back, the truck was run over scales and was found to be overweight. Needless to say he arrived back on the site a few hours after he was sent. In this case and with this pipe the contractor was able to finish the pull very successfully; however, if he would have been dealing with a firm-setting, cement grout I’m not sure the results would have been so favorable!
Also, if you are talking about using or adding e.g. exothermic/setting cements (e.g. Portland cement concrete or grout) and particularly around plastic pipes, it is also possible heat generated as Portland cement sets wouldn’t help the strength and deformation/ collapse/buckling resisting moduli of the plastic pipe, at least over the time the temperature (and external pressure?), is applied.
[I have given escrowe a star for I think some quite good points made;>)]

 

[escrowe]

I think specifying a sleeve for installations below roadways (and other structures) is certainly a good idea, and has been required on my projects (especially with that squishy HDPE). And grouting within the sleeve does seem redundant, or even counterproductive. :]