Rail Crossings – Auger Bore versus Horizontal Directional Bore (HDB) 1

[auzie5]

In Canada, we typically follow TC E-10 guidelines when crossing a railways with our pipelines. These guidelines require that, “pipelines shall be installed under tracks by boring and/or jacking, if practicable”.

I have always interpreted this to require that “auger boring” or “pipe hammering” be used to install the crossing since as the crossing path is drilled or hammered-in below the railway, the hole that is left behind is immediately replaced with the carrier pipe (or casing) to ensure there is no risk of hole collapse (and potentially track settlement).

However, many people in the industry refer to a straight “horizontal directional drill” (HDD) as a “horizontal directional bore” (HBD). “Horizontal directional drills” or “horizontal directional bores” rely on drilling mud on the empty bore walls to prevent a potential collapse. A collapse of a shallow drill path below a rail crossing can lead to settlement of the tracks which is why I always assumed rail companies insisted that HDDs or HDBs be reviewed in greater detail and only be proposed when auger boring or pipe jacking is not practical. Even if the HDD or HDB drill path does not collapse before getting the drag section installed, the final ream pass for an HDD or HDB is oversized so there is a chance for the annular space between the bore diameter and the carrier pipe (or casing) to settle over time.

I have completed a number of approved crossings using auger bore equipment. But more frequently, contractors are preferring to use a Ditch Witch HDD rig to complete a directional drill crossing. I have successfully completed many rail crossings using the HDB method as well but I have always stated in the crossing agreement that the crossing method is a "directional drill" (i.e. not a "bore"). The crossing request is usually scrutinize by the rail companies when the crossing method is listed as a "directional drill".

The more I speak with other operating companies, the more I notice that everyone is listing the crossing method as "bore" in their crossing agreements but then are using the "HDB" method to install the line. Is this common for you guys? If so, I would prefer to list the crossing method as "bore" rather than "HDB" in my crossing agreements since it tends to get approved quicker. Of course I'll still design the crossing to be a HDB if that is in fact how I intend to cross (e.g. deeper drill path and minimizing the final ream pass diameter).

Any general feelings out there on rail crossings using auger bore versus HDB? In each case work pits (entry/exit) will facilitate a ~3.05m (minimum) burial depth below tracks.

Reference to TC E-10 Guidelines:

https://www.tc.gc.ca/eng/railsafety/standards-tce10-236.htm

Thanks in advance for any comments.

 

[BigInch]

"The more I speak with other operating companies, the more I notice that everyone is listing the crossing method as "bore" in their crossing agreements but then are using the "HDB" method to install the line. Is this common for you guys?" Yes.

This complicated by the fact that non-engineers, or non-pipeline engineers, often fill out the permit application and they don't know that there is any difference at all between any kind of boring, whether by auger, hammer, HDD, or HDB. Furthermore the contractor can propose a different method to what you put as well unless you are the contractor of course.

A horizontal directional drill is longer, but can avoid digging and safety issues with the installation pits. The directional drill will normally have a much deeper depth, set to avoid unfavorable soil layers, cave in and blowouts. Much more than the +/- 3m that are more typical for shallow bore methods.

 

[auzie5]

Thanks for the prompt reply BigInch.

I guess my concern is that in our crossing detail, when we state “bore” and have a fairly shallow crossing (~3m), we are planning for the hole to be reinforced with carrier pipe. If I knew the crossing would be installed at ~3m below the tracks via a straight shot HDB (from entry pit to exit pit), I would typically spend a little more effort convincing myself that there was no geotechnical feasibility concerns since integrity of the empty bore hole might be an issue at that depth during the short time after it is reamed and before the pipe is pulled into place. Maybe I’m being overly cautious. In fact, I have never heard of an actual case of track settlement from a bore hole sluffing in before the pipe is pulled into place.

If ever we move to a real HDD for crossings, we're usually so much deeper under the tracks that I'm less concerned than when someone changes a "bore" to a "HDB".

Thanks again for comment. Have you ever heard of a track settling due to a bore hole collapsing at a relatively shallow depth (~3m)?

 

[bimr]

You should also verify with the railroad what their standard procedures call out.

HDD and HDB are different methods. HDD maximum pipe size is generally less than 24-Inch. HDB is used for largers pipes.

HDD uses a directional drill and pumps the drilling mud back to the drill rig on the outside of the drilling string. Slurry is prepared to stabilize the borehole and to lubricate the surface of borehole. Once the hole is drilled, the drilled borehole is reamed and the carrier pipe is pulled back through the drilled borehole. For a crossing where the carrier pipe is to be installed within a casing pipe, it is common for the carrier pipe to be preloaded inside the casing prior to pullback and the two pipes are installed together in one pass.

Auger, hammer, or HDB are used to install a casing pipe. The cutting head of the HDB micro tunneling head pumps most of the slurry fluid inside of the casing pipe backed to the slurry settlement tank. A hydraulic drive is used to push the casing inside of the borehole. Once the casing pipe is installed, the carrier pipe is then installed inside of the casing pipe.

https://www.youtube.com/watch?v=zr6pgRv6RDo

The HDB method will be preferred by the railroads over HDD because the HDD slurry pressures are much higher and there is a risk of blowout with the HDD method because of the high pressures.

Auger and hammer boring are probably more practical for railroad crossings because of the shorter crossing distances.

 

[bimr]

For HDD:

"Existing surface and subsurface features that could be damaged by settlement must be identified, through a combination of site visits, researching as-built drawings and aerial photographs for other projects and utilities in the area, using the one-call system to identify subsurface utilities and potholing. Features that may be at risk from settlement include buildings, highways, railroads, levees, utilities, piles, piers, sound walls and culverts. Risk for a given feature depends primarily on ground clearance between the feature and bore, annular space, and condition, age and contractor workmanship."

http://trenchlessonline.com/design-issues-for-hdd-projects-part-2/

http://www.brierleyassociates.com/w...-This-Project_Strater_Brierley_Associates.pdf

 

[LittleInch]

What's in a word eh?

I had to go look up HDB to make sure I wasn't missing something and initially I though HDB and HDD were in fact exactly the same thing, but then you said
"If I knew the crossing would be installed at ~3m below the tracks via a straight shot HDB (from entry pit to exit pit), I would typically spend a little more effort convincing myself that there was no geotechnical feasibility concerns since integrity of the empty bore hole might be an issue at that depth during the short time after it is reamed and before the pipe is pulled into place"

This implies that you dig a pit about 3m deep, line up an HDD rig horizontally and then start drilling in a horizontal line?? Please confirm as I'm not sure how you don't flood the pit with drilling mud and cuttings, but a little diagram would help a lot here. Also the key point of an HDD is that the mud pressure and density is such that it keeps the hole open while you pull in the pipe, be it a casing or the main carrier pipe. What you describe sounds like an empty open hole for a time while you insert (push?) the pipe into the open hole?? That's sounds like it entails greater risk than any other crossing method you describe.

I feel that using the word "bore" to describe such an operation is deliberately misleading the owner of the railway to get you an easier time on approvals, which will come back to bite as some point and in my mind isn't the right thing to do.

Size matters here. Anything up to about 12" I would say even if it did collapse you won't see much, if anything on the surface, but as you get bigger the risks increase.

The devil is always in the detail - size, distance and most importantly geo-technical data. I've just been involved in a directional augur bore in very poor and wet ground conditions which resulted in very significant ground slumping and movement and I estimate about twice as much material flowed out of the pipe as was replaced by the casing. So augur boring isn't perfect and if I had my time again I would try and use a TBM to maintain the hole and a much more experienced crew.

Bimr - I disagree about max pipe size - HDD has installed many pipes bigger than 24".

What you refer to as HDB I would term micro tunneling and uses a TBM which in sizes from 900mm upwards installs pipe by pushing through as you describe. The attached video is an interesting mix between different technologies which I would describe more as a steerable augur or microtunneling. The interesting bit was that they noted specifically that they keep the annulus pressure high to prevent collapse and excess flow of soil into spoil removal system. The issue here is that if the soil is more liquid / silty / wet than anticipated, then the soil in front of the cutter can flow before it is "cut" and hence induce collapse at the surface.

Railways normally guard their ROW very seriously and any depression in the tracks creates a considerable safety hazard and cost a lot of money to put right. Therefore, despite the "grief factor" involved in crossing railways, I would make sure the design is the best it can be and reduce risk of settlement or hole collapse to as low as possible.

I agree though that HDD needs careful thought

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

LittleInch - Did not say that 24-Inch was the max pipe size.

Micro tunneling and HDB are different. HDB is also known as Direct Pipe. Direct Pipe is a one-pass technology that combines aspects of horizontal directional drilling and tunneling to install pipes. It is particularly effective for crossing under levees, waterway, rail tracks and roadways because the borehole is continually supported and because of a greatly reduced risk of inadvertent fluid returns.

Example of HDD:

http://napipelines.com/michels-completes-record-intersect-beneath-houston-ship-channel/

Example of HDB:

https://www.youtube.com/watch?v=7MQAO4lJWi8

 

[LittleInch]

bimr,

given that we all have different interpretations of exactly what HDB actually means it isn't easy to say what it compares to.

The Direct pipe thing as said before is a mixture of a couple of different technologies and deserves its own name. Certainly looks interesting.

Those guys also do a version of HDD where they push and pull at the same time and reckon they can do 4km in a straight line....

you implied HDD was normally < 24" , but I'll take your last statement as correct and move on.

I take the point that more HDDs are relatively small bore than > 24".

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[auzie5]

Hi LittleInch,

Sorry for the confusion. I am almost certainly misusing the term HDB. By HDB, I mean when we use HDD equipment to drill a level horizontal drill path for the carrier pipe rather than a drill path with a big belly (as we typically would to traverse across a deep river crossing). We still keep the Ditch Witch at surface (not lowered into an entry pit like during auger boring) but we set it back a short distance (~20m). This allows us to spud into the ground from the surface and get the drill bit onto a level horizontal drill path as soon as possible at a shallow burial depth. We dig a disconnect pit some distance ahead of the HDD entry point (~20m) and only pull back the drag section to the disconnect pit. Disconnect the drag section from the reamer and leave it in the disconnect pit for final tie-in to mainline.

So the pipe ends up crossing under the tracks horizontally at a minimum burial depth. 3.05m below the tracks is the commonly referenced minimum depth of cover but that assumes the pipe is installed via an auger bore installation method (or equivalent). When I know the contractor is planning on using a small HDD rig, I like to specify a deeper drill path (unless a shallow drill path can be supported by our geotechnical investigation).

Sorry for the delayed reply. Hope that adds some clarity? If not, please do not hesitate to comment.

I appreciate all the comments guys.

Also, stumbled on a good reference read on the topic of trenchless installations. Short but sweet. With all of the misused terminology surrounding trenchless installations, I feel Figure 6 (Classification systems for trenchless methods) is a handy tool. Please only use this link as a preview. If you find value and plan on retaining a copy, support the author by securing a copy off Amazon.

Trenchless Installation of Conduits Beneath Roadways

http://libraryarchives.metro.net/DPGTL/studies/1997-nchrp-242-trenchless-conduit-installation.pdf