Sanitary Sewer Creek Crossing

[johnyt79]

I've encountered a design challenge where we are required to cross under a low flow creek with a 200mm sanitary sewer main. The usual method is to cross the creek with a segment of ductile iron pipe, encase it in concrete, and then place 200mm di. cobbles on top.

However, the enviromental team wont allow us to pour, or use concrete around the creek area.

One path is to embed rip-rap, somewhat building an underground rock-stack and placing the pipe behind to support to prevent it from moving...

Anyone out there with experience or thoughts?

thanks!

 

[TDAA]

In our area, mostly to fix scour situations, we install level control to the creek, downstream of the pipe. This sort of a drop structure will help raise/maintain the creek bed elevation at the pipe location to allow protection of the pipe. Different cutoffs can be used to keep that from washing away.

That may not be viable, if they wont even let you use concrete.

Why will they not allow it?

 

[rconner]

While probably not as preferable in general hydraulic and maintenece sense as an at-grade sanitary crossing (if that can somehow be done), I guess you could alternatively consider seeking approval to design the crossing as a "siphon", and thereby go deep enough that the powers-that-be may not be worried about scour.
If using siphon principle, some authorities for various reasons like to utilize "double-barreled" siphons(two or more parallel lines).
For various deeper crossings, suitable flexible restrained joint ductile iron pipelines are incidentally/frequently nowadays being installed by horizontal directional drilling (HDD), if open cut with regular pipes or ball joint is less feasible due to conditions etc.

 

[bimr]

It is difficult to get a permit to open cut a stream.

As rconner suggests, directional drilling (trenchless construcion) is at present the preferred construction technique.

It is not feasible to install an inverted siphon in such a small sewer.

You should investigate using a different route for your sewer or perhaps a lift station. While an open cut is a simple solution, it no longer acceptable.

 

[CPENG78]

johnyt79,
While working on on a previous project, I had to cross an existing creek with a 12 inch sewer line. To do this, our concept was to encase the 12 inch sewer line (Poetntially PVC) in either a 24" steel or DI pipe. In order to minimize movement, spacers would be placed an even distance apart to prevent movement of the 12" line within the 24" carrier pipe. I have come across various manufacturers that produce spacers for such an application.

I did swith to a different company before I saw this project to completion. However are there any pipe material limitations when direction drilling is utlized as a technique? In other words, could a DI pipe be installed with directional drilling?

 

[jgailla]

CPENG78,
The method you describe would be more appropriate for a jack and bore.

Directional drilling would not use encasement. The material is almost always smooth profile HDPE. There is fusible PVC available for the application, but it is not used in my area (NE Florida). Some contractors will also use glued PVC for small (3" and under) applications.

If encasement is required, a jack and bore method would typically be used due to cost and consructability issues. Rigid pipe such as ductile iron cannot be used in directional drilling due to the stiffness of the material and the necessity for bending along the profile, especially at the entrance and exit.

 

[coloeng]

We have used directional boring to install steel casing pipe up to 24-inch diameter. We have done this for highway, railroad and canal crossings. You just need a long enough entrance pit for the casing pipe.

 

[CPENG78]

Jgailla,
I do remember Jack and bore been explored as a technique and I imagined that stiff pipe would be a problem for the directional drilling.

However, like coloeng mentions if the pit is long enough, it sounds like it may be feasible.

Coloeng,
Any problems encountered once the operation started due to the length of the entrance pit?

Thanks everyone for your input

 

[jgailla]

coloeng,
I think we are using different terms.
In my area, directional drilling would be done from the surface. There is a small pit, but only the size one would trench to connect the pipe. The equipment is not in the trench. An auger bit is drilled from one side of the obstruction to the other with a diameter slightly less than the pipe to be pulled. Drilling mud is used. The profile is shaped like a flattened "U". When the suger bit reaches the other side, the pipe is attached the the auger and pulled back through the hole.
A jack and bore would be done from a large pit. The pit contains the jacking equipment and the pipe sections. The pipe is pushed horizontally under the obstruction at a constant depth. Depending on soil conditions, the hole may be prebored and then the pipe jacked in.

I believe the term directional boring that you are using I would use the term jack and bore for. I'm no expert in this. My experience is in the best way to install utilities from one side of a road, stream, or wetland to the other for pipes ranging form 3" to 16" diameter. Normally we would call for directional drilling if no casing is required and use HDPE or call out jack and bore if a casing is required. The casing would be jacked in and then the pipe installed with the spacers.

 

[coloeng]

CPENG78
Our utility hasn't had any problems with the steel casing pipe installation. A neighboring utility had a piece of steel casing jam under an Interstate highway and it broke from the drill head. They had to redrill and pull a new piece of steel. In another instance where they directionally drilled and pulled back a 20-inch gas pipeline under the Interstate highway, a void was left above the pipe that caused settling of the lanes of the interstate. They believe the cause was because of the 6-inch minus fill of the roadbed that collapsed while the reaming was occuring.

We have had problems with both HDPE and Certa-loc C900 with the pipe getting jammed and having to redrill it. The HDPE got jammed in a cobbly-gravel formation and the Certa-loc got jammed while pulling it through sandy conditions.

jgailla
The directional drilling I am refering to is the same as you describe. For the steel pipe you can either dig a pit that is as long as the steel casing (usually if it is less than 40-feet) or pull the steel back from the surface through the pilot hole after using a reamer to enlarge the hole. The radius of the arc of the drill steel is much greater when you pull steel than when you pull either HDPE or PVC so you will end up with a longer bore. We have found this method to be more cost effective for casing pipes that are 24-inch and smaller, although we have used it for up to 48-inch casing pipe that was less than 40-feet long.

 

[bimr]

Not sure why you fellows are talking about trenchless installation as it is off topic.

A reliable installer will probably inform you that at the present time, directional drilling equipment is not capable of the accuracy that is required for the installation of gravity sewers.

 

[CPENG78]

Bimr,
Doesn't it fall in the responsability of the engineer/designer to understand the level of accuracy that it is involved with each construction technique? That is the purpose of forums like this one. I would not specify directional drilling for a pipe system whose inverts need to be installed to the nearest hundreth of a foot. In the case of the concept installation I previously mentioned, there were substantial allowances in placement of the sewer line to account for changes due to the installation process or factors encountered in the field. Like jgaille pointed out to me, the jack and bore may more appropriate for the installation I was describing and that technique of course comes with its own set of inacuracies.

To say that it is off topic, well it may just be a matter of a specific project requiring more accuracy but not a fair blanket statement that can be made about the construction technique itself. In your experience, what is the level of accuracy attained with directional drilling?

Does anyone know the level of accuracy that can be attained with other techniques such as Jack and Bore?

 

[bimr]

Ted Dimitroff of Trenchless Flowline, Inc. states in this article published in April 2008 that he can not get enough accuracy fot gravity sewers.

"The Metropolitan St. Louis Sewer District (MSD) and many other sewer districts worldwide look forward to using HDD for gravity flow installations, but they want the HDD industry to match the open-excavation specifications. Engineers do not want to make design changes simply because HDD cannot consistently install tight grades below .50 percent. We must give some credit to those HDD firms that have an employee or owner who has developed the special touch for installing gravity flow sewer mainlines. The volume of work that the sewer industry offers will require many crews installing mainline sewer pipes and the construction method of using HDD must be consistent and trainable."

http://www.trenchlessonline.com/index/webapp-stories-action?id=512

 

[CPENG78]

Bimr,
Very interesting article. It looks like there is some promise with the Arrowbore method. It will be interesting to see how this technology further develops or new technologies are also developed in years to come.