Draining a landslide, help needed designing push pipe

Greetings,

I am in need of some advice. I am with a group of homeowners who find ourselves in a landslide. To date the slide has moved a couple of feet in the last 3 months. The slide has been studied by 2 geotechnical firms. They feel without mitigation, the slide will continue, possibly catastrophically.

Resources are limited, the terrain is challenging. After much research by the geotechnical engineers, the consensus is that we need to get the water off the slide area. Of the various methods to accomplish that, horizontal drains are very efficient for the cost. Normally horizontal drains are metal or PVC pipes that are placed in the slope with a slight up angle. They are placed in bored holes. The pipes are slotted with screen material to keep the soil out. They provide a path for the groundwater to move out of the ground and into collection systems.

Normal horizontal drains will not work in this situation because the ground is still moving. The movement shears off the pipes. After more research we have found a system that uses “wick drains” in place of rigid pipe. Wick drains are tubes made of geotextile fabric around a plastic core. Normally they are placed vertically.

The method that has been developed uses drill pipe that has an ID of at least an 1⅜” and an OD of about 1¾”. The wick drain is threaded into the bore of the drill pipe and fastened to a metal plate with a piece of rebar welded to it. The metal plate is placed over the end of the drill pipe. This push plate keeps soil out of the pipe and acts as an anchor for the end of the wick drain

The pipe is then pushed into the slope with an excavator bucket or a dozer. As each section is advanced, a new section of pipe is added with more wick drain. This is pushed in and repeated until somewhere between 75 and 150 feet of pipe has been placed. Once the pipe is deep enough, a short section of pipe with chain hooks welded to the body is attached, and the pipe withdrawn. The plate with the wick attached to the rebar is anchored in the soil and stays as the pipe is withdrawn. As each section is withdrawn it is removed from the string. When completed the wick provides a path for the water that has been “filtered” by the geotextile cloth tube.

Typically a group of these are placed in a fan pattern from the same place. The operation is moved to the next site. We may need to place as many as 500 drains total.

The problem we have encountered is the drill pipe. We had anticipated locating a used string of an appropriate size drill pipe. Unfortunately the size we need, (something like AQ wire-line drill pipe with an inner diameter of 1 ⅜”) is not very popular. The small sizes are primarily used in geologic core sampling.

We now feel the best course of action is to fabricate a ‘Push pipe system”. We have access to a college machining, metal fabrication, and welding programs. The machining shop has Haas CNC turning centers, as well as the normal manual shop machines.

Our need is the design of the threaded connections. The connections do not have to withstand the rotational forces of drilling since the will be driven into the soil. They do have to withstand the force of driving a 150’ of pipe into the soil, and then the force of withdrawal. We expect the total string to be +-160 feet, made up with 20’ pipes. They also need to be able to be connected and disconnected in difficult conditions. I would think a robust thread is needed. The connections need to have a bore of at least 1 ⅜” The outside diameter could have a diameter of 2 ¼”. I would think the connector should have a larger diameter than the tube.

We have the ability to manufacture the connectors on the CNC lathes, and then weld them to steel tube. Timing is important. A major rain event could trigger catastrophic movement.

Can someone point us to a connector design that is in the public domain (many drill pipe connectors are licenced) that would accomplish what we need?

I have links to engineering articles of the process, if needed. Thanks for your suggestions.
Elliott
Photo of the slide scarpe.

Any thread will work , even tapered pipe. the wall of the pipe is the target stress value. Pushing pipe 150 feet into the ground sounds like a real challenge.

You need a rotary shoulder connection

Find a Vermeer dealer and buy the connections or at least a sub saver and copy thread.

Buy inserts and threading tools from tool Flo.

I have something that may work for you! PM sent.

Elliott:

I'm pretty skeptical that you'd be able to push a small diameter pipe 150'into the ground - even a small cobble would stop you dead. And the skin friction goes up pretty fast as you get deep into the ground. Did you ask the geotechs if this specific method would work with your soil profile?

I can offer a couple ideas though. First, you might want to post this question over on the Eng Tips Foundation Engineering Forum. They have some very good people there - both geotechnical engineers and contractors. This would give you a quick reality check if this idea is even going to work. Foundation engineering Forum - Eng-Tips

Second, have you looked at directional boring? These machines are commonly used to install power, and phone lines in built-up areas. They typically use a boring string that is about the same size you mentioned. Normally they are working in a horizontal plane but could probably direct the boring vertically as well. The machines are pretty fast - undoubtedly faster than pushing pipe in with an excavator.

Regards,

DB

D Bronson said:

Elliott:

I'm pretty skeptical that you'd be able to push a small diameter pipe 150'into the ground - even a small cobble would stop you dead. And the skin friction goes up pretty fast as you get deep into the ground. Did you ask the geotechs if this specific method would work with your soil profile?

I can offer a couple ideas though. First, you might want to post this question over on the Eng Tips Foundation Engineering Forum. They have some very good people there - both geotechnical engineers and contractors. This would give you a quick reality check if this idea is even going to work. Foundation engineering Forum - Eng-Tips

Second, have you looked at directional boring? These machines are commonly used to install power, and phone lines in built-up areas. They typically use a boring string that is about the same size you mentioned. Normally they are working in a horizontal plane but could probably direct the boring vertically as well. The machines are pretty fast - undoubtedly faster than pushing pipe in with an excavator.

Regards,

DB

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Thank you for the response. The technique is proven. The depth is certainly the limit for the drill pipe of the size I listed. The drains do not have to be that deep. It would just mean that more drains would have to be placed.

Just a point of clarification. These wick drains are installed horizontally. Actually they are pointed up 5 to 10 degrees from horizontal for drainage.

The normal method of horizontal drains will not work at this time. The hill is actively moving. The geotech borings were lined with pvc pipe. They lasted 3 days before they sheared. Wick drains have demonstrated that they can deform and still function.

Here are some links to a couple of articles.
http://onlinepubs.trb.org/onlinepubs/archive/studies/idea/finalreports/highway/NCHRP057_Final_Report.pdf

http://inside.mines.edu/~psanti/paper_pdfs/Cook%20Horizontal%20Drain%20State%20of%20the%20Art.pdf

Elliott

Thanks for the quick response. I will contact a Vermeer dealer to see what might work.

Thank you for replying. I think a tapered pipe thread in a coupling will be a weak point. Flexing is a problem described by people who have done this. A standard pipe coupling is going to flex and break.

I think you need to revist your pipe size, over here this approach is a common one for soil stabilisation on swampier building sites, lets the water drain down to the water table faster, this massively speeds up the rate the soils will naturally compact. Sure you have seen the youtube videos of this by now.

The system we were involved with simply had the hose - tube on a roll, the machine used a drive foot the tube kinda tucked in. Don't think it was even a round drill pipe, but the tube was ran inside the "drill tube" Simply pushed down with the drill tube then the drill tube pulled back out leaveing the drain behind. and the drain cut off just above surface. Because the drains ran inside the pipe it does not really matter what size the pipe is so long as its big enough for the drain to go inside.

Im curious with who came up with your 150' depth number? We never use to go near that deep, you don't need to push it as far as the water table, just far enough to get it into a dry zone above the water table some 2-3 meters but more so in your case punch well through the impermeable layer your slide is happening on. So long as you drain that porous - impermeable barrier your slide is happening on, you should achieve the desired effect.

That said i would go very carefull playing at fixing this, all the more so moving heavy plant onto a active slide, depending on the area it might make a lot more sense by also tackling this from the surface, making the site well drained with a simple open ditch network may gain you a lot more than the seap drainage will. If the rain can run off rather than soak in your massively reducing your problems to start with. Its a lot easier to do and all you needs some kinda digger. Equally you can do this fast ditches need not be big or deep, Just say 6" deep and equally wide just slope em down hill in a herringbone pattern, that improves the run off and reduces the rate water meats you problem layer.

Based on the pic, you need to some vegetation too, makes a massive difference, big rout plants are your friends, they naturally stabilise the soil as well as suck up a lot of moisture, if its all barren mud like your pic, its almost a man made problem!

Sounds like theres people living on this? If so stabilising it to be safe cheaply is a big ask, personally i would be moving, at the very least evacuating every time there’s heavy rain and probably for a few days after wards. IMHO its just not a safe place to live long term. Short term, you may stabilise it enough to stop it, long term complacency will set in and generally with mud - landslides there’s few survivors.

Some processes cannot be stopped- I would have plans in the works with the assumption that remediation will fail.
I don't have time to do a literature review just now but will read up- just off the cuff I would be asking question along the line of how much precipitation is needed on your site to overcome the stabilization effort and keep the site active...

I don't understand why you have talked to multiple engineers about this issue, and then come ask for such critical design answers from a bunch of unknown strangers on the internet of unknown credentials.

SirElliott said:

Thank you for replying. I think a tapered pipe thread in a coupling will be a weak point. Flexing is a problem described by people who have done this. A standard pipe coupling is going to flex and break.

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Why use a standard coupling? You said you have machining facilities.

You are talking about non standard stuff, make couplings with a supporting counter bore that gives support to the joint.
All I'm saying is that the threads are not going to be the limiting issue. Use straight threads if you want. They don't need to be something special that you have "developed' through comments off the internet.

Don't fish yourself into too small a box.
Considering the description of the situation, and what is at risk, I suggest you get what ever proven piping system is available and get the work done before the winter rains.

SirElliott said:

Thank you for the response. The technique is proven. The depth is certainly the limit for the drill pipe of the size I listed. The drains do not have to be that deep. It would just mean that more drains would have to be placed.

Just a point of clarification. These wick drains are installed horizontally. Actually they are pointed up 5 to 10 degrees from horizontal for drainage.




The normal method of horizontal drains will not work at this time. The hill is actively moving. The geotech borings were lined with pvc pipe. They lasted 3 days before they sheared. Wick drains have demonstrated that they can deform and still function.

Here are some links to a couple of articles.
http://onlinepubs.trb.org/onlinepubs/archive/studies/idea/finalreports/highway/NCHRP057_Final_Report.pdf

http://inside.mines.edu/~psanti/paper_pdfs/Cook%20Horizontal%20Drain%20State%20of%20the%20Art.pdf

Elliott

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I understand what is trying to be accomplished but I have some serious reservations if the suggested methods of pushing the pipe that far is going to be doable for the number of pushes needed.

First of all, using a Cat or excavator is not going to work. Been there, done that and I can tell you, it will not work. With the 20 ft. sections of pipe, the push force will cause the pipe to bow and bend. You need a dead on straight pushing force.

What you do need is a hydraulic pusher. This will require using 4ft. sections of pipe but you will be able to get the pipe through to the length you need and retrieve it easily. This will allow you to do a push and retrieve in minutes not hours.

Another challenge is the depth of the bores. The shallower a bore is, the more difficult it is to keep on grade. Without knowing the exact details of your push I would also suggest using a steerable head on the end of the pipe. This allows you to steer the push by rotating the pipe and tool as you advance the bore.

I know this project sounds simple but to actually pull this off will take a degree of experience and practical knowledge. Attempting your first bore in these type of conditions is going to be frustrating at best and more than likely quite expensive for you in time and money. I would strongly recommend getting a contractor to at least do the pushes for you. It will save you time and money in the end. The contractor might have suggestions to control the cost and even might be willing for you to supply the bore pipe if you construct to the way he tells you. The local Vermeer or Ditch Witch dealer can help with locating a good contractor. These two companies have developed most of the current technology for doing this type of work.

I would also take TRboatworks advice to heart. It might not be what you want to hear but some of these geological events are very difficult to stop. You do need to ask yourself what is Plan B and does Plan A's cost justify the possible failure probability.

JNieman said:

I don't understand why you have talked to multiple engineers about this issue, and then come ask for such critical design answers from a bunch of unknown strangers on the internet of unknown credentials.

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The engineers have given us a small number of mitigation projects they believe will stabilize the slope. Of the methods, horizontal drainage is the most cost effective. They have told us the method of using hard pipe will not work here. The wick drain system has been successfully used and studied at other landslide sites.

The only advice we are seeking is how to manufacture a substitute for the drill pipe normally used.

We are a small group of homeowners and orchardist who have to try mitigate this ourselves. We are private citizens trying to save our homes and property. There is no insurance for landslides. No government is funding this.

The geotechnical engineers are under contract to the county and the public utility district. They were not funded to develop or perform mitigation. The engineers have been "loaned" to us for a couple of hours. They will be giving us locations to place drains in based on their boreholes and knowledge. Other than that, this is all on us. Every penny comes out of our pockets, and those pockets have been hit hard already.

We have been fortunate to have some things donated to us. Besides the engineers time, we have been allowed to use the college. A local excavator is loaning us his equipment. We have volunteers ready to work on the placement of the pipes.

As to asking a group of strangers for advice, I am looking for people with expertise in fabrication to help us. I have seen posts from people in the drilling field. I have already gotten some ideas from what has been posted in response.

I hope that explains what and why we are doing this.
Elliott

Elliott

Thanks for your reply. I believe you are talking about the usual practice of placing wick drains vertically. What we are doing is a modification based on projects that have been done with good success.

As you describe, the vertical drains are typically installed by a machine purposely built to install vertical wick drains. I understand it works well and is very fast. The problem is that we need to drive these into the side of a hill.

Getting a truck based system to where we need to go isn't going to work. It must be a tracked vehicle. It must be able to go horizontal. Actually the drains are placed at 5 to 10 degrees up for drainage. As far as 150' that is the farthest. Most will be shorter than that. How far the drain needs to be placed is a function of topography and water table location. We are seeking to drain a water table traped in soils that create perched levels.

You may be right about pipe size. The requirements are that the wick has to be able to fit into the bore without much resistance. It has to be strong enough to be pushed into soil and then withdrawn. It has to be movable and in sections that connect to each other.

The 1 photo I posted just happened to be on a site that had been excavated. The slide is vegetated. A portion has cherry orchards on it.

We know it it has risk. Landslides here are not covered by insurance. We simply can not afford to walk away from our only homes. I am retired firefighter on a fixed income. I have house payments, I can't afford to pay for another place to live. We monitor the weather very closely. The US weather service even provides a special forecast for the slide area.

We understand that this is not the whole solution. Additional mitigation will have to take place. The next phase is very complicated and very expensive. We are seeking FEMA grants but the soonest construction could start is next spring. That's assuming the grant gets funded. Not a given.

I have attached another photo. I will have to post a link to the other photos because they are too large for this forum.

I think your best bet is to copy the DCDMA thread design for W series casing. I do not think I still have data on the thread designs but will check. From memory, it is a 4 pitch modified square thread. You will probably have to create a special size to fit your application. According to memory the Q rods were made with a heat treated 4140 tool joint welded to carbon steel tubing. You will probably need to follow that practice for durable tool joints. I was just checking the internet and the smallest casing size I found listed was BW. That is a lot bigger than what you want.

SirElliott said:

Thanks for your reply. I believe you are talking about the usual practice of placing wick drains vertically. What we are doing is a modification based on projects that have been done with good success.

As you describe, the vertical drains are typically installed by a machine purposely built to install vertical wick drains. I understand it works well and is very fast. The problem is that we need to drive these into the side of a hill.

Getting a truck based system to where we need to go isn't going to work. It must be a tracked vehicle. It must be able to go horizontal. Actually the drains are placed at 5 to 10 degrees up for drainage. As far as 150' that is the farthest. Most will be shorter than that. How far the drain needs to be placed is a function of topography and water table location. We are seeking to drain a water table traped in soils that create perched levels.

You may be right about pipe size. The requirements are that the wick has to be able to fit into the bore without much resistance. It has to be strong enough to be pushed into soil and then withdrawn. It has to be movable and in sections that connect to each other.

The 1 photo I posted just happened to be on a site that had been excavated. The slide is vegetated. A portion has cherry orchards on it.

We know it it has risk. Landslides here are not covered by insurance. We simply can not afford to walk away from our only homes. I am retired firefighter on a fixed income. I have house payments, I can't afford to pay for another place to live. We monitor the weather very closely. The US weather service even provides a special forecast for the slide area.

We understand that this is not the whole solution. Additional mitigation will have to take place. The next phase is very complicated and very expensive. We are seeking FEMA grants but the soonest construction could start is next spring. That's assuming the grant gets funded. Not a given.

I have attached another photo. I will have to post a link to the other photos because they are too large for this forum.

View attachment 172179

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I see there is standing water in the one picture. Has anyone investigate why the water is there now and not before the slide event. You might get immediate relief if you were to expose the area and allow it to drain down hill through a pipe or even on the ground. It appears that you have a subsurface spring that is hydraulically raising the slide mass. If you can immediately reduce the hydrostatic pressure, you might at least be able to stabilize the area to some degree and buy you time.

Not to say that the wick drains aren't needed but it seems that this is at least a month away or more. It seems to me that at least slowing the slide would be the first order of business. In fact, if this were my project, I would not even start the boring operation until there was at least some degree of stability. That means no appreciable soil movement over several days (week). This site condition has a significant risk for doing a boring operation as the very act of pushing the pipe is going to disturb the soil structure and can likely trigger a slide event.

Excavating a cutoff trench at the top of slide area might be enough to stabilize the hill side. The goal is to expose the spring and reduce the hydrostatic pressure. Then you can start the remediation work.

Just out of curiosity, what is the approx grade of this slide area? Are we talking "shallow and unnerving to see things slide sideways", or "if it goes, everything slides off a 250ft cliff"? Keep in mind, most people killed in slides are crushed when they are sleeping, so if it does go, there may be no practical way to escape. I'm not trying to be dramatic, I was a home builder for a while, my point is: remember that keeping a house isn't worth you life.

I have experience as an excavator operator and the owner of the company has agreed to help. He has a ton of experience. I understand the concern, Please read the articles I have attached in this and other replies.

The pipe bending has been identified as an issue. There are several ways to manage it. The best seems to be fabricating a pipe gripper that can be moved along the length, We will be fabricating one. They were also able to guide the pipe with a larger guide pipe and with timbers.

Please take a look at this article. This process had been done successfully in different parts of the world.

http://onlinepubs.trb.org/onlinepubs/archive/studies/idea/finalreports/highway/NCHRP076_Final_Report.pdf

The next step in mitigation is cutoff french drains across the slope. At some points the drains will need to be 30 feet deep. The terrain is steep. The soil is wet. The trenches will be 200' long. Multiple trench boxes will be needed. That is a disaster. We plan to use biopolymer trenching techniques. That is going to be expensive and will require a grant.

As a bit of context for folks not experienced with the portion of the Pacific Northwest in, and to the west of, the Cascade Range, there are large areas of weak soils on slopes which are prone to landslides when saturated with water. These landslides range from affecting a house or two, to cutting highways, to obliterating entire neighborhoods and killing dozens (e.g., 22 March 2014, Oso, Washington). Before the advent of LIDAR, many, many of the historical landslides were not identified. Even the clearly identified landslides, and their hazards, were not well communicated to the general public.

We now know that a historical landslide is quite likely to slide again. We also know that we've developed on top of, or directly in the path of, many geologically active landslides. There is no program to buy out or relocate families currently living in these zones. In fact, the current discouragement of (re-)building in known floodplains, weak and feeble as it is, is a strong and vigorous program compared to keeping people off the landslide areas. People who have settled in such a zone have to dealt with their situations as best as practical.

In most cases, the source of soil-saturating water is not springs but rainfall and drainage from further upslope. And this part of the PNW is known for long periods of rainfall. I don't live near the OP, but had standing water on my lawn for weeks in late winter and the first part of spring this year.

(We also know that Seattle area development has moved well into the zone of pyroclastic flows and mud lahars originating from volcanic Mt. Rainer, but that's another story.)