Horizontal drain alternatives 8

[Hbone]

I have a fairly steep cut embankment, stability problem.

The embankment consists of varying sized cobblestones and alternating layers of varved clay--glacial till soils--with an underlying layer of firm gray clay. The slide is of a circular / rotational type and is visually heaving at the toe almost four feet high. The top of the embankment has a 4-5 foot scarp, and accompanied by tension cracks throughout the upper portion of the bank.

At the bottom of the embankment is a truck terminal lot, light poles--which are now leaning at 15 degrees--and a water main about 15 feet ahead of the tip of the slide. Through test boring, and utilizing the PASTABLE Slope Stability Analysis Program, we have determined that the major cause of the instability is due to a large amount of underground (an artesian head) and surface water.

Recommendations include the installation of horizontal drains, and from what I have been reading throughout the threads here, that horizontal drains in this type of soil may not be feasible. Could this endeavor be an expensive risk and unnecessary risk?

Limiting possible solutions is the property boundary at the top of the embankment--eliminating the ability to cut a shallower bank.

Any ideas or thoughts would be greatly appreciated.

 

[VAD]

I would presume that this slide occurred sometime after the slope was cut. If this is the case then the re-establishment of the water level within the soil mass has contributed to the problem. Before the cut, the site was likely stable. This slide has been occurring gradually and was probably observed before the failure described occurred.

The recommendation of horizontal drains is to reduce pore water pressure. This should have perhaps been done earlier based on my interpretation of problem. I could be off track in my evaluation. I am trying to piece together the case based on your info provided.

Your site appears to be constrained at the top and bottom. The toe has shown itself as well as the scarp, hence the slide is well defined probably sliding on the hard layer. This is often the case when gravelly materials overlie clay strata and seepage is involved. The reduction in weight above hard layer is a contributory factor here as well.

Horizontal drains may very well work as you can tap into source and drawdown water levels. Varved clay should allow reasonable drainage. Be careful that you do not cause a problem upslope especially if residences are present.

You will need to rebuild area and this will require moving away some slide material. Drains my get destroyed and would need to be located, manifolded and protected so that water can be led out from fill without creating new instability. This is done by use of drainage pipes/conduits from manifold into trench drain.

Another option is to use drilled piers at the toe. These may not have to be deep. These will prevent further toe movement. In some cases this may be required at the head of slide if there is danger of losing some thing important upslope. Slide material can then be removed as required and drainage measures implemented. Gravel blanket drain at the head scarp my be of benefit if seepage is observed.

Generally, one can see this effect if source of water is not deep. Testpitting can assist in defining seepage zones. Some info has been obtained from your drill holes which may be useful in your evaluation.

In the end you may end up using a toe restraint and drainage measures.

I guess one can come up with many possible solutions. the challenge is to decide on one that is not too costly and would give a lasting solution. Not forever, but at least for 20 years. The problem in deciding which way to go is the uncertainty of being defeated by failure reoccuring in a short time. This often happens to all of us.

How to remove slide material without causing further distress is also one of the issues you will also need to address in the overall scheme.

With some more feedback such as the height of cut, actual water level observations etc, upslope constraints and ideas from others I may be able to provide some more thoughts.

An interesting problem.

Good Luck

 

[Myoho]

THe cause of the instability is most definitley the varved (laminated)clay. THis has been triggered by the ingress of water into the varved clay - probably due to a recent excavation within or above the slope.

Installing horizontal drains, or any type of drainage would further hinder the problem and probably cause a total collapse.

I have recently stabilised two cuttings with very similar conditions in the varved (laminated)clay. I designed soil nails at 20 degrees to the horizontal. This has the effect of utilsing the inherent strength of the stiff clay. As you probaly know the Varved clay has very little strength in the horizontal plane, where the slip is occuring C' = 0, phi' = 16 degrees.

I find your problem very interesting - if your require any help with the stabilty design I would be glad to help.

Regards,

Rob Kitchen.

 

[Hbone]

I greatly appreciate the help so far on this issue. Every bit of information learned and ingested helps me in making a solid decision. I would like to answer the questions asked by VAD and Myoho and possibly clarify the situation some more.

The cut is almost 6 years old, and the 300 foot affected section has been sliding for the past 4 that I am aware of. It is approximately 46 feet high with a 21-degree slope and has a center swale.

We regrettably tried to "fix" the problem a couple of years ago by digging a deep trench at the base of the hill--where the drainage ditch is--and filled it with large stone. The idea at that time was that it would provide a base for the embankment to sit on. It made the situation worse and by the next spring our pavement was heaved up in that area three feet.

There is no danger at the top at least buildings or populace wise. There is however, an old pond, now a swamp, directly above the area affected. A small stream, which runs most of the year, runs along the top of the embankment directed by a swale. There is observable seepage throughout the bank.

VAD, is the gravel blanket drain designed to head off shallow spring water or to collect surface water from above?

We bored six test holes and used three of them to monitor the water levels. One on the bottom (1), one in the center (2), and one up near the top (3). The water levels were as
1- +1' high in the tube
2- 7' deep
3- 13.5' deep

Myoho,
You seem to take an opposite stance on the horizontal drains. Why do you feel that any drainage scenarios here would only promote further distress? Wouldn’t getting rid of the water in some way help to stabilize the bank? I am curious to learn your reasoning here. I would also like to thank you for your offer, and I will keep updating this thread as the project progresses.

VAD, I also like the way you describe a solution. I am sure there are people in my company that believe this will be a permanent fix, --forever!

Thanks again VAD and Myoho

 

[Focht3]

Your problem is entirely too big for soil nails - in many ways.

The pond and creek are your sources of water - that's pretty obvious now. I think VAD is generally correct in his approach, although I haven't had much luck using drainage to stabilize slopes.

I do differ with VAD on one item: the piers may have to be pretty deep for three reasons: 1) the cut for the water line has reduced the available resistance behind the piers to a depth equal to the depth of the trench cut for the water line, 2) the water line can't take too much force, and 3) the height of your slope (which VAD didn't know.)

 

[VAD]

Many thanks for comments and additional information. The blanket drain is to collect water from subsurface seepage upslope. I note that the depth of water is 13.5 feet. Reason why horizontal drains were recommended.

However it may be possible to have a drain dug within head of the slide which could act as an interceptor. You may need to have an impermeable cutoff on the downhill side so as to prevent seepage to enter slide. This is often done by stitching an impermeable liner to a geotextile and using this to form the sides of a trench drain. This concept has to be looked at carefully to see if it fits the site problem. This does not work at all times.

Well, the above are just suggestions. I have learned over the years that understanding the site through a site visit is utmost important to the solution to be used. You are the only one that can put the system in perspective and decide which of the alternatives you should try.

Did the gravel drain have a pipe in it?. If the stream has to run at the head of the slide,is it possible to line the channel with an impermeable liner to prevent percolation into the slope. Can you drain the swamp?. These may not be practical solutions.

I would advise that you look at all alternatives as eventually you will have to make a decision.

I hope that you receive more ideas as this problem may not be an easy one to solve.

Cheers and keep us posted.

 

[VAD]

Hbone: Just a few additional questions if you do not mind as I am trying to get a clearer picture of the slide.

In para 3 of your response, I gather that a drainage ditch was at the toe of the cutslope which is also a roadway ditch. This ditch was excavated to some depth and backfilled with large stones. Was a pipe placed in this excavation? Where did this ditch drain to?.

This deep ditch would no doubt have received water and should have been constructed to drain. Can you daylight this stone ditch if it was not constructed to drain?. Was the concept one of the stone ditch being a shear key?. When you speak of "base of embankment to sit on" para 3,is the embankment here the cutslope as you previously referred to as the "steep cut embankment"

I am going to put forward one thought. If the deep ditch you created is not draining then with the slope on the move and the ditch being a reservoir for water then the movement of the pavement is of no surprise. The idea of a "toe drain" was not a bad idea but could have been if it was not constructed to drain. The plus 1 ft rise in the lower borehole may be providing this indication of poor drainage.
in the toe area.

You may wish to do some probing with a backhoe within and adjacent to the slide area to determine more what is happenning so that you can develop some more ideas for remediation.


Cheers

 

[Hbone]

VAD,

Thanks again for your insight.

The cut embankment runs the entire length of the facilities, some 2,500' or so. At the bottom of the embankment is the ditch, which was designed to drain both directions and the area being discussed is mid-way where the direction changes. On the other side of the ditch is the blacktop lot or truck terminal where the tractor trailers pull up to the loading docks. The first 5 feet of the black top which runs along this section has curved up three feet high or so, actually creating a small pond when it rains since it cannot drain. The water main is about 15 feet in under the black top.

The stone ditch that was built a couple years ago does not have a pipe in it, neither did the original. The ditch has since heaved up so high it stands clearly 2- 3 feet above the black top. Originally it set at least 2-3 feet lower than the black top surface.

Yes the main idea as I understand it--I have since become more involved--was that the stone would provide a base for the embankment to sit on. You bring up an excellent point though when you mention the drainage of this stone ditch. As it was constructed the stone section was much deeper than either side where it would have to drain out, so, there must be standing water in the ditch under the stone.

The ditch is open, so yes we can daylight it (if I interpret this question correctly)

Focht3,

Thank you also, for your help.

As you probably well know horizontal well drillers are not readily available,--around Upstate N.Y. at least--and the recommendation to install horizontal drains has been a catalyst for debates among the contractors chosen to look at and bid the job. They are eager to try the piers or retaining walls versus getting into something they do not fully understand or have any experience in.

Hope this additional information is helpful, Thanks again.

P.S. What are your feelings about tie-backs here?

 

[Focht3]

You can construct a tied-back wall. But I'd be concerned that the slide would pass beneath the wall. And what would you use for anchors? Where would they be located? Are they "sufficiently" outside the slide zone?

With the slide geometry you have described, it would be one hell of a wall!

Another approach: construct a drilled shaft wall behind the slide (at the top of the slope.) Use tie-backs to cut down on the wall's embedment and section. Now remove "some" portion of the soil at the top of the slide to reduce the driving force. Install a drain against the drilled piers to intercept some of the seepage. Rebuild the slope with lightweight aggregate (kiln fired clay) or styrofoam blocks (preferred.) The idea is to reduce the driving force as well as reduce the piezometric level. I used this approach on a project on Clear Lake near NASA's Johnson Space Center.

This will give you a permanent, if unusual, repair.

 

[VAD]

Thanks for the additional information. Focht3 has provided you with a solution. There are many possibilities but in the end one has to make a choice and this is often difficult and if not careful could be expensive. For this site you will have to spend some money. In my experience if we tackle the problem when it is observed by instrumentation etc we could save money but this is not always easy to sell as most people like to wait until they can observe something dramatic. Then its is too late as the soil is all broken up and you are faced with a "dog's breakfast".

The solution I propose is of the same type as Focht3 but instead I will opt for placing the piles at the toe of the slide and without the use of tiebacks. The location will be on the inside of the cobble drain and I have presumed this will be about 8 to 10 ft inslope from the original toe of slope which can be judged from the non failed sections. I will propose the use of 600 mm diameter drilled shafts reinforced with Steel H-sections at least 10 inch by 10 inch. The depth of embedment is proposed to be 10 metres minimum from the top of the road level and piles spaced 1 to 1.5 m on centres. I would use 1m for this site. Again the non-heaved road elevation is used as a bench mark. This will allow at the proposed location of the pile some 4-5 ft of pile stick up with the rest below ground.

The description provided re heave etc has allowed me to determine that the depth of sliding at the location proposed for the pile is around 8-10 ft maximum. I do not know the stratigraphy in relation to depths and you may wish to check on these assumptions.

I will utilize the piles as free standing as I believe that the slide is in a resting position at this time and does not wish to move unless the heaved portion is excavated.

After installing piles, I would excavate on the road side of the pile and backfill with large sized granular material. Your excavation may encounter water as you have indicated that the cobble ditch was deeper at this location. You should check this out re water and if this is the case then you should either drain that section across the road if feasible (perhaps not). The other way would be along the 2500 length of the toe of cut. These schemes need to be explored to reduce costs on trenching etc.

On the other hand, a new ditch could and perhaps be better to be installed on the upslope side of the piles and running along the length of the piles. The outlet ends are to be allowed to drain either on ground surface or to some underground system perhaps the storm sewer if this is feasible and can be allowed. The outlet location would depend on your grades longitudinal to piles or transverse and what topography exists. I am not sure of these aspects and you are the best judge.

The upslope side of the piles can be lagged but may not be completely necessary but this can be done with timber to reduce costs. If you have extra money you can connect the pile heads with a grade beam. The depth of this can be made to provide support for the slide mass and provide additional resistance and keep the heads of the piles from distorting.

The soil mass that has slid may not be required to be removed en-masse and regrading with some drainage at the head or herringbone drainage connected to a main ditch that connects with the longitudinal drain at the pile location may all that may be required for this site.

The above are my thoughts. Very often such thoughts can change when the site is reviewed. Again you will have to be the judge whether these thoughts have any merit.

There are other systems as well such as screw piles but these would have to be at least 10 inch diameter piles. This system allows quick installation but spacings have to be closer and tops should be capped with a concrete beam. I have used gabions infront and around these for soil support etc insted of the capping beam.

Would be interested to know what will be finally done.

Good Luck.

Some pointers

1.This work could lead to a nice practical paper for publication. There is certainly a need for this type of information in the literature for practitioners and graduates.

2. Have you asked yourself why this section has moved and not the adjacent one. Well you described one aspect - the pond on top. Is the situation the same at the non failed areas.

3. You should by now know the depths of movement and must use this along with understanding of the subsurface stratigraphy to determine the depth of your piles. Failure to do this and some analysis can lead to failures despite piles. Hence what I or Focht 3 have provided must be checked by you to ensure that you are satisfied before taking the plunge.

 

[Focht3]

Hbone originally said (May 29,2003),
The slide is of a circular / rotational type and is visually heaving at the toe almost four feet high. The top of the embankment has a 4-5 foot scarp, and accompanied by tension cracks throughout the upper portion of the bank.

And in his second posting, Hbone added (May 30, 2003),
The cut is almost 6 years old, and the 300 foot affected section has been sliding for the past 4 that I am aware of. It is approximately 46 feet high with a 21-degree slope and has a center swale.

I disagree with VAD when he declares that the slide is only 8 to 10 feet deep at a distance "about 8 to 10 ft inslope from the original toe of slope." Your description sounds like deep-seated movement to me; my best guess is that the slide is at least 20 feet deep that far in from the original slope toe; after all, the slope has risen 4 feet just due to the slope's 2.6:1 (H:V) original declination.

Trying to brace the toe of this deep-seated slide will require at least 36 inch shafts (~0.9 m) with a minimum of 2% steel in a rebar cage (and probably more.) You will only have 6 inch (0.15 m) gaps between the shafts, and a cap beam will be a requirement (regardless of the wall's location.) And they'll need to be at least 14.9 meters (45 feet) deep, since you can't count on the water line's back fill to 4.9 meters (15 feet) to offer much passive resistance. (My gut - when I look at the cross-section you described - says the slide surface wants to pass through that water line's trench.) And numerical analyses may dictate the need for much deeper piers, since the failed materials offer significantly reduced resistance. The piers will have to pick up the difference. And that's hard to do for tall, steep slopes like the one you have described.


I strongly disagree with the notion of putting gravel on top of the slope. Why would you want to increase the driving force?! The gravel will do the following: 1) increase the driving force, 2) offer little sliding resistance, 3) allow easy infiltration of surface water and debris, and 4) improve drainage in the zone where the gravel is placed. I see three strong negatives and one (weak) positive. Drainage is a weak advantage because it's where it is does the least good: the top of the slope.

On the subject of horizontal drains: your experience is typical. You will have to contact a specialty geotechnical contractor (Hayward Baker, Schnabel, etc.) to find someone who understands horizontal drains - and has the right equipment. Get out your checkbook!

VAD is dead-on correct when he says, You should by now know the depths of movement and must use this along with understanding of the subsurface stratigraphy to determine the depth of your piles. Failure to do this and some analysis can lead to failures despite piles. Hence what I or Focht 3 have provided must be checked by you to ensure that you are satisfied before taking the plunge. Truer words were never spoken!

You have a very interesting problem on your hands; let us know what you decide!

 

[VAD]

Well,we can go on to further discussion on this topic but it is at the stage where the thoughts provided on solutions have to be addressed by Hbone. Further debate unless additional information is provided will in my opinion be counterproductive.

I will rest my mouse and take a

Interesting debate and project.

 

[Hbone]

I thank you all for the great amount of information you have supplied for me.

I will definately post up-dates to this discusion as to the decisions and results.

Thanks again you were all very helpful!

 

[BigH]

I just saw this - and, wow, it is like a live debate!! I want to read up on it more seriously, then may have some to offer. I am going to add some early thoughts - and, I apologise if I ramble a bit - it is late and I am sweltering under 105deg and 95% humidity!! - no aircon!!

Do you have room at the bottom of the slope to install a stabilizing berm? I know it may eat away at some of your parking, but, modifications to the slope angle can be important. One way, though, is that you could provide a benched parking area - put on a 10 ft high 'bench" onto which you could park 2 or three lanes of trucks, then have the lower bench for other parking. Good site contouring might make this a possibility.

Can you turn your 21 deg slope into a benched affair? Can you cut off the top 8 ft or so, cut it back behind the cracks (or the day light?) - how much room do you have at the top - you say it is limited but how much. Surely your cracks on a neighbour's property is not being a good neighbour, eh? This is the sad part of the site - the inability of providing any graphics to help those understand a bit better. But, in the end, geometric adjustments only add a marginal increase (assuming realistic ones).

I note your point that you have "artesian" heads; yet your water levels did not suggest this - at least on my first quick reads through the various posts.

You've indicated that the slope is a glacial till varved clay. It is my experience that the varved clay is a glacial phenomenon (infilling 'holes' by alternating winter and summer depositions) but I would not characterize varved clay as a glacial till. There is a lot of good work in the Canadian Geotechnical Journal on varved clays and Terzaghi, Peck and Mesri have a nice discussion of it. Varved clays are alternating layers of "clay" - winter deposition when the ice on the lake is formed and "silty" - summer deposition when the larger particles fall faster. This leads to (1) anisotropic permeabilities and (2) strange undrained shear strengths. Regarding (1), the horizontal permeability will govern - the water will want to "run out" the seam rather than go down through it. So, any drainage should be with a view to providing horizontal collection and I don't see horizontal drains doing this.

Vertical drains, now that might be the rub! I might suggest you consider to use something like stone columns with a central drainage chamber (vertical holes filled with stone and then with a central slotted pipe inside). Installed at, say, 20 ft spacings along the top (or better yet a mid-point bench), they could act as a drain for the natural groundwater to enter. Then you use a siphon or automatic sumps (sumps on demand) to keep the water level low. You then draw down the water table to a much lower level and this will help in your effective stress analyses.

I'd love to see your cross section with geodata on the site as I am sure Ron and Focht3 would too. Then, perhaps some added, useful, comments could be given.

Suggestions of soil nails, secant piles, tie-backs will all be very expensive and also frought with some uncertainties. I remember Chuck Brawner saying that control of water is the key - it can increase your FS by 30%; geometry, less than 15%.

More later - sorry for the rambling.

 

[bbarrett33]

We have had recent success with combination soil nails and horizontal drains installed with the Soil Nail Launcher (

http://www.soilnaillauncher.com)

Installing hollow steel bars on a tight pattern provides tensile capacity of the steel bar plus the drainage capability of close spaced drains. Very impressive to see in the field. Cost for us has been half or less of other methods we have used in past years. Bob

 

[Hbone]

BigH,

Thanks for your insight here. It certainly is a privilage speaking with you all [smilejap]

I have a question on the vertical drains, which you describe: "stone columns with a central drainage chamber (vertical holes filled with stone and then with a central slotted pipe inside). Installed at, say, 20 ft spacings along the top (or better yet a mid-point bench), they could act as a drain for the natural groundwater to enter."

Where you mention using a pump scenario to drain the wells is it possible to put a drain at the bottom and use solid pipe going to the ditch system at the foot of the embankment; or do you see problems arising because of the disturbance to the soil while digging the trench for the pipe?

Your explanation of varved clay is very interesting. We are located just below the Finger Lakes, on the glacial front, --as you probably well know. The soil you describe is much like the soil make-up of the embankment.

There is a right-of-way at the top of the embankment of about 15'; so far,
the damage is restricted to our property. There is a bench in the middle, but no room to make the slope shallower.

I am going to work on setting up a web site where you and anyone else can view some pictures of the site and data logs, if you so wish. I will post the site with the site address when I get it made.

 

[BigH]

The problem I see with other than pumping down the well is how to connect vertical drains to it. If you want to tap into it at the bottom, it will be a job to execute the accuracy. To "dig a trnech" would be difficult and frought with support to get "people" into it; directional drilling would be the only way I would see but how accurate can they be?

Remember, why I suggested a vertical drain set-up is that the varved nature of the clayey deposit drains more readily horizontally than vertically - so you try to tap into the "column" rather than in a single line - for in this you might only tap a limited zone of varves. 20 ft spacings was my guestimate - but could be more widely spaced - this is something to look into - the basic gist is that the vertical drain acts as the "back end" of a horizontal drain - or finger drains if you are constructing, say, a tailings dam and want bottom drainage. You might have enough water entering to keep a syphon going which is why I brought it up in the earlier post.

Where in lower Finger Lakes - I am a "High Above Cuyuga's Waters" - man!! Loved the area but limited opportunities as I saw it unless tied into the University.

Best to you all - look forward to Ron and Focht3's thoughts - and will look forward to your web site posting.

 

[VAD]

Just to add a few thoughts on the vertical drains/wells based on my interpretation.

Vertical wells for drainage are generally installed using drilled shafts/bored pile holes which are interconnected at their base by belling to be effective. There are situations in which belling cannot be done. A large diameter slotted screen is used along with gravel to infill the drains. Casing may have to be used if the holes are slumping. A pump or pumps can be installed with trip switches to allow pumping when water levels reach a certain height. System has to be connected to power which should not be a problem.

Generally, stability analysis is undertaken and the level maintained where there would not be a chance for instability to occur if it is determined that pore water pressure was the cause for the problem. The best approach if one contemplates using large diameter vertical drains is to undertake a trial hole or two to see if the intended scheme will be worthwhile.

Alternatively, horizontal drains have been drilled to tap into the vertical drainage columns and allow the water accumulated in the columns to be taken out by gravity flow.

My personal view on stabilization of slides is that there are many possible solutions but many of these have to be eliminated for one reason or the other. The site conditions that exist, what is expected to be done to repair the area etc are important in determining the solution or sometimes combinations of solutions to be eventually chosen.

Wicks can also be installed horizontally to assist in removal of water etc. Do not forget also total excavation and replacement of slope is a stabilization technique as well, if it can be undertaken.

I think that when Hbone puts up his website the problem may become clearer to us, but nothing will come as close as the feel that one gets by walking over the site, reflecting on the terrain, topography, subsoil conditions etc and doing some serious thinking. Take a good contractor with you and let him understand what you are thinking. He may have answers to what you may think are impossible.

As it is often said, 90% of the answer to a foundation design is obtained through a personal site review of the drilling. The remainder is for the office paperwork.

Well, I better stop before Focht3 takes out his hammer.

Regards and

 

[Focht3]

Too late!



JUST KIDDING! Besides, the hammer often applies to me, too!

I look forward to the web site with photos, drawings, etc. - it will really help.

Good thread!

 

[Myoho]

Focht3 you state "Your problem is entirely too big for soil nails - in many ways". Then you state "my best guess is that the slide is at least 20 feet deep".

15.0m (50 feet)long soil nails at 22 degrees to the horizontal should be adequate length to stabilise a slope failure only 20 feet deep. So size is definitley not a problem. I have used soil nails up to 30m long (100 feet).

I think if you look carefully at the U4 samples you would be able to pick up some of the multiple failure planes. THis is how varved clay fails.

Varved clay is sometimes called Bible clay because when you take a sample out of a U4 it just opens up in your hand like pages in a book.

It has virtually no lateral strength at all and when water is introduced it sucks it up readily and softens the surrounding clay. The result being any slope greter than about 10 degrees just starts falling to pieces.

Unless you have specifically designed remedial measures for a slope of this kind before, you wont even begin to be on the right track.

One solution would be weight the toe with a gabion wall and reduce the slope angle with granular material. Build a 3.0m high Gabion wall at the toe of the slope. Lay terram and regrade to a shallow angle with granular fill (reducing the angle of the slope).

THis is quite expensive and not as effective as installing 5 rows of 50 feet long soil nails at 22 degrees to the horizontal at 2.0m centres horizontally and vertically.

Any other solution will not be totally effective.

Now hands up everyone - whos specifically dealt with varved clay before?