Slope Stabilization Techniques 2

[mraye77]

Hi, I am brainstorming for ideas to stabilize an overthrust developing on a dam abutment slope. It is a cut slope (8H:1V) in a clay/shale material, naturally given to slumping. The slope was constructed in the early '60s and is vegetated with local weeds and grasses.

Slope indicators in the area show a shear zone approximately 20 m below the top of the slope. The slope is creeping at a slow rate (generally a few mm a year) and no structures are in danger. The overthrust was first identified in 1996, along with some upland cracking. The cracking was filled and clay capped and has not reappeared.

If anyone has any ideas or suggestions for remedial techniques, or can point me to any on topic articles or web-sites I would appreciate it.

Thanks.

 

[Focht3]

Look at the Waco Dam failure studies. (near Waco, Texas - home of Baylor University.) As I recall, the problem was worse than you have described - but should give you some insight and ideas.

 

[BigH]

Also check out papers on Bearpaw Shale - from Saskatchewan, I believe - at least it is Canadian. Some of these shales have local shear zones (ancient) and you might be slipping on one such zone. Can you relieve the foundation pore pressures? Check out some work, too, of Chuck Brawner of University of British Columbia - great guy (home is in North Vancouver). He's done quite a bit on rock slope movements/failures and has always favoured porewater pressure relief. Keep us posted, por favor!

 

[georam]

How much access do you get for research paper in geotechnical field ?

I did some research work a while back and the following papers from Can geotech journal maybe helpful.

Also , an excellent book
Landslides: Investigation and Mitigation, transportation Research Board, Special report 247
editors A. Keith Turner and Robert Schuster
It costs US $45 but it is worthwhile (300 pp+ ?)

http://www2.nas.edu/trbbooks

Hope this helps

PS :
A number of landslides in midwest of Canada (prairies region from Manitoba to Alberta) are landslides in clay shale bedrock. The papers below are generally related to this type of failures.

Canadian Geotech. Journal 1989-1998
Topic: Landslide

Wetmiller, R.J., and Evans, S.G. 1989 “Analysis of the earthquakes associated with
the 1965 Hope landslide and their effects on slope stability at the site”. Canadian
Geotechnical Journal, Vol. 26, No. 3, August 1989, pp 484-490.

Hunter, Dal 1989 “A hydrochemical study of urban landslides caused by heavy rain:
Scarborough Bluffs, Ontario, Canada: Discussion”.
Eyles, N. and Howard, K.W.F 1989 “Reply”. Canadian Geotechnical Journal, Vol. 26,
No. 4, November 1989, pp 756-757.

Lefebvre, Guy, Rosenberg, Peter, Paquette, Jean, and Lavallee, J.G. 1991 “The
September 5, 1987, landslide on the La Grande River, James Bay, Quebec, Canada”.
Canadian Geotechnical Journal, Vol. 28, No. 2, April 1991, pp 263-275.

Pritchard, M.A., and Savigny, K.W. 1991 “The Heather Hill landslide: an example of
a large scale topping failure in a natural slope”. Canadian Geotechnical Journal, Vol.
28, No. 3, June 1991, pp 410-422.

Misfeldt, G.A., Sauer, E. Karl, and Christiansen, E. A. 1991 “The Hepburn landslide:
an interactive slope-stability and seepage analysis”. Canadian Geotechnical Journal,
Vol. 28, No. 4, August 1991, pp 556-573.

Shaller, Philip J. 1991 “ Analysis of a large moist landslide, Lost River Range, Idaho,
U.S.A.”. Canadian Geotechnical Journal, Vol. 28, No. 4, August 1991, pp 584-600.


Cruden, D.M., Keegan, T.R., Thomson, S. 1993 "The landslide dam on the Saddle
River near Rycroft, Alberta". Canadian Geotechnical Journal, Vol. 30, No. 6, December
1993, pp. 1003-1015.

Fell, Robin 1994 "Landslide risk assessment and acceptable risk". Canadian
Geotechnical Journal, Vol. 31, No. 2, April 1994, pp 261-272.

Kelly, A.J., Sauer, E. Karl, Christiansen, E.A., Barbour, S.L., Widger, R.A. 1995
"Deformation of the Deer Creek bridge by an active landslide in clay shale". Canadian
Geotechnical Journal, Vol. 32, No. 4, August 1995, pp 701-724.

Cruden, D.M., Thomson, S., Kim, H.J., and Peterson, A.E. 1995 "The Edgerton
landslides". Canadian Geotechnical Journal, Vol. 32, No. 6, December 1995, pp 989-
1001.

Corominas, J. 1996 "The angle of reach as a mobility index for small and large
landslides". Canadian Geotechnical Journal, Vol. 33, No. 2, April 1996, pp 260-271.

McAffee, R.P.,and Cruden, D.M. 1996 "Landslides at Rock Glacier site, Highwood
Pass, Alberta". Canadian Geotechnical Journal, Vol. 33, No. 5, October 1996, pp 685-
695.

Hansen, D. 1996 "The angle of reach as a mobility index for small land large
landslides: Discussion".
Corominas, J. "Reply". Canadian Geotechnical Journal, Vol. 33, No. 6, December 1996,
pp 1027-1031.

Finlay, P.J., and Fell, Robin. 1997 “Landslides : risk perception and acceptance”.
Canadian Geotechnical Journal, Volume 34, No. 2, April 1997, pp 169-188.

Cruden, D.M., Lu, Z-Y., and Thomson, S. 1997 “The 1939 Montagneuse River
landslide, Alberta”. Canadian Geotechnical Journal, Volume 34, No. 5, October 1997,

Finlay, P.J., Fell,R., and Maguire. P.K. 1997 “The relationship between the probability
of landslide occurrence and rainfall”. Canadian Geotechnical Journal, Volume 34, No.
6, December 1997, pp 811-824.

Wong, H.N., and Ho, K.K.S. 1997 “The 23 July 1994 landslide at Kwun Lung Lau,
Hong Kong”. Canadian Geotechnical Journal, Volume 34, No. 6, December 1997, pp
825-840.

 

[BigH]

Good list of references - Canadian Geotechnical Journal is still a great publication! By the way - the Scarborough Bluffs are not in rock - but in sands and tills as I recall. I've had a couple of projects on it as did our company. One of "reasons" for some of the landslides there is that dips of lower permeability seams is has a declivity "back" away from exposed face, building up an underground reservoir behind the outbreak of the clayey zone on the Bluff face. Then, as the Bluffs are undercut and a portion "falls" in, the reservoir is released and a much bigger instability can occur. I would still check out Brawner's work.

 

[VAD]

One of the problems with clay shales is that they often have bentonitic seams. If such material is present then failure plane can be at a relatively flat angle. While flattening the slope of such cuts is often used as a stabilization approach, this method also allows a wider area of soil to be exposed to the effects of rainfall etc and coupled with rebound which tends to allow the bentonite and shale to become softened. Stabilization of these types of slides is very interesting and often one has to live with the problem. You may wish to review the paper by PFRA on the Gardiner Dam published in the Canadian Geotech Journal. There is also the Devon Landslide by Eignbrod and Morgenstern some time in the early 70's. Geotechnical Practice for Stability in Open Pit Mining by C.O. Brawner and V. Milligan, AIME 1972, p 233-236.

In the mid 1980s in the Devon Area in Alberta for a 20 m deep approach cut for a highway bridge,we debated about cutting back the slopes at 6H to 1V and finally decided to leave it at 3H to 1 vertical because the bentonitic seams were becoming very softened as a result of water infiltration. It was conceived that a flatter slope would add to the problem.

Detailed geotechnical investigation allowed us to decide on the use of horizontal drains to remove seepage from the slope. The drains were fanned out and the outlet ends connected together to discharge in a pipe that was sunk in the highway ditch and taken out away from the problem area.

Here are some other references that you may find useful

1. NCG Technical Report NO 15 Empirical Study of Behaviour of Clay Shale Slopes

2. The Devon Landslide, Edmonton, Alberta bu Broscoe and Thomson

3. The Grierson Hill Slide, Edmonton, Alberta by Martin et al from a Canadian Geotechnical Conference Proceeding around mid 1980's.

If possible, I would be interested in seeing some background information on this problem as this area is one in which I have spent many years. There is a great joy in tackling and providing effective stabilizing measures for such problems areas. However, some are quite difficult and expensive to fix and one has to determine if this is the case with your site.

Interesting problem. Hope that the references help.

Cheers

 

[BigH]

VAD - Were your horizontal drains the vacuum type that Brawner was involved with??

 

[VAD]

BigH: No they were not. The type is the same used by the Claifornia Dept of Highways. Drill into source and stick in slotted PVC pipes which then bring water out of slope, then maifold and lead to trench drain outlet. Some people allow it to flow on surface into a roadside ditch depending on site conditions. I have not used the vacuum type but it may be similar to system used for consolidation of soft ground but may have different materials. I am guessing here.

Cheers

 

[mraye77]

VAD: I am intrigued by your suggestion of horizontal drains. My recollections from university suggested that horizontal drains would not be effective in draining a low-permeability material like the clay/shale I am dealing with, particularly because there is not well defined aquifer in the material (at least not one we have found yet). In such a case, would we install the drains below, at, or above the shear zone?

FYI - In 1962, construction activities, predominantly excavation, north of the Coteau Creek embankment (Gardiner Dam west abutment) reactivated an ancient shear zone. An overthrust and upland cracking developed. A toe berm was constructed over the overthrust to stabilize the movement.

In 1994, post-construction downslope movement of the Coteau abutment in the vicinity of a concrete, surface drainage flume led to the developement of a small overthrust bulge and structural damage to the flume. A joint gap was constructed in the flume to mitigate the effects of future slope movement. Upland cracking was located in 1996. The creacks were sealed and clay capped and have not reappeared. Continued downslope movements effectivley closed the flume expansion joint by August 1997. The joint was reconstructed in June 1998 with an opening of 100 mm, and has since closed about 80 mm.

Stratigraphy in the slide area consists of clay/shale overlain by 0.6 - 3.0 m of shaley sandstone and 0.15 - 3.0 m of Till. The sandstone layer may not be present north of the slide area.

Deformations in the vicinity of the flume are recorded at several movement lkuine pins and SIs. Deformation of the flume structure is monitored by recording the expansion joint gap width, and by surveying the elevations of the pag pins. Soil moisture testing in the vicinity of the flume begain in the spring of 2000. In November 2001, 13 standpipe (6" outer diam.) and 3 pnuematic piezometers were installed to monitor ground water levels and shear zone piezometric pressure in the movement area.

Chainage line surveys adjacent to the flume show maximum suface movement occurring near the toe of the overthrust (approximately 140 mm since 1996). Minimal surface movement occurred in 2001/2002 - attributed to the ongoing drought being experienced in Saskatchewan.

Subsurface movement in the Coteau abutment are recorded at several SIs. Inclinometers in the movement area show in the neighbourhood of 220 mm of downslope movement since the 1970's. Movement is occurring between elvevations 518 m and 520 m. To contast, an SI located south of the slide area has shown less than 35 mm of movement since 1964.

Shallow standpipe piezometers (6.5 m to 8 m) were installed in permeable materials across the slide area. Most stadpipes were installed dry or quickly became dry. Only three piezometers maintained a readable water level. One year of observations have been obtained. Additional readings are required to develope a response pattern for the piezometers.

Pneumatic piezometers were installed adjacent to SIs showing significant shear zone movement. Tips were placed in the shale material within the shear zone. Again, additional piezometer readings are required to develope a piezometer response pattern.

A comparison on SI and piezometer readings for the shear zone appears to suggest a positive relationsop between downslope movement and proewater pressure. Preliminary data tentatively confirms speculation that increased downslope movemnt during winter months is a result of increased porewater pressure.

 

[BigH]

VAD - I used the vacuum drains at Tumbler Ridge Table Tunnel portal exit - we stabilized a silty excavation slope using them. The work was done by one of Brawner's students and they have used them (1980s) in Malibu CA. We drilled 4 drains from same spot fanning them out into the hill. The vacuum part permits the equilibrium water pressures to stabilize much much quicker than by horizontal "gravity" drains. Mraye77 - you might want to consider the vacuum system.

 

[VAD]

Thanks BigH for the info on vacuum drains. Mraye77, well you are holding on to the tail of a tiger. Several years ago I was intrigued by the dramatic pore pressure response of piezometers installed in clay shale on which we were constructing a 15 m bridge approach fill. As a result we did intensive field work by installing multiple piezometers etc and taking care to install such and have controls etc.

This work confirmed that high pore pressures could develop in clay shale with moderate loads. I then began to look at the structure of the clay shale. I am a backhoe fan as one can see alot more than with a drill hole. If I am looking for water and want to understand the subtle stratifications in a soil then if feasible I always use a backhoe testpit so that I can get my hands dirty and be able to reflect on the nature of the soil. I found that the shale was fissured and as I broke the pieces I could see thin films of water, if one looked intently when breaking pieces. I had this material examined at the University level under a scanning X ray microscope? and the micro-structure was such that water was able to pass freely although one often thinks that that shales are impervious.

The fissuring etc results from rebound etc. I proposed a research program into the understanding of the pore water response in clay shales but it was not funded. Inspite of this I gained valuable field experience which has helped me to examine problems related to slope instability in this material.

Your problem is, however, one of great magnitude and after reading your response I dug out my file to find the paper on the Gardiner Dam by Jasper and Peters which you know all about and in the same file was my piezometer plots of the approach fill that I worked on several years ago. Interesting stuff.

I am not sure that horizontal drains will work in your situation as several ideas were looked at on the Gardiner Dam. One of the things that I see with your site is that the shale is overlain by sandstone and in other areas the shale is overlain by sand.

Based on the fact that water from the reservoir is in communication with the sandstone and shale the drainage approach is probably not going to be the answer. Movements can perhaps be minimized by keeping the reservoir level low but this in not a very desirable approach as water is needed, hence the dam.

Here is and idea, I do not know how practical it is but you can give it some thought. It seems that to reduce the pore pressure caused by seepsge from the reservoir then a possible solution is to have some sort of deep grout curtain to block the seepagefrom the reservoir and hence reducing the pwp downstream. This can be explored with deep soil mixing technology. This would be expensive but the expense may be warranted for this scale of project. Practicality would have to be examined and discussed with speciality contractors. Just a thought.

I once tried to have that concept used on a site with similar deep seated movement and multiple slide zones that caused movement first at 20 m depth when less than 1 m of fill was placed for construction of a bridge approach fill. However the cost of getting the equipment down was prohibitive. Luckily, we had instrumented the area and as a result the bridge structure was not constructed. The foundation material was clay shale with bentonitic intrusions. Another great site with interesting geology.

Well do not pull all the hair out of your head only some. I know the feeling very well. Your problem is not simple and as Dr. Morgenstern once said - can't remember exactly but I think it goes - despite our efforts the "landslide devil" often laughs at us. In some cases we yearn desperately for a solution to such problems but we may have to live with the situation.

I do not like to be defeated by such problems so I can see your end as well.

Cheers

 

[BigH]

and a complementary observation I heard years ago: "No matter how well you think you've designed for water; it is GOING TO GET YOU in the end."

VAD - great on your recent inputs. Histories of experienced engineers is what, I believe, these threads should showcase.

 

[mraye77]

VAD & BigH: minimization of movement is definitely what we are going for. Because slope movements aren't endangering the dam they are viewed mainly as a nuisance, so extreme measures aren't warranted - no retaining structure, and probably no grout curtain. I was leaning towards drainage of some kind to relieve pore-water pressures in the shale. What are your views on trench drains?

Thanks for the great info. It has been very interesting.

 

[Focht3]

Another excellent post by VAD. I am particularly fond of his use of a backhoe to look at problems; I have also found pits and trenches to provide enlightenment on difficult problems. All too often, engineers use borings to investigate problems that are not amenable to investigation using small samples. Sometimes we need to think outside the box -

Like VAD, I don't think that drainage will provide you answer, unless there is something about the problem geometry that we don't quite grasp.

I would like the full Morgenstern quote about the "landslide devil" - if anyone has it. I'd like to use that quote on a few clients...

 

[VAD]

Focht3 and BigH many thanks for the comments made.

To answer Mraye77's question regarding trench drains. You can probably achieve drainage by this process but I am not sure that in your particular situation it would prove to be very beneficial. However, you are living with the problem and I defer to your site appreciation. I have used trench drains in this material type and once they get going they can provide a continuous flow. Again the topography, source, etc need to be looked at. Would the drains act as a conduit that may be undesirable is a question that you should look at as well?.

Cheers