Use of Inclinometers to prove slope stability 1

[dlustri]

I am in a situation where counsel from an opposing side wants to use inclinometers to prove/disprove a slope stability problem. Is the following statement defendable?
Using inclinometers over short periods of time(i.e. 3 months) does not indicate the true extent of movement.
A brief background, a contractor dumped fill on a virgin slope (over 100 years with heavy tree growth)took down the trees, buried them at the base of the slope and dumped fill on top of the organics. This happened three years prior to the request to use inclinometers to monitor the slope. I maintain that using it now for three months will not show the true extent of movement because the majority of movement would happen immediately after the fill was placed and then taper off over time. The decomposition of organics would cause this movement to extend over a longer period of time. Can anyone shoot any holes in this argument?

 

[dgillette]

There is A SMALL CHANCE that it would show further movement IF the three months were timed exactly right, catching a spell of particularly wet weather that would cause a little more movement of a slide. (I've seen a number of slides that moved a little every spring when the snow was melting, then stopped in the summer when things dried up.) A far more likely outcome in three months of monitoring is that it would show no further movement, as you expect, regardless of what happened three years ago. Therefore, I agree with you about 99 and 44/100 percent. Depending on who is making the decisions (a judge?), the data showing no movement could be a red herring. I think you are smart to try to head it off, rather than argue about the data later, although the opposition might argue that you want to hide something.

 

[dlustri]

I appreciate the percentage certainty, I am not a practicing geotech, but I was hoping that I didn't lose all of my common sense! I am against it for the same reasons you stated. I have to find a way to defend NOT using them for the very same reason. More background, my house is at the top of this slope, my garage is now cracking, and I caught them burying the trees and destroying the slope. I told them to fix it and they didn't. The only 'objective' evidence so far is soil bore tests proving there was no compaction and there was organics in the fill. Is there a way to use the inclinometers in my favor?

 

[dgillette]

Just like Ivory Soap, I'm 99 44/100 percent pure.

Right off, I can't think of any way to use the inclinometers in your favor unless something were to destabilize the slope slightly, like a long spell of wet weather, or an excavation at the toe of the slope to get the trees out or verify their presence. It's possible that the movement is generalized strain distributed over a large thickness, rather than a distinct sliding surface. If the former, it's even less likely that inclinometers would see much. You may get more benefit from reporting the timing and location of the settlement, any signs of a scarp or more general vertical offset at the top of slide, etc. Without knowing details of the geology or geometry, I can't say much more.

Were any of the old trees tilted or curved in such a way that they might show movements of the slope prior to the work on the slope? Too bad there's no way to post pictures here.

If the garage is on the fill, the poor compaction could explain it pretty easily.

Rephrasing some of what I said before, data from the inclinometers would mean nothing about previous movement of the slope UNLESS they show movement again, in which case they are on your side. If no movement is detected, the data are neutral. Inclinometers often show very small generalized tilting movements (as opposed to local shearing) when they are first installed, before they stabilize. If that happened, I suppose they could end up testifying falsely for your side if the tilt was downslope.

 

[dlustri]

Thank you for that! I have numerous photos of the site before, during and after. I have a detailed topo of the site before the fill was placed and a topo after.
Your comment about the trees is great! I will forward that to my attorney. The house was built in 1911, the garage was built in the 1940's. There were cracks in the garage before this. But they have been there for decades(according to prior owner who lived there for 50 years). Now there are new cracks in the cinder block of the garage. (The entire garage is block) The garage was approximately 15' away from the edge of the slope, which then fell approx. 25' to the bottom. The bottom is another flat area before reaching a creek some 150' away. The new cracks appeared approx. 2-3 months after they did their work, and have become larger since. All of this info points to a favorable conclusion for me. I think opposing counsel, who has done numerous trials like this, is trying to pull something. Thank you for your comments, they definitely help.

 

[DRC1]

Some slopes creep over time . Others go in fits and stops some are fine for years and suddenly go, usually, but not always in a period of heavy rains. I agree that slope inclinometers could very well be inconclusive. However a good geotechnical firm, esp. one with trial experience, could review the site, perform some stability calculations, hopefully, conclude the factor of safety is marginal, that the movements you are seeing could be expected from such a slope, and the poor construction practices are the cause of the low factor of safety. On that note dumping fill and burying trees does not provide a stable platform to build on, however, it is not likely that it would effect the stability of the existing slope. That is the dumped soil and trees would simply move down the existing slope, so if your structure is founded in the old slope, the recent activities may not be the cause of the movements, although it is hard to say for sure with out seeing the site.

 

[dlustri]

Thank you DRC1 for commenting. The contractor bulldozed the slope in order to knock the trees down, pushed them to the bottom and then brought fill in on top of them, extending the top surface an additional 10'. They basically compromized the slope by "stripping" it of vegetation. In addition to that, they place a tremendous amount of weight/fill on the slope that hasn't seen anything in over 100 years.
My soil engineer expressed that this added weight alone could cause a slope failure, but is more than likely a supporting cause.
I'm in the middle of depositions for this problem. They are trying to build a case against me. My initial reaction to now placing inclinometers was that it was an inappropriate timeline. Placing them three years after and for only three months won't prove anything, but it may be considered as reduced damages because the movement has stopped. Unfortunately, I am not dealing with other engineers in this case. So I'm sure someone will think that just because it stopped(or reduced to negligible movement), that everything is ok.
I am looking for a hard fact as to why inclinometers are inappropriate at this juncture. Two sets of soil bore tests were performed, one by me and another by the defendants. Their results mirrored mine. What other tests would be 'fair'? And what other facts are there to search for?

Thank you for responding.

 

[Zoomzoo]

Okay, let me wade in...

Inclinometers tell you where the slip plane is (if there is slippage). Frankly, I've never heard of anyone in their right mind who would propose using a set of inclinometer readings over a three month period to prove a slope is safe. I don't care what the past history of the slope is - it's ridiculous. There are so many unknowns: 1) Where to place the inclinometers to get readings of what is really happening in the subsurface? 2) What frequency of readings is appropriate (one reading at the beginning and one reading 3 months later, or?, 3) What is an insignificant reading for this type of case in the limited time frame?, 4) Will the weather effects during the 3 month test period mirror that of the next 50 years (which is a typical engineering design period)?

You could challenge opposing council to produce evidence of three other projects where inclinometers were used over a limited time period (three months) in a similar manner as your site to prove that a hillside slope was safe for a design life of 50 years. That will probably send them scampering.

The fact is the contractor, who destroyed the slope while most probably ignoring every applicable code, and then backfilled with who knows what while meeting none of the regulatory requirements, has to prove to you that the slope is safe. Perhaps, as a good-faith measure, he could take out a fifty-year hillside slope insurance policy to insure your property against any slope failure due to his crass stupidity...

Zoom

 

[dlustri]

Zoomzoo, it is beginning to look like opposing counsel is using a tactic to instill doubt in the fact that the slope is actually failing or has failed. If I don't allow it, they will claim I am hiding something, If I allow them to do this exercise, then I will have the burden of disproving their bogus biased results. Even if I prove them wrong, a jury will hear that it may not be failing anymore, and that could stick in a lay person's mind.

 

[dgillette]

Obviously, we can't judge from afar exactly what the cause of your settlements is, without knowing geology, topography, climate, timing of movements, etc. However, we all seem to agree that the inclinometers are, INTENTIONALLY or NOT, a smoke screen, red herring, or whatever your favorite cliche might be, especially if they are only to be in place for three months. You and your attorney might want to take the offensive against the IMs on this basis, explaining in great detail how they work and why they cannot measure what's already happened prior to their installation, and that the results are likely to be misleading. I glanced through a couple of books I have on instrumentation (USBR's "Embankment Dam Instrumentation Manual" and T.H. Hanna "Foundation Instrumentation") for good direct quotes you could use, but had no success. I thought that would be more helpful than just citing your internet buddies who you have (as far as you know) never seen face to face. (Plaintiff's attorney: "What basis do you have for opposing installation of the instruments?" You: "ZoomZoo and dgillette were pretty adamant that they were a bad idea." PA: "Who are these arrogant, opinionated people and what are their qualifications?" You: "Well, they're these two guys, or maybe two ladies, who...")

It occured to me today that the IMs would, with at least partial validity, support the case against you if the settlements and building distress continue and the IMs don't show movement. (I say "partial" here because they would have to be located right to measure deformation.) Then, you would have to be looking at other mechanisms for the damage. If no further distress occurs in the 3 months, then you're back to zero net value from the IMs.

Bon chance!

 

[dlustri]

Oh, you can't PE stamp through the net?! I have soil and structural engineers for this lawsuit. The only reason I posed the question is to make sure that I haven't forgotten everything I learned! I thank both of you for reaffirming my opposition to the defense counsel.
This begs the next question, is there anything that can be measured after the fact that would be of any use to this lawsuit?

 

[dgillette]

Evidence of a scarp or offset in the areas not directly disturbed by the construction. A visible scarp next to your garage would be pretty strong evidence

New or old tilting of trees, associated with new movements caused by the recent work or older ones that could have been reactivated by the excavation.

Old photos you might have that can be compared against new photos to show ground movements.

That's about all I can think of.

 

[Dirtguy4587]

You mentioned earlier about soil boring results confirming uncompacted/organic material within the fill the contractor placed. Personally, I think that would be a pretty good indication that the slope had a high probability of deformation.. Other than that, dgillette's thoughts on scarps, cracks, toe heave, etc. would be the other 'visual' signs of failure.

 

[BigH]

Keep in mind too is that while the slope might 'show' stability at present, it might be a false stability in that slight environmental changes in the near future could restart any movements. As pointed out, the SI's only give you the 'snapshot' scenario - not the 'movie'.

I don't know if this work was done for you on your property or done for someone else on downhill property that is causing an affect on your siutation. This would be interesting to be filled in on this.

If the latter, then you should be pushing the opposing side to show that the slope is in fact stable by investigation, site characterization and stability analysis and what kind of factor of safety that you have - even in the best of time, you want to have a computed factor of safety of at least 1.3 (assuming no risk of life, etc.). And, better yet, that he did the necessary homework to confirm this prior to starting the work. If the former, it might be trickier.

 

[dlustri]

Thank you all for your comments, be forewarned, the following is a summarized history of the events, with as many major points that I can think while writing this...

A contractor signs a waste agreement with me to bring the spoils of a school project across the street to me(in exchange for some work around my home, no money exchanged). I live at the top of a 35' slope (approx. 2:1) My house was built in 1911, garage built in 1940's. Garage is cinder block and was about 12' from slope edge before work began. The slope was heavily wooded. Basically virgin ground for at least 50 years, no signs of dumping on the slope. In 2000 I bought the house, inspected the structure, found old cracks in the garage. The prior owner stated that they were there for decades. (Prior owner lived there for 50 years). The fill was to be placed on the vacant lot next to me. The school project had a mistake which required 2' more excavation over the entire footprint. The contractor began filling closer to my home. Our agreement stated that a wrap around drive was to be installed around the back of my house(between slope edge and garage) Without my approval, contractor bulldozed the slope, trees and all, and placed the brush and trees at the toe, then proceeded to fill on top of it. I caught them at the end of the workday and told them that the organics must be removed from the toe or my drive would fail. I was told they would. Next day, everything is finished. I invite the owner of the contracting co., to come out and inspect before I pour my concrete drive. I repeat the question of whether or not the organics were removed from the toe of the slope, he said they were. Approx. 6 months after concrete is poured, and after rains, the slope has visibly moved, with a 'belly' protruding from the lower third(just above the trees).
This is occuring in 2003, the house was completely painted in 2001. It went from white to gray. The old cracks were filled completely on the garage and then painted. Now, those cracks have opened up and there are new cracks all around the garage. They are very visible due to the white paint through the gray.
I told the contractor verbally before the project, during the project, and after, that the area under my drive needed to be compacted. I was assured it was.
SPT test right at the top of the slope resulted in between 2-6 blows per foot for about 10 feet. Split spoon from the deeper bore had large chunks of tree.

You would think that I have a strong case. All of my verbal warnings to the contractor can not be proven. Only the results from the soil bore tests in conjunction with the new cracks in the garage prove any movement. Some additional evidence includes a detailed topo. done before the work and then an asbuilt after. This only proves where fill went. Photos from before of the garage and slope from before and after. (I have these photos because I have been restoring this 1911 house for the last five years and have been documenting it)

Some more info, the toe of the slope is my property line, the slope is sliding onto my neigbors property now. My structural engineer and soil engineer determined that caisons are what would be necessary to remedy this at a tune of $800,000. (Gravity retaining wall would move too much and not support immediately the current structure) I do not want a lottery ticket here. I want the costs for pursuing this and someone to either determine or insure the slope from future failure. They sent a soil engineer out and he found the same results as mine did.

I am $20K out of pocket so far, and depositions are next week. This is why I am responding to this at 1am!

Thank you for taking the time to read this.

 

[chlharry]

I was browsing through this field and read all the comments but failed to see any mention that denuding a slope of vegetation, ie canopy cover as well as ground cover, will significantly increase waters ability to enterthe slope. Seen this many times in the mining industry. Net result, pore pressure up, effective stresses down. As we all know, water is the problem with any slope failure

 

[bandraoi]

put telltales on the crack in the garage wall - this will prove that movement is occuring.

If you slope is still sliding into the neighbours property then movement is still occuring. Since it's surface movement, repeated topo surveys should prove slippage.
Cheaper and more effective than inclinometers would be a series of monitoring points on the slope that could be surveyed in on a monthly basis over a one - two year period.

As for remedying the slope chlharry is getting at it there - forget engineering, what you need is nature. Roots to hold the soil together and also to dry out the slope. They worked before and they'll work again.

 

[bandraoi]

Also get your hands on the codes of practice the contractor should have used and start quoting them at him.

You gave him verbal warnings and you say that your case is weaker because you've no records of them. That's not true.
The contractor has an obligation to carry out work that will not damage you home.

He has, so it's he's been negligent. Doesn't matter whether you told him or not.

 

[VAD]

dlustri:

I have read your history of March 3 and from what you have said I would think that you are to be blamed rather than the Contractor. Sorry to be so blunt. In fact, I would submit that the Contractor could be vindicated if the judge is on the ball and the Contractor has a good lawyer.

First you made an agreement for him to place fill between the garage and top edge of a 35 ft deep slope. In order for him to place fill at the location he needs to place material along the slope as he has to attain a sideslope. What he did would be what any earthmover would want to do. He denuded the slope and probably benched same to get the new fill to bond. His equipment operator could care less about slope instability. How much fill was to be placed to create this roadway. This would have dictated whether he had to place fill on the slope.

You have erred in my opinion by agreeing to this roadway especially that it is within a zone that I am sure is within the setback distance of the property. In fact, your garage is likely within the setback distance i.e distance from top of slope within which no structure should be placed.

You cannot assume that because the building was built in 1911 when such an aspect(setback)was not considered that the building would maintain its integrity throughout its life. This is not the case in geotechnical practice today. It does not matter whether the building was performing well over the last 50 years.

The fact is that your building is likely in a high risk zone and you should have been aware of this as sufficient information is available from your local building permitting office for constructing buildings adjacent to slopes.

The Contractor does not have to know about that unless he has been advised accordingly. It should have been your call to have the scheme of placing fill on the slope checked by a geotechnical engineer before agreeing to building a road in exchange.

As for your remedy of $800,000, I think this is on the high side and that both your structural and geotech are using the CYA approach as if anything goes wrong they will be sued as well. I do not blame them. Some freestanding drilled piles reinforced with h-beams between the top edge of slope and the garage with removal of the additional roadway fill could suffice. I am guessing here as I do not know the stratigraphy. I presume that you have a borehole extending below a depth of 35 feet.

Another solution is to create a toe berm at the toe of slope using fill placed for driveway/roadway. What constraints are existing on your neighbour's property to creating a berm at the toe of slope.

In summary, I think you have a weak case.

 

[dlustri]

VAD, I appreciate your position, you make some very valid points and they will help me model my argument against the defendant. Your second paragraph is misguided. If I told the contractor to place the fill at that location then this whole issue is moot and it is most certanly my fault.
The fact is, I specifically told the contactor to stay away from this area for the very same reasons you stated and more. As a civil engineer with sitework experience and enough geotechnical knowledge to be dangerous, I know to stay away from something that could prove to be a major problem, and made them aware of this before, during, and after the project.

In your second paragraph, you give this contractor way too much credit, he denuded the slope from top to bottom in order to facilitate his dumping, not for anything as noble as doing his job properly! I am curious to know what contractors you deal with and what part of the country?! They certainly sound more ethical than those in my area. (Do you have a need for inspectors in your neck of the woods?!)
Futhermore, any competent operator wouldn't disturb a slope unless told to do so by the project foreman, otherwise receive the wrath of the boss.
I am curious to know what philosophy of embankment includes 'benching' with trees, stumps, brush, and other organics? The last I checked these were considered deleterious materials to be removed from structural supporting areas.

Now your third paragraph, the dirt driveway existed, for 50+ years, so did the garage. I didn't propose it there, it was there. What I proposed was a continuation of the drive on the adjacent parcel where the grade difference was only 4 feet. This was some 20-30 feet away from the slope and garage. I beg to differ on whether or not 50 years of history carries any weight. Are you suggesting that this is a coincidence? Is it possible? Sure, is it a competent assessment of the situation with the known facts described herein? Hardly. You are ignoring select evidence that suits your argument, that would be shot down with a handful of cross exam questions.

I am not dismissing your opinion completely, but as I am sure you are aware, we have to defend our professional opinions with sound engineering principals, not "the operator could care less". Well guess what, the operator is required to care more per our laws. It is called "workmanlike" manner. Even a contract that simply says, 'dump there' carries numerous implied responsibilities, inlcuding but not limited to "don't bury organics under sructural entities"!

Perhaps you could answer this question if proposed to the contractor; What projects have you buried organics under roadway or foundation areas and not compacted under the same? (Assuming you say none) Then the next question would be "Why did you bury them under the plaintiff's driveway? Now if you have examples of projects where you purposely buried trees and the like under areas you knew were going to bear structural entities, then you are welcoming a slew of additional lawsuits.

Not to burst your contractor friendly bubble, but this is a simple case of contractor greed. All contractors who do a substantial amount of earthwork projects know that their 'shortcuts' will not be noticed until a substantial time afterwards, usually when the project is over and proving that they are at fault is virtually impossible. This contractor treated me like an average Joe, unfortunately for him, I know what would cause these problems and I stopped them while they were doing these things. They promised to fix the problem the next day, and they didn't and lied about it. Contractor greed.

Finally, the constraint on the neighbors property is taking up extremely valuable real estate for a retaining structure. The toe of the slope was approximately my property line. Now the toe is 15 feet into the adjoining property. Don't you think that owner has a worthy argument agains't me?

Thank you Vad, you helped me realize that I have a stonger argument then I originally thought.