Sudden, localised, settlement 6

[CELinOttawa]

Hello All,

I'm stumped. Just got called to site where we've been working on a condo with a single storey basement garage which needs midlife repair due to delaminations typical of a garage where tires are constantly tracking in salts and water, etc. We did a preliminary report some weeks ago about the main bars and confinement steel being corroded and in need of replacement in a number of columns. Pretty typical and ho-hum.

This evening and tonight are where this gets interesting. I got a call stating that an owner in one of the condos was complaining of cracks in their walls and that their door was sticking. I dropped everything and reported to site, fearful of seeing distinct pattern cracking in the slabs, evidence of rotation, or other evidence of structural issues.

What I have found looked for all the world like differential settlement, the type I'd expect in a timber structure due to bad frost heave or terrible compaction of the granular subgrade. Doors are titled and jambing, cracks have formed where gypsum infill (non-load bearing) wall meet structural beams, some minor pattern cracking in the CMU walls, all pointing at settlement of a discrete number of INTERNAL columns along two grid lines.

The clients who have been affected stated that the troubles with doors was worse on the week-end and has improved.

Here's the rub: In forty years, this settlement is new, with evidence appearing in just the last two to three weeks. I'm stumped, and so is my Geotech consultant. He agrees it is differential settlement, and has no idea why it would have started suddenly. Where do we go from here? I was so tempted to order temporary propping, but there is simply nowhere I would think to prop. The columns and slabs above look fine; The settlement is happening far enough apart that the slabs are accommodating the change, it is the gypsum and CMU infill walls that can't take the delta...

The plan, so far:

- Get original drawings from the city and review load paths in detail.
- Do a preliminary information search based geotech assessment (by others)
- Tape ends of all cracks and observe for change.
- Re-review on Thursday to see if any changes can be noted.

I don't mind telling you all that this one gives me the heebee geebees. I don't like *really* not knowing what's going on....

 

[Canuck67]

I have been working on a similar situation and agree it can be difficult to pinpoint. My project involves 40 year old townhouses built on conventional footings. For 39 years, things were mostly fine, then within the last year 3" of settlement at one end of one block. Boreholes show footings on competent soil, but underlain with quite a bit of soil/clay with n values equal to zero. A localized water main break combined with a wet season seem to be the current theory as to why the movement occurred. Any water table issues in your area?

Sometimes best to work at eliminating potential reasons first and narrowing the list before going looking for the root cause. Given that there is deterioration of the reinforced concrete structure, perhaps start with the exposed areas and try and eliminate localized failure of the structure. An elevation survey might also significantly narrow the search for areas that might have settled. Often elevation shots on the soffit of the underground parking area are more accurate than top of slab on grade.

 

[hokie66]

Incipient sinkhole due to something happening below like broken pipes, tunnelling, natural erosion, or whatever?

Or could it be on piles which are corroding?

 

[steellion]

Major, sudden settlement for a structure that has been in place for 40 years seems to imply some triggering event (sinkhole, pipe rupture, soil liquefaction, mine subsidence, etc.)

 

[CELinOttawa]

I agree with you all, but how to determine "what" triggering event is now the problem...

I've had the clients tape the ends of the cracks to see if they continue. I was more assured that we weren't looking at a possible structural issue as the situation has already started to abate... The question is now three fold:

-Will it start again?
-What caused the settlement, if indeed it is shown to be settlement (looks likely)
-What, if any, repairs are required for safety.

The cosmetic repairs only concern me in so far as I would like to be able to advise when they can go ahead. I am still working to rule out any major structural problem. The Admin is going to run into the city today to try and obtain the original structural drawings.

And the baby is sick again. *sigh*

 

[EngineeringEric]

You may want to investigate the possibility of a broken water pipe. We recently had a building break a main, their bill went up 2000%, they then noticed that the building was settling in at a given location. the water flowed downhill under the building washing away the subgrade.

If i recall we had a geotech testing agency go around looking for sinkholes below grade and for underground channels. It was not a clean clear solution. Ours was easy and cheap enough to just add large diameter helical in the effected area and specified a rater deep install. Yours sounds harder, sorry.

 

[hawkaz]

I join the consensus. Sounds like a pipe break. Check water pipes, sprinkler lines, sewer lines...

 

[dcarr82775]

Were the footing (I assume shallow footings) exposed? Was the original foundation design adequate? As silly as it sounds I have seen the removal of a slab-on-grade along with a couple feet of overburden cause issues with footings that were woefully under designed. That being said, I agree the more likely cause if waterline or something of its ilk.

 

[BAretired]

Several years ago, I found a similar problem in a twenty year old medium rise apartment building in Edmonton, Alberta. Everything performed satisfactorily until someone noticed that the second floor, a flat plate, had suddenly settled (or so it was thought) and was crushing the partitions in the lower floor. The City was called in and did a check but found no problems with the water lines, so I was asked to inspect the site and determine the cause of the problem.

On the asphalt surface outside the building, I found two white circles more than two feet in diameter about ten feet away from the exterior wall. Two trees had been removed and were cut flush with the asphalt surface. My client advised that they had been removed, along with two other trees in the landscaped area in the previous year because the branches were brushing against the building and disturbing the tenants on windy days and nights.

My conclusion was that the roots of the tress had been ingesting rainwater for many years and that after they were removed, the water permeated the soil under the building. Typical of Edmonton clay, it was an expansive soil and the damage occurred, not from settlement but from heaving due to swelling of the clay soil.

The partitions were repaired, leaving a small gap at the underside of second floor slab and the problem was resolved.


BA

 

[SlideRuleEra]

how to determine "what" triggering event is now the problem...

Another possible trigger, besides a broken water line, etc., could be intentional subsurface dewatering at a nearby location. For at least a 1/4 mile radius, check to see if there are construction projects, or similar activities, where dewatering has occurred in the past few weeks or months. More than once, I have been on projects where dewatering was the likely cause of sinkhole development some distance away.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[CELinOttawa]

Well everyone, I'm going to work on a preliminary report and have another look tomorrow. I've advised the owner that we should have a survey of all conditions, inverts, cracks, etc, prepared as quickly as possible in order to have a baseline for modeling.

Note: This is a midrise as well. Three storeys above a basement parking level. The building was subject to major repairs some ten years after opening due to a gas explosion. The slab on grade was subject to grout injection to eliminate any voids created, and a number of additional members were added as well as extensive slab replaced. The fact remains, however, that was more than twenty-five years ago and these issues are new.

The affected area also had very large trees removed some ten years ago in order to avoid any troubles with roots. Ironically we now think (as per BA's suggestion above) that the remaining roots and voids might be causing this issue.

So many variables. *sigh* I'm still recommending detailed observation, and am very much hoping not to find another batch of new cracks when I'm back on site tomorrow evening. Fingers crossed.

Another question for you all: When is cracking and settlement too much for a structure? I'm been asked this before in a blast context and fully expect most answers to mirror my own: It is a distinct matter for each individual structure, and in large part professional judgement. Anyone able to do better?

 

[jayrod12]

I would have to agree with you. Depends on the structure, the use, and the actual impact of said cracking and settlement. There's no right answer I wouldn't think.

 

[oldestguy]

In your area I think there is high plasticity clay that changes in volume as season change, from grout to rainy. There is a classic paper written about volume change in Ottawa. As a result of the seasons, volume change occurs. Adding to the problem is trees taking water. When present they can significantly reduce volume to deep depths and result in major settlement of structures. Is there any chance that the grouting back then was to fill voids due to trees causing settlement? And with them gone now, is the soil taking on water and expanding? That leaking water pipe thing may be involved then. I've seen several cases where the soil shrinkage due to trees has been mitigated by adding water.

 

[oldestguy]

Take a look at this web site paper. I see a map there that may help.

http://www.ottawastructural.com/cmhc/About your house, sensitive clay soils.pdf

 

[oldestguy]

One more comment. It is not always settlement that occurs. Heaving may be present, masked as settlement. Is the outside of the area receiving water while the inside is not? Remember the depth of drying can be many feet, 15 feet depth of shrinkage is very common in some areas. Area of effect can also go far into building areas. The examples shown in the above mentioned article are unrealistically shallow in my opinion as to depth and area extent of the shrinkage.

 

[TLHS]

There's also the possibility that something you did broke a secondary load path and your loads shifted. For instance, if you cut into the slab on grade and it wasn't properly isolated from the footings, it could have been taking load. Then if there was poor compaction or erosion under the footing you get settlement when the load shifts back to where it was originally expected to go.

You can get the same thing if there was some sort of moment frame action happening in the overall structure, bridging over one support that was softer and then you disrupted it somehow.

 

[CELinOttawa]

THLS: Quite right, only we've not done anything to date. Other than sounding the columns, removing loose (delaminated) cover from one column (which is actually quite far from the apparent settlement at approx 30m away), and propping the worst two columns with single shores quite close to the column face, nothing has changed.

The cracking also appears to have stopped. No new cracks have appeared in the past two days, and as such we will be taking a detailed benchmark and continue to monitor for some time. The Geotech and I advised the clients tonight that we hope to never be able to tell them why this occurred, but rather to be able to advise to simply go on and maintain records. With luck this was a transient condition and has arrested itself. Otherwise, should it continue, it is likely going to need a great deal of investigative/forensic work to figure this problem out.

 

[SolidPE]

Sadly, I do not believe that the problem will just go away. It is more likely that voids below the slab and footings just got larger, and the soil above them was unable to bridge over them, soils above collapsed a little, leaving the above elements that the soil supports to settle. I think it has just come to a new (temporary) equilibrium.
I would suggest, as others had, checking into the records for underground abandoned pipes, wells that have their cover removed, history of tree stump dumping. My best guess is that it's a old landfill and things are just decomposing. The rate of decomposition picks up after about 50 years. The fact that void spaces had to be filled in the past is not minor issue. It's a red flag.
Have your geotech propose to do borings and measure penetration resistance with depth. I bet you'll a find some Serious layer of zero blowcount material. The cheapest fastest way to do the explorations In a confined space is with a Geoprobe type machine. You don't really need blow counts. Count the number of seconds to advance 1 ft and I bet you'll have your answer in a morning. Good luck.

 

[oldestguy]

Now let's suppose the borings that Solid PE suggests show nothing unusual or perhaps very solid stuff. That has happened on one of the jobs I have investigated after they were done. The expansive clay sites, as are common in Ottawa, then will pose questions in the minds of those expecting to find soft stuff, wondering how the heck that stiff stuff settled, yet still replacing that with concrete.

If the site is in one of the areas mapped as expansive clays,then the first thing to check out is moisture transfer possibilities.

 

[Canuck67]

Just an FYI for my project mentioned above. Due to the poor soil layer with n values equal to zero and the amount of settlement that had occurred, we decided to underpin using helical piles installed down 40 ft. to bedrock. In our case, and with a pretty good picture of the soils from the geotech investigation, we felt this was the best option. This may or may not be the case in your project.

In my past I have come across lots of other reasons for behavior like this, including a building that was constructed over a relatively large diameter 1880's brick sewer line. The building started settling once the brick sewer line deteriorated to the point soil migration into the empty sewer line was occurring.