Sudden, localised, settlement
Sudden, localised, settlement
(OP)
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....
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....






RE: Sudden, localised, settlement
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.
RE: Sudden, localised, settlement
Or could it be on piles which are corroding?
RE: Sudden, localised, settlement
RE: Sudden, localised, settlement
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*
RE: Sudden, localised, settlement
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.
RE: Sudden, localised, settlement
RE: Sudden, localised, settlement
RE: Sudden, localised, settlement
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
RE: Sudden, localised, settlement
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.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Sudden, localised, settlement
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?
RE: Sudden, localised, settlement
RE: Sudden, localised, settlement
RE: Sudden, localised, settlement
http://www.ottawastructural.com/cmhc/About%20your%...
RE: Sudden, localised, settlement
RE: Sudden, localised, settlement
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.
RE: Sudden, localised, settlement
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.
RE: Sudden, localised, settlement
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.
RE: Sudden, localised, settlement
If the site is in one of the areas mapped as expansive clays,then the first thing to check out is moisture transfer possibilities.
RE: Sudden, localised, settlement
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.
RE: Sudden, localised, settlement
RE: Sudden, localised, settlement
RE: Sudden, localised, settlement
RE: Sudden, localised, settlement
This column is nowhere near the areas where the cracking has occurred above, and the areas that have cracks are on columns which show no deterioration. It is perplexing, but I continue to believe this is corrosion of the steel popping the concrete, and the cracking is likely geotechnical in origin.
The plot has thickened as to cracking: I have found other condo owners who have had similar cracking in the past, may over a good number of years. It seems that we just got a batch all at once, so again this indicates geotechnical as likely. Cracking between the concrete structural elements and linings is an old symptom, the cracking in the columns is very new.
I think the confinement ties may have rusted through, and the column may in fact be failing. It may also be the supporting thickened concrete slab which is failing. Either way, first thing tomorrow is the fastest anyone can prop the column.
I've never seen one this bad, and I am not impressed. It is not a good situation. I'm specifying shoring for six of the columns, six props each. It will make the repairs a touch harder to do without re-shoring, but it will also make me feel a great deal better.
RE: Sudden, localised, settlement
Your masons hammer is on the top shelf, right next to your slide rule
RE: Sudden, localised, settlement
I'll keep observing and recording changes. This column is going to get heavily propped this morning, and repaired as quickly as possible.
RE: Sudden, localised, settlement
RE: Sudden, localised, settlement
The propping has been ordered because of the number of unknowns, the 1970s flat slab amongst them, and because I want to be able to safeguard the residents while figuring this out.
I called in another engineer for a consult by phone and he agrees the props are prudent (thanks Hokkie for confirming my pull of the trigger there) and that he couldn't think of anything else that should be done. Now I just hope the city gets back to me with the original plans so I can do a good check of the columns. With the framing of the post-explosion steel beams, I suspect they may be near their Max and the deterioration is the straw on the proverbial back.
RE: Sudden, localised, settlement
I can count my blessings:
- The Owner is taking it seriously.
- The City is giving us the time to address the issue.
- Shoring is in place and either this, or coincidence, has arrested the propagation of cracks.
- Cathodic protection is in. Actually I had advised this be ordered to minimize lead time, and they got it to us in two days.
- Ground water analysis has shown that there is *not* a deleterious element to the native location (at least not when my Geotech took samples)
To do/Remaining:
- We have found reference to the original Geotechnical Report. Despite having been done by one of the "big boys" who are still around (Patterson), I've not been able to find just *who* at that company I need to speak with to get an old report.
- The client now wants us to issue and administer the CCDC-2 contract. I don't mind, but it wasn't originally in our scope, so I want to try and get this done without causing any delay(s).
- Find a source of Galvanized Welded Wire Mesh. I don't want to use black bars when this structure has already been repaired on two occasions. I feel it would be a disservice to the client to omit the galvanizing.
Hope that interests someone... I just hate it when we put all this time into helping someone out and the thread simply goes cold and silent. At least this way the thread may be of some "lessons learned" value.
RE: Sudden, localised, settlement
Maine EIT, Civil/Structural.
RE: Sudden, localised, settlement
Interesting, indeed.
Curious, what is the intended use of the Galvanized Welded Wire Mesh? For slab repairs?
What type of catholic protection are you specifying?
RE: Sudden, localised, settlement
In this case I'm specifying Vector XPT anodes.