Sinkholes
Sinkholes
(OP)
I am looking for some ideas on what may be causing manholes and sewer mains to sink. I was recently called to check out the repair operation of a manhole and replacement of the sewer line where the line had been videoed and the pipe was so badly seperated near the manhole that the video camera could not pass the seperation. The sewer line is a 27 inch main. The depth of the sewer line is about 22 feet. The soils are clay (CH) and also some clayey silts / silty clays ML/CL that fall along the A line. The clays were fairly soft. Groundwater is about 6 to 10 feet below grade. There are occasional lenses of sand, though in the area of the sewer it appered to be mostly the clay that was present. The sediments are lake deposits. The clays are generally slightly overconsolidated. The sewer pipe is clay pipe encased in about 12 inches of unreinforced concrete. There was no other bedding material below the pipe. The sewer system is old probobly installed between 1900 and 1930. The area is subject to many earthquakes and strong ground shaking. This particular manhole was not in a high traffic area.
I am thinking about liquefaction, bearing failures of the clay, or infiltration of the fines into the line and errosion of soils surrounding and supporting the line. What else should i consider?
I am thinking about liquefaction, bearing failures of the clay, or infiltration of the fines into the line and errosion of soils surrounding and supporting the line. What else should i consider?





RE: Sinkholes
RE: Sinkholes
RE: Sinkholes
RE: Sinkholes
RE: Sinkholes
Are the manhole and pipe actually settling downward relative to a known datum? They probably aren't significantly heavier than the soil they "replace," or maybe not even as heavy, which makes me think it may not be a clay consolidation problem, particularly since you mention the clay being lightly OC. If not for the concrete encasement, the pipe would be a lot lighter than the soil it replaces, even when full. Any chance it's a flotation problem? Do you know the geometry of the separation (pipe higher than manhole, pipe lower than manhole, pipe pulled away from manhole approx horizontally)?
The title is "Sinkholes," but you didn't describe them. Where, when, and what did they look like? If material was lost into the sewer, that might create voids around the pipe.
Water wells near by?
Any idea on when the separation occurred?
RE: Sinkholes
RE: Sinkholes
What has me puzzled now is why they would get a break in the pipe that was encased in about 12 inches of concrete. It was heavy clay pipe, and should be able to withstand significant loading. They also told me that this section currently under repair, is a bypass put in place to fix an adjacent line that had previously failed as well. They did not have any info on when that occured but suspect it was a long time ago, more than 50 years.
I wander if maybe the concrete encased pipe and manholes were such a rigid system that during earthquakes when the waves are passing by it was enough to break the pipe from relative motions of the soft ground around the stiff sewer system? Maybe pore pressures built up enough to float portions of the pipe as well? Maybe your on to something Dgillette. Maybe the pipe and concrete encasement cracked enought to allow infitration of the fine soil to occur and we ended up with a piping problem? What are your guys thought on any of these theories? I am getting some pictures of the repair operation and will try to post some of them when they come in. Thanks guys for your help.
RE: Sinkholes
RE: Sinkholes
RE: Sinkholes
htt
htt
RE: Sinkholes
http://fil
http://fil
http://fil
RE: Sinkholes
http://fil
http://fil
RE: Sinkholes
It looks like you're in California, it could also be that the current drought caused the fat clays to shrink - but you'd think that would have impacted the whole street.
RE: Sinkholes
i am in a low seismic area so that can be ruled out. it was just the slow patience of water over a long enough period of time.
RE: Sinkholes
The CL-ML and sand lenses you describe are apparently erodible. Look at the slurry flowing in the excavation. You said the sewer is at 22 feet and the water table at 6-10 feet. That is a lot of differential head to drive infiltration, boiling and piping. The bottom of the excavation in your photos is heaving and unstable. It's tough to build any thing well under those conditions. Probably wasn't much better 50 years ago, so it's not surprising that there was not a water tight seal at the pipe-manhole connection.
You need a water tight installation, and you will probably need to dewater the soil to stabilize the excavations so the contractor can get high quality construction. The soils are now disturbed, and replacement construction can be expected to settle some, so you need flexible connections at the manholes and gasketed pipe joints to tolerate movement without creating leakage. Geotextile around the pipe and manholes to filter any moving soil can serve as a back-up measure.
You need a really good dewatering contractor and good sewer contractor. Good luck.
RE: Sinkholes
The patches in the street indicates this is not the first time this has happened - is this an ongoing thing? Has there been recent construction downstream? Has the groundwater table been lowered locally for some other construction recently? Has the grade been raised there recently and the manhole extended?
Interesting situation.
RE: Sinkholes
1. The top end portion of at least a small paper roll inside a stiffer, larger encasement will be damaged inward when there is sufficient bending (negative bending moment) of an initially very close-fit (not allowing rotation or joint deflection) connection.
While I'm not sure of any relevance to whatever you are dealing with, this may kind of beg the question, was the crown crush by any chance in the basic location where "the pipe that was encased in about 12 inches of concrete", and/or are you by any chance talking about the general location where the pipe end was rather rigidly grouted into a larger hole in the large manhole structure?
If so, I think it is possible a more flexible connection and or stronger, more ductile pipe (like ductile iron) might be advantageous where there might be some relative vertical movement of a structure like a manhole and the connecting piping.
RE: Sinkholes
One bit of confusion is that I am getting some of the information on what has occured second or third hand, so I am not completely sure at which location some of what I have been hearing occured. Through out the city there is more than one location with similar problems so some of the info on how the pipe was crushed, or offset, etc. may be for some of the different locations. One reason the city is so keen on trying to figure out what is going on. We may have more than one cause.
The design engineer is planning on using an in-situ liner to rehabilitate some of the area of some nearby streets with degraded pipe that maybe not completely sinking but is deteriorating to some point. They videoed the line and it needs rehab. How good would you think a liner would be if my first hypothesis about pipe stiffness relative to ground stiffenes were the cause for some of the distress, and how good do you think it would be at rehabing areas that may have problems caused from the backfill, assuming they can get it past any obstructions? I understand that these liners are steam cured in place, so how ductile/ brittle will they be?
Msucog
- What kept the area of distress around the manhole from not expanding beyond about 3 or 4 feet, was it just due to the depth?
Lcruiser
- I believe the area in the pictures was a repair for a bypass line from the original failed one. But don't know when, that is why I think maybe they used concrete but maybe they broke the bypass line first, then they tried to repair with concrete? But the concrete encasement extened from the manhole all along the pipe to the next manhole so it looked to me like it was by design for some reason. I don't believe this is an ongoing thing. So there is some uncertainty in my mind as to why they have a concrete encasement. Any if this concrete encasement if only for this bypass line, or if maybe they used similar type encasement for other sewer lines in the city. I also don't have any data on the groundwater fluctuations, but suspect that it has been fairly steady at 6 to 10 feet for the last 100 years.
aeoliantexan
- what you are describing with the "flow failure" of a sand lense sound intersting to me. I did CPTs along several of the sewer lines in the city that they are looking to repair and definetly would go through some cleaner sand lenses, or some buried channels. That is one reason why I was suspecting some differential settlement / floation may have influenced the system in the past. If we had a large increae in pore pressures from liquefaction then you could speed up the piping or flow of the sand into a sytem preaty quickly. Or maybe just enought to get the process started, and then the rest slowly degrade over time with the hydraulic differences between groundwater head and the deep sewer. This area had major earthquakes in 1940, and 1979.
Rconner
- I am not certain of the exact location of the failure in relation to the manhole. But I am definetly looking to introduce more flexiblity into the system.
Howardoak
- the streets in the area are very poor so they may be getting torn up all the time by shink and swell of the clays in the area, so I think this was something different they noticed. Your pointing out the level of construction quality seemed to hit the target. I could just see them dumping a full load in the trench when the concrete was still green and something just tearing up the pipe.
Thanks everyone let me know what else you think.
RE: Sinkholes