Sinkhole in an open field. 3

[pelelo]

Engineers,

I got a question about a depression that occurred in a piece of land.

There is nothing around, the depression ocurred after consecutive heavy rainy days.

Dimensions are 40 ft long, 33 ft wide and 15 ft deep. Per the borings, rock was detected at 20 ft.

The idea is to back fill the depression and grout in angles around the perimeter of the depression. Then, perform a grid of vertical grouts on top of the depression.

My question is, do you think a grid of 5 ft by 5 ft is ok or is it very conservative?. Same question is related to the angled grout around the perimeter. I plan to propose grout locstions every 5 ft around the perimeter.

Please let me know your thoughts, thanks.

 

[oldestguy]

Location please. Geology info if any. Similar situations in the areas?

 

[concretemasonry]

Must be sedimentary rock under the area (Florida??).

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[pelelo]

yes, it is sedimentary rock.

The depression is in Lakeland, FL.

We don't have detailed information other than blowcounts. Usually for properties, we recommend 8 to 10 ft center for Limited mobility displacement (LMD) grout. But not sure if for this situation 5 ft on centers, we are too conservative.

 

[L0k]

my experience in grouting the limestone and basalt layer was required lots of grouting material, si it is required a detail investigation.

 

[Okiryu]

If there is nothing around and large excavations are allowed, you can use the below method:

http://www.ei.lehigh.edu/envirosci/enviroissue/sinkholes/repair.html

Since you operate in a karst area, I think that may already know about that repair method.

 

[pelelo]

Okiryu,

That solution would work if it is a shallow depression and if rock is shallow.

In my case, per the borings, rock is found at 50 feet deep. At the same time, the depression showed up on a parking lot.

 

[Okiryu]

I was thinking that nothing was there so you can do something simple and economical. Anyways, you mentioned that there is a depression of 15' and the rock (limestone?) is at 50', what is on top of the rock? Depth of water table? Sands and silts are more prone to erosion so perhaps you have this above the rock? Any signs of bad grading and draining in the area, so soils may be washing out thru the limestone? Also, if the LMD grout is typical in your area, you may just use it. I just thought that it is expensive method. I may be wrong.

Let us know how it goes. I deal with karstic geology also and am curious how people deals with it in other locations.

 

[pelelo]

Okiryu,

Sure, I will let you know, thanks fro your input.

On top of the limestone the borings show loose sands (SP), there is not Water table. There is a pipe, which was leaking, so yes, the pipe is part of the cause. LMD is what we use typically around here, what i was not sure of is if 5 feet spacing was very conservative.

 

[Okiryu]

pelelo, I think that the problem was bad drainage so the loose sands (which are very prone to erosion) got washed out thru the limestone... ensure that you have a good grading and drainage after the LMD repair work. Good luck !

 

[pelelo]

Okiryu,

Thanks. Yes, i agree with you.

 

[GoMizzou99]

I deal with sinkholes routinely in Missouri. Ours are typically formed in in limestone and dolomite.

This method

http://www.ei.lehigh.edu/envirosci/enviroissue/sin

does work if you can find the opening and the opening is relatively small. To get down to the opening (sinkhole throat), we have had to use double track-hoes...one in the bottom of the hole digging "up" and another grabbing the lower track-hoes spoil and getting it out of the hole. The excavations can get very large especially following OSHA trench safety guidelines. We have dug to 48 feet below grade before using this method. A single sinkhole repair using this method on the 48 feet deep excavation took 7 full days with 2 track-hoes and a D-9 dozer. We also used 50 cyds of boulders and 20 cyds of concrete.

Sometimes, when the opening (sinkhole throat) is too large we will place a layer of boulders and concrete at the base of the hole. Then repeat - essentially gluing the boulders together with the concrete - but there are still openings in this repair method that will allow water to continue to seep into the sinkhole.

We have also used structural mats doweled into sound bedrock designed to hold the soil and live loads above - we do place weep holes in the structural mat to prevent water from ponding on the repair.

 

[pelelo]

GoMizzou99,

Thanks for your reply.

At least here in Florida, when sinkhole occurs, no one is allowed to go down to the bottom of the depression and do any type of works. This is more for safety purposes.

Given that condition, in my case my remediation plans starts around the depression (to avoid it from continuing moving sideways) and then, after depression is backfilled, we proceed with grouting on top of it.

 

[GoMizzou99]

pelelo,

Sinkholes in Florida are a whole different animal than what we deal with in Missouri. I am amazed how they can grow laterally over a short time.

Pressure grouting followed by compaction grouting, in my Missouri experience, is normally done when "filled sinkholes" are present (either ancient ones filled with younger weak rock or soft shale or filled by man with "whatever") in an attempt to stabilize them without performing any excavation. This method is very expensive, significantly more so than excavation and repair, and is typically only used when excavation and repair can't be done.

Thanks for sharing your repair method. No two sinkholes are alike and I am always learning.

 

[morgwreck243]

I have a project in North Alabama (Huntsville area) where we are encountering multiple sinkholes within a 200 acre site. We are currently repairing one of the sinkholes with the rock backfill remediation plan (similar to the LeHigh link). It is also about 40' below ground surface.

Does anyone have any sources for other remediation methods? I'm just trying to see if there is a more economical method (aside from building in another site).

this is for a new office campus which will have up to 4-story office buildings and parking lots / decks.

--morgwreck243
KN4DHC

 

[fattdad]

Just a bit confused. . .

The OP describes rock at 20 ft and the longest dimension at 40 ft. In that instance, I would also consider a reverse graded filter first.

Later, there is a description that the rock is at 50 ft. So, that would suggest a reverse graded filter not likely.

Grout seems expensive, but. . . It may be what you need to do. It's not obvious whether 5 ft centers would be all you need; however. So, there'd need to be some consideration for primary, secondary and tertiary grout holes. That would require some performance level be written into the specs.

Will it be perfect? I mean at what point will you be confident enough to build a hospital or other such critical structure? Don't know. That said, the suspenders would be to complete the building pad by constructing the backfill as a soil raft. You know figuring out how thick and the demands that may be placed on the reinforcement.

I'm also curious about epikarst soils - you know the really soft lean clay that results from the inplace weathering of the limestone. We have that stuff and it'll trigger settlements as it's essentially normally consolidated.

On the more pedantic side. DOLOMITE IS NOT A ROCK! <rant over>

Dolostone is a rock, just as limestone is a rock. The principal mineral in limestone being calcite and the principal mineral in dolostone being dolomite.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Ron]

pelelo....this thread is 5 months old, so I assume you have likely made some repairs by now, but since you are in Polk County, another sinkhole is a high probability so I'll offer my $0.02 worth for investigation.

Once a cavity dome collapse occurs, drilling in the center or near the center of the depression will only tell you where the fractured rock is now or, if a clean collapse, where the bottom rock is. Going outside the perimeter of the sinkhole you can more likely tell where the rock WAS prior to the dome collapse. This is important. If you compare the inside boring and the outside borings and plot a profile in several directions, you can likely tell if there is still a void to be filled (by grouting). If there is not, and you feel reasonably certain of that, then you can fill the depression and be done with it.

You have a good foundation contractor, Hayward Baker, in Tampa. They have a variety of techniques that can be used for these situations.

For disclosure.....I am not an employee of Hayward Baker nor do I have any affiliation with them. As a consultant, I'm just familiar with their capabilities.

 

[GoMizzou99]

Clarification:

Dolomite is indeed a rock. Limestone is calcium carbonate, dolomite is magnesium carbonate...both rock types are extensive here in the great state of Missouri. For instance, the Jefferson City formation is largely comprised of dolomite. Here is the Wiki link -

https://en.wikipedia.org/wiki/Jefferson_City_Formation

the rock types are on the right in the box.

Dolostone and dolomite terminology is largely a function of geography with different parts of the country preferring one over the other.

 

[Ron]

In Florida we refer to it as dolomite. Actually, dolomite is calcium magnesium carbonate. It is rarely pure in natural formations here....usually intermingled with limerock (calcium carbonate...limestone). In the area where pelelo's problem exists, it is common in its conglomerate form and is the primary phosphate bearing rock that is mined in that area.