Questions about Surcharging

[NoWittyHandle]

Our firm is representing a homeowner's association for a development that was built on a deposit of miscellaneous fill and organic silt. The fill was as much as thirty feet thick, and the organic silt was as much as forty feet thick. The project consists of two-story attached units supported on shallow spread footings. Dynamic compaction was used to consolidate the fill, and drainage wicks were installed in the organic silt during surcharging.

I am in the process of going through the documents related to the site, and I have identified some problems with how the work was constructed, but I have several questions relating to surcharging. Given these soil conditions, how well could the best surcharge program be expected to perform? How much additional settlement can be expected if the surcharge is removed before the organic silt has had a chance to consolidate under the surchage load, even if all of the expected primary and one cycle of secondary settlement has occured? Thank you for your responses.

 

[garrettk]

Wow, several things come to mind right off the bat. Beyond the initial reaction of what a junky site to build on.

After the dynamic compaction, was the compaction of the upper fill material redrilled and tested to confirm results? It wouldn't be the first time that a dynamic compaction program did not achieve the desired results at depth.

Was the surcharge program monitored with settlement plates during and after the placement of surcharge material? If not, how did the original geotech know when settlement was substantially complete? The lab tests should give a ball park length of time, but field monitoring should have been done also.

The load from the 2-story structures shouldn't be impacting the organic silts, but the consolidation of the silts from the overlying fill could be responsible for differential settlement over the site. That makes the assumption the existing fill was properly compacted - or the fill may be settling as well.

As far as how well could a surcharge program work? It can work - but I expect that in this scenerio it would take a fairly long time. Not something a developer is ususally willing to wait around for.

Sort of a rambling answer. Hope it helps.

 

[msucog]

i've never seen a residential developer go through such trouble. i'm wondering how much the geotech was actually involved...based on my first comment. the answer to your question depends on the soils you're dealing with and/or geology you're dealing with. if you're concerned about how the work was performed, that (in my opinion) falls back on the contractor since they are typically responsible for the fill they place.

most of the settlement analysis scenarios i've seen always try to calculate to the slightly conservative side based on the available data/information. if the site conditions/geology are erratic, it's entirely possible that the borings simply did not happen to obtain the "crappy" soils. mother nature is hard to "completely" sample and test without deep pockets from the owner...in other words, the owner will typically get what they pay for.

if you happen to be in an area being hit hard by the drought, the drop in groundwater may be attributed to additional settlement that was not foreseeable. as a matter of fact, i spoke with someone in another state that mentioned the significant drop in groundwater being the culprit in excessive settlement on a site with thick fill(without me priming them). if you've read another thread of mine regarding the issue, then you know my opinions on the subject. pretty much...mother nature is a biotch sometimes and there's no way to guess what she'll do next.

as i previously stated, i'd be surprised a residential developer would go through such troubles much less wait for an engineer's opinion.

 

[motorbiketocrank]

First…and no offense intended by this…I hope your final conclusions will be as impartial as possible. Certainly I don’t know anything about the case but often I see folks try to support one side of an argument based on who is paying them, more than based on sound engineering. I mention it because you ask questions like “how well could the program be expected to perform.” and “how much additional settlement can be expected…”. If you are having a tough time with these questions having seen how materials actually behaved at the site after construction, then keep in mind that determining these answers during the design is not such an easy task. Again, not a criticism as for all I know the contractor is fully to blame….just an encouragement to strive for fairness in your judgements.

Having said that, it sounds like you are asking how much long term secondary consolidation can be expected. My answer to that would be that it mainly depends upon the amount and structure of the organics in the silts (and I’m assuming there wasn’t organics in the fill?). Were organic content tests done…over a range of locations and depths? One of my biggest questions would be how long did it take for settlement problems to show up. I’d have a tendency to fault the contractor more if the problems showed up relatively quickly (say over a couple years) than if they took substantially longer. We’re settlements monitored during construction (with settlement plates or similar) and did the monitoring data support reasonable conclusions that settlements were effectively complete?

In my mind, if a significant surcharge load was added along with wick drains, and then the load removed after settlements were effectively complete, I’d be scratching my head. While continued secondary consolidation could still occur, I’d be inclined to consider other causes such as groundwater table fluctuations likes msucog mentioned. Since there are a variety of poor materials at the site, has it been conclusively determined where the main problems are occurring? You mention the fill is as much as 30’ thick but what is the variability……do the building foundation stresses reach the silt layer in some places and not in others……could the settlement just be due to variability in the fill even after dynamic compaction etc, etc?

Not sure this helps but just some thoughts.

 

[NoWittyHandle]

Further information about the site:

Check borings were drilled to test the dynamic compaction on the site; no obvious problems were evident with the dynamic compaction. Settlement plates were installed to monitor settlement during surcharging, though piezometers were not installed to monitor porewater pressure. The geotechnical engineer was extensively involved in the project, and the borings appear to accurately describe the soil conditions. The site is located along a river in a major metropolitan area, which it is why this site was developed, and dynamic compaction and surcharging were undoubtedly chosen because they were cheaper than driving piles. The project was completed eight or nine years ago.

My first question concerned how well such a site could be expected to perform with a well designed and executed dynamic compaction and surcharging program. For example, theoretically you could construct a ten or twelve-story building on a mat foundation if you used a heavy surcharge load and removed enough fill from beneath the foundation, assuming that the soil was strong enough. However, such a building would probably perform very poorly, and I wouldn't want to try it. Part of the problem is that the company the developer hired to perform repairs has ignored two court orders to turn over repair records, so we don't know how extensive the problems on the site are.

To expand on my second question, if the surcharge load is removed before the soil has had a chance to fully consolidate, the soil will still have excess porewater pressure. As this excess porewater pressure dissipates, the ground may continue to settle. We hav a copy of Plaxis in the office, and it would be interesting to model this situation, but I was wondering if anyone had ever encountered this problem.

Thank you for your responses.

 

[dgillette]

Just for my curiosity, EricinNJ:

How much surcharge was used on top of the 30 feet of fill, and how long was it left in place?

Thanks,
DRG

 

[NoWittyHandle]

The height of surcharge ranged from about five to nine feet, and the length of time for surcharging ranged from about five to about nine months. Again, wick drains were also installed prior to surcharging.

 

[motorbiketocrank]

On your first question, I'd think a properly designed and executed program could basically be completely successful with settlements < 1". Only caveat would be that organic content in silts not be at a % high enough to result in significant long term secondary consolidation.

Since you mentioned piles....just a thought that driven piling probably wouldn't have resulted in a much better outcome. Negative skin friction would probably wreak havoc on performance.

Tough site but surprised that the surcharging/drains/dynamic compaction wasn't successful. Not sure what I'd have tried instead.......maybe excavation and replacement with lightweight foam blocks (if groundwater/uplift didn't prevent their use).

Curious...what magnitude of total settlements did they get on the settlement plates during the monitoring period?

 

[msucog]

it does sound like someone went through some of the necessary headaches to develop the site. i doubt anyone here can answer your question since it will depend on the lab data, field data, and overall project specific expectations. hopefully, the settlement monitoring program indicated that the settlement had slowed sufficiently...but there again, depending on the year and time of time, the groundwater table may have been higher so perhaps it dropped over the timeframe you're dealing with and kick started the settlement again. maybe the settlement never slowed enough to begin with...again, i don't know. trying to picture myself being the geotech on the job, the owner would have to openly accept the risks and "unknowns" associated with performing the remediation you mention. if they couldn't accept the risks, then put it on piles or undercut it all. sorry for the non-answer answer since i wish i could provide more assistance to you.

by the way, are you working in a litigation capacity or remediation capacity for the HOA?

 

[DRC1]

It sounds to me like the fill overlies the organic silt. I assume the water table is at or near the top of the organic silt. The problems I see with the approach you describe are:
1.) The organic silt can be highly variable in terms of both primary and secondary consolidation, as well as permiability, which will drive time of consolidation. Furthermore Deep dynamic compaction will have caused some consolidation of the silt, which may not have effected total settlement, but would have reduced permiability. It does not take a lot to signifcantly alter permiability.

2.) If the DDC was done on top of the silt, the fill may have compacted locally, but with out a firm base, it is hard to say how much effect it had globally. i would be afraid I would be afraid that soft spots may have been left between the compaction locations and as the silt settled, the compacted areas loosened.

3.) How misc. was the misc fill? that may have areas that were effectively treated and others that were not. It really depends alot on how good the confirmation problem is.

4.) I have trouble seeing how 5 feet of fill for 5 months would be an addequate surcharge to acheive a substantial amount of settlement.

5.) Without a lot of data on the silt or the fill, it is hard to say how effective this program would be.

 

[NoWittyHandle]

The fill overlies the organic silt. DRC1's suggestion that dynamic compaction may have effected the permeability is a good one. Piles could have been used on the site, provided the capacity was increased to account for dragdown.

 

[moe333]

How long has the fill been on the silt? The silt may still be consolidating from the fill load.

 

[fattdad]

Wow! What a series of questions and replies. Here's what I'd like to know:

Where's the ground water table?
What's the moisture content of the organic silt?
What's the USC description of the fill?
What's the wick spacing?
What are the Atterberg limits (wet and dry method) for the organic silt?
How was the surcharge designed.
etc. . . .

Regarding the following quote:

if the surcharge load is removed before the soil has had a chance to fully consolidate, the soil will still have excess porewater pressure.

This is not a universal truth. A surcharge can be designed to allow (for example) 50 or 60 percent of the dissipation of excess pore pressure recognizing that when the surcharge is removed, 100 percent of the excess pore pressure will be dissipated for the final loading condition.

Surcharges can also be designed to "drive" out a measured component of secondary compression.

I'm not sure I'm helping other than to say these types of engineering problems can be complex and it's hard to reply without real engineering data to go on. Further, even if you posted all the real engineering data, it would take quite a bit of effort (and I'm not in for the work) to really work through the problem.

f-d

¡papá gordo ain’t no madre flaca!