Fill change of property with time 9

[pelelo]

Experts,

Do you think it is possible that a fill (placed and compacted in layers) could change its properties with time?.

There is a case in which a house was built on a fill that was placed and propertly compacted, about 10 years ago. The fill (SP-SM) thickness is 10 ft, below the fill, there is natural material (loose to very loose sand), down to 35 feet. Water level is about 15 feet below surface.

We are doing some additional geotech investigation to the same property and the SPTs show N values in the "properly compacted fill" of between 0 (weight of hammer) to 9 blows per foot. These results were shown in 6 borings around the property.

My question is,

1 - Do you think fill properties can change their compaction level or field density with time?. I have never seen something like this before. I have records of the field density tests, of 10 years ago, and they show adequate compaction results.

2- Or do you think the fact that this fill material was placed on top of a thick (> 20 ft thick) layer of loose / very loose sands (SP and SM) (SPT-N < 10), might change with time?, as the fill weight might be contributing to long term settlement on the loose natural sands therefore SPT values have decreased?

Please let me know your thoughts. Thanks

 

[oldestguy]

This looks like another post where settlement of the house took place. My comment was this job may have been done in winter. Even with low moisture content, sand below freezing temp can have frozen water coating the particles and making them look larger. Test results also can be affected. It is common in northern climates.

 

[PEinc]

Since the fill was placed 10 years ago, has there been any change in the level of the ground water within the loose sand?

http://www.PeirceEngineering.com

 

[SlideRuleEra]

pelelo - Some of your other posts have involved sinkholes in Florida. If this project is in the same area, perhaps there is an underground cavity nearby that has not (and may never) raveled to the surface (creating a sinkhole).

http://www.SlideRuleEra.net

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[pelelo]

Gents,

Thanks for your reply.

OG, I didn't know that situation happens in northern climate areas, I'm my case, this project is located in FL, so no cold weather around here.

PEinc, is there any problem is water have fluctuated of varied in the last 10 years in the loose fill?, yes, in here we have seasonal water level changes, it varies from 3 to 6 feet.

SlideRuleEra, this project not located on a sinkhole area.

 

[fattdad]

to the OP: No. Properly compacted granular soils have no basis to change their void ratio over time. It strikes me that the fill may have included a bridge lift (i.e., where N=0), the technician may have been committed to the wrong Proctor, or the technician may not have had good ground control (i.e., location and lift number).

In the realm of cohesive soils used in earthwork, sure - softening will occur owing to wetting/drying, freezing/thawing, etc.

I just don't think the strength of a granular soil (think embankment dam or the hundreds of projects I've designed over the last 35 years) is temporal.

f-d

ípapß gordo ainÆt no madre flaca!

 

[pelelo]

I agree with you fd. Thanks a lot for your reponse.

Do you think fluctuation in water level might have something to do?. I highly doubt, but just asking.

In clays and silts, i do understand that fluctuation with time might affect the properties but I don;t forsee this happening to granular soils.

 

[oldestguy]

If the house has not settled noticeably I'd question the test borings. Significant loosening by drilling ahead of SPT might be part of the reason. Did any experienced engineer watch the work of the testing crew? If you are not well experienced, you can be fooled by the drillers. it is easy to speed up the work and drastically affect the SPT

 

[PEinc]

pelelo, if the water has dropped significantly (more than the seasonal 3' - 6' variation), the formerly buoyant soil becomes heavier and settlement can occur.

http://www.PeirceEngineering.com

 

[pelelo]

Thanks again for your answers.

OG: The house is showing cracks on the walls. Actually, one of the SPT, detected WOH between 6 - 8 feet, that SPT boring goes well in line with the location of that crack. Other than that, not noticeable settlements have been seen.

Drill works were supervised by a qualified / experienced engineeer.

PEinc, thanks, yes, i agree with you.

 

[oldestguy]

What are the properties of this fill? Is is something that can beak down in time? Light specific gravity? An industrial waste? Coal ashes? Coal ashes break down it seems forever in my experience. Is there any chance it is dumped fill and the compaction reports were falsified? That would not be the first time this has happened. What would former job people remember? Any leaking sewers (leaking in) that can erode soil from a large area? I'd run relative density tests as well as gradations. I have seen uniform sands with low blow counts, yet very small range of relative density, but then settlements were minimal. My bet would be a dumped fill that never was compacted.

 

[pelelo]

OG:

The fill is considered as a SP (fine sand). There was an excavation and the same material was used as backfill.

Since it was over 11 years ago, i havent been able to contact anyone who was present at the site at that time.

About falsified reports, wow, i can not believe this could happen in usa. I have never seen this here. In few other countries i have seen it though.

Something that raises a flag to me is they used a hand held compactor in order to compact such a big area (70 ft x 70 ft). I guess a roller would be more suitable for these purposes.

No sewer under the property BUT the property is next to the sea, so yes, water lever is somewhere between 3 and 6 ft.

 

[oldestguy]

You got it with the small compactor. Chances are layers were very thick. Possibly the material is very sensitive to moisture content differences.

As to possible falsified reports, welcome to the club. I've run across that sort of thing with some technicians. Usually the excuse was "We got behind". When out where test borings are being run, set back a ways with a telescope mounted on the car window. You think those low counts were found correctly? How's about one or two borings done on the job and the rest of the "holes" show up and the rest of the logs drawn up in the truck. I could go on, but get that scope mounted and watch. Also check how lab work is done, since corners can be done to shorten the time it takes, etc.

 

[EireChch]

I think i would agree with the general consensus discussed by OG and PEinc.

Bad compaction/dodgy compaction reports would be my first guess. If you have got 6 borings around the property all showing Ns from 0-9 i think that speaks for itself. If i was writing a report on it i would just say it as a matter of fact. Construction records indicate that fill was compacted appropriately but the borings suggest otherwise.

PEinc makes a good point too. If the groundwater has dropped dramatically the there will be an increase in effective stress and thus potentially settlement.

 

[geomane]

When did the cracking show up? Where in Florida and how close to the Gulf?

I would check to see if any type of construction that involved dewatering and vibrations has occurred in the area. I would also check to see if the cracks showed up after severe weather that could have lifted the house, as opposed to settlement.

Do you have borings in the footprint of the house? Your original post stated around the property. They could have filled the site and only compacted the areas in the footprint. The contractor is not going to do more than they have to or more than what they get paid to.

 

[Okiryu]

This is a house so perhaps the bottom of footings are at shallow depths. You may be able to do some proctors and field density tests by yourself so you can have other information than SPTs and proof that compaction is not appropriate. If footings are small you can also do some plate bearing test to check bearing capacities.

 

[oldestguy]

Now what is next? I'd look at compaction grouting. Not a heck of a lot is needed if only the foundations need a little extra support and perhaps some re-leveling. However, only a very experienced contractor should be used because damage can happen if done wrong. Certainly no slab jackers should be used.

 

[pelelo]

Thanks for your replies, very valuable.

OG, I will keep all that in mind as I had no clue this could occur in USA, thanks for the heads up. About the solution, I understand that underpinning will be the best option. Grouting would be too expensive.

jmcc3265, the house is right by the gulf. I don't have records of construction of any vibration activities around but will do some research. About the borings around the property, probably the best option and I will do this the next time is to drill the borings INSIDE the house. This time, the borings were around the property (4 to 6 feet off the perimeter), due to access reasons that was the best the field crew was able to do.

Okiryu, thanks for your input. The bearing tests might take some time.

 

[oldestguy]

More from the OG. Hold on. Not so fast on what to do. I'd look at several possible treatments and their cost as well as possible success. As to underpinning we usually use that for bypassing weak stuff down to better support. In loose sands, preventing caving of excavation sides would be difficult. Where is that better deeper support? How about mini piles? They could transfer the loads well down there with light friction over significant depth of pile as another option. Contacting a well experienced soil improvement contractor, such as those advertising in Engineering News Record Special editions might be money well spent.

 

[pelelo]

OK,

Thanks a lot OG. I was thinking about helical piles BUT competent strata is around 40 ft. Per my experience, helical piles don;t get that deep, that's why I was thinking about underpinning as a solution.

I will do more research about this.

thanks