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Fill change of property with time

Fill change of property with time

(OP)
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

RE: Fill change of property with time

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.

RE: Fill change of property with time

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

www.PeirceEngineering.com

RE: Fill change of property with time

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).

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Fill change of property with time

(OP)
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.

RE: Fill change of property with time

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!

RE: Fill change of property with time

(OP)
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.

RE: Fill change of property with time

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

RE: Fill change of property with time

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

www.PeirceEngineering.com

RE: Fill change of property with time

(OP)
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.

RE: Fill change of property with time

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.

RE: Fill change of property with time

(OP)
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.


RE: Fill change of property with time

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.

RE: Fill change of property with time

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.

RE: Fill change of property with time

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.

RE: Fill change of property with time

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.

RE: Fill change of property with time

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.

RE: Fill change of property with time

(OP)
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.

RE: Fill change of property with time

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.

RE: Fill change of property with time

(OP)
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

RE: Fill change of property with time

I think what OG is getting at is that for underpinning as well you will need a competent stratum

RE: Fill change of property with time

(OP)
Yes, i completely agree that at some point, I will need to reach the competent stratum.

RE: Fill change of property with time

Competent stratum?? Let's say you compact the zone under footings down to 6 feet. Settlements come about dependent on the ratio of added pressure to existing pressure as one factor. The deeper you load the existing ground with a given pressure, the less settlement you get. So with compaction grouting versus underpinning you start with a lower pressure (wide area) than with underpinning. Thus you can get by without a denser layer to transfer loads to. In addition compaction grouting pre-loads that lower zone and can be used to raise footings at the same time. BUT it has to be done by a competent contractor or unexpected things happen!!! Likewise piling of one type or another can be used in loose sand by adding light friction loads per foot through a deep depth.

RE: Fill change of property with time

OG, I have reservations about the use of compaction grouting (LMG) for ground improvement. See extract below from the Ministry of Business, Innovation and Employment (MBIE)guidelines that are the standard for repairing houses that have suffered liquefaction induced settlement. I know liquefaction isnt the issue but the OP is left with the same problem of a house that has settled.

The EQC trials showed that where LMG is injected as a ground improvement method at shallow depths with little control (which can result in heave and dilation of the ground), then little or no improvement results, and in fact there may be a slight increase in the liquefaction vulnerability of the ground. However where it is injected for lifting purposes (refer Appendix A1.9 of the guidelines), then for typical house foundation loads, lifts up to about 75 - 100mm are possible without adversely affecting the ground. MBIE recommends that, in the absence of additional specialist input and analysis, lifting of houses with LMG should be limited to about 100mm. Greater lifts are possible where there is additional confinement from a heavy building, or the lifting is carried out at depths of greater than 4m.

In summary my thoughts would be that compaction grouting isnt that effective in improving ground conditions but it can be used to re-level the OPs house. Although, the OP has stated that there are walls cracked etc. If footings are also cracked then re-levelling through LMG may not be a goer and the only option may be to demo and rebuild. Or you could just re-cast the internal concrete slabs and leave the walls out of whack and learn to live with it.

At the risk of potentially opening another can of worms, would the house insurance cover the repair? If the ground conditions were never appropriate for the house/applied loads, well then the foundations were probably never fit for purpose. Surely someone has to be held responsible for the house settling? The designer of the foundations, the contractor for not compacting properly....who knows.

I know if it was my house i would be pretty pi553ed....

RE: Fill change of property with time

Yes there can be problems with comp0action grouting. However I've seen some great precision work re-leveling foundations by AN EXPERIENCED contractor. So if interested, get references first.

RE: Fill change of property with time

To get back to your original questions:

Yes fill properties can change with time, usually because the water content has changed. Plastic clay can shrink and settle when it dries, or swell and heave when it gets wetter. This is probably not happening with your SP-SM.

Most fill materials can soften and compress when wetted. If properly compacted and lightly loaded, there may be no significant settlement even when wetted to saturation. Well-compacted fills more than about 25 feet thick can compress significantly under their own weight when wetted. see "Review of Wetting-Induced Collapse in Compacted Soils" by Lawton, Fragaszy, and Hetherington, in ASCE Journal of Geotechnical Engineering, Vol. 118, No. 9, Sept. 1992.

It seems unlikely that a properly compacted silty sand would become loose to very loose on wetting. Like several other posters here, I would suspect poor construction (little or no compaction) combined with poor testing. I have seen cases where deep backfills were compacted in lifts, as demonstrated by photos of equipment on the backfill, and documented with tests to meet the specifications, but settled many inches when they became wet. In both cases I suspect dry soil and thick lifts. One was tested by sampling with a drill rig at 5-foot intervals. Pushing Shelby tubes in a loose fill can produce a dense sample due to friction along the sides of the tube.

The weight of the fill could have compressed the underlying very loose to loose sand, but the settlement would not be expected to loosen the fill unless the settlement was so differential as to cause cracks and voids due to arching. The sand should have compressed mostly as it was loaded, so that explanation is unlikely.

I would check the house for settlement by shooting elevations of the floor and of some perimeter feature such as a mortar joint or soffit that was probably built level. There will be some random variations, but significant settlement should show patterns.

Areas such as Texas, where the house perimeters tend to settle due to drying and shrinkage of the fat clays, have many firms that underpin homes. Many jack 3-inch tubes to refusal by reacting against the weight of the house with a bracket attached to edge of the footing. Some dig pits under the footings and jack concrete cylinders into the ground until refusal is encountered. These methods may be more economical than compaction grouting. You probably need to consult a local geotechnical engineer.

Good luck.

RE: Fill change of property with time

It would seem highly unlikely for a granular fill to soften (loosen) so anomalously with time.

I think the timing of the cracking (even anecdotal evidence) could provide insight- if the SP-SM fill was compacted negligibly I would expect significant cracking to have occurred within the first year post-construction. Is there any possibility of the sandy fill having been eroded internally (piping) through damaged service pipes?

OG's comment regarding the SPTn = 0 values being recorded around the structure as opposed to beneath the structure is important- I have mistakenly jumped to conclusions based on offset testing on several occasions- you could have intersected an animal burrow or some similar anomaly!

The commonest means of underpinning in thick deposits of loose sands in my area is on jacked piles- hydraulically-driven small diameter steel tube piles. Though there is no "competent" stratum at depth the tube piles will pick up good friction in the sands and, because they are jacked to a set resistance with a hydraulic gauge, they are effectively load-tested during installation.

All the best.

RE: Fill change of property with time

I eschew the notion that clay fills soften when made wet. The benchmark for strength is contingent on sample saturation. So, the only softening that could possibly occur in a properly engineered fill is the restoration of saturated strength. When actual fill materials are placed and compacted, they are not saturated. They include negative pore pressure, capillary suction, and GREATER strength then one would determine in the laboratory.

We are using the laboratory strength values, eh? Okay, let's say we are not - we are using correlations from the fathers of geotechnical engineering! Those fathers related the index properties to saturated (i.e., laboratory) strength.

When actual clay fill is placed, we can only hope there was some measure of material verification in measuring relative compaction. If the compaction did not achieve the critical void ratio, then there will be problems. The technician may have perceived dry strength, thought the fill was strong and little basis to question the finding. If the soil was actually governed by dry strength (i.e., in part), making that interval of fill saturated would negate all dry strength and the soil would behave as mud.

Step 1: reference the correct proctor.
Step 2: get proper compaction

Clay cores, embankment dams, etc. do not fail when they get saturated. The state of stresses change, but we'd have considered that in design.

Elastic compression (i.e., immediate settlement) does occur as the fill is placed (primarily). There is a long-term parameter, "Ct" that informs the likelihood of continued movement. Over a span of 30 years, that time-frame term can return 20 to 30 percent more settlement then calculated using elastic methods. So, if there is some anticipation of 9 inches elastic compression (i.e., a ridiculous amount of immediate settlement), over the next 30 years you may realize another 3 inches.

Just a few thoughts.

f-d

ípapß gordo ainÆt no madre flaca!

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