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STP Borings in compacted fill

STP Borings in compacted fill

STP Borings in compacted fill

(OP)
Is there anyway to evaluate the compaction of fills using SPTs? The engineer of record has done multiple SPT borings, the material ranges from a SM to ML with anywhere between 23% to 65% fines. The engineer of record is now saying the material was not compacted properly based on the N values. There was a foundation failure after a large rain event 6.5 inches in two days and almost 8.8 in 9 days (which supposedly had nothing to do with the failure). Fills ranged from 5-11 feet.

My feeling is the N-values are an index test, and the values listed for sands are for clean sands not silty sands and sandy silts, the material definitely has cohesion as a 10 foot shear cut held up for 5 days with compaction equipment ran up against it for 5 days.

Any ideas???

Thanks in advance  

RE: STP Borings in compacted fill

Welcome to Eng-Tips.  Hope that the site will be useful to you.  I notice that you posted this in both this forum and the soil testing Engineering forum.  By doing so, you will get disjointed responses.  I've red-flagged the one in the soil testing forum and asked that it be removed leaving this one for responses.  Double posting is not appropriate to the site. Most geotechs review all the geotechnical forums.  I'll give your situation some thought and other will too.  Do a search - for there has been a similar question recently.  That was for confirming, or not, compliance with the specification.  Why there wasn't any testing done during the construction phase is a mystery - as with yours.

RE: STP Borings in compacted fill

Aw dangit, Big H, I just replied to the other one (which happened to be directly above the one you were referring to).  I said the same thing as before: test pits and sand cones.

RE: STP Borings in compacted fill

me too, dgillette...I copied my reply before it get the inevitable can....



"I agree with dgillette.  The "near surface" soils can be better tested directly with appropriate methods...SPT is ok for deeper stuff by correlation, but when other methods are readily available that will give more accurate data...use them."

RE: STP Borings in compacted fill

Sorry guys - I had to pick one . . .  Maybe when a thread is double posted and one is removed, the replies could be transferred?

RE: STP Borings in compacted fill

In the early 70's an article by Bazarra in the ASCE geotechnical journal presented new correlations between SPT and relative density using tests in compacted fill.  After some discussion, i believe the geotech community generally agreed that Bazarra's correlations appied to compacted fill and the old Gibbs and Holtz correlations still applied to sedimentary sands.  Compacted fill seemed to produce higher SPTs for a given relative density than natural deposits because compaction locked in high lateral stresses.  There has been a lot more published on SPT since then.

You can develop a correlation between SPT and relative density for a given material; it will depend on depth and compaction method as well.  It will take a lot of work.  Others can argue with your results.

If a dispute over money is involved, I suggest using test methods that all parties will agree to accept.  Otherwise, a resolution will be held up by disagreements over interpretation of data.

RE: STP Borings in compacted fill

what does a "foundation failure" consist of? was there excessive settlement or sinkholes or did the building heave over to one side like a sinking ship or what?

also, what sort of blowcounts were found? and you meantioned that relative to the blowcounts, "the values listed for sands are for clean sands not silty sands and sandy silts". what values are you referring to and wouldn't a blowcount be a blowcount? the blowcount doesn't care what the material is...only we care what the material is. so in other words, a 5 is s 5 although our interpreted consistency would be different if it were a silt versus a sand. even the topic of a "silt" or "sand" is fuzzy, varies dramatically with the available test data and depending on which way you want the result to fall. for instance, i've seen a wash #200 go from 35% to 60% simply because dispersing agent wasn't used by the testing firm...so that supposed SM then become a ML.

i'm rambling a bit to make a point...
1. there's not enough info to make any sort of reasonable guestimate about what's going on
2. a simple data point or two likely doesn't tell you anything unless it happens to fall on the indiviual reader's point of view
3. a data is useless if it wasn't obtained correctly to begin with
4. if you are the geotechnical engineer, who is this EOR? are they the geotechnical EOR or a structural or what? are you stamping the project or working for the contractor or another client?
5. what degree of compact was desired?
6. loads?
7. area of loading?
8. etc
9. etc

RE: STP Borings in compacted fill

SPT are an index tests of sorts that yield relative density. They are applicable to Silty sands as well as clean sands. They are not useful for cohesive soils. The chief use of SPT's is to estimate the internal friction angle of the soil, from which an estimate bearing capacity can be computed. Unfortunately, most compaction jobs are controlled by specifying a dry density Proctor or Modified Proctor value. These values do not correlate to SPT tests (unless you do the site specific testing aeoliontexan suggested.
I would point out that the SPT test is highly variable and is not precise. Laboratory testing of field samples would be the best way to determine soil properties or bearing capacity. fills with fine contents ranging from 23-65% wouldn't be expected to perform well in 9 inches of rain. Could you provide some more details on the nature of the failure?
Also note that unless you really now you have a solid clay running compaction equipment along the base of  a 10 foot shear cut is not a good idea. These faces can colapse without warning

RE: STP Borings in compacted fill

If we think about the reason for specifiying soil density as a control mechanism for compacted fills, we might find that the SPT can be useful. The compacted fill will probably have to support a load even if it is its own weight.  To support a load, the soil will have to possess a minimum shear strength. It can be expensive to test the shear strength of a fill with a frequency that would provide reasonable confidence.  We know that there is a rather dependable correlation between strength and density.  And density tests are less expensive to perform in the field.  Over many years and many projects we have found that relative density values of say 95% density (Standard or Modified depending on the project requirements)typically provide adequate soil strength.  Therefore, I think that performing SPT would be useful if you are convinced of this arguement and you can convince the Engr of Record of the same.  I have done this with both SPT and Shelby Tube sampling on certain projects with acceptable outcomes.

   

RE: STP Borings in compacted fill

That's fine from a pragmatic and practical point of view.  But if this is for judging that a contractor has lived up to his stipulated requirements in the contract - it doesn't wash.  It is now contractual vs practice.

RE: STP Borings in compacted fill

(OP)
To shed some more light on the situation. The geotech of record recommended the removal of soils within the building pad. The soils removed consisted of organic clays, fill materials, tree roots, old pipes, etc... The report recommended the removal down to bedrock (sandstone and shale) with the perimeters being cut a further 10 feet beyond the footing lines. The materials from one end of the site were approve by the geotech for use as fill (SM-ML). The fills were placed and tested and compaction was attained. However the material went in slightly drier than +-2% and in lifts of roughly 12-15 inches, which the contractor was advised outside of the specs. Density testing revealed that the recommended compaction was attained.
During footing excavation the footings were tested with a drop bar and favorable blow counts were attained. After a 6.5 inches of rain in two days the footers in one corner of the building had dropped 3 inches, there was no load on the footers. It should be noted that the site sat under 2 feet of water for at least 2 days after the rainfall. Test pits were dug in the failure area and the moisture contents revealed that fills were then above optimum moisture, and that the contractor had not removed the required 10 foot perimeter excavation. The geotech of record then performed SPT borings, which revealed blow counts ranging from woh to in excess of 50. The area in which the failure occurred was removed and backfilled with recycled material.Subsequently the eor has submitted an opinion that the materials were not impacted by the rainfall, improperly compacted, and that the whole building should be pressure grouted.
To me discounting the impact of the rainfall is way off the mark, as pictures of the fill going in and afterward is like night and day. He has discounted the washing of fines, but how would you tell if the material already contained varying amounts of fines? Is it possible that the rainfall event combined with the material being placed dry of optimum caused the grains to go through some type of reorganization and caused settlement or a loose or soft condition? Also is it possible that the rainfall event sufficiently raised the water table and decreased the bearing capacity of the soils?          

RE: STP Borings in compacted fill

1. Somebody should have records of construction control, such as density testing and weather or not the contractor achieved density during construction.
2. Obviously the lack of excavation at the perimeter will be an issue.
3. Optimum moisture only applies during the compaction operation. The water content can increase after that with no increase in DRY density.
4. getting to the meat of the matter, the problem appears to be two fold: a) the fill soils seem to be highly variable, and probably some have high fine contents and low sand contents and others are just the opposite and b) there does not appear to be any mention of drainage provisions, which based on the flooding are non existent or inadequate.
Soils with high fine contents and granular soils do better with different types of compaction, so that would explain why some parts of the site were good and others are not. The other problem is that fine soils do not drain quickly. as the weight of the fill rises due the saturation by the rain, the water trapped by the clay particles can not bleed off fast enough, the internal pressure rises and the clay or silt soil structure can fail.
*** Obviously I have not seen the site or any data beyond what you have mentioned. This is just my feeling based on your notes. You may want to bring in an independent geotech to review the site and data to make recommendations. If the soils are not coarse and drainage is an issue, compaction grouting may cause more problems than it cures.
Hope this helps

RE: STP Borings in compacted fill

the fact that the contractor put the fill in 12-15 inches thick will kill any argument. running compaction at the top of that tells you nothing. with the rain you mention, it sounds like someone is putting in a loose rocky-soil fill that then washes down in to the voids since it was put in way too thick/loose. i don't know the geology but just assuming the problem is in the fill, then the EOR has my vote. if it went in compacted in the first place, it doesn't magically become uncompacted with a rainfall. and raising the groundwater doesn't necessarily lower the bearing pressure...in fact, it could increase the "bearing pressure". bearing pressure is not a simple test...it is much more and has multiple inputs. again, the EOR has my vote.

good luck.

RE: STP Borings in compacted fill

(OP)
Not arguing that the material becomes "uncompacted". When the water table is raised if you calculate the theoretical acceptable bearing pressures you essentially cut the calculated pressures by half.
I have had situations where material went into trenches or underneath pavements, the material was fine grained in nature, as it went it it was below the optimum moisture and compaction requirements were met, after rainfalls the material lost its strength. That is the argument, its no different then if you have a clay that is dry and strong, then its saturated and it looses all its strength. The contractor has to answer for its errors, pertaining to lift thickness and not removing the necessary perimeter cut. However, the EOR needs to accept responsibility for approving a fine grained soil as structural fill for a building pad.   

RE: STP Borings in compacted fill

    With respect to the last post's "theoretical acceptable bearing pressures" - you are not actually describing the acceptable bearing pressure but the acceptable (allowable) bearing capacity based on shear.  The "acceptable" bearing pressure is not the same as the "acceptable" bearing capacity - in most cases it is less due to the service limits on movements set by the structure design criteria.
    I doubt that the footing settled because of the lack of "strength" - but it is possible that the compaction, being drier than optimum caused a metastable state of the grains to be developed and that the subsequent inundation of the material gave "lubrication" to allow the metastable state to rearrange to a more dense state - and this was significant.  Others have postulated, correctly, that for most fills, the dry density doesn't change on inundation - I agree but obviously with the settlement, the volume is now smaller for the same mass of soil - so the dry density would be higher.
    Do you have records of where the compaction testing was carried out?  Was it done on a random basis or was the testing selected in a biased fashion?  Who selected the locations?  Was there any actual tests run in the area where the footings experienced the distress? Perhaps, as the contractor did not "extend" the excavation out beyond the footprint as stipulated, perhaps he didn't extend it to the perimeter of the footing either?  In this case, the footing might not be founded partly or wholly on the placed fill.  
     I find it difficult to believe that any compacted fill would have a WOH "blow count" This suggests the postulation noted above. Just a thought.

RE: STP Borings in compacted fill

A caveat on my last statement about WOH - I have seen this (or a situation that would be so) but this was when compacting a loess soil and it was allowed to dry out completely - the upper 300 to 500 mm was "powder".

RE: STP Borings in compacted fill

Sounds like a classic case of "hydrocollapse".
Your lift hights were to much, and the soil was not completly compacted. Once it was inundated it collapsed.
Also not completing the removal as specified probably contributed as well. There is no reason to blame the EOR for using a finer grained soil for structural fill.  If there was then a lot of dams would not exist.
If I was the EOR I would make the contractor remove the fill entirely, to the proper limits of the removal as specified, and start over, that would be cheaper anyway than compaction grouting. (unless you let the contractor continue with the construction of the building?)


Collapse looks like this on the consol diagram.
__________________ Log P
|\
| \
|  \
|   | <- H2O added
|   |
|   |
|    \
|     \
|
Percent
Consolidation


 

RE: STP Borings in compacted fill

I agree with msucog and muddfun.  It looks like a fairly simple case of a very inadequate job of overexcavation and replacement.  A contractor who doesn't excavate outside the perimeter as required and places 15-inch lifts probably doesn't do a very uniform job of placement and compaction, so there will be hard and soft zones after water gets to it.  There is nothing wrong with using ML and SM for a building pad if the fill is properly compacted.  Yes, it will absorb a little water and soften somewhat if inundated, but it won't become "woh"; it won't settle 3% of its thickness under its own weight; and the bearing capacity won't become so low that the footings settle under their weight, no matter where the water table is.

I agree, the contractor should remove everthing and start over.  And the testing lab should watch every lift being placed and test it with judgement applied.

My comments are based on 25 years of geotech investigations and inspection where thousands of buildings were founded on spread footings on structural fill, usually CL, but also ML, SM, and SP.

RE: STP Borings in compacted fill

I agree with Aeoliantexan on several of his points.  Silts are OK if you can compact them properly.  Thick lifts in silt are really a problem because of the narrow moisture band at the proper density range.  Plus, there can be some amount of bridging that does not show up with thicker lifts.  The lower portion of the bridged soil can still be loose and settle with the introduction of water.  I read two important items in geo1976 information.  First, only one corner of the site settled indicating to me perhaps a local problem with different compactive effort, soil type, etc.  Second,SPT values in the problem area ranged form WOH to 50?  Has anyone ever seen that large of a range in properly compacted soil in a small area of the site?  If the rest of the site has performed well, does that need to be removed?  Just asking?   

RE: STP Borings in compacted fill

{quote]Second,SPT values in the problem area ranged form WOH to 50?  Has anyone ever seen that large of a range in properly compacted soil in a small area of the site? [/quote]

No, and that is why I agree with aeoliantexan as far as removing and starting over.  There seems to be a lack of control for the fill, and therefor, I would consider all of it to be suspect.   

RE: STP Borings in compacted fill

geo1976 - You indicated that testing showed the spec'd dry density was reached, but what was spec'd, 95%?

RE: STP Borings in compacted fill

Regarding TDAA's comment to remove and replace the fill, it would seem to me that this suggestion is discounting the adequate performance of the remainder of the foundation on the fill.  I guess I don't see as straight forward as y'all do.  

RE: STP Borings in compacted fill

But can you say the rest of the fill has really been tested, jrm73?  The problem apparently resulted from the fill in that area getting wet.  If you knew with confidence that the whole thing had been tested by wetting or that the remainder never could get wet, I'd be inclined to agree with you.  However, longer spells of wet weather, rising groundwater, broken pipes, etc. could hit areas that have not been tested already.

In an area that did not settle, dig a shallow pit, or build a low sandbag wall to contain a little standing water.  Dig or drill a few deep holes to conduct the water down into the lower parts of the fill.  Add water, stand back, and see what happens in a few days.  Alternatively, get some block samples or tube samples and do collapse testing on them in an oedometer.

RE: STP Borings in compacted fill

A few of you have hit on something that, in my experience, has been the main cause of very significant differential settlement in many cases.  Also, seldom do big problems only occur due to one factor.

That main factor commonly is the inadequate width and sometimes also inadequate depth  of the replacement fill materials.  The mere fact that it is known the perimeter replacement was not done should be a significant red flag.  Getting WOH in a zone that has slipped sideways into a nearby crappy zone also fits this situation.

To resolve the thing when replacing the work, have someone on site do a careful documented survey of what is really found. I'd lay my bets on the presence of the old soft junk being too close horizontally and possibly vertically to the loaded area.  Water likely has softened that stuff to some extent, in addition to adding weight to the site.

For some dumb reason many an excavating job is done by staking out on the ground surface the horizontal location for the bottom corner of the proposed excavation.  The excavator,sitting inside the job site, leaves that stake there and digs with the bucket going down and in towards the bottom of the required "hole".  That frequently leaves a zone of unexcavated stuff in the worst possible place for a nearby footing sitting on the new fill.  Waiting for "some time" after the excavation work before filling also allows sides of excavation to cave in also into the worst possible place for it, with contractors not bothering to remove that material either before placing "compacted" fill on it.
It isn't always the stuff underneath the foundation that causes it to fail.  What happens along side it frequently is at fault.

RE: STP Borings in compacted fill

I have seen exactly what oldestguy described.  A three-story structure was built on footings on several feet of recompacted silty clay.  The back wall settled badly after construction was completed.  A test pit revealed that the excavation began near the wall line and sloped into the building area, so the old uncontrolled fill that was to be removed remained just beneath the footing.  

I have seen several cases where the overexcavation was laid out too small, the wrong shape, the wrong location on the site, or omitted an external building feature, such as outboard columns or a bell tower.

If you believe the fines in the soil rendered it susceptible to drastic loss of strength upon saturation, you can run soaked CBR on properly compacted samples.  In my experience, silty clay or silt compacted to 95% of standard Proctor softens to a depth of about 12 - 18 inches when water is ponded on it for a week or so.  Below that depth it may change from very stiff to stiff, but is still suitable for allowable bearing pressures of 3000 psf or so.

The remainder of the site has not truly been "tested" unless the footings were loaded by the building and the water was there long enough to saturate it all.

It has been shown that the contractor did a poor job.  Why should any one else take on his liability at this point by trying to determine which parts of the fill need replacing and which can be left in place?  I agree with the eor, except I am guessing that replacement will be less expensive and more reliable than grouting.  It will also be in the contractor's interest to redo his work rather than pay someone else to grout it.

RE: STP Borings in compacted fill

Very interesting case and comments. Lawyers love these cases when soil below floor slabs settle and footings undergoe settlement. This issue has been with us for years and would not go away unless we are able to have full time inspection of all backfill to be placed on sites. Often this is done on an "on call" basis. No good, since as soon as  one's back is turned the crazies continue their program of placing large lift thicknesses etc. It is akin to putting Dracula to be in charge of the Blood Bank.

Quite a process to monitor and often this is left to a technician in many jurisdictions who most likely would have been briefed for half an hour in the use of the nuke and does not have a clue on how to evaluate soils.

Today's quality control is based on how to select low paid staff to make the most money. Sorry to say this but the profession has moved in this direction and appears to be more amenable to this type of business. Not to say that there are some, but a minority, who would do good work but would fail to get those jobs because of price.   

Such a person checks only density and forgets the relationship of moisture content to the soil behaviour when the soil is wetted up in time.

I agree that raifall on compacted soil can create much difficulties, and this is not carefully addressed. It is always a surprise to many to see wetter and weaker soils after construction than what was supposed to have been observed during construction. The answer to that lies in a number of issues that are likely inherent during construction.

eotechnical Engineers should take a good look at Unsaturated Soil Mechanics by Fredlund and to recognize the influence of water on so called strong soils that we are confortable with as their SPT blow counts are high. These are generally not staurated soils if they are within many shallow foundation depths. These soils lose their strength rapidly and not necessarily from total innundation but by moisture movement even in the vapour stage. The british have done considerable work in this ares in the 1960's

The answers given, grouting etc are typical since in many cases we are unable if a building and floor salb were present to determine the reason for the issues we face and no one wants youi to spend large sums of money on investigation but would spend larger sums on a fix. If the fix does not work you are on the hook then our friendly lawyers are ready to feast.

In my experience these problems are as a result of many factors but all may be associated with water in some fashion which is a friend and foe. We also have to recognize that their is movement/settlement of a fill resulting from its self weight which can be small but adds to the issue.

Geo1976 has made many worthwhile comments and my vote leans on his side. Aloeian texan has also made some good comments. However in regard to the ponding of water and deterioration of a specific depth this may be true, but have we addressed this observation as the fill is placed when very often mother nature allows it to pour and immediately after fill is placed by the dozer operator especially if no one is looking.

This is part of the issue and as well if we have a building foundation in place the ability of water to seep into the foundation as the exterior is not sloped at the time or other measures are in place to prevent wetting up after construction due to moisture movement by rainfall.

Regarding Jrm73 statement that silts are okay if properly compacted, one has to be careful that water does not get into compacted silt via a broken water main etc.

There is a need to carefully look beyond compaction when materiasl of fine grained nature are to be used for engineerd fills. Of course the system today is constrooled by truckers who are only too eager to transport fill from one site to the other without any proper engineering evaluation of the material except to have an environmental certificate indicating the soil is acceptable for use. It is interesting to observe where geotechnical engineering is going. The environmental scientists have taken over the roost, but this is just as important.

I note that geotechnical engineers are pressed to use fill that is at a reasonable cost to the Client and as such we tend to use clayey fill rather than gravel fill. The latter also has its issues if allowed to be exposed to precipitation by rainfall or snow melt.    
    
Please excuse the length but this thread is of valued importance to the building construction industry that many comments made here indicate that we are far from resolving the issues of post construction building performance.It is a subject that requires careful thought by the geotechnical engineer but likewise to the technician his lot is no better today where money supercedes the desire for a quality product.

I can go on but will leave for now   
 

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