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
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
RE: STP Borings in compacted fill
RE: STP Borings in compacted fill
"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
RE: STP Borings in compacted fill
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
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
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
RE: STP Borings in compacted fill
RE: STP Borings in compacted fill
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
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
good luck.
RE: STP Borings in compacted fill
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
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
RE: STP Borings in compacted fill
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, 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
RE: STP Borings in compacted fill
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
RE: STP Borings in compacted fill
RE: STP Borings in compacted fill
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
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 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
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