STP Borings in compacted fill

[geo1976]

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

 

[dgillette]

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

 

[jrm73]

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.

 

[dgillette]

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.

 

[oldestguy]

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.

 

[aeoliantexan]

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.

 

[VAD]

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