SPT and grade of compaction for foundation of metal bulidng on fill

[Okiryu]

Hi,

I have an old fill and would like to know its grade of compaction. We have SPT information also, so are there any correlations between SPT and grade of compaction?

Happen that I have a site with a thick layer (8 meters) of fill with clay/silt type soils. Occasionally construction debris is mixed (pieces of concrete and asphalt). The native soils are soft silt and clays (STP betweeen 1 and 4) with the groundwater table around the bottom of the fill (layer thick is 5 meters). Below these layers medium dense/dense gravels were encountered. A light 1-story metal building structure is planned here.

I am still waiting for the lab results for these fill and native soft soils but in the meantime I am thinking on how the foundation can be. Since there is some flexibility for the site grading, perhaps cut the site in order to "compensate" the weight of the structure with the cut volume is the best option. Also, there is an existing metal building similar to the proposed new one just adjacent to our site and it is performing well (no signs of settlement).

Any other thoughts?

 

[GeoPaveTraffic]

First there is no coorelation between SPT and compaction.

As for placing a building on a site with 8 meters of uncontrolled fill that contains some amount of construction debris; you and the owner will be taking a lot of risk. Unless you dig up all of the fill, you don't know what is right next to your explorations. Could be well compacted gravel or could be a bunch of trash.

If the owner of the building is willing to take the risk and for the type of contruction that you are describing, I would probrably excavate the upper 3 to 5 feet of material within and about 5 feet past the building footprint and place a well compacted and controlled fill. That way you have a zone of good material to help bridge future settlements that could occur. You could also add a couple of layers of geogrid in this fill to add some tensile capacity.

Mike Lambert

 

[oldestguy]

Mike may be right. However, if you wish to be a little more conservative, look in this "room" for my comments regarding the subject title "Wet, Very Loose Sand". Preloading of dumped fill has worked well for me. One of my sites sounds similar to yours.

 

[Okiryu]

Thanks for your replies.

Problem is that I do not have consolidation parameters so I will not be able to calculate the time required for preloading. However, I do have atterberg values so I can calculate settlements. The fll was not homogeneous so we decided to not take undisturbed samples for consolidation or shear strength tests. In this case, perhaps a settlement monitoring program may be needed. I will take a look at the thread OG recommended, also.

Thanks again!

 

[GeoPaveTraffic]

Pre-loading often works, but I don't like it with uncontrolled fill. Too many variables that you really have no idea of.

To me it is all a mater of risk and what the owner is willing to accept. Just make sure that the owner accepts the risk in writting and that you keep a copy in your file. Owners have a tendency to forget when thing start moving around.

Best of luck.

Mike Lambert

 

[Okiryu]

Mike, what do you think about my original idea about partial remove of the uncontrolled fill in order to compensate the removed soil weight with the weight of the structure? I was thinking that may be an option also.

 

[oldestguy]

Sometimes when questions come up as to what preloading will do, I have tun a test fill, instrumented to monitor settlements to get a handle on what the proposed surcharging job would do. Keep in mind this is not fully representative of what a full scale pre-load surcharge would do, because of rapid load spread pressure reduction with depth using a small loaded area. A typical test fill would be 6 or 8 feet high and measuring about 10 x 10 ft. on the top. In my past jobs I never had even Atterburg limits and very few if any moisture content tests on the fill. For most sites a "rolling" surcharge windrow was gradually moved across the site. Undercut depth was such that pressure distribution from footings would not exceed the surcharge pressure at the base of that undercut depth. I little "cheating" could be done recognizing that with added depth the footing pressure distribution would start going less than the preload pressures there, and even better with depth.

I've never tried to predict the time required for surcharging, via lab tests, etc. since my luck in that regard has been bad, always (except for two jobs) with settling going more rapid than I computed, sometimes by a bunch. Settlement platforms are the way to go if in doubt.

Your cutting down to a lower final surface elevations accomplishes the same idea as a surcharge, perhaps even better due to time effect.

 

[Okiryu]

OG, thanks for the advice. For this project due to time constraints, preloading may be difficult to implement. Cutting down in order to allow for a "floating slab" type foundation can be an option. Or, remove the entire fill, but it will be costly... I will get the lab results by next week, so I will be posting again once I get more info from the lab...thanks again !

 

[aeoliantexan]

I would approach your project more or less as follows:
1. Will your building have heavy distributed floor loads, like a warehouse? If so, consolidation of the soft natural soils may be the biggest issue.
2. Was the fill placed as part of a planned development, perhaps with specified compaction, or just dumped? A few test pits can help. You can run in-place densities and take associated samples for Proctor compaction tests. (Remember that pushed tube samples can be compressed during sampling, making the compaction appear higher than it actually is.) Test pits can also reveal trash that would not have been permitted in a controlled fill. Look for open voids between clods or under pieces of trash. Look for the imprints of a sheepsfoot roller.
3. Is the water content of the fill likely to increase after construction? Replacing a weedy vacant lot with a lot of grassy lawn and sprinklers can have a big effect. Plumbing can leak, too.
4. How much differential settlement can the building tolerate? How much can the floor tolerate? Sometimes it make sense to protect the building with a deep foundation to the dense gravel and let the floor move.
5. How old is the fill? If just a few years, it may still be settling from compression of the soft natural soils.
6. Poorly compacted fill can compress a lot if it becomes saturated or nearly so. You can't always be sure that won't happen. Preloading can bring the fill into equilibrium with the preload at the present water content, but if the fill gets saturated, it may compress anyway.

If you can convince yourself that the fill is well compacted and has been there a decade or so, spread footings and a slab on grade may be appropriate. Otherwise, there is money to be spent or risks to be taken.

 

[oldestguy]

One more item. For buildings typically found in small warehouse areas and where the ground is not the best, as apparently here, I add one added recommendation. Assume any area in the building measuring 10 ft. by 10 ft, will have zero support. I have not had a structural engineer come back and say he can't design for that. Waffle slabs seem to work out for these cases I understand.

 

[Okiryu]

Thanks again for the responses.

I got the lab results. The native soft silt/clays soils are normally consolidated. Natural water content is close to LL, so OCR could be confirmed. Shear strength is low, around 35 kPa.

About the loads, I have column loads (DL+LL) of 180 kN and a floor live load of 8 kPa. The building is flexible as it is a pre-engineered building. I will limit the total settlement to the typical 25 mm and the differential settlement to 10 mm.

Also, I found that the fill has been dumped there in the last years with the latest placement in 2013. No compaction control during placement.

Based on these conditions, I feel that the fill has to be removed. I have to check with Civil about the proposed grade elevations to check the required amount of fill and check consolidation settlements due to the fill and structure loads.

BTW, very close to our site, there is a similar building which was constructed in 2000. The shallow footings were designed for a low allowable bearing pressure (35 kPa). I inspected that building and the slab on grade was in good shape. No signs of settlement was noted.

OG, the 10'x10' unsupported portion of the slab, suggest me a waffle type SOG...

Thanks again for your all you input.

 

[aeoliantexan]

Congratulations on obtaining some crucial information!

I think you have to assume that the soft natural soil is still compressing under the fill loads. Even if consolidation is now complete, it could compress significantly under the floor loads if they cover a sizeable area. You are correct, the relationship between present and final grade is very important. And, if you replace the fill, the new fill will likely be heavier than the existing uncompacted fill.

Removing and replacing 8M of fill will not be cheap. With a good bearing formation at 13 M, you should consider a pile foundation and structural floor. And leave room to add another step at the front door in the future.

The building next door is irrelevant unless you know the soil conditions there and the construction history.

 

[oldestguy]

The form of slab and foundation used on jobs I have seen, including two story apartments and slab on grade industrial buildings, usually is not just a thick slab, but a form of waffle configuration. Replacing the current fill with better fill has never been done on the many similar jobs I have seen including over organic soft soils. The closest would be where some undercut was done at footing locations to not exceed the surcharge pressure (from footings), which dissipates rapidly with depth. Usually the jobs required a raise of grade. In that case the new fill is compacted to allow reasonable footing pressures on it.

 

[Okiryu]

Thanks again for your advise. How about to cut the site to an elevation where the weight of the removed fill soils will be compensated with the weight of the structure (a floating mat slab building, so it can bridge differential settlements)? This will be similar to OG's idea of undercutting at footing locations to not exceed surchage pressures.

Also, I think soil improvements (soil-cement columns or geopiers)can be considered as well.

Will talk with the designers and post again here once I get more information.

Thanks again !!

 

[Okiryu]

One question, do you consider earth passive pressure for the resistant parameter for lateral loading in such as uncontrolled cohesive soft fill layer?

 

[Okiryu]

Just as an update, I am looking at a helical pile option installed to a depth where the dense gravel layer is located. I needed to increase the pile diameter due to I am assuming very low lateral resistance for the top cohesive uncontrolled fill. The piles will be approx. 13 m, 1 per each column. I was worried if 1 pile under the column can create eccentricity problems, but the building is very symmetric so 1 pile under the columns might work fine.

I decided to explore the helical pile option to decrease risks as suggested in some posts above.

Thanks for your help!