Large Fill Site

[hollowstemhead]

I am working on a large site with several proposed new single story buildings and roadways. Construction will be phased in conjunction with keeping existing buildings (several of which are within footprint of proposed structures) on site open. Native soils consist of dense to very dense glacial till with a high fines content (up to 50-60% passing #200) and frequent cobbles and boulders. On top of the native till is up to 10 to 15 feet of fill soils. The fill soils primarily consist of reuse glacial till soils cut from activity near the site. The fill soils visually are very difficult to differentiate from the native soils. The SPT blow counts are very erratic in the fill, ranging from 15 per 6 inches all the way to 1 blow for 12 inches in building locations. The native material is on the order of 20 to 50 blows for 6 inches. The fill soils are very wet, on the order of 20% moisture content. A small number of borings identified debris such as ash and glass within the fill. Water table is on the order of 10 feet and bedrock is 200 feet. Based on the erratic blow counts within the fill stratum and hints of debris we have determined that the fill soils are not ideal for building support. We are not in favor of overexcavating the fill for several reasons: Visual inspection in determining if overex has reached bottom of fill will be very difficult since the fill/native look very similar, overex will go below water table, reuse of suitable overex material will be extremely difficult since moisture is so high requiring a large quantity of import, and overex will require undermining existing structures. We are leaning in the direction of some type of stone column or rammed aggregate system to reinforce the erratic fill stratum for building support. I am shying away from a driven pile since end bearing will not be feasible and friction may be hard to install with all the boulders/cobbles. I am just looking for some opinions on my thought process or similar experience from others. Do you agree with our decision of unsuitability of existing fill? Is overex as poor of an option as I have evaluated? Any recommendations for other options here?

 

[ishvaaag]

The alternative of auger piles shouldn't be rejected without further consideration for the machine will be normally able to engage enough in the glacial till to provide real support and still be less disruptive to the soil than trying to ameliorate it by altering huge amounts of soil. If a modern machine can't penetrate further in to some extent removable soil at the given diameter, proportionate pile loads wouldn't go further. That is what they call refusal, they know that is enough for what asked from the pile. It may also be cheaper and provide more consistent support.

 

[BigH]

Could we enquire about the approximate location? Is it is Ontario - the Peel Till Plain? or . . .

 

[hokie66]

Sounds like a very difficult/expensive site for single storey buildings. If these were multistorey buildings, I think driven piles or perhaps Frankipiles would be my options, and even with only one storey, you still may have to look to deep foundations.

 

[oldestguy]

For single story buildings I have been involved with several sites over dumped fill, including concrete slabs, but generally not old landfill. Fill thickness varied from 10 to 30 ft. Fill age generally not old.

Each was then surcharged, some with rolling surcharge, generally 10 or more feet high. No settlement platforms on most, but where they have been used, usually the most time required for practically no settlement rate is two weeks. Sites include clays as fill also. Water tables usually deep, but if there is fill over marsh (peat, etc.), it may take months per location.

Recommendations to designers is to use either a reinforced raft foundation or a grillage of grade beams over footings. No design soil pressure higher than 1,000 psf.

The rule for the structural guy is to assume that any area of 10 x 10 feet could have nothing under it. This includes building corners. Never had a structural engineer complain about that.

All interior walls and exterior walls have control joints every 20 feet or less and every opening has a joint in the walls there. Wood or metal siding need no joints outside.

For your site, you probably have a lot of soil that can serve as surcharge, but it might be a good idea to use settlement platforms. They may indicate a higher surcharge is needed, say 15 ft. Top of surcharge at least along wall locations or farther out.

I also ran this rolling surcharge at all utility runs where differential settlement may cause them problems later. Just keep shoving that pile along, never letting it drop below 10 ft. ht.

I've later visited these structures (stores, gas stations, warehouses, etc.) and note nothing in the way of noticeable differential settlement. Never had a client complain about results.

Of course no grade raising after the surcharge is off.

 

[ncarolinageo]

I think that you might be onto a good idea with the rammed-aggregate pier system. Not only do you get some end bearing from the piers - but you will see improvement in the surrounding soils from the compaction process.

 

[aeoliantexan]

My experiene is similar to Oldestguy's. Dumped or poorly compacted fill may support light loads if it has consolidated under its own weight, but can compress several percent under new load, such as an area fill, or if the moisture content increases. I have seen buildings several decades old sitting on thick fill that began to settle several inches when broken storm sewers or poor roof drainage began to saturate the soil. It is good policy to avoid bearing on any fill that has not been thoroughly inspected, tested and documented during placement.

That said, if the fill is wet enough, say at least several percent above the plastic limit, it may respond to preloading with a temporary earth surcharge. Often the top few feet can be removed and recompacted to provide a competent layer to distribute footing loads. The surcharge should impose pressures within the fill that will exceed the final condition under new grades and foundation loads by at least 15 to 25 percent. Compression under the surcharge is often complete in just a few weeks. It is important that the fill be protected from future moisture content increases. A layer of compacted clay fill as a seal and good surface drainage can help keep water out.

This approach is intended to save the owner money as compared with more positive solutions, such as removal and replacement or deep foundations. It is important that the owner understand and accept the risk involved.

 

[mwinaz]

Oldestguy - How exactly have you seen the rolling surcharge done - I'm having trouble visualizing this? When you say keep shoving the 15 ft pile along ~ Is this typically done with a dozer? Thanks!

 

[davep043]

I'm a geotechnical engineer working for a ground improvement contractor. We do dynamic compaction, rammed aggregate piers, vibrocompaction, and vertical wick drains, so I have familiarity with most of the reasonable options. My first choice in this situation, if feasable, would be dynamic compaction. Based on the fill depth, the soil types, and low-rise nature of the planned structures, dynamic compaction would offer the most cost-effective method of improving the existing fill soils. The most significant drawback is the level of vibration caused by the dynamic compaction. Whether this eliminates dynamic compaction as an option will depend on the distance between any current construction phase and any existing building which must remain in service. We have conducted dynamic compaction within 100 feet of existing homes without damage, although the vibrations are noticable in the houses. When we get within about 50 feet, we anticipate the beginning of cracking to drywall. If the existing buildings are to be demolished relatively quickly, such cosmetic damage may be tolerable.

The next level of cost would be rammed aggregate piers, but there are some potential issues. For any kind of drilled rammed aggregate pier, the presence of cobbles and boulders can severely hinder the installation. The presence of groundwater may require the use of casing, which can significantly slow the process and add to expense.

The rolling surcharge idea also has merit. The only drawback I see is that while a surcharge will overconsolidate the existing fill beyond the stress to be imposed by the eventual development, minimizing future settlement, it may leave localized areas of still-soft or weak soils. This can certainly be handled by limited undercut/replacement of such pockets.

 

[JedClampett]

Has the soil exploration been thorough enough to eliminate the possibility of hazardous waste (or even dumped material)in the backfill? If you have potential hazardous waste mitigation the project could get extremely expensive.

 

[theCorkster]

Rapid Impact Compaction may provide another approach. This involves an excavator mounted hydraulically operated compactor (3 to 4 feet in diameter) that is repeatedly impacted to the ground surface. Treatment depths are on the order of 10 to 15 feet. The method moves very quickly. When completed, the surface has to be regraded to fill in craters left by compaction. The depth of cratering depends on the looseness of the soil. The depth of treatment should address the loose soil in the zone of influence of the foundations for lightly loaded single story structures.

The only likely restriction could be the groundwater; if at 10 feet, the process shouldn't be significantly affect by groundwater.

 

[davep043]

Corkster, do you have direct experience with the Rapid Impact Compaction? We've talked to Pennine in the UK, who build the RIC units, and they've said they only count on maximum of 10 feet of influence, and then only in very clean coarse granular soils. Where the fines content is higher, the depth of improvement is substantially less. Even so, 5 or 6 feet of improvement may be adequate for this site.

 

[jnimens]

Continuous shaft helical piles can be installed to support the buildings, be they 1 or 10 stories, very inexpensively and their torque monitored installation takes all the guess work out of their capacities. Joe Nimens

 

[epongra2]

With regards to the option of removing and recompacting the existing fill, is there not a soil profile at the top of the native glacial till? If so, that is a particularly easy way to verify you are through the fill and into the native below.