Foundation Engineering Challenge

[jdmm]

Question: Does anyone have any suggestions to stabilize or transfer load through garbage consisting of woodwaste, concrete rubble, and other garbage overlying hogfuel and peat.

The problem is a proposed shopping centre on a large lot that was once used for topsoil production and disposal of construction waste including wood and concrete. Hogfuel (tree bark) was used as a working platform over the peat that wasn't removed. The woodwaste hogfuel and peat is up to 40 feet thick.

Normally we would just pile through the garbage but in this case the peat and garbage is underlain by a discontinous layer of sand that varies from 0 to 30 feet thick at a depth of about 60 feet. From the peat to the sand is a low plastic silt that has a moisture content of about 60% to 80% and is highly compressible. Beneath the sand is a marine clayey silt that is close to normally consolidated and extends to a depth of about200 feet. (we haven't drilled to the bottom of the marine silt but seismic surveys suggest that it is below 60 metres 197 feet).

The surface has been filled with between 5 and 10 feet of mineral fill. The mineral fill and garbage has resulted in settlements of the underlying silt and marine clayey silt of up to 6 feet where the sand layer is missing and substantially less where the sand exists.

We have experience on other sites nearby where we preloaded the silt and underlying marine silt but this site has a problem with the garbage waste at the surface where I expect that there may be long term (secondary consolidation or creep) settlement in the garbage, hogfuel and peat. I have been thinking about some sort of vertical reinforcement through the garbage and peat. Something like soil-cement mixing, jet grouting, soil cement stone columns, expanded base columns.....

The concept would be to prevent secondary consolidation of the garbage and peat by transfering load from the garbage to the columns through skin friction and by transferring the load from the overlying mineral fill through to the top of the silt. This would be done after the site was heavily preloaded.

Does anyone have any experience with this kind of problem?

 

[VAD]

Hello jdmm:

An interesting and challenging problem indeed. At face value your concept of vertical reinforcement would be one thought especially the soil mixing which would result in the reconstitution of material that is potentially problematic.

In further reflecting on some of your statements I have the following questions or present some of my thoughts based on what I am conceiving from your information. First, it is of interest that the area with little or no sand has settled less than those with larger sand depths. This seems to suggest that the sand was acting as a bridging layer rather than aiding consolidation. The magnitude of the loading above existing at this time perhaps has some part to play in relation to this aspect.

I presume that the reason for the proposed heavy preloading is to remove the primary and secondary settlement of the subsoils. Should this scheme not have an impact on the garbage,hog fuel and peat as well. I also presume that the reason is to achieve good ground conditions for both surface utilities such as roadways and perhaps the use of shallower foundations for the shopping centre.

On the other hand, the use of deep foundation units may be conceived but would be subject to significant dragloads and may not be ideal, unless of course the concept of deep foundations is still needed despite the proposed heavy preload. This combination can be seen to be plausible depending on the structure loadings etc. Could be a costly project.

Would dynamic compaction before placing preload help in modifying the garbage or removing some of the undesirable characteristics of the garbage. I know that Landva (I hope I have spelled his name correctly) has done some work some moons ago on garbage re settlement etc and this may be of some value to review if you have not already done so.

I do not know the proposed structural scheme of the shopping centre but would a bouyant foundation be reasonable to think about so that some of the garbage can be displaced by excavation. Just a thought to effect better performance. Is excavation of some of the undesirable surficial material out of the question. I presume that this woill be expensive as the site is covered with fill already.

The hogfuel and peat can pose some interesting issues at this site. Degradation of the hogfuel can be expected in time when not encapsulated. Both peat and hogfuel are good conductors for underground fires. As you most likely know one of the issues of burning in peat areas is control of underground fires. This however may be of no concern to your site but are you constructing on a site that has the potential for this problem.

Back to geotechnical, it is often difficult to remove all the secondary compression of the peat despite preloading. We have done this on highway grades but problem still persists. Perhaps the preloads were not on for a sufficiently long time.

Looks like I will be rambling from here on and quit by saying that this problem would have its solution based on some good ground information and back to basics on consolidation, stress distribution etc, and of course imagination.

I presume as well that this site has to be used. Just curious to know if the site conditions were known before by the Client.

All for now and good luck. Others may have some better or additional ideas. Will return after some feedback.


need a few of these

 

[jheidt2543]

jdmm,

One question I would add to your list is: How do you plan to handle the methane gas produced as the garbage decays? With a building over the top of it, the gas has to go somewhere.

 

[BigH]

I will review the question and VAD's comments in more detail in a couple of days after vacation is over - but, I agree with jheidt - there may be a methane gas problem - but this could be handled, I suppose with a venting system installed.
By the way, is this in the Burnaby Peat? Your use of the term "hog fuel" suggests you are from BC. If so, you might check out city of Burnaby and get an old report (1960ish) by FENCO on development in the Burnaby Peat - this is an oldie, but was, I think, rather well done in many aspects - and good from an historical point of view.

 

[jdmm]

Methane is always a problem in organic soils below the water table. Test pits dug in 1987 had methane bubbling from the bottom of the pits. Methane is handled either with a passive or active system. In the passive system, perforated pipes are laid beneath the building floor at 10 to 20 foot intervals (depending on the permeability of the surficial mineral soil closer spacing for sand wider spacing for siltier soils). The pipes are connected to risers that eminate at the edges of the building. Methane being lighter than air is allowed to vent outside the building. In the active system the pipes are connected to a ignition proof fan system that vents usually at the roof. In enclosed areas within the building that don't have good ventilation methane detectors are often used.

Further information:

Test pits dug through the garbage indicate that it consists of about 90% wood consisting of cut ends of dimensional lumber, but there is lots of plastic sheeting, plastic pails and bottles, wire rope, conveyor belts etc. Some zones have lots of voids so I think that soil mixing would use a lot of grout and would be difficult to accomplish. Expanded base piles would be more expensive than precast concrete so I think that long, low capacity precast concrete piles will have to do. I think that preloading in advance to minimize differential settlement between the exterior grade and the building floor will also help to improve the capacity of the piles and prevent settlement in the case of adjacent development.

Thanks

 

[cap4000]

I hope for the EOR and Owners sake, that VERY DEEP dynamic compaction works at least for the slab-on-grade and parking lot loadings. My pick would be steel H-Piles based on a solid point of refusal and skin friction combo for the interior columns with pile caps of course. The major problem with any pile is What Is The "Unbraced Length" for its design.Hopefully, all the loads transferred are not high. Get an E&O policy from the all Engineers involved Definitely get a Maintenance Bond from the Site Contractor. Good Luck!!

 

[Daryoush]

We have had a few projects with very similar features. Since your overburden material (garbage) is erratic in nature and the problems of implementing an appropriate deep foundation are cost prohibitive, you may approach the problem from a settlement evaluation standpoint, designing with the consideration of the anticipated settlements. The first phase would involve improving the subsurface conditions by all the above noted methods, as well as introducing piles or piers (driven piles with partial predrilling) coupled with leveling means. Periodic relevelling/jacking is done to maintain the super structures. This may be a project savior in terms of the cost reduction.

 

[fndn]

What a challenge! I would try to abandon the site due to extra foundation cost. However, if the client can develop a high-end retail and recover the extra cost then go ahead. I would erect a sheetpile system and make a perimeter around the building layout with 15 ft blowout. Then we could excavate the unsuitable and dewater simultaneously to 20 ft deep. Then assuming the excavated materials wheighs 100 pcf and we're at 20' deep that gives us 2000 psf of overburden removed. Then I would place 4 ft sand cushion and then drop steel balls to densify the area. Then relevel the sand back to -16 ft. Then compact it with a plate connected to extended reach backhoe. Then place 10 ft of geofoam and cap it 2ft of flowable fill. Now we're at -4ft. Then I would design a rigid box foundation with qa of 2000 psf. Make the T/F 3" higher to allow for seating settlement. For the Pk lots, we can do the same process but with 2 ft sand, 3ft geofoam and 1 ft of 3/4" gravel with fines.