Foundation on fill

20 m of peat is pretty thick - actually damn thick. You have only 4m of sandy fill onto which your footing will be 2.5m in minimum size. For spread fdns, the pressure bulb will go some 2 to 2.5 x footing width (for 10% or so of applied pressure). For strip footings, it will go 3 to 3.5 x. This means that you will have relatively significant pressures extending into your peat. This is assuming that the fill has been in place for a significant period of time and you are well into secondary consolidation of the peat due to the fill.

This is tricky and should be handled with care. An experienced geotechnical engineer should be approached - without question. I might assume you are in Canada, if so - and you need some contacts, please advise.

Some initial thoughts:

Properties of the peat, of course, is the nature of the peat - whether fibrous or amorphous. What is the compression characteristics - i.e., primary and especially secondary. Peat will have significant secondary settlements. What are the strength parameters.

What is the nature of the fill - old/new? Was it an engineered fill or just loose fill placed and dozed as the occasions warranted??

If you are planning to add any more areal fills to raise grade - this will create muchas problemas. These fills - if done at the time of the construction - will lead to further significant primary and secondary settlements. I would, even suggest that a rigid raft fdn might be a better choice in that the structural behaviour would be more uniform - tilting, maybe, but scant to little differential between bearing walls/adjacent footings.

I was involved in a project in Burnaby, BC, Canada where some 2.5m of fill was placed atop 25ft of peat and 25ft of soft clay. The building was on piles and they used wood piles for a later expansion. As the expansion was in a traffic area, a problem was encountered in that the fill would settle leaving the butt ends of the wood piles peering up proud to the ground. They would cut off the butts to "ground level". Then, a year later, same thing - again they would cut off the butts. They did this for several cycles. So - you see, settlement ongoes.

Consideration might be given to preloading. The more the better although you will have to watch for edge instability. Plus, though, you need time.

Hope these early thoughts have been of some use to you. Check out some of the other threads given at this site regarding peat. There are some good comments in them too.

Best regards and Good Luck.

Thanks for your response.
I am from Vietnam and I am working at a project for building a new factory.
The designer of the project made foundation solution for this building site.
They improve soil using geogrid/geotextile, place 4m thk. compacted sandy fill on top.
For major foundations they use piles, for foundation of pipe-rack, and minor structure
they use spread foundation with the less width up to 2.5m, laying in the fill layer.
They require that the sandy fill compact to the bearing pressure of 5 ton/sq.m for
bearing the foundations. I am not satified with their solution and want to prove that the
solution is unfit. (I have not soil data yet, but looking for typical soil characteristic
for a quick judgement).
Thanks.

Peat performance is highly unpredictable and should be avoided as a bearing soil. If the building is light, you may be able to doo a compensated or "floating" foundation in the sand. It should be noted that fine sand does not make a great fill and may have low bearing capacity orinduce settlements. If you found a structure on a shallow footing, the facility should be designed to accomidate differenttial settlements. A high quality site investigation is probably required. Deep foundation elements such as piles or shafts should be considered.

Here in the States, depending on the location, we may very well mine the peat, condition with other soils and sell it, and use engineered fill for the foundation design for the building you discuss.

Probably no shortage of peat in your area.

On a project involving capping a pond/wetland, I ued a geogrid material, placed 2-foot medium/coarse (USCS) sand and another geogrid material and more sand, covered by general fill (mixture of sand, silt, clay). Was very stable. Contact a vendor of geogrid materials and follow Geotechnical recommendations. It can be done. Good source of published properties of sand - Bearing Capacity of Soils and Settlement of Soils, both Army Corps of Engineers publications.

Good luck, please keep us posted on what you use and results!

Mixing the foundation types, piles and spread footings over soils that you describe is asking for trouble. Unless the peat has been preloaded for an extensive time differential movement will occur, even with preloading peats are subject to secondary consolidation. Provisions for jacking up the pipe racks and minor structures should be incorporated in the design.

one may consider raft foundation instead of spread footings in order to minimize the bearing stress and lower the settlements.

20 meters of peat is awfully risky material to build on. The amount of secondary compression (from decomposition) is highly unpredictable and is usually very high. Do you have any alternate foundation types from which to choose, such as piles?