Expert: Bruce E. Johnson - 10/15/2009

Question
QUESTION: I am having a reinforced concrete raft foundation laid on poor ground conditions.  It is 35 feet square. The raft platform is 6 inches thick and the beams are a further 12 inches deep with bottom widths varying from 3 feet  to 18 inches and their top widths being another 12 inches.  i.e  45 degree angles for base of beam. I thought the best way to lay it would be to excavate the entire area of the raft to 15 inches; infill with required material; compact that infill; then excavate the beam regions (removing both infill and underlying clay as required);  infill and compact under the beam bottoms; place reinforcement cages (pre-fabricated with bent bars and mesh) securely on "chairs"; place the required mesh securely in the platform area;  then pump the concrete in.  I foresee a possible practical problem in levelling the surface over such a wide area but cannot see how the beam regions can be laid  other than at the same time ( problems with sectioning off the reinforced sections.)

My groundworker wants to excavate the beam regions before laying the infill and pour the concrete in to the first 12 inches of of the beams using shuttering (which will leave 5 inches of the cages protruding upwards) and then put in the main platform infill ; place the platform reinforcement mesh and pour the concrete in one half of the raft at a time, enabling him to level the surface more easily.

Can you advise me please?

Many thanks,


Derek

ANSWER: Hi Derek, I have done this type of foundation both ways but I prefer the first method and this is the method I generally use today.  First the sub-grade is cleared and compacted and then the fill is brought in and compacted. ( I usually keep the grade of the fill an inch or two lower than the finish grade depending on the size of my footings, this way the fill excavated from the footings is used to bring the slab to final grade.) At this point I bring in my plumbing and electric contractors for any underground work.  When the trades are done and their trenches backfilled and compacted I begin my final foundation preparation.  The footings are dug and compacted. The excavated fill is redistributed and the slab is brought to final grade then compacted one last time.  At this point, prior to installing any steel I have the entire slab area pretreated for termites. Immediately following the termite spray I lay 6-8 mil visqueen down over the entire area as a vapor barrier, tape all the seams and around all plumbing and electrical penetrations and then, using the proper chairs install my steel and mesh.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson...bejohnsonconsulting.com

---------- FOLLOW-UP ----------

QUESTION: Many thanks for very prompt and helpful reply.   One more question.   How do you level  ("tamping" I call it)  the raft platform over such a large area?  Bear in mind the platform area is also reinforced with steel mesh.

Thanks.


Answer
Hi again Derek, in this country we usually use a laser level to level and grade large areas.  Tamping is the process of compacting soil.  It is usually done with a plate compactor.  On large areas rolling vibratory compactors are used during the initial build up of fill but the plate compactors are used to tighten up the earth prior to slab pours.I hope this information helps, please feel free to write again regarding this or other matters,
sincerely bruce e johnson...bejohnsonconsulting.com