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RE: Caissons

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Kenneth  Liu

> ----------
> From: 	Tom Long[SMTP:tomlong(--nospam--at)]
> Reply To: 	seaoc(--nospam--at)
> Sent: 	Tuesday, March 10, 1998 4:47 PM
> To: 	'seaoc(--nospam--at)'
> Subject: 	Caissionse
> To All:
> We are designing a USD$400 million airport in Manila.  The foundation
> system will be caissions bored into natural bedrock is about 3m.  The
> existing ground above the bedrock has a large amount a expansive
> properties.  Therefore we are using a structural slab system for thr
> first level of a five level airport.  The distancd to the bedrock from
> level 1 varies from 1/2m to 2-1/2m.  The structural slab is a two-way
> flat plate with drop panels, the thickness of the slab is 300mm (12").
> The UBC94 code is concerned, Section 1807.2,  states that we will need
> tie beams for our caisson foundation scheme. The code states that:
> Individual pile caps and caissons of every structure subjected to
> seismic forces shall be interconnected by ties. Such ties shall be
> capable of resisting, in tension or compression, a minimum horizontal
> force equal to 10 percent of the larger column vertical load.
> EXCEPTION: Other approved methods may be used where it can be
> demonstrated that equivalent restraint can be provided.
> I think that the flat slab alone on level 1 could meet the above
> restraint requirement mentioned above? What do you think? Please
> comment
> on this. Does anybody have any feedback.
> Best Regards
> Tom Long
> PCI Asia
> Dear Tom :
> I would agree that the 12 inch slab can serve as a interconnecting
> member to meet the requirement of UBC Sect. 1807.2  except you have to
> design the  slab strip as a interconnecting members( add some
> continuous rebars, or a deeper section along column grid lines) .
> Is it too late to discuss any alternate approach to design the
> foundations?  Caissons or Pile Foundations normally are more
> expensive. Besides,  the project site has shallow bedrock,  0.5 m to
> 2.5 m is very shallow to me. The main purposes of your Caissons or
> Piles are for vertical loads and reduce settlement only.  Since the
> expansive soil is not very far from the finished grade, replacing the
> entire bad layers with a better graded soil probably feasible.
> Another method is using sand piles.  The material used in sand piles
> can be graded  gravel and coarse sand.  This methods has been applied
> to several major projects very successfully.
> The most recent report that I read was : Kansai Airport in Osaka,
> Japan.  They made a manmade island, 5 km offshore, about 2 square mile
> ( 5,200,000 square meter ) and built airport on the island. Sand Piles
> was applied to the foundations. The airport has opened for commercial
> since 1994.  There is an article about the manmade island in
> Structural Engineering International January/ 1998.  It is very
> interesting report. 
> Best regards, 
> Kenneth  Liu