Friday, July 27, 2012

Zero skin friction zones

These are general rules of thumb, first approximations, and are often broken for various reasons.

The upper portion of a skin friction has no reliable skin friction due to drying and shrinkage of the soil away from the pile, frost penetration, and potential for some fill.

Fill is unreliable, as it has two characteristics, it has unreliable strength, and unreliable settlement characteristics, most of the time. Shallow engineered fill, after one year may be suitable for light structures.

Typical recommendations for skin friction piles, drilled, cast in place concrete, are from 0 to 1.5 metres below final grade, 0 skin friction. This provides protection from drying and/or frost action. This requires a relative level site after stripping, plus/minus 1 metre, and competent native soil.

Occasionally we use 0 to x.x metres below current grade, where we have fill, or a significant slope to the site. The cause of this requirement should also be noted in the report. Occasionally elevation may be used of odd cases.

Generally, we prefer only one condition for a site, however two or more may be used if there is an clear reason, such as an old basement, utility line fill, or other special situation.

In the case where there is fill, there may also be down drag. Down drag is a load placed on the pile from fill settlement. Typically for old competent shallow fill, -10 kPa is the downdrag value assigned. New loose fill should be atleast -25, typically -20 kPa is used for fresh compacted fill. These are typical for 1 to 3 metres of fill.

Often, where less than one metre of fill below floor slab elevation, the fill is just part of the zero friction allowance and is ignored. With say 1 to perhaps 3 metres of fill, the first 2 or 3 metres of pile friction resistance, below the fill, is discounted to deal with a bit of down drag. This requires a fairly uniform condition across the site.  This requires good friction in the upper soils. Note that the surface soils can also be soft, and have lower value for other reasons, or it is specified and must be included in the structural design. That is useful  where the site has know fill areas.

Down drag can be reduced to -5 kPa by using a double polly rapped sone-tube as a friction reducing smooth surface.

Crap fill, as truck dumped winter fill, uncontrolled truck fills should all be considered as poor, and are unreliable for pile friction and slab support.  

Values and recommendations from previous projects, found in the boiler plate report, must be customized to the current situation. Any missing recommendation sections must be added. 

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