wrote
BB,
Piles don't have to reach "bedrock" to work. Friction can hold them
in place.
Just spotted something.
Both New Orleans & Lake Pontchartrain are on the Mississippi delta,
with Lake Pontchartrain being on the North side of New Orleans.
Sediment under Lake Pontchartrain is reportedly 20,000 feet
deep .... the bedrock is under that, as the oil drillers know.
I don't recall any mountain-building processes in the area that
would have created a 20,000 foot tall mountain under the site of
New Orleans.
So I'd guess that your 80 foot number just applies to how deep
pilings are driven for some purposes and has nothing to do with
your assumed bedrock depth.
This being the case, my recall of things would not have been
far off at all ..... nor my qualified statement much in error,
if at all. Unlike your lint, assumptions & hasty conclusions G.
I think there is a misunderstanding of the term "bedrock". Bedrock is any
consolidated material underlying a loose or poorly consolidated surface
material. It does not have to be some massive igneous or metamorphic layer.
Mudstone, claystone and sandstone can all be bedrock as long as the strata
has not been broken up and intruded into by looser material. Given enough
time and pressure alluvial soil(silt, clay and sand) will aggregate into a
solid mass. It is entirely probable that at about the80 to 100' level the
alluvial material has compacted enough to be called "bedrock". New Orleans
would not be like a city built on an upland or piedmont geology.where there
is a definite boundary between soil and bedrock. It is a more gradual
change so pilings would have to be driven well into the "bed rock" to
support the tall buildings on Canal Street.
This is also a major reason for the land sinking. As the organics decompose
and the finer materials fill voids between the larger ones the volume
decreases. Unless flooding is allowed to add more material on top the
delta will sink.
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at:
http://www.rutuonline.com
Shameless Commercial Division:
http://www.spade-anchor-us.com