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Need advise for special foundation and soils problems

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I have a problem that has been ongoing for about four years. I was called in
to evaluate and resolve a problem with a post and beam building in a private
Golf resort in La Quinta California. Here is some history.

The land was developed over fifty years ago -- dedicated by then President
Dwight Eisenhower. From what I am able to tell, the site consisted of groves
of Tamarisk trees. Tamarisks were not indigenous to this area and were brought
in to be used as wind breaks. I also suspect that the site had a mixture of
more indigenous varieties such as Palo Verde and Mesquites.
I was told by one of the club members that little was done to ensure that
clean fill was used during the creation of the golf course and I believe it
was possible that not only roots were left, but that stumps where simply
turned under.

The Club House was constructed in the early sixties -- at a time when the area
was unincorporated. The structure is a one story post and beam (heavy timber)
with plenty of glass and, in my opinion, lacking in lateral resistance.

The foundations, from what I can remember of the few original drawings I sway,
consisted of a continuous shallow grade beam in the center of the building
which carried the main 12x12 wood columns. The outer pads supporting the
exterior wall columns are, I believe, tied from column to column, but are not
tied back to the corridor foundation.

For about 25 years the building used a septic system until the city installed
sewers in the mid 1990s.  Subsequent to the installation of the sewers,
changes in the form of differential settlement started to be noticed
throughout the building.  The property lies in the "flats" at the base of a
residential section constructed in a Cove (surrounded on three sides by
mountains). The cove is approximately one mile wide by three miles deep and
contains approximately 1,500 homes. Sewers were installed within the last six
years.

A culvert separates the Country club from the Cove and flooding has not been
an issue in this area since the installation of the sewers / storm drains.

A soils company was brought in to investigate and try to find the cause of the
settlement. Cores to 15 feet were taken throughout the building and at
locations where damage was most notable.  A Manometer study was done and
subsequent measurements were taken over the last three or four years.

Investigation of the sprinkling system and the existing plumbing and sewage
system was done, but there was no indication (by pressure testing) of leakage.
The leach field used by the old septic system is south of the building and far
enough away from the structure to (by the soils engineers conclusion) not have
been the cause of subsidence when the septic system was removed.

Soils
The soil study indicated mostly sand. There is little if any clays on the site
except those brought in to line a the golf course ponds which are more than
100 yards away from the building.  There are also natural wells on the
property which we just learned about and we plan to do some static level
monitoring to see if there is a dramatic changes.
For the most part, the soil under the structure was confirmed to be moderate
to high in organic which we are confident is causing the settlement.

A plan was created to pressure grout portions of the building and the phasing
was done to coordinate both the clients budget and the seasonal timing around
the members and tourists.  

The main corridor was demolished in the 1997 in order to expose the
foundations and to install some new footings under columns that became load
bearing due to the settlement. Up until the time we pressure grouted, the tile
along the main corridor would simply crack each time it was patched.
Two years after the grouting of the corridor, the area has stayed level with
little if any settlement.
One exterior column was also pressure grouted. At the time, the soil below the
column was so loose that almost three times the expected volume of gout was
needed. Three years later, we are again noticing movement in this area.

Problem:
We have plans to complete the pressure grouting of the building this spring
and summer. My main problem is the following:
The building site and surrounding grounds are level grade, however, the club
has been noticing horizontal movement to the North and south of the corridor
AND the exterior sidewalks and drives for golf cart use. Following the
exterior curb lines, the displacement laterally is noticeable at each
expansion joint. I do not see vertical displacements occurring on the exterior
flatwork, however, I have suggested Manometer studies outside the structure
and within say 25 feet of the building. The exterior flatwork is not
continuously attached to the flatwork adjacent to the building as a planting
area exists for grass. This would eliminate the idea that the with the
subsidence of the soil below the column foundations that the columns and
foundations are moving both vertically and horizontally (vector paths).  
I cannot find justification for the lateral movement when no major seismic
events have occurred in the area, we are not on sloped or hillside property. 

The movement is noticeable - 3/4" in less than three years.

I'm missing something here and can't figure out what it is. My only conclusion
is that subsidence is occurring due to reduced compaction by the organic found
in the soil. This would not account for the lateral movement.  The pond is too
far from the structure to affect it and the soil between the building and pond
is mostly sand -- very little clay was found.

I would appreciate any suggestions as to what I can do to search for a
definitive answer for the lateral movement. Until then I am afraid to invest
the Country Clubs money into more than resolving vertical subsidence.

Any suggestions, recommendations or comments would be greatly appreciated.

Sincerely,
Dennis S. Wish PE