Case Studies
Eagle Crest
Situation: Eagle Crest is a group of multi-family properties located in North Irving,
Texas. This is an area near the DFW Airport and close to the high-end
development of Las Colinas. North Irving is on the Eagle Ford geological
formation. These soils have an extremely volatile shrink-swell potential
when any change in soil moisture content is introduced. The property is
approximately twenty years old and still in good enough shape to be rated
as a strong ‘B’ rated property. We have no history on this
property and no specific geotechnical information. On this type of property,
we can always assume total regarding of the top soils and all cut and
fill type site development. “Cut and fill” simply means soil
cut from higher elevations and moved to lower elevations to provide level
plots for constructing buildings.
Solution: To be certain that the water drains downhill and away from the building
foundations, all top soil to at least 12” should be removed from
the site except if used for landscaping. The top 3-4 feet of all soils
should be homogenously mixed, graded to the desired elevation and compacted
in 6” lifts. The soil is compacted with equipment that will deliver
the desired amount of energy into the compaction process at the correct
moisture content for each location and for each soil layer or lift. In
order to achieve proper compaction, there needs to be a geotechnical engineer
employed on site to insure that each of the soil layers is compacted to
proper density. Density is measured by unit of dry density weight and
in the United States it is customary to use measurements calibrated to
pounds per cubic foot. Often times, rubber-tired equipment is employed
for this task. Weights added to the compactor or taken off the compactor
as well as regulating tire pressure will control the amount of energy
used to consolidate each lift. If the soils are under-compacted, over
time, the soil will consolidate and allow the foundation to settle. This
process is accelerated by repeated wetting and drying through the changing
seasons. The soils might be over-compacted and maybe a little on the dry
side since the foundation was probably poured in the summer. When the
rains come and the irrigation system is turned on, the soils begin to
swell and the foundation heaves up. The realities of the construction
business are that the geotech has his recommendations, the civil engineers
have their recommendations, the owners take the cheapest bid, the general
contractor takes his cheapest bid and so on. The soils that are cut to
a lower grade get moved on top of top soil full of organics and are compacted
with a dozer in whatever lifts that are convenient to shove, spread and
compact with dozer tracks. Some of the cut soils are much too dense and
most of the fill soils are under-compacted. At Eagle Crest, improper site
preparation led to remedial foundation repair some twenty years after
the buildings were built. By this time, they have undergone several remodeling
and maintenance phases to patch cracks, re-lay brick, realign doors and
so on. Jim Roberson, President of RJT Commercial (a sister company of
Ram Jack of Texas), got a call to look at the property with the new owners
from California. Jim McNamee, P.E. had done an engineering report on the
property earlier. Mr. McNamee and Mr. Roberson have worked through many
very difficult jobs through the years. The Eagle Crest job would be no
exception. The differential elevations on the building to be repaired
showed the building to be 19” out of level. It is quite possible
that the high side of the building heaved up within the first few years
after construction and the low side of the building settled a little each
year over the life of the structure. The settlement is initiated each
wet and dry weather cycle with moisture acting on the expansive clay soils.
The repair offered some unique challenges. The foundation is a post-tensioned
monolithic slab on grade. The slab has a perimeter foundation but no interior
grade beams so it could be considered a semi-structural slab. Ram Jack®
hydraulically-driven steel-pressed piers were driven on the perimeter
and on the interior load-bearing walls wherever practical. Most of the
piers were driven to between 40 and 45 feet. Final driving forces on the
piers drove them with 70,000 pounds of force. The heavy loads, the soft
soils and high driving pressures required the use of 8 ft. guide sleeves
in order to relieve the lateral forces being exerted on the piers. The
slab floor did not have sufficient strength to completely bridge between
the piers on the load-bearing walls. A wide flange steel beam (see picture
to right) was welded to the horizontal arm of the pier bracket. This was
welded in place after the pier was set up but before pier driving began.
Post-tension cables can be observed through many of the access holes in
the slab floor (see picture at left). These cables should be tensioned
to within 90% of their yield strength or around 27,000 lbs. for a 1/2”
cable. Some of the cables can be seen to be loose and have slack in them.
The purpose of the cables is to keep the concrete rigid by exerting compression
forces into the concrete through tension. A rotary drill and pavement
breaker were used to create the access holes through the slab. This method
was chosen to prevent the possibility of cutting through the cables while
creating the access holes. The interior floor slab and the interior non-load-bearing
walls are supported on helix piers using floor slab brackets that utilize
hand operated hydraulic jacks for lifting.
Conclusion: The lift on the 60 ft. by 150 ft. building was accomplished with 135
hydraulic jacking systems while being carefully raised simultaneously
as required for a uniform lift. During the lifting process elevations
were carefully monitored and documented after each selective lift phase.
Needless to say, the apartment had to be vacated prior to the work and
the plumbing and electrical services needed some attention. The differential
elevations were corrected within 2 inches across the entire building.
Jim Roberson has developed lasting relationships through the years by
working trade shows, prospecting and networking with engineers and property
managers throughout the Dallas-Ft. Worth Metroplex. Jim is responsible
for almost all of the Ram Jack commercial revenue produced in North Texas.
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