DESCRIPTION

The project consists of Geopier Foundation Support for a 230,000 sf, 4-story headquarters building and a 6-story parking garage for the Navy Federal Credit Union.  Column loads ranged from 200 to 1000 kips and shear wall mats had combined loads of over 3300 kips.

CHALLENGE

Very soft to stiff Piedmont Residual silts were encountered to depths of approximately 15 to 20 feet below bottom-of-footing elevations for the structures. These soils were underlain by very stiff silts transitioning into weathered rock at depths of 35 to 40 feet below footing bottom elevations. Ground improvement was required because of the compressible soft silts.

The closely-spaced footing layout (shown below) resulted in a significant amount of overlapping stress on individual footings from adjacent footings.

SOLUTION

Geopier® elements were used to control settlement within the soft silts and elements were installed deeper to overcome the effects of overlapping stress on the lower zone settlement of individual footings. By using high modulus Geopier elements installed using an X1® mandrel, larger 36-inch diameter elements could be installed to depths of 25 feet or more.  The X1 mandrel allowed for larger diameter Geopier elements to be built that were approximately 15 to 20 percent stiffer than conventional Geopier elements.

The modulus tests results showed that the Geopier X1® element had a stiffness approximately 33% stiffer than needed to meet the design requirements.   At the maximum top of pier design stress of 19 ksf the compression of the pier was only 0.34 inches.

The use of the X1 mandrel is the next generation of Geopier Technology that allows for stiffer piers to be built faster and deeper.  This was a perfect fit for the soft soils at the Navy Federal Credit Union site.  The stiffer Geopier X1 elements provided the needed settlement control for the heavily loaded footings and shear walls and allowed for deeper piers to be installed to control the overlapping stress associated with the large adjacent footings.