Under a multiphase $2.4 billion plan, the Department of Homeland Security is consolidating a staff of 14,000 from 40+ locations into a single location in southeast Washington, DC. The plan’s ambitious infrastructure program includes the addition of an off-site access road. To help accomplish this, ZGF, the prime architect under the General Services Administration for campus development, brought on Schnabel as the geotechnical Engineer of Record.

The new location is set in the west campus of the former St. Elizabeths Hospital. Connecting the road to the 176-acre campus required improvements to the I-295/Malcolm X Avenue/South Capitol Street interchange, with:

  • 1.5 miles of roadway widening
  • Six new bridges
  • The widening of one existing bridge
  • Several miles of new retaining walls, and
  • Stormwater management features.

Our team’s initial task involved a comprehensive subsurface exploration to support the interchange improvements. During this phase, we addressed design solutions for pavements and pavement subgrade stabilization, new and expanded bridge foundations, and new mechanically stabilized earth walls supporting roadways and slopes. We also analyzed and created a detailed design of new post and panel retaining walls with tie-back anchors, and delivered cost estimates and construction documents for geotechnical and geostructural elements.

This subsurface exploration revealed that the entire project site is located along a tall slope underlain by Potomac Clay, a highly plastic, extremely over-consolidated clay from the Cretaceous age. Potomac Clay formation has historically been the source of many slope failures in the region.

Several new MSE retaining walls were planned along the middle of the slope which required a stabilization system below the wall foundations to improve global instability of the walls and to bring the overall slope to an acceptable level. To stabilize the Potomac Clay slopes that supported new MSE walls, we designed an innovative system using continuous flight auger (CFA) piles nearly 100 feet deep. The use of CFA piles saved more than $1 million in construction costs compared with an alternative plan utilizing traditional drilled shafts.

The Challenge: Designing in Potomac Clay

Potomac Group Clays have high undrained strengths (> 4ksf) but exhibits very low residual shear strengths strength (<10 deg.) in long-term drained conditions. The material reaches peak strength at low strain, then its strength steadily reduces to a residual level at higher strain levels. Too often, stability analyses do not account for the material’s long-term residual shear strength being much lower, which makes it susceptible to significant slope creep. This has historically been a source of slope failures in the region. Since the clays at the site had historically undergone slope movements, we based our design on the residual shear strength, as opposed to peak strength.

The Plan: Engineering for Extreme Shear Resistance

Our design included one to three rows of 24-inch diameter CFA piles that were between 55 and 95 feet long. These piles would essentially act as shear pins, providing the necessary shear resistance to improve the global stability of the system at low strain levels, thereby limiting the deflection of the walls or the slope. The stiff piles would have full length cages to mobilize shear resistance far below the ground surface and would be embedded a sufficient depth below the failure plane to develop sufficient resistance. The location of the critical failure planes varied from as shallow as 25 feet below the ground surface to as deep as 65 feet.

Because the slope and wall height varied, the resulting shear demand in each pile varied widely. It was necessary to mobilize the resistance in all the piles effectively, not just the front row of piles. In some areas, it was more effective to have piles in front of the wall, with a pile cap assisting with load transfer between the piles. In other areas, it was more efficient to have the piles below the MSE wall and an aggregate load transfer platform between the pile and the wall. (In some areas, due to space constraints and the heavy load demand, the latter approach was the only option.)

The Execution: A Specialized Construction Approach

CFA piles are typically installed through soft soils with partial depth reinforcing cages. To our knowledge, no one has installed CFA piles in such hard material, so deep, with full length cages anywhere in the US. There were some doubts if it could be done – which is why the drilled shaft alternative was developed.

Prior to construction, several contractors expressed concerns over the ability to auger through such hard clay (N values greater than 100) and wet-stick full-length cages. During construction, we found that the drill rigs were able to drill through the clay without an issue. However, some issues with the grout were encountered, as the grout would “pack” and densify, causing wet-sticking on the cages near the bottom of the shaft.

Through onsite observation and testing, we determined that grout pressures were spiking to much higher pressures than were expected. Our investigation concluded that this was due to the hard clay not expanding under pressure and preventing the grout from escaping past the augers. In our case, we did not need grout pressure to provide expansion of the surrounding clay. This is typically done to achieve greater side friction axial capacity, no axial capacity was needed for the stabilization piles. Through the use of specific grout additives and by limiting the grout pump pressures, the contractor was able to eliminate this occurrence and successfully install the piles.

There were also several cases of “communication” between a pile being grouted and a recently grouted pile about 6 diameters away. We realized that this was caused by the high grout pressures and the presence of existing fractures in the clay that allowed the grout to travel between the piles. Reducing the grout pump pressure also eliminated this problem.

Because the loading demand was shear and not axial, load testing could not be performed to evaluate the effectiveness of the piles. This made construction quality control absolutely critical. We provided observation and testing services during installation that allowed us to detect issues quickly and resolve them in the field. Integrity testing was performed on the piles using thermal integrity profiling, which was very effective. Testing and observation indicated all piles were successfully installed with sufficient cover and grout uniformity. The hard clay helped with maintaining hole stability with minimal bulging or necking, helping to avoid issues with concrete cover over the cages, or grout washout from groundwater intrusion.