Undoubtedly, the regional successes of National Harbor have influenced the designers, contractors and owners of new projects planned in the Washington, D.C., metro area, par- ticularly plans for redeveloping southwest and southeast D.C., where soil conditions are similar to National Harbor. The new Washington National’s stadium is supported on driven precast concrete piles, and planned building developments in Washington’s old “Navy Yard” will also be supported on driven precast concrete piles. Neighboring Arlington’s “Potomac Yard” re-development, under construction since 2004, has several buildings supported by higher capacity, precast concrete piles.
ECS began a new building project in 2008, termed “Waterfront” located in southwest D.C. which is supported on 150-ton, relatively short (lengths on the order of 45 feet), 14-inch square precast concrete piles.
Vice-President and project representative, Herb Faling of Vornado said, “I have built buildings supported on drilled shafts (caissons), driven steel piles and auger cast-in-place piles, but never driven concrete piles.”
In 10 more years, as the pile driving industry continues to flourish alongside Washington’s building boom, we don’t expect to hear these sentiments repeated.
Updating Pile Capacity Assumptions for the 21st century
The success of National Harbor was born out of the idea of matching the structural pile capacity with the geotechni- cal capacity, for an efficient design. This approach has been repeated successfully since. Section 1809.2.3.3 of IBC 2003 states that the maximum allowable pile stress for prestressed piles is f’c = 0.33 f’c-0.27 fpc (f’c is the 28-day concrete com- pressive strength, fpc is effective prestress). For example, for a concrete compressive strength, f’c, of 5,000 psi and 6,000 psi, the maximum allowable piles stresses are 1,461 psi and 1,797 psi, respectively (assuming fpc is 700 psi). As such, allowable compression working loads for 14-inch precast concrete piles could range from 140 to 175 tons (approximately). The author recalls seeing many late 20th century geotechnical reports, industry training guides, or text book examples with suggested working loads for 14-inch piles in the 75- to 90-ton range.
Increasing the geotechnical design capacities, nearer to the code derived structural limits will produce a more efficient design. A more efficient design is likely to be more cost com- petitive with other deep foundation alternatives. This strategy is why the author believes precast concrete piles have seen a rise in popularity in Washington, D.C., over the past several years.
Improved Capacities’ Impacts on Cost
Utilizing higher capacity precast concrete piles makes them more attractive financially. Beginning about 2004, when higher capacity piles in the DC metro area were first consid- ered, proving their cost effectiveness to owners and structural engineers was important. The Potomac Yard redevelopment project in Arlington presented a great opportunity to establish our anticipation of precast concrete piles’ cost advantage.
Potomac Yard developers initially expected drilled shafts (caissons) to be the least costly foundation. Our initial assess- ment was similar; however, once higher capacity concrete piles were considered, their cost advantage became evident. Below
is an example cost evaluation that was prepared for Potomac Yard in 2004-5. Precast concrete piles only became the less expensive alternative (on a cost per ton basis) after their capacities were increased, and the pile group sizes diminished.
During the same 2005-06 time frame, ECS made several other similar cost evaluations for buildings supported on Auger Cast-in-Place (ACP) Piles. Like drilled shafts, ACP piles did possess price superiority, but only when late 20th century PPC pile capacities were applied. Once higher PPC pile capaci- ties were applied, they became the less expensive alternative. Below is an example cost evaluation utilizing the higher range of PPC pile capacities.
Impacts to the Industry
Driving similarly sized piles to higher capacities does impact the pile contractors. Larger ram weights of 1.5 to 2 percent of the test load may be needed to establish minimum pile tip elevations, suitable terminating blowcount criteria, or “prove” the higher capacities during a dynamic testing program. For the 150-ton (300-kip) example mentioned several times above, the required ultimate test load would be about 600 kips (F.S.=2.0). The suggested ram weight may need to be in the 9.0- to 12.0-kip range and larger hammers require larger cranes. In some cases, deeper pile embedments require longer drive times and more total blows, which impact hammer and pile cushions. Hydraulic and air hammers are best suited due to larger ram weights and stroke height control. Large diesel hammers can be utilized if stroke height control can be ensured.
Q2 • 2008