Drilled shafts make superlative bridge foundations, as many state DOTs have grown to appreciate. They can carry huge vertical loads, can effectively carry large lateral and seismic loads, are, in most cases, easier to install than driven piles, can be readily installed offshore and are highly resistant to scour. They also appear to be the economical choice.
According to the Federal Highway Administration (FHWA), the use of drilled shaft foundations in highway construction has increased significantly over the past decade. Drilled shaft construction and associated equipment is a $2 billion industry today.
Drilled shafts and temporary earth retention systems have been used regularly in the private sector for many years. However, the strong collaboration between FHWA, state DOTs and ADSC?The International Association of Foundation Drilling?has been the catalyst for their increased popularity in the transportation industry.
Early efforts by FHWA and ADSC in developing design manuals, funding research and sponsoring short courses has ultimately led to sophisticated design tools, measuring devices and new applications. Now, at least 30 state DOTs routinely specify drilled shafts for their bridges.
Ongoing efforts by ADSC and others are bringing the real-world application of this technology to the engineering and construction industry.
Progress with meaning
Understanding a drilled shaft means understanding both its structural and geotechnical capacities, and more importantly, the interaction between soil and structure.
"In the mid-?60s no one really knew how to accurately calculate the capacity or settlement of a drilled shaft, although the structural strength of concrete and steel was well defined," said Dr. Michael W. O?Neill, professor of civil engineering at the University of Houston and a leading deep foundations researcher.
Early research brought understanding, which brought a level of comfort in using drilled shafts as structural foundations. As drilled shafts have become more popular, additional research has added to the breadth of knowledge about their performance.
A real workhorse
Depending on the soil conditions, drilled shafts can be built in almost any size. Although there are some examples of mega-shafts, up to around 16 ft in diameter, the majority of drilled shafts are 8-ft-diam. or less. The lateral and vertical capacity of these larger drilled shafts is unparalleled.
The foundations for the new cast-in-place concrete segmental bridge to be built by the North Carolina Department of Transportation over the Northeast Cape Fear River in Wilmington, N.C., for example, were designed to carry very high vertical loads to support the 478-ft-long main span.
Groups of 15 drilled shafts with 8-ft-diam. will be used on both ends of the main span to provide an ultimate axial capacity of nearly 4,000 tons per shaft. The shafts will be drilled through muck and alluvial deposits and embedded 35 ft into the cemented silt and sand of the Peedee formation, which anchors the shafts and provides the necessary capacity.
A $2 million load test program for the Northeast Cape Fear River bridge saved over $6 million in costs of the bridge by decreasing the number of shafts, the lengths of shafts and the required diameter.
Drilled shaft territory
The California Department of Transportation (Caltrans) regularly uses drilled (which they call cast-in-drilled-hole) shafts because of their excellent seismic performance. The below-ground shafts are one with the above-ground columns, creating a continuous massive structural member.
Caltrans is currently funding research to evaluate the lateral capacity and predict the amount of deflection that a drilled shaft will experience under earthquake loading. As part of the research, a single 6-ft-diam. shaft, 48 ft below grade and 40 ft above grade, was loaded back and forth, simulating the lateral loading from earthquakes, until the concrete at the "plastic hinge" was crushed to rubble.
Researchers at UCLA will use the performance information to compare against Caltrans standard design procedures and other more sophisticated procedures.
Over-water construction is another condition where drilled shafts excel. Often, a single large-diameter combined shaft and column can be constructed in lieu of a group of driven piles with a pile cap. This precludes the need for expensive cofferdams and dewatering, and has dramatic savings in cost and schedule.
Drilled shafts were the preferred foundation choice for a bridge over the Lake of the Ozarks in Missouri. Case Foundation Co. of Roselle, Ill., worked from barges to construct 24 drilled shafts/columns that support the bridge at eight bents.
The water was up to 100 ft deep and the foundations extended up to 35 ft into the lakebed deposits and bedrock. Each shaft supported vertical loads of 2,000 to 4,000 kips.
Drilled shafts stand up to the most demanding environmental conditions, including severe river scour and hurricane-level wind loading, conditions in which the performance of driven piles or other foundations may fall short.
The Arizona Department of Transportation learned about the limitations of driven piles first-hand on the Salt River. Winter storms in the years 1978 to 1980 caused the failure of 14 pile-supported bridges along a stretch in Phoenix.
Most of the replacement bridges were designed and constructed with drilled shaft foundations. The foundations must withstand peak flows of 250,000 cu ft per second and up to 60 ft of scour.
"The shafts extend through the alluvium and are anchored into the hard clay below," said Robert Schock of Case Foundation, who installed the drilled shafts. "We performed a $550,000 load testing program during design, which demonstrated the feasibility of large-diameter deep shafts, and saved over $4 million in construction costs."
The typical embedded length of shaft is 130 ft, including 25 ft embedded in the hard clay. "You simply couldn?t drive piles into the clay," said Schock.
The preferred choice
In fact, there are many locations throughout the country in which driving piles isn?t feasible. Newer structures within the limestone belt between Knoxville, Tenn., and Birmingham, Ala., for example, are almost entirely founded on drilled shafts bearing on the limestone bedrock.
In this region, the depth to the top of the "pinnacled" limestone varies dramatically and unpredictably, typically between 10 and over 100 ft. These variations can sometimes occur over a space of a few feet, making the below-ground rock surface very steep.
At the new Knoxville Convention Center, for example, Long Foundation Drilling Co. is installing over 200 drilled shafts ranging in between 3 and 7 ft in diameter. The thickness of soil covering over the rock ranges from 0 - 23 ft. The depth from the top of weathered rock to the competent load-bearing rock strata ranges from 1 - 82 ft.
"Using driven piles is impractical because of the large variations in length," said Bruce Long of Long Foundation Drilling. "The tips of the driven piles sometimes have the tendency to slide along the steep rock surface as opposed to embedding firmly upon it."
One perceived disadvantage of drilled shafts is that, because the shaft is built in-place (as opposed to formed in a casting yard as with piles), it is difficult to monitor the shafts constructed integrity. In the early 1990s, owners and engineers started routinely performing non-destructive testing on their drilled shafts to evaluate the in-place integrity.
The testing occasionally indicated the presence of some anomalies (i.e., voids in the concrete), despite the highest quality construction workmanship and materials. This led to more questions about the impacts of these anomalies on the shafts? load-bearing characteristics.
Current research funded by FHWA, ADSC and various state DOTs is exploring this issue. Preliminary results indicate that the largest void that would go undetected by a well-conceived testing program is one that occupies about 15% of the gross cross-sectional area of the shaft.
The ongoing research will be used to develop realistic "capacity reduction factors" to be used during design to account for the minor decrease in capacity caused by these anomalies.
One of the beauties of drilled shafts is their versatility, ease of accommodating a wide range of conditions and relatively straightforward installation techniques. Drilled shaft equipment has advanced dramatically over the past decades and drilled shaft contractors have become more knowledgeable, skilled and sophisticated in their approaches to projects.
Due to the training and leadership provided by ADSC, there are many very competent drilling contractors in the market today. More bidders mean the price goes down. A higher level of skill means the quality goes up. Continued research drives the technology forward.