|A Scheffler crew is approximately mid-way through drilling a CFA pile at the Pacific City Development in Huntington Beach, Calif.|
In an age of ever-increasing costs for construction materials, labor, and fuel, the greatest challenge facing engineers is often finding a foundation and shoring system that meets or exceeds the technical requirements of a project while still being affordable for the owner. This has become a constant issue for the designers of large-scale commercial developments, and is becoming increasingly important on public works projects as well. To counteract increasing costs, engineers must integrate new technologies into their designs to provide real savings for owners.
Taking advantage of the continuous flight auger (CFA) method to construct drilled piles and shafts is one way engineers can provide cost benefits to projects without sacrificing quality or performance. CFA technology is not new. It has been successfully used in Europe for nearly three decades, and is part of the United Kingdom Highways Agency’s specification. Nevertheless, it is still somewhat of an emerging technology in the United States.
In the CFA method, shafts are drilled using a continuous flight auger with a hollow stem through which concrete is placed under pressure during extraction of the auger. CFA shafts can be constructed as large as 48-inches in diameter and more than 90 feet deep, although most are 24 to 36 inches in diameter and 30 to 60 feet deep.
|Workers prepare to install a rebar cage in a completed CFA pile. Note the perfect circle of fresh concrete that has been exposed after cleaning off the spoils.|
The construction process of a CFA shaft is similar to augered cast-in-place (ACIP) pile technology that is still being used in parts of the United States. However, unlike ACIP piles, the CFA method relies on use of powerful European-style tracked rigs with fixed masts and full computer instrumentation to enable construction of higher-quality shafts with larger diameters. In addition, CFA shafts use standard concrete with aggregate as large as 3/4 inch and full-length rebar cages rather than the grout and top-cage/center-bar combination that is typical of ACIP piles. The combination of these differences allow CFA shafts to be treated from a design standpoint in the same way as a conventional drilled shaft, with the significant difference only being in the method by which the shaft is constructed.
A typical CFA shaft is installed using the following steps:
Step 1 — Before drilling, the operator enters critical shaft information such as shaft length, diameter, and desired concrete oversupply percentage into the onboard computer. This information can also be entered directly into the drill rig’s computer from the contractor’s office using cell phone and satellite technology.
Step 2—Pumping lines connecting the concrete pump to the rig’s auger are primed and pre-charged with concrete.
Step 3—The drill rig is positioned with its auger carefully centered over the pin marking the shaft center. The mast and the auger are checked for verticality.
Step 4—The auger is drilled into the earth.
Step 5—The operator monitors the drilling process on the overhead screen by way of real-time information, including drill resistance, torque, depth, and penetration per revolution.
Step 6—The drilling process continues until reaching the preset tip elevation. During this time, a small amount of spoils, roughly equivalent to the volume of the auger, reach the surface. The remainder of the spoils are held below the ground surface on the flights of the auger. Using this process, there is no open hole while drilling.
Step 7—Upon reaching tip elevation, concrete is immediately pumped through the auger, releasing a bung that protects against contamination during the drilling process.
Step 8—With the operator monitoring the concrete pressure and oversupply to ensure complete replacement, the auger is slowly raised as concrete continues to flow. During this part of the process, soil is continuously removed from the auger as it is extracted above the ground surface.
Step 9—After the auger is completely extracted, a support excavator clears away the remaining spoils and the crew carefully hand excavates any spoils or contaminated concrete at the top of the shaft until a perfect circle of concrete is exposed to the full diameter of the pile.
Step 10—A support crane lowers either a rebar cage or steel beam into the fluid concrete of the shaft. Centralizers ensure that the steel is centered in the shaft. Depending on depth, a small amount of vibration may be applied to the steel to aid insertion.
Step 11—The steel pile or rebar is supported at the ground surface to maintain its position and the crew moves to the next shaft location.
The result of this process is a single shaft that is drilled to depth, including placement of concrete and reinforcement, in a short period of time. Depending on the diameter and depth of the shaft, the entire process takes 20 to 60 minutes. In soft wet soils, this may be as little as 25 percent of the time it would take to complete an equivalent shaft using conventional drilling methods with casing and slurry. The advantage is obvious: Faster production means shorter project durations and reduced labor, equipment, and fuel costs.
The Pacific City Development in Huntington Beach, Calif., is an excellent example of the savings that CFA can bring to a project. The project includes construction of residential, mixed-use, and hotel structures on a 34-acre site. During the initial design phase, engineers evaluated several options for the foundation system. Drilled shafts were completely ruled out because of high cost and slow production inherent to shaft construction in soft, saturated, sandy soils. As a result, the foundation was designed to be supported by approximately 1,200 driven, prestressed concrete piles with lengths of 40 to 55 feet and with allowable loads of 340 kips downward, 48 kips upward, and fixed head lateral capacity of 13.3 kips.
While this system was technically feasible and fell within the budget and schedule requirements of the project, it presented a high element of risk because the piles would be driven in close proximity to existing homes and a large hotel. The developer of the project, concerned about possible vibration damage claims and noise disturbances to the hotel, contacted D.J. Scheffler, a full service drilling company located in Pomona, Calif., to evaluate alternative foundation systems that would mitigate this risk while still being within range of the original budget. D.J. Scheffler determined that while conventional drilled shafts would cost two to three times that of the current driven pile system, CFA drilled shafts could be constructed at or below the budget of the driven pile system.
The obvious benefit of a CFA drilled shaft, compared with a conventional shaft drilled with concrete and slurry, is a higher production rate. CFA drilled shafts also can provide greater capacity than driven piles, even in ground that is considered prime for pile driving. In fact, while CFA drilled shafts can generally be designed at an equivalent capacity to that of a conventional drilled shaft, there is some evidence that they develop a slightly greater capacity than standard drilled shafts because of limited disturbance of the shaft sidewalls and placement of concrete under pressure.
In the Pacific City project, this was verified through a rigorous testing program in which CFA test piles were subjected to compression, tension, and lateral loads. The results of the testing indicated that a 24-inch pile drilled to a depth of 55 feet had an ultimate capacity of 1,433 kips and an allowable capacity of nearly 600 kips in compression—almost twice that of the driven piles and 10 percent greater than anticipated using traditional drilled shaft design techniques (see Figure 1).
|Figure 1: Compression test results from the 30-inch-diameter test pile, installed to a depth of 55 feet at the Pacific City Project|
Testing was then repeated with reduced-length (50-foot), 30-inch-diameter CFA shafts, which yielded an allowable capacity of approximately 700 kips. This was significant, considering the poor quality of the upper 35 feet of soft, saturated silts through which the shafts were installed. Equally important, testing indicated that allowable tension capacity of the piles was more than 300 kips for 24-inch and 30-inch shafts, allowing a replacement ratio as high as 6:1 in areas of the foundation controlled by uplift. While the final design is still under way, it is likely that the total number of CFA shafts required will be between 600 and 700, compared with roughly 1,200 driven piles. This should yield a cost savings for the project of about $350,000, while also shortening the project schedule by approximately 20 days.
While this foundation project demonstrates the cost and time savings that CFA technology can provide, the benefits are also clear for shoring projects. The Scheffler companies, including D.J. Scheffler, Scheffler Northwest, Scheffler Nevada, and D.J. Scheffler Canada, have constructed a number shoring walls using the CFA method to install either secant or soldier piles. This includes roughly 20,000 square feet of soldier pile and tieback shoring at the Block 31 Mirabella project in Portland, Ore.; secant piles and tiebacks for CalTrans at the PCH Emergency Stabilization project in Malibu, Calif.; and numerous others along the coastline of California. In each case, CFA piles/shafts were selected in part because of cost and time savings, and in part because of other benefits inherent to this method of shaft installation. These benefits include the following:
- reduced installation costs;
- reduced schedule duration;
- increased safety with no open hole;
- negligible vibration;
- low noise;
- ease of design (similar to conventional drilled pile/shaft);
- immediate replacement that reduces the risk of subsidence;
- concrete placed under pressure;
- no need for casing or slurry;
- limited disturbance of sidewalls;
- no free water in the shaft, which reduces the risk of segregation/poor concrete;
- real-time monitoring of pertinent drilling and concrete-placement data; and
- permanent record of construction, including an idealized cross-section of the shaft.
The advantages of the CFA method for constructing drilled shafts and shoring piles are clear. Given the rapidly increasing costs of steel, concrete, fuel, and labor, designers must not only evaluate systems to meet the complex structural requirements of a project, but also find innovative technologies to offset the rising costs of construction. Allowing—and in some cases specifying—the use of CFA is one way this can be accomplished while also providing the best quality product for the project.
Michael Zeman is president of Scheffler Northwest, Vancouver, Wash. He can be contacted at 360-818-0070 or by e-mail at firstname.lastname@example.org. Scheffler Northwest is part of the Scheffler family of companies, which includes D.J. Scheffler, Inc.; Scheffler Nevada; and D.J. Scheffler Canada. Dale Scheffler, president of D.J. Scheffler, has long advocated use of CFA technology in North America. He can be contacted at 909-595-2924 or by e-mail at email@example.com.