Deep Dynamic Compaction Drives Subgrade Improvements for Port
By: Mariusz P. Sieradzki, Ph.D., P.E., G.E., Principal Geotechnical Engineer, Kleinfelder
Bartlett W. Patton, P.E., G.E., Senior Principal Geotechnical Engineer, Kleinfelder
Douglas J. Sereno, P.E., Director of Program Management, Port of Long Beach
The Port of Long Beach in Southern California, the second busiest U.S. port and one of the world’s largest seaports, contains more than 7,600 acres of wharves, cargo terminals, rail yards and shipping channels. To continue its growth, the port authority plans to construct a new maintenance facility as a part of a 17-acre land development project (Figure 1). The facility will include a two-story structure with a footprint area of approximately 70,000 square feet.
Figure 1: Site for New Maintenance Facility at the Port of Long Beach
Prior to construction, geotechnical uncertainties emerged in this seismically active environment. It was confirmed that the site was reclaimed from the Pacific Ocean and filled with dredged, potentially compressible and liquefiable materials. In addition, rock slope protection and construction debris (rubble) were known to have been deposited within the fill. To mitigate the impacts of soil liquefaction during a seismic event and considering the challenges presented by the buried rock and rubble, Kleinfelder, the geotechnical engineer-of-record for the project, recommended a deep dynamic compaction (DDC) solution as an affordable ground densification method.
While DDC has been used for ground improvement at the Port of Long Beach in the past, the maintenance facility location posed some unique challenges. The site is close to existing structures and active underground oil lines. The Port of Long Beach was concerned that ground vibrations from DDC activities could impact the nearby facilities.
Convinced that DDC offered the most efficient solution, the Port of Long Beach and Kleinfelder put in place a pilot study and an associated continuous monitoring system for the project site and at nearby existing structures to evaluate the effectiveness of DDC, confirm that induced vibrations were at safe levels and confirm the procedure was the right choice.
High Energy Options
DDC uses high-energy waves created by the repeated impact of heavy weights to compact areas of loose soils and uncontrolled fills, increase density and stiffness of soil, and reduce foundation settlement. This ground improvement process is simple, fast, cost-effective and has been used in numerous projects in the U.S. since 1978.
During the site investigation phase of the project, Kleinfelder and the Port analyzed several other options besides DDC, including vibro-displacement stone columns, driven steel H-piles, and a reinforced soil mat to improve the liquefaction resistance of the on-site soils. The use of vibro-displacement stone columns or driven piles was questionable and even impractical due to the presence of the rubble fill (up to approximately 40 feet deep) in the central portion of project site and a potential for very difficult penetration of this material. A reinforced soil mat would not be expected to eliminate the potential for liquefaction-induced settlement and reduce it to a magnitude acceptable from the structural standpoint.
Using the DDC ground improvement process, the proposed maintenance building could be supported on conventional spread, shallow footings—assuming the vibrations would not adversely impact the surrounding structures.
Pilot Test
The DDC pilot test area covered 50 feet by 50 feet, using a 30-ton tamper and an 80-foot drop.
The tamping points were spaced approximately 9 feet apart in a square arrangement, and the number of drops at each location ranged from six to 10. In addition, to evaluate means to reduce the levels of vibrations reaching neighboring facilities, a temporary “isolation” trench, approximately 12 feet deep and 5 feet wide, was constructed adjacent to the test area. Field monitoring included ground vibration measurements at different distances from the tamping point, depths of crater formation following each series of drops, average ground loss, and ground heave at selected locations during the densification process. Photographs of DDC equipment (Figure 2) and craters at tamping points (Figure 3) are shown below.
Figure 2: A 130-ton crawler crane with a 30-ton tamper
Figure 3: Crater at tamping point
To evaluate the effectiveness of ground improvement and to estimate anticipated total and differential seismic-induced settlement, the team performed cone penetrometer tests (CPTs) within the DDC pilot test area before and after DDC.
Collected results indicated significant ground improvement within the upper 30 to 35 feet, resulting in reduction of seismic-induced settlement from approximately 6 inches (prior to DDC) to 2 inches (following completion of DDC). Test results also showed that the isolation trench significantly reduced levels of vibrations leaving the site.
Through this pilot test, Kleinfelder developed a constructible ground improvement design in a short time frame and provided on-site construction support consulting services during implementation of this cost-effective ground densification solution.
Full Scale DDC
The full DDC operation within the approximately 70,000-square-foot building pad area was performed in two phases. The first phase, dubbed “high energy,” was designed to achieve the required densification criteria utilizing a 130-ton crawler crane with a 30-ton tamper weight and falling height of 80 feet, with multiple drops and passes.
The second phase, dubbed “ironing,” smoothed the ground surface for future site grading. A 15-ton tamper with a drop of 40 feet was utilized for the ironing pass. The average energy applied on grid systems across the pad area during the high energy and ironing phases were over 300 ton-feet/square foot and 30 ton-feet/square foot, respectively.
As a part of quality assurance testing, engineers conducted CPT soundings at selected locations across the project site to confirm densification criteria within the improved site. In general, post-DDC uncorrected tip resistances over an effective interval of 5 feet to the depth of at least 30 feet met or exceeded the specified minimum value of 120 tons/square foot.
Ground vibrations caused by dynamic compaction can be potentially damaging to nearby structures, utility lines and other improvements, as well as annoying to people in the immediate vicinity of the DDC operations. Several velocity sensors were installed across the site and near the existing structures to record peak ground velocities at various distances from the tamping points and evaluate vibration impact to nearby improvements. To reduce the potential impacts to the existing oil lines, a temporary isolation open trench, approximately 12 feet deep and 5 feet wide, was constructed along the northwest boundary of the project site. Based on the collected vibration data, the trench was effective, reducing vibration levels by as much as 40 percent. Records from the nearly continuous monitoring show that vibration levels were at all times within accepted limits at and beyond the property line of the maintenance building site.
This project is a great example of successful planning, design and teamwork by identifying and implementing practical and cost-efficient ground improvement options based on site-specific conditions and local construction practices.
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