Running along much of the coastline in southern New Jersey, the Garden State Parkway (GSP) is a critical thoroughfare for residents, businesses and visitors throughout the year.
The New Jersey Turnpike Authority’s (NJTA) ongoing GSP Interchange 30 to 80 widening program between Somers Point and Toms River was intended to improve the roadway to meet current and projected capacity and allow travelers an easier commute.
The widening project included construction of a new bridge to take the GSP over the Mullica River in Bass River Township and Port Republic. The new bridge was constructed parallel to the existing structure, using large-diameter drilled shafts with self-consolidating concrete (SCC) that enabled the project to be finished within the required time frame and with ample foundation capacity, as confirmed by testing and a thorough quality-assurance and quality-control (QA/QC) program.
With the aim of improving capacity, the new Mullica River Bridge was designed by Parsons Brinckerhoff to accommodate a future configuration of three 12-ft lanes with 5-ft inside shoulders and 12-ft outside shoulders in each direction. The new bridge consists of seven spans varying in length from 175 to 220 ft with new abutments supported on concrete piles and a total of five piers (four in the water and one on land) founded on 8-ft-diam. drilled shafts extending from 142 to 182 ft below the mean high-water elevation.
A subsurface investigation program was developed to verify the geologic conditions, finding a generalized soil profile within the Mullica River composed of approximately 60 ft of dense sand overlying 55 ft of fine sand with silt and clay. This in turn was underlain by 80 ft of stiff silt and clay, 15 to 30 ft of dense sand and 25 ft of very dense sand intermixed with silt and clay.
Going for a dip
While a dense sand layer near the surface would usually provide ideal conditions for shallow foundations or pile foundations, scour presented a major concern at the bridge location. During design, scour from the 100-year design storm was estimated to range from 28.5 ft at the land pier to a maximum of 39 ft for the in-water piers. As a result, shallow foundations were not an option. Moreover, due to corrosion concerns from the saltwater conditions, only concrete piles were a viable pile alternative. However, concrete piles were precluded from use at the pier locations based on past difficulties experienced with splicing concrete piles, which would have been necessary to achieve sufficient lateral resistance of the pile group.
Furthermore, due to the close proximity of the existing structure to the proposed bridge, vibrations produced from pile driving also were a concern. As such, drilled-shaft foundations were considered the optimal solution for the design of the bridge piers. Several shaft configurations were analyzed; however, environmental constraints ultimately proved the deciding factor in selecting the final drilled-shaft alternative. Due to regulatory requirements, all in-water construction could be performed only between April and October. Protected shellfish beds and designated wetland areas adjacent to the project site also required that disturbance to the river environment be kept to a minimum.
Considering these environmental conditions, three 8-ft-diam. drilled shafts at each pier location were selected to ensure in-water construction would be completed within the designated time period. Not only would the 8-ft-diam. option limit the number of shafts required within the waterway, but this alternative also would allow the pier columns to extend upward from the drilled shafts without the need for expensive and time-consuming in-water construction of a pier cap for the shafts. The 8-ft-diam. drilled-shaft tips were designed to be embedded into either the stiff silt/clay layer or dense sand layer to carry the required design load. The depth of scour also was considered when determining how much soil could be relied on for both vertical and lateral capacity. By designing the shafts to be resistant to scour, the need for scour countermeasures was eliminated.
Based on the drilled-shaft calculations, a maximum ultimate capacity of 6,000 kips with a corresponding maximum allowable capacity of approximately 2,800 kips was established. The minimum and estimated tip elevations were established based on the shaft length required for lateral resistance and axial capacity, respectively.
No defects
Due to the large size of the drilled shafts, the potential for concrete defects was a major construction concern. In an effort to mitigate this concern, a SCC mix was selected by the NJTA—the first time for large-diameter drilled shafts on a GSP bridge project—to minimize potential defects due to the segregation of the concrete between the inside and outside of the reinforcing cage and to improve the workability of the mix for the duration of the pour.
The 4,000-psi SCC mix was verified for slump loss and pumpability prior to approval and production use. Since the specialty drilling contractor, Case Foundation, was planning to use two pump trucks to deliver the concrete to Pier 3, approximately 400 ft from land, concrete flow retention was simulated by circulating the test batch through the pump truck twice before beginning the pre-excavation slump flow test to confirm workability of the SCC mix. A slump flow of 18 to 24 in. was maintained during the 10-hour test, and average 28-day strength results were slightly over 7,000 psi. These results were typical of those encountered during construction.
First a demonstration
In an effort to verify both the design parameters and the specialty contractor’s means and methods, a sacrificial demonstration drilled shaft was constructed prior to the start of production shaft installation, as the performance of drilled-shaft foundations can be greatly affected by the method of construction. The demonstration shaft was originally located in the vicinity of Pier 2 within the waterway but was moved adjacent to Pier 1, closer to the shoreline, within a water-tight cofferdam in order to avoid in-water construction restrictions and provide an earlier start for drilled-shaft excavation.
During demonstration shaft installation, contamination due to overflow of soil and slurry during drilling and soil/slurry disposal were major concerns due to the presence of protected shellfish beds downstream of the project site and the adjacent wetlands. The issue of overflow was adequately addressed by covering the bottom of the working platforms with plastic and installing wooden boards around the platform perimeter. Disposal was achieved through use of a spoils barge adjacent to the working barge, with spoils thereafter removed using a sealed clamshell bucket to prevent soil from dropping into the wetlands.
Survives in fresh water
A Caldwell 200 CH drill rig attachment mounted on a 230-ton Manitowoc 4100 crane was used by Case Foundation to excavate the drilled shafts. In order to ensure the integrity of the shaft sidewalls during excavation, vented buckets were used to reduce suction during tool removal, and polymer slurry was selected for use in order to stabilize the excavation. A total of 10 slurry tanks, each with a capacity of 20,000 gal, were set up on-site to accommodate slurry production and recycling. The tanks were staged in a central location. Four-in. supply and return slurry piping were mounted with brackets alongside the existing bridge girders to each pier.
During initial slurry use at the demonstration shaft, which used river water in the mix, the saltwater was observed to be affecting the composition of the polymer slurry, resulting in lowered viscosity and “corn-flake” material to appear floating at the top of the slurry. As a result, a freshwater well was located approximately one-half mile from the slurry mixing tanks and piping was installed to convey the freshwater to the tanks for slurry production. Once freshwater was added to the mix, polymer slurry properties met or exceeded the project requirements and the “corn flakes” were no longer observed. Permanent steel casing also was installed within the template to allow in-water concrete placement, prevent overflow of slurry during excavation and provide a greater slurry head to counteract the hydrostatic pressure imposed on the excavation.
Due to the relatively high axial loading, ensuring a clean bottom of excavation immediately prior to pouring concrete was essential. As a result, mini-SID testing was specified to verify bottom cleanliness with criteria requiring less than a ½ in. of sediment over at least 50% of the shaft base at the time of concrete placement and no greater than 1.5 in. at any location. Initial bottom cleanout was attempted using a standard cleanout bucket. However, the thickness of the bottom plate on the bucket, which was approximately 1.5 in. thick, prevented the cleanout criteria from being met. As a result, an airlift was used to complete the bottom cleaning and successfully meet the cleanout criteria. Based on its successful use on the demonstration shaft, an airlift was thereafter used at each production shaft location.
Concrete was installed via a 10-in.-diam. tremie pipe, with pours of 400 cu yd lasting approximately five hours. Even after the concrete pour at the demonstration shaft, which lasted approximately eight hours, the SCC remained highly workable and free-flowing, with a slump retention test showing a drop in slump flow of only 1.5 in.
The only issue encountered with the SCC was with regard to sounding the top of concrete. Since the normally used weighted tape would simply sink into the fluid SCC, a weighted plate was fabricated on-site to allow the top of concrete to be determined during the pour and was used successfully on all production shafts.
O-fficial savings
In order to assess the actual in-situ capacity of the drilled shaft and compare the measured and design capacities, an Osterberg-Cell load test was performed using three 21-in.-diam. O-Cells. Load-test results indicated a maximum sustained O-Cell load of 7,000 psi, which was significantly higher than the calculated maximum design capacity of 2,800 kips.
While maintaining safety factors, the drilled-shaft design was reinvestigated using skin friction and end-bearing values obtained from the test results. Re-analysis of the production drilled shafts resulted in drilled-shaft lengths being reduced a total of 660 ft to the minimum tip elevation, thereby providing significant savings to the client.
NJTA’s Elizabeth Trimpin said: “The design successfully provided an innovative solution to the challenge of completing the pier foundations within a six-month window. The design strategy included consideration for reducing the number of piers with the trade-off of longer, bigger shafts balanced with the use of self-consolidating concrete and the opportunity to use a cofferdam to extend the in-water window by working within the cofferdam if necessary. An $800,000 O-Cell load testing for the demonstration shaft saved over $1.3 million in cost of the bridge foundation by reducing the lengths of shafts. Furthermore, reduced shaft lengths also provided a significant construction time savings in the water and minimized possible construction difficulties due to the longer shafts. There were so many interesting facets of the project; it was a thrill to be involved in it.”
Half the time
In addition to the savings provided by the O-Cell test, the demonstration drilled shaft allowed Parsons Brinckerhoff and the specialty contractor to find the optimum installation procedure for project-specific conditions. Based on the performance of the polymer slurry, 24-hour continuous drilling was no longer necessary, provided the slurry properties were kept to acceptable values and monitored regularly. Allowing the shaft to sit for at least 12 hours enabled any suspended particles to fall out of suspension and provided for an expedited final bottom cleaning, as confirmed by mini-SID test results. Based on the demonstration shaft, the following general installation procedure was established:
- Permanent casing installation;
- Shaft excavation;
- Shaft allowed to sit overnight following excavation (post-excavation Day 1 p.m.);
- Final bottom cleaning via airlift (Day 2 a.m.);
- Final slurry testing, mini-SID testing, sonar caliper testing (Day 2);
- Lift steel reinforcing cages (Day 2 p.m.);
- Splice reinforcing cages and install in shaft (Day 2 p.m.–Day 3 a.m.); and
- Pour shaft SCC (Day 3).
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Using this procedure, the specialty contractor constructed the drilled shafts within the aforementioned in-water construction window and without disturbance to nearby environmentally sensitive areas. In addition to the O-Cell load test, cross-hole sonic logging (CSL) and mini-SID inspection were performed for all drilled shafts, with sonar caliper testing also performed on select drilled shafts to ensure the in-place integrity of drilled shafts. Additional QA/QC included testing of the polymer slurry and SCC. Widely accepted The drilled-shaft installation procedures and QA/QC program implemented on the Mullica River Bridge project resulted in acceptable drilled shafts without the need for concrete coring or remediation. Based on this project, the following conclusions were drawn from the drilled-shaft process:
- The SCC performed well under construction conditions that presented difficulties for concrete placement and reduced concrete pouring time;
- The excellent flow of SCC proved especially useful for the construction of long, large-diameter drilled shafts, which required a congested cage, tremie placement and extensive concrete pouring times. Based on the results of this SCC performance, NJTA adopted SCC as a standard concrete for drilled-shaft concrete;
- Using the ultimate capacity obtained from O-Cell testing, the production shaft lengths were reduced at a savings to the client;
- The selected installation methods and drilling specifications were successfully implemented to finish in-water construction within the specified time and without adversely impacting the surrounding area and protected shellfish beds;
- The large-diameter drilled shafts presented a viable and successful alternative to a driven pile foundation under the environmentally and vibration-sensitive conditions; and
- Large-diameter drilled shafts can be installed successfully in a marine and soil environment in lieu of driven piles with a project-specific QA/QC program implemented.
