The New Jersey Department of Transportation (NJDOT) is replacing the two-mile causeway that serves as the main route connecting the seashore resort town of Ocean City with mainland Somers Point. Using a technique relatively new to North America, contractor George Harms Construction Co. Inc., Howell, N.J., is replacing more than 2000 of the 2700 piles originally called for with vibro cast-in-place concrete columns (VCIPCC). The result is a savings in layout time as well as increased accuracy.
VCIPCC as a method for constructing concrete columns is more popular in Europe and other parts of the world than in North America. And although this is Harms' first prime contract using the technique, some very successful VCIPCC projects have been carried out in North America during the last six years. These include the Route 9 Chestnut Neck Project (N.J.), the Atlantic Bridge Project (Fla.), the Victory Bridge Project (N.J.), and State Road 10 over Pablo Creek Project (Fla.).
A major undertaking
Located 12 miles southwest of Atlantic City, the new causeway will replace a 70-year-old causeway that has seen a substantial increase in marine and highway traffic in recent years. That increase has led to more accidents among vacationers and weekenders visiting Ocean City, while structural deterioration has also become an issue. The $400 million Route 52 Causeway Bridge Replacement Project will address both problems.
In this project, the four existing bridges—two fixed and two moveable—will be replaced by two new bridges. The $140 million first phase now under way calls for building the low-clearance sections of the two bridges. Both are fixed, continuous-span, bulb-T bridges featuring four 12-foot-wide lanes, two 8-foot-wide outside shoulders, and two 5-foot-wide inside shoulders each. Additionally, a 10-foot-wide walkway for pedestrians and bicycles will be added on one side of the highway.
Starting at Somers Point and traveling southeast toward Ocean City, the 3600-foot Bridge-I spans over the Ship Channel and Elbow Thorofare and terminates on Rainbow Island. From here a new 1600-foot roadway will link Bridge-I to Bridge-II.
Two construction methods, VCIPCC and surplus-load/wicking, are being used to increase the ground support at both bridge abutments on Rainbow Island and under the new highway connecting them. At the Bridge-I abutment on Rainbow Island, the vibro-cast columns will support the entire abutment.
Where the roadway starts at the end of the abutment, surplus-load/wicking ground stabilization is being used. After water-wick drains are installed in the ground, an overburden 14 feet high by slightly wider than the width of the highway is constructed. This temporary surcharge consolidates the ground beneath it so the new roadway will not cause significant settlement.
Vibro-cast columns also will support the Bridge-II abutment. All bridge sections, except the abutments, will be supported by poured-in-place concrete piers built atop jetted/driven precast concrete piles.
How it's done
VCIPCC is a refinement of the vibro displacement-densification (VDD) method. VDD consists of creating a deep, narrow hole in the ground by lowering a vibrating soil penetrator attachment mounted on a drill rig. As the vibrator advances downward, it laterally compresses the surrounding soil. None of the soil is removed; it is only displaced by the vibrator.
Once the specified depth is reached, the vibrator is incrementally withdrawn in a repeated short-stroke, up-down-up action. Meanwhile, the hole is gradually filled through the vibrator with either gravel or crushed rock and densified by the vibration. The entire procedure is performed until the compacted stone reaches the top of the hole. The end result is a hole filled with stone called a compacted stone column. Such columns provide suitable bearing characteristics for building some foundation types.
Installing vibro-cast columns also uses the VDD method but, instead of filling the hole with stone, it is filled with ready-mixed concrete. The strength of the concrete specified, as well as any admixtures added to the concrete mix, depends on the soil conditions and the specified end bearing capacity of the column. Suitable soil conditions for applying VCIPCC usually include organic and cohesive types that generally show very poor load capacities.
For the VCIPCC work, Harms is using its Bauer BG 40 drilling rig fitted with a Bauer TR 17 vibro-soil-penetrator. The BG 40 is a versatile drilling rig that Harms has used on other construction projects albeit using various soil-improvement methods. Its versatility is a very important attribute for Harms because versatility increases the machine's utilization, which makes it more cost-effective than other drilling rigs that are not as versatile.
Kevin Harms, the company's vice president of construction, reports the BG 40 is providing outstanding productivity on this project. “We already have completed making 1064 cast-in-place concrete shafts very efficiently,” he says, “pumping 6070 cubic yards of concrete into the holes and inserting 25-foot #11 rebar for reinforcement. There are another 1000 shafts to make and we do not anticipate any major problems.”
VCIPCC was not specified originally for this project. It was Harms who, after the fact, recommended to NJDOT that it should be used. Harms' engineers contended this method of ground improvement would effectively transfer loads to deep, more stable ground and thus be a very reliable and adequate support system upon which to build the abutments. VCIPCC functions much like traditional precast-concrete piles. However, the cast-in-place concrete columns do not depend as much on the surrounding soils for their load-bearing capacity.
Generally, VCIPCC does not include inserting steel rebar. Because in this case rebar is being inserted into the fresh cast-in-place concrete, Harms' engineers are calling it a shaft instead of a column. This designation, they say, makes it clear to all on the project that steel reinforcement is part of the concrete structure.
Ground penetration with the VSP is made about 70 feet deep (the point of more stable ground) and takes from five to seven minutes, depending on the ground conditions. An 18-inch-diameter hole is made to all but a few feet from the bottom where it is expanded to a 24-inch diameter. This bottom-bulb effect is accomplished by repeatedly raising and lowering the VSP slightly in the hole. Once the bulb is completed, concrete is pressure-pumped by a Putzmeister truck-mounted concrete boom pump through the TR 17 VSP. Gradually, as the TR 17 VSP is withdrawn from the hole, the void is filled to grade with the concrete.
Traditional surveying and layout methods have been circumvented while carrying out VCIPCC using a Leica Geosystems GPS 3-D system mounted on the drilling rig. The GPS system guides the drilling rig operator to precisely position the VSP prior to commencing the vibration-soil-penetration.
Justin LaBarca, technical field representative for Atlantic Laser Specialists, Matawan, N.J., worked closely with Harms' project superintendent Jason Hardell to select the Leica GPS components that would optimize the VCIPCC operation. According to LaBarca, the mast on the Bauer BG 40 is designed such that the Leica antenna can be installed atop with no offset compensation required when setting up the software program.
Increased precision is one advantage of using GPS instead of conventional surveying for positioning the vibro-soil penetrator. Usually, three or more stakes are placed around the specified penetration spot as a guide for the drilling rig operator. Using the GPS method enables the operator to position the VSP point to within ± 1 inch of the specified location, well within the NJDOT specifications of ± 3 inches.
Using GPS also saved time. With more than 2000 cast-in-place concrete columns being built here, it would have taken 800 more man-hours to use conventional surveying and staking methods, based on a traditional two-man survey crew.
On this job a surveyor also used the GPS system with a Leica rover to direct the crane operator for placing the precast concrete piles at precise locations, again saving time and money.
The new causeway's northbound lanes are expected to be opened to traffic by Memorial Day 2008. The southbound lanes will then be constructed and the old causeway and drawbridges removed. Project completion is expected in late 2012.
— Rodney Garrett is an international technical photojournalist based in Bernville, Pa.
What's in a name?
“Vibro cast-in-place concrete columns” is neither a household phrase nor is this construction method commonly used in North America. Still, it is certain to become more widely recognized and used here by contractors and engineers involved in ground-improvement related to construction.
Various acronyms are used to designate this method. For example, the construction managers presently engaged in the Route 52 Causeway Bridge Replacement Project in New Jersey refer to the method as VCS (vibration concrete shaft). They have adopted the term “shaft “ instead of “column” to acknowledge the use of rebar, which is not always part of the process. Some NJDOT engineers overseeing the project informally are referring to it as VCC, for vibro concrete column.
The description VCIPCC is used by Bauer Maschinen—a German drilling and piling equipment manufacturer that advocates this construction method for certain types of foundation projects. It is a good way to refer to the construction method because the acronym represents exactly what it is: vibro cast-in-place concrete columns.