Before starting pile driving, the contractor performed sonic core borings to 110 to 150 ft (34 to 46 m) below mudline at each pipe pile location to search for obstructions. The sonic cores obtained continuous recovery of the entire length of boring, and no boulders greater than 2 ft (0.6 m) thick were seen. The contractor installed the plate insert piles in two to three 50 to 75 ft (15 to 23 m) long sections, using a hydraulic hammer that delivered approximately 375,000 ft-lbs (508,430 N-m) to the top of the pile. Dynamic testing was performed on each pile for the entire driven length to monitor stresses. The team installed two indicator piles to completion at each river pier prior to production pile driving for the remaining eight piles at each pier. Production pile driving commenced following acceptance of the indicator piles. To confirm capacity, the contractor performed restrikes on the piles three and seven days following end-of-drive. Per specifications, final driving criteria consisted of driving the pile to either a blow count resistance relative to hammer energy, to refusal or to a design tip elevation, whichever occurred first. However, final pile acceptance was based on achieving the required factored resistance as measured using CAPWAP on restrikes. The actual driven depth below mudline ranged from 130 to 200 ft (40 to 61 m). Following installation, the piles were filled with sand to approximately scour elevation, and then with concrete to pile cutoff elevation. As an added precaution against corrosion, the contractor installed a passive cathodic protection system, in the form of aluminum-zinc anodes, on the exterior of the piles below the river level. Detail and photo of plate insert pipe pile A total of 457, 14x117 H-section piles support the five new bridge land piers. The contractor outfitted all piles with a pile tip coated with coal tar epoxy on the top 30 to 40 ft (9 to 12 m). Piles at Piers 1 and 8 were driven to end-bearing to the required factored resistance of 495 to 568 kips (225 to 258 tonnes). Final driven lengths varied between 50 and 190 ft (15 and 58 m). Piles at Piers 2, 3 and 7 were friction piles driven to an embedment depth of approximately 120 ft (37 m). Required factored resistance of the friction piles ranged from approximately 193 to 230 kips (88 to 104 tonnes). The contractor used a hydraulic hammer that delivered approximately 70,000 to 120,000 ft-lbs (95,000 to 163,000 N-m) to the top of the piles. Dynamic testing was done on indicator piles to monitor driving stresses and confirm capacity during restrikes, as assessed by CAPWAP. Out of 457 piles, only three piles encountered conditions that required adjustment during construction. Maintaining Existing Bridge Service Vertical settlement data at Bent 2 prior to and after jacking The existing bridge remained in service during construction. The project team was particularly concerned about potential settlement of the existing bridge due to nearby pile installation, pile cap construction and abutment earthwork filling. During the design phase test programs, settlement extensometers, piezometers and inclinometers were installed in the ground and strain gages were installed on several of the steel bents of the existing bridge to monitor effects of pile driving on the ground and existing bridge. Data generated from this instrumentation in combination with empirical assessments led the project team to discuss contingency plans in the event the existing bridge settled during foundation construction. As a result, RIDOT authorized installing an extensive instrumentation program of strain gages, vertical survey points and seismographs on the existing bridge. Each instrument could be monitored remotely and nearly continuously. As a complement to the instrumentation program and prior to the start of foundation construction, RIDOT included in the contract a jacking system at many of the existing bridge’s steel bents supported on shallow foundations. The forethought regarding the instrumentation and jacking system turned out to be critical in ensuring the continued service of the existing bridge. On several occasions during construction, instrumentation alerts indicated that a pier’s settlement and truss member’s strain/stress were approaching action limits set by the project team. Almost immediately, the engineers halted the work temporarily while the contractor jacked the pier back to near its original height and stress state (In each instance, existing bridge service was not impacted, and construction resumed shortly after jacking was completed.) If the instrumentation and jacking systems had not been in place and ready to employ at a moment’s notice, the existing bridge would have been put out of service and foundation construction halted indefinitely at a large economic loss and inconvenience to the public. DEEP FOUNDATIONS • JAN/FEB 2015 • 15
