FEATURE ARTICLE Suction Pile Technology and Low Noise Marine Pile Driving Suction caisson concept Regulatory bodies continue to tighten restrictions on marine pile driving due to potential dangers to marine life. Adapting suction caisson techniques to drive permanent piles is proving environ- mentally superior to traditional impact driving. Typical impact driving results in noise pollution levels reaching the 200dBA range. Suction driving operates at a range of only 100dBA, well below the 120dBA threshold considered as behavioral harassment of marine mammals, according to CALTRANS. There are noise mitigation measures for impact driving, such as bubble curtains, and jackets have been designed to help minimize noise pollution; however, they only reduce the noise by about 15-20dBA. Other such techniques include cofferdams and vibratory ham- mers, which are more effective in reducing noise, but have limitations with regard to cost, site or soil specific applicability, and construction time. The suction pile driving technique strives to reduce noise creation at the outset as opposed to existing measures, which seek to intercept existing noise. At this stage, suction pile driving is a concept, but one that relates to well-established suction caisson technology, used in the offshore oil and gas industry. Before discus- sing suction pile driving, we need to review the fundamental principles of suction caissons and the method’s development. Suction Caisson Principles Suction caissons are large diameter pipe piles with waterproof pressure heads, which are stiffened plates with a suction nozzle for a submersible pump. Power for the pump is provided by an umbilical from a surface support vessel. This same vessel lowers the caisson to the seabed, allowing it to penetrate the soil under its self weight. The self weight penetration is highly dependent on the geometry of the caisson and the soil properties at the site. In most cases, the self weight penetration will create a pressure tight seal necessary for the suction phase of installation. The next phase utilizes the pump mounted on the pile head to create a pressure differential between the inside and outside of the caisson. By pumping water out of the caisson, decreasing the internal pressure, the external pressure (overburden) pushes the caisson deeper into the soil. Additional soil failure mechanisms occur at the microscopic level, decreasing the soil strength locally and allowing the pile to be driven with less resistance. Once the caisson is driven completely into the soil, the operator then removes the pump, allowing the internal and external pressures to equalize. By the time the pump is removed, the caisson has developed sufficient tensile capacity to keep it in place due to the friction force on the pile walls. At this point, we can use the installed caissons to support floating structures connected by mooring lines or as foundations to resist tension, compression and horizontal forces from gravity base or jacket type structures. Background A brief review of suction caisson develop- ment with project examples follows. In the late 1950s, a researcher secured a piston corer to the bottom of a lake bed using the principles of suction to hold it in place while work was completed. Using inverted cans sunk to the mudline, a vacuum was created by pumping water out of the can, locking it in place. Once the work was completed, the device was retrieved by pumping water back into the can releasing the suction force holding it in place. Using an active pressure differential, the vacuum, as a temporary anchorage was further investigated through the 1960s and 70s by various researchers as a means of anchoring offshore structures. Using active suction for all load cases throughout service seemed unreasonable for permanent structures, and this view delayed the introduction of suction technology for several years. How- ever, for offshore structures, others took advantage of suction for temporary stability prior to installing piles by adding skirts to mudmats. These helped resist tension and sliding forces exerted on the structure. AUTHORS Gerard Houlahan, Vice President and Matt Wickens, Civil Engineer, Moffatt & Nichol DEEP FOUNDATIONS • JAN/FEB 2013 • 53
