The first major capital improvement on the canal was the construction of the Madden Dam and Power Project, which was completed in 1935. That not only stemmed and controlled the flow of water moving into Gatún Lake to a rate of some 200 billion cubic feet (6 billion cubic metres) per year but also created a large reservoir, Lake Madden (now Alajuela Lake). It also increased the production of electric power in the region. The Boyd-Roosevelt Highway was then built across the isthmus, thereby adding a third means of transportation to the waterway and the railroad. In 1955 the Thatcher Ferry Bridge (now called the Bridge of the Americas) was built, which connected Panama City and Balboa to the west side of the canal. From 1957 to 1971 Gaillard Cut (also called the Culebra Cut) was widened from its original 300 feet (90 metres) to 492 feet (150 metres).

In 1991, within two years of the final transition of power, the ACP began its first expansion program, a $219 million project to widen the nearly 8.5-mile- (14-km-) long Gaillard Cut from 500 feet (152 metres) to a maximum of 728 feet (222 metres). Completed in 2001, that ambitious development allowed the two-way passage of so-called Panamax ships (the then largest ships allowable in the canal) and decreased the average canal travel time by about 6 hours to about 10 hours total. The ACP also invested $54 million in new lock locomotives, new tracks and tugboats, conversion of mitre gate locks to hydraulics, and a $30 million GPS vessel tracking system.

Despite such improvements, many supertankers and large naval vessels were still too large to pass through the canal. There was much study of the feasibility of either widening the existing canal and locks or building a larger sea-level canal at another location. Cost and environmental concerns eliminated the latter option, and in 2006 the Panamanian government and voters backed the Third Set of Locks Project, a $5.2 billion expansion program to increase the width of Gatún Lake’s navigational channels to 920 feet (280 metres) in the straight sections and 1,200 feet (366 metres) at the turning points to facilitate cross-navigation. The project, completed in June 2016, raised Gatún Lake’s maximum operating level to 89 feet (27 metres), with the goal of increasing Gatún Lake’s usable water reserves by a daily average of 165 million gallons (625 million litres). Furthermore, the project included four phases of dry excavation that created a new 3.8-mile- (6.1-km-) long access channel connecting the new Pacific locks with the Gaillard Cut and also widened and deepened the existing navigational channels and deepened the cut. Freshwater dredging totaled more 37.9 million cubic yards (29 million cubic metres) of earth taken from the lake, 11.4 million cubic yards (8.7 million cubic metres) as part of the Pacific access channel, and 23.5 million cubic yards (18 million cubic metres) from the Atlantic entrance. Each sea-entrance navigation channel was widened to 738 feet (225 metres) and deepened to at least 18 feet (5.5 metres) below the lowest tide levels. The project doubled the canal’s capacity by adding two new steps of three-step locks, one at either end of the canal, and to allow the passage of a new generation of supersized ships, dubbed “neo-Panamax,” that can carry 13,000 TEUs (twenty-foot equivalent units; 1 TEU is the capacity of a container 20 by 8 by 9 feet [6.1 by 2.4 by 2.7 metres]).

After approval of the Third Set of Locks Project, ACP began awarding major contracts to firms from around the world, including an environmental-impact study conducted by the American firm URS and two Panamanian universities in 2007. To address concerns about the project’s effect on wildlife, the builders conducted extensive monitoring, both in person and with cameras, to determine what types of traps and how many traps would be required to capture and relocate animals to safe locations during construction.

On September 3, 2007, Panamanians celebrated groundbreaking on the first dry excavation project on the Pacific side. By December ACP had qualified four global consortia to bid on the main component of the program—the construction of the third pair of locks. In July 2009 ACP selected Grupo Unidos por el Canal—led by the Spanish firm Sacyr and including companies from Italy, Panama, Belgium, the Netherlands, and the United States—to design and build the new locks, which the group proposed to do at a cost of $3.12 billion. The final cost of the expansion totaled over $5.25 billion.

The canal expansion required a significant concrete-procuring effort. Two concrete plants operated 24 hours a day, six days a week, and were supported by a system of trucks, barges, conveyor belts, stockpiles, crushers, and coolers. At the height of construction, 8,000 tons of aggregate a day were transported from the Pacific side to the Atlantic side by barge and then carried by as many as 60 trucks to the site. Aggregates varying from coarse rock to fine sand were added to various types of concrete mixes and applied to different sections of the locks. An on-site quarry operation produced basalt for the concrete mixes, though some material was reused from existing excavation if it was of sufficiently high quality.

In 2012 ACP announced an eight-month delay due to the extensive challenges associated with procuring concrete with a 100-year design life, a weeklong strike by one of Panama’s biggest construction labour unions, and bad weather. With the anticipated completion of Third Set of Locks Project pushed back to 2016, the expansion was still underway during the canal’s centennial in 2014. The expansion opened on June 26, 2016.

In August 2019 a third bridge, known as the Atlantic Bridge, was opened for road traffic over the canal’s Atlantic entrance. The new bridge replaced a nearby bridge that had been constructed in 1942 and dismantled in 2018 as well as the Panama Canal Ferry. It is the only bridge north of the Culebra Cut.

Effects of expansion

The construction of the third set of locks inspired numerous articles, reports, and studies speculating on how the passage of post-Panamax ships through the canal would have an impact on global shipping patterns. In the United States, many East Coast ports began ramping up expansion and modernization plans in anticipation of increasing amounts of those large ships, which generally require channels with depths of more than 50 feet (15 metres) if fully loaded. However, the global recession in 2008 brought a long-term pattern of yearly U.S. import growth to a halt, implying increased trade uncertainty and much slower growth rates.

Despite uncertainties in future shipping patterns, the Third Set of Locks Project brought global attention from the engineering industry to Panama and the ACP. Unlike the original construction of the canal, the Panamanians hold proprietorship over the expansion, and the ACP has signed multiple partnership agreements with port authorities and other entities throughout the Americas and the world. In 2012 the ACP hosted an inaugural Engineering and Infrastructure Congress, which drew hundreds of geotechnical, electrical, structural and civil engineering practitioners, as well as exhibitors and vendors, and featured multiple sessions to address the ongoing issues associated with the canal expansion.