The Panama Canal Locks, which lift ships up 25.9 m (85 ft) to the main elevation of the Panama Canal, were one of the greatest engineering works ever to be undertaken at the time, eclipsed only by other parts of the canal project. No other concrete construction of comparable size was undertaken until the Hoover Dam in the 1930s. The total length of the lock structures, including the approach walls, is over 3 kilometres (nearly two miles).
The locks, which have a total of six steps, limit the maximum size of ships which can transit the canal, known as Panamax. Each of these steps has two lock chambers, doubling the amount of traffic that can be handled; together they raise ships from sea level to a height of 25.9 m (85 ft).
The locks are to be expanded in the near future to allow more and larger ships to use the canal.
There are three sets of locks in the canal. A two-step flight at Miraflores, and a single flight at Pedro Miguel, lift ships from the Pacific up to Lake Gatun; then a triple flight at Gatun lowers them to the Atlantic side. All three sets of locks are paired; that is, there are two parallel flights of locks at each of the three lock sites. This, in principle, allows ships to pass in opposite directions simultaneously; however, large ships cannot cross safely at speed in the Gaillard Cut, so in practice ships pass in one direction for a time, then in the other, using both "lanes" of the locks in one direction at a time.
The lock chambers are 33.53 meters (110 ft) wide by 320.0 meters (1050 ft) long, with a usable length of 304.8 metres (1000 ft). These dimensions determine the maximum size of ships which can use the canal; this size is known as Panamax. The total lift (the amount by which a ship is raised or lowered) in the three steps of the Gatun locks is 25.9 m (85 ft); the lift of the two-step Miraflores locks is 16.5 m (54 ft). The single-step Pedro Miguel lock has a lift of 9.5 m (31 ft). The lift at Miraflores actually varies due to the extreme tides on the Pacific side, between 13.1 m (43 ft) at extreme high tide and 19.7 m (64.5 ft) at extreme low tide; the tides on the Atlantic side, however, are very small.
The lock chambers are massive concrete structures. The side walls are from 13.7 to 15.2 metres (45 to 55 feet) thick at the bases; towards the top, where less strength is required, they taper down in steps to 2.4 m (8 ft). The centre wall between the chambers is 18.3 m (60 ft) thick, and houses three long galleries which run the full length of the centre wall. The lowest of these is a drainage tunnel; above this is a gallery for electrical cabling; and towards the top is a passageway which allows operators to gain access to the lock machinery.
Each lock chamber requires 101,000 m3 (26,680,000 US gal; 22,220,000 imp gal) to fill it from the lowered position to raised; the same amount of water must be drained from the chamber to lower it again. Embedded in the side and centre walls are three large water culverts, which are used to carry water from the lake into the chambers to raise them, and from each chamber down to the next, or to the sea, to lower them. These culverts start at a diameter of 22 ft (6.71 m), and reduce 18 ft (5.49 m) in diameter — large enough to accommodate a train. Cross culverts branch off from these main culverts, and run underneath the lock chambers to openings in the floors. There are fourteen cross culverts in each chamber, each with five openings; seven cross culverts from the sidewall main culverts alternate with seven from the centre wall culvert.
The water is moved by gravity, and is controlled by huge valves in the culverts; each cross culvert is independently controlled. A lock chamber can be filled in as little as eight minutes; there is significant turbulence in the lock chamber during this process.
The gates which separate the chambers in each flight of locks must hold back a considerable weight of water, and must be both reliable and strong enough to withstand accidents, as the failure of a gate could unleash a catastrophic flood of water downstream.
These gates are of enormous size, ranging from 47 to 82 ft (14.33 to 24.99 m) high, depending on position, and are 7 ft (2.13 m) thick; the tallest gates are required at Miraflores, due to the large tidal range there. Each gate has two leaves, 65 ft (19.81 m) wide, which close to a V shape with the point upstream; this arrangement means that the force of water pushes the ends of the gates together firmly. The heaviest leaves weigh 662 t (730 ST; 652 LT); the hinges themselves each weigh 16.7 t (36,817 lb).
The original gate machinery consisted of a huge drive wheel, powered by an electric motor, to which was attached a connecting rod, which in turn attached to the middle of the gate. These mechanisms were replaced with hydraulic struts beginning in January 1998, after 84 years of service. The gates are hollow and buoyant, much like the hull of a ship, and are so well balanced that two 19 kW (25 hp) motors are enough to move each gate leaf; if one motor fails, the other can still operate the gate at reduced speed.
Each chamber also contains a pair of auxiliary gates which can be used to divide the chamber in two; this is designed to allow for the transit of smaller vessels — such as canal tugs — without using the full quantity of water. They were originally incorporated due to the fact that the overwhelming majority of all ships of the early 1900s were less than 600 ft (183 m) in length, and would therefore not need the full length of the lock chamber. Nowadays these gates are rarely used; instead, small boats such as tour boats, tugs, and yachts are passed in groups.
From the outset, it was considered an important safety feature that ships were guided though the lock chambers by electric locomotives, known as mulas (mules, named after the animals traditionally used to pull barges), on the lock walls. These mules are used for side-to-side and braking control in the rather narrow locks (narrow relative to modern day ships). Forward motion into and through the locks is actually provided by the ships engines and not the "mules". A ship approaching the locks first pulls up to the guide wall, which is an extension of the centre wall of the locks, where she is taken under control by the mules on the wall before proceeding into the lock. As she moves forward, additional lines are taken to mules on the other wall. With large ships, there are two mules on each side at the bow, and two each side at the stern — eight in total, allowing for precise control of the ship.
The mules themselves run on rack tracks, to which they are geared. Each mule has a powerful winch, operated by the driver; these are used to take the line in or pay it out, to keep the ship centred in the lock while moving it from chamber to chamber. With as little as 60 cm (2 ft) of space on each side of a ship, considerable skill is required on the part of the operators.
Smaller vessels, such as small tour boats and private yachts, are taken as handline transits, where mooring lines to the lock walls are handled manually by line handlers on the ship.
A failure of the lock gates — for example, caused by a runaway ship hitting a gate — could unleash a flood on the lands downstream of the locks, as the lake above the locks (Gatun Lake or Miraflores Lake) drains through the lock system. Extra safety against this is provided by doubling the gates at both ends of the upper chamber in each flight of locks; hence, there are four gates in each flight of locks which would have to fail to allow the higher level of water to pass downstream. The additional gates are 21 m (70 ft) away from the operating gates.
Originally, the locks also featured chain barriers, which were stretched across the lock chambers to prevent a ship from running out of control and ramming a gate, and which were lowered into the lock floor to allow the ship to pass. These fender chains featured elaborate braking mechanisms to allow a ship up to 10,000 tons to be safely stopped; however, given the precise control of ships made possible by the mules, it was very unlikely that these chains would ever be required. With many modern canal users being over 60,000 tons, and given the expense of maintaining them, the fender chains were reduced in number in 1976 and finally removed in 1980.
Beyond this, the original design of the locks had yet another safety feature — emergency dams which could be swung across the locks at the upper end of every flight. These consisted of swinging bridges, from which girders were lowered to the lock floor; steel shutters could then be run down these girders to block the flow of water. Monthly drills were held, by night and day, to make sure that these dams could be deployed in an emergency.
In the late 1930s, the original dams were replaced by new dams, which were raised out of slots in the bottom of the lock chambers, either hydraulically or by compressed air. The new dams were themselves retired in the late 1980s, and today, no emergency dams are in place.
Since all the equipment of the locks is operated electrically, the whole process of locking a ship up or down can be controlled from a central control room, which is located on the centre wall of the upper flight of locks. The controls were designed from the outset to minimise the chances of operator error, and include a complete model of the locks, with moving components which mirror the states of the real lock gates and valves. In this way, the operator can see exactly what state the locks and water valves are in.
Mechanical interlocks are built into the controls to make sure that no component can be moved while another is in an incorrect state; for example, opening the drain and fill valves of a lock chamber simultaneously.
The project of building the locks began with the first concrete laid at Gatun, on August 24, 1909.
The locks at Gatun are built into a cutting made in a hill bordering the lake, which required the excavation of 3,800,000 m³ (5,000,000 cubic yards) of material, mostly rock. The locks themselves were made of 1,564,400 m³ (2,046,100 cubic yards) of concrete.
The quantity of material needed to construct the locks required extensive measures to be put in place to handle the stone and cement. Stone was brought from Portobelo to the Gatun locks; the work on the Pacific side used stone quarried from Ancon Hill.
Huge overhead cableways were constructed to transport concrete into the construction at Gatun. 26 metre (85 ft) high towers were built on the banks of the canal, and cables of 6 cm (2.5 inch) steel wire were strung between them to span the locks. Buckets running on these cables carried up to six tons of concrete at a time into the locks. Electric railways were constructed to take stone, sand and cement from the docks to the concrete mixing machines, from where another electric railway carried two 6-ton buckets at a time to the cableways. The smaller constructions at Pedro Miguel and Miraflores used cranes and steam locomotives in a similar manner.
Concrete is normally moulded in formwork, temporary structures which give shape to the concrete as it sets. For a simple construction, these would normally be made quite simply of wood, but the scale of the locks demanded extraordinary forms.
The forms for the walls consisted of towers, fronted with braced vertical sheets, 19 cm (7½ inches) thick, mounted on rails to allow the locks to be constructed in sections; a section of lock would be poured behind the form, and when it was set, the form would be moved to do the next section. Each of the twelve towers was 23.8 m (78 ft) high by 11.0 m (36 ft) wide. The forms for the culverts were made of steel, and were collapsible so they could be removed and moved along after each section of culvert had set. There were, in all, 33 forms for the centre and side-wall culverts, each 3.7 m (12 ft) long; and 100 smaller forms for the lateral culverts.
The Pacific-side locks were finished first; the single flight at Pedro Miguel in 1911 and Miraflores in May, 1913.
The seagoing tug Gatun, an Atlantic entrance working tug used for hauling barges, had the honor on September 26, 1913, of making the first trial lockage of Gatun Locks. The lockage went perfectly, although all valves were controlled manually since the central control board was still not ready.