Almost everyone is familiar with the impressive sea locks at IJmuiden, Terneuzen and Antwerp and the iconic lock complexes that separate the famous Panama Canal from the Atlantic and Pacific Oceans. There are around sixty sea locks worldwide, which is few compared to the number of locks on the world's waterways. The exact number is unknown but is probably in the hundreds of thousands. The Netherlands alone has almost two thousand locks, of which some 140 form part of the main waterway network and are managed by the Directorate-General for Public Works and Water Management (Rijkswaterstaat).
 
In all cases, locks play a crucial role in water management and navigation by providing a level separation (and therefore water retaining) function in combination with the lock function, enabling vessels to navigate through differences in water levels or neutralising the effect of tides for them.
 
For this purpose, locks are equipped with lock gates, of which the most common types are lift gates, rolling gates and mitre gates. Lift and rolling gates can be single or double retaining, which means that high water levels can be accommodated on both sides of the lock gate. Mitre gates, on the other hand, are mostly single retaining.
 
Regular inspection and maintenance of these lock gates and the associated operating mechanisms are crucial, as otherwise very undesirable (socially) or even dangerous situations can arise. An example of this is the incident at the Eefde lock in 2012, where a lift gate fell in front of a vessel, fortunately not causing any injuries but causing considerable damage and disruption to navigation for a long time. Occasionally, a lock gate can be involved in a collision, resulting in navigational disruption and significant economic damage. The reliability and availability of locks are essential for both navigation and the economy. This applies not only to unexpected incidents but also to planned maintenance. Therefore, any disruption to navigation during maintenance work on the lock gates must be kept to a minimum. To achieve this, Iv-Infra has applied its many years of experience in the underwater measurement of the vital components that must result in a first-time perfectly fitting new or renovated lock gate.

From a sustainability perspective, repairing or renovating the gates is preferable to replacing them. In doing so, we reduce the circle of the Ellen MacArthur Foundation's well-known circularity model (the 'butterfly diagram'), thereby extending the lifespan of the lock. Iv-Infra uses this circularity principle as a key principle for its designs and asset management advice. In addition, we have already demonstrated in a previous study that replacing lock components, including lock gates and their operating mechanisms, up to five years before the end of the civil structure's technical life is not only preferable from a sustainability point of view but also pays off in terms of lifespan costs.

Underwater measurement and 3D modelling of the relevant lock gate components is no easy task and requires a great deal of knowledge and expertise. We call this 'knowledge is measurement'. Thorough knowledge of the different gate types is essential for smart lock gate measurement.

Lift gates
A lock with lift gates is easily recognised by its impressive tall lift towers. The lift gate is a vertically movable lock gate that rises with each lock movement. A vertical guide rail system is mounted on the lift towers on either side of the lock head. These, together with the guide roller mounted on the gate, ensure smooth up and down movement. The lift gate operating mechanism is located at the top of the traverse connecting the two lift towers. Watertightness is secured by the correct sealing of the lower end post positioned on top of the cill and the vertical end posts in the recesses on either side of the cill. In the case of double retaining gates, these recesses are on either side of the lock gate. It is, therefore, necessary to determine the exact position of the various components of the lock head and lock gates. For the lock head, these components are the cill, the vertical bearings, the guide rails, the operating mechanism and any guides for the counterweight. For the lock gates: the lower and vertical end posts, the guide rollers and the engagement points of the operating mechanism.

Rolling gates
The key feature of a lock with rolling gates is the presence of gate recesses adjacent to the lock chamber. The gate moves into this side chamber in the open position. The operating mechanism is primarily located in the mechanism chamber at the far end of the gate chamber. The watertightness of rolling gates is comparable to that of lift gates but is only guaranteed at the bottom by a raised cill. The principle of double-retaining is also common to this type of lock gate. It is also necessary to accurately determine the position of the various components of the lock head and lock gates. For the lock head, these are the cill, the vertical bearings, the lower rails and any upper rails, the operating mechanism and the associated guide systems. For the lock gates: the lower and vertical end posts, the lower and upper rollers, if any, and the engagement points of the operating mechanism.

Mitre gates
Locks with mitre gates can be identified by the open-swinging gates, which are positioned at an angle of one-third in the lock chamber when closed. The hinged side of the gate is integrated into the chamber wall allowing it to be concealed in the gate recess when open. The rotational axis of this type of gate consists of a lower bearing, called the pintle, and an upper bearing, called the gudgeon.
There are two types: mitre gates with a fixed pivot point and gates with a free pivot point. The watertightness of mitre gates is ensured by the so-called 'watertight frame', which consists of the cill, side and front bearings. In addition, force transmission is an essential element of mitre gates. For fixed pivot points, these are the pintle and gudgeon pin and for free pivot points, the rear bearings.

To obtain a correctly functioning lock head, it is even more critical to determine the position of the various components of the lock head and the lock gates. For the lock head, these components include the cill, the exact position of the cill edge, the rear and side bearings, the pintle and gudgeon and, to a lesser extent, the operating mechanism. For the lock gates, these components are the lower, side and rear end posts in relation to the axis of rotation formed by the centres of the pintle and gudgeon pin.
 

Iv-Infra has developed a 3D data capture method with millimetre accuracy for all components underwater. Highly accurate surveying equipment is used, Leica TDRA6000/TS60 industrial tachymeters with an angular accuracy of 0.5'' and a distance accuracy of < 1 mm. Auxiliary equipment in the form of a measuring frame is also used to measure the cill of all gate types. By accurately measuring the part of the frame above the water, its underwater position can be determined, providing the 3D position of the cill with millimetre accuracy.

A high precision industrial spirit level determines the position of all vertical components underwater. The level is positioned underwater by a diver, and the measurements are recorded above water by the surveyor, thereby providing the 3D position of the vertical components.

During the execution of these measurements, continuous support is provided by a dive team consisting of divers with many years of experience in lock gate measurement.

The measurements from the lock head are processed into a 3D model. This 3D model is then used to determine the extent to which the corresponding lock gates can be made to fit and requires a complete 3D survey to be carried out before the lock gates can be installed.
The 3D survey is performed with the lock gates set up vertically in the gate storage area or at a temporary storage location. In the latter case, the gates are positioned horizontally, requiring torsion-free operation. The latter is particularly relevant for mitre and lift gates.
By integrating the 3D models of the gates into the model of the lock head, the end posts of the various types of lock gates can be manufactured to size. The best way to do this is to build the timber into the lock gate with additional thickness so that it can be planed to size after marking.

As everything has been measured in advance and the gates have been made to measure, any hindrances in replacing the lock gates can be kept to a minimum. In addition, any defects and/or damage are identified during the measurement. For example, it can be decided in advance whether drying or partial drying using (maintenance) limpet dams is necessary.

To ensure that renovated lock gates and all associated components continue to function properly for years after renovation, Iv-Infra has developed a control system to monitor them. The key factor is the correct functioning of the gate movement during opening and closing.

Although this is different for each type of lock gate, the principle is always the same: the lock gate must make the correct 3D movement during the gate movement in accordance with the original design. If it does not, long-term damage is inevitable. All the different components can be mapped by performing a high-frequency 3D measurement of the gate movement. This includes the opening and closing of rolling gates and their upward thrust due to water pressure, the opening and closing of lift gates and their upward thrust, and the pivoting movement of mitre gates and their upward thrust in the case of a free pivot point.

For us, measurement and monitoring are not limited to the abovementioned checks. We have developed the DREAM® method to make the best possible contribution to smart circular locks: Data-driven and Risk-driven Enterprise Asset Management. Through this method, we analyse all possible failure mechanisms of the lock. Based on the measured data, we can closely monitor the lock gates' condition and (failure) behaviour and predict, albeit with some uncertainty, the expected failure moment for each of the possible failure mechanisms. We then use this information to optimise the inspection, monitoring and maintenance of the locks, taking into account the well-known triangle of performance, risk and cost.