monitoring systems in maritime works

Carles Navarro Freixas

Civil EngineerTechnical Surveying Engineer

Xavier Lledó Fernández

Survey Systems Technician

MONITORING SYSTEMS IN MARITIME WORKS


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1. INTRODUCTION

2. BREAKWATER DIKE TYPES

a) Vertical Breakwaters

b) Mound Breakwaters

3. INITIAL STUDY

4. DIAGNOSIS METHODS

5. LAND MONITORING

6. INSTRUMENTS

a) GPS TRIMBLE R8

b) TRIMBLE S8 Total Station

c) TRIMBLE TX5 Laser Scanner

7. OBSERVATION METHODOLOGY

8. DATA PROCESSING

INDEX


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1-INTRODUCTION

The protection works of a port or coastal area bear on stone structures that shelter the internal waters from the energy of the waves. A structural failing on their part could entail material damage and even the loss of human lives, thus making it necessary to periodically check their state.

A thorough control requires determining what movements may have taken place over a certain time. These movements may be generated by possible settlements, such as foundations on clay areas, the degradation of the elements the breakwater is composed of or sea storms.

The latter, on average, constitute the greatest danger as they may lead to the absolute destruction of the breakwater without prior notice, as it were.

Thanks to new monitoring techniques, it is now possible to assess the state of preservation of the dike and thus foretell the maintenance of the infrastructure more efficiently.

Execution of the Works of the Protection Breakwater Dikes of Blanes Port (2010)


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2- BREAKWATER DIKE TYPES

a)Vertical breakwaters

These are composed of prefabricated concrete caissons and anchored in their definitive locations. Once they are anchored, the capping slab and shoulder wall are executed so as to prevent wave overtopping.

Cross-section of Vertical Breakwater

b) Mound breakwaters

These are protection works consisting in an embankment whose core is composed, in most cases, of ele-ments of loose materials while its outer coats are made of filters with increasingly larger diameters until the coats of the main layer are created. This layer consists of rockfills or concrete blocks of diverse geometries.

Cross-section of Mound Breakwater

SHOULDER

MAIN LAYERConcrete layers

ROCK FILLING

DREDGING AND FILLING

CORE


SLR

Hs, TpRc

d hs

SHOULDER


ROCK FILLING


CORE


SLR

Hs, TpRc

d hs


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3- INITIAL STUDY

All of the breakwater dikes are sized on the basis of the Wave energy that will impact its main body. Except for extraordinary storms and outports, the energy must not exceed the limits that may lead to a failing in the structure.

Even so, there might still be small tilting movements in Vertical Breakwaters and slight movements of the main layer in Mound Breakwaters. The sum of repeated impacts causes deterioration which, if not corrected, can weaken the structure until it ceases to function.

Effects of the hydrodynamic pushes against the Vertical Breakwater

When it comes to Vertical Breakwaters, the failing results from the hydrodynamic pushes against the main body of the dike. The effect of the energy of the waves on the vertical face may lead to the sliding, tilting or even sinking of the whole caisson.

Maritime engineering informs us that, in the case of Mound Breakwaters, a structural failing takes place on account of a space in the outer coat of the main layer that is large enough for an element of the second coat to exit. This would directly affect the outer filters, creating a hemorrhage-like effect, until the structure is made to collapse.

Structural failing of the Mound Breakwater

Due to this, it is necessary to carry out periodical check-ups of the elements the breakwater is composed of so as to establish the state of the protective elements. All of these interventions lie within the preventive actions performed to prevent possible collapses of the maritime works.

SHOULDER


ROCK FILLING


CORE


SLR

Hs, TpRc

d hs

SHOULDER


ROCK FILLING


CORE


SLR

Hs, TpRc

d hs


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4. DIAGNOSIS METHODS

There are different methods for assessing the state of preservation of the breakwater. These methods range from visual observation to modern control systems using pressure cells.

Thanks to the new geometric monitoring systems and the use of land laser scanning techniques, it is possible to obtain a high-precision “photograph” of the geometric state of the non-submerged element. The pur-pose is that of obtaining a high number of geometric data on the breakwater dike in order to determine if there have been partial or general movements between two series of measurements separated by a certain period of time.

Also of interest for the land survey are the bathymetries for offering topographic information on the sub-merged area and light detection and ranging (LIDAR) for completing part of the land survey.

5- LAND MONITORING

Land monitoring consists in the observation, measurement and analysis of the emerged area of the protec-tion works.

The purpose is that of detecting whatever movements have taken place on the part of the breakwater ele-ments above the sea by means of land survey methods.

Since many data are required for determining the partial movements of some of the blocks the breakwater is composed of, the laser scanning method is included.

Laser scanning makes it possible to detect possible movements in some of the blocks or even a global con-solidation process.

Once the scanning has been obtained, one can combine its information with other data such as the geome-try of the bottom of the sea, the type of terrain, photogrammetric surveys, among others.

3D survey of the East Breakwater head of the Port of Barcelona


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6- INSTRUMENTS

GPS TRIMBLE R8 TRIMBLE S8 Total Station TRIMBLE TXS Laser Scanner

a) GPS TRIMBLE R8

One must establish starting points and end points by means of Post-Processing mode techniques to deter-mine possible global movements of the whole. These bases will serve as reference points for the network of bases to be established on the breakwater.

B) TRIMBLE S8 1” TOTAL STATION

A high-range instrument is required, one such instrument being the 1” angular-precision Trimble S8 Total Station for determining the polygonal network of the points of support for the Laser Scanner.

The accuracies of this instrument are at a sexagesimal second angular accuracy and 1 mm plus 1 ppm (part per million) of the distance taken, which means that, if a reading is made at a length of 100 m, one will most probably obtain a target of points as follows:

• (Angular) transverse deviation = 100m x Tg (1”) x √2 ≈ 0.7mm

• (Distance meter) longitudinal deviation = 1mm + 1 x 0.1 ≈ 1.1mm

C)TRIMBLE TX5 LASER SCANNER

The TRIMBLE TX5 Laser is a high-performance scanner that can record up to 976,000 points per second, ob-taining precisions of 1 mm at 10 m and 2 mm at 25 m, up to a range of 120 m.

This device includes additional sensors that make it easy to fit the point clouds in their corresponding coor-dinates, such as the dual-axis compensator, altimeter and electronic compass.

It also has a photographic camera in the optics for capturing images and applying textures to the 3D models generated.

The laser scanner is undoubtedly the ideal device for metrically digitalizing reality as quickly as possible.


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7- OBSERVATION METHODOLOGY

First, one must establish a network of reference bases to determine if there has been a global movement in the overall breakwater.

In order to ensure an absolute positioning, the references must be established by means of a GPS from a reference antenna, located in a stable area, reaching the limits of the breakwater to be controlled.

It is advisable, at each one of the extremes, to implant a total 6 references that will serve as a support for the reverse intersection* to resolve the closure of the polygon. The bases will be set up in Post-Processing mode with a minimum reading of 30 minutes and a standard deviation of less than 1 cm.

Once the references have been calculated, one will start with the Total Station at a reverse intersection of the polygon and then carry out the itinerary of the intermediate points with sections of approximately 200 m until reaching its other extreme, thus closing with the other reverse intersection. The closure deviations must be of less than 1.5 cm√ Km.

Once the main polygon is finished, one will have to establish a set of radiated points distributed into groups of three with the same profile and these will serve as an orientation for the Laser Scanner. Each group must have a spacing of 25 meters and will be used for obtaining the points for supporting the scanning.

With the bases referenced, the next step will be that of surveying by means of laser scanning, positioning the instruments between the support points.

Main Network of Bases

Support Points of the Laser Scanning

*Reverse intersection: A survey method using the Total Station and consisting in taking angular and dis-tance readings of a set of established points located around the observation point in order to determine the position and orientation of the instrument.

200 m. 200 m.200 m. 200 m. 200 m. 200 m.200 m.200 m.

200 m. 25 m. 25 m. 25 m.200 m.200 m. 25 m. 25 m.

200 m. 200 m.200 m. 200 m. 200 m. 200 m.200 m.200 m.

200 m. 25 m. 25 m. 25 m.200 m.200 m. 25 m. 25 m.


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8-DATA PROCESSING

The main objective of the processing is that of obtaining refined and referenced point clouds in order to analyze and inspect them so as to quantify and document the changes that may have taken place in the structure during the period between the two Monitoring campaigns of the Breakwater.

In order to carry out the fitting and referencing of the multiple scannings in their corresponding coordi-nates, one must use the Trimble SCENE application. Thanks to this, one can improve the point clouds by applying filters based on the intensity of the measurement recorded and on the scattering of the point. One may also apply color to the points by using the images that have been automatically recorded by the Trimble TX5 scanner.

Moreover, Trimble SCENE makes it possible to automatically detect reference objectives, such as targets or spheres located on known points, captured during the reading of the surroundings, and thus assign the coordinates obtained of the radiated points from the polygon. Each scanning, which will be treated inde-pendently, will be located in its corresponding coordinate by using an average of four objectives, two being the minimum for carrying out the fitting.

Image of original point cloud

Once the refined point cloud has been obtained, one must then analyze and inspect it by means of the potent office application Trimble RealWorks, which was especially designed for treating laser scanner data.

By means of the 3D inspection tool, a comparison will be made between the different Monitoring campaigns so as to see if movements have taken place. This tool generates 3D color maps with a legend, on which one will be able to appreciate the movements in accordance with the color changes. The legend will help one to quantify these movements with the added advantage of quickly and accurately analyzing large surfaces.


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Image of 3D inspection map

Once the areas with noticeable movements have been determined, one will have to use the tool for inspec-ting twin surfaces. This tool makes it possible to carry out a more detailed analysis of the variations of the elements the breakwater is composed of. For this purpose, datum planes will be used to generate transverse and length profiles, ground plans and isometric curve maps. It will thus be possible to detect and quantify the movements of the breakwater.

Image of map for inspecting twin surfaces


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Additionally, it will be possible to individually deal with the singular elements the Breakwater is composed of, such as the concrete blocks or the rockfills of the main layer. In order to determine these movements, the software has a 3D modeling tool with which to adjust basic models, such as the cube in relation to the total or partial area of a block that has been scanned. Once the 3D models of the “before and after” of the block have been obtained, the differences will be demarcated by calculating the directions of the movements.

Image of a comparison between 3D models

Finally, the results obtained will be exported to compatible formats in the interests of a detailed study of each one of the part of the Breakwater, generating general reports of comparative profiles, demarcations, 3D meshes, point clouds, orthophotos, videos, CAD and ASCII files, among others, in order to determine the actual state of the Maritime Works as accurately as possible.

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monitoring systems in maritime works

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