Comparison of different dumping methods for breakwater ... · Comparison of different dumping...

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Comparison of different dumping methods for breakwater construction P. Rouault, M. Nehrig, J. Meng & T. Stückrath Department of Hydraulic Engineering, Technical University Berlin, Germany Abstract Breakwaters are essential constructions in coastal engineering for the protection of coasts, human lives and infrastructure and are key components in port design. This contribution will focus on rubble mound breakwaters. There are different methods to construct rubble mound breakwaters by dumping rocks of varying rock size distribution. The choice of the construction method mainly depends on the local site conditions, that is the workflow of the construction. In general practice, a homogenous grain-size distribution of the breakwater core is assumed and the workflow of the construction is not considered. In this paper, three different methods of construction are compared, taking into consideration the actual heterogeneous grain-size distributions in the breakwater cores as well as the workflow of the construction. These three methods are: Land-based construction by dump truck Sea-borne construction by barges Sea-borne construction with fall-pipes by barges The three methods have been examined intensively, experimentally and theoretically at the Department of Hydraulic Engineering, TU Berlin. The projects were granted by the German Research Foundation DFG. A large number of physical experiments were carried out on different scales with focus on the dumping process and on the resulting grain-size distribution in the structure’s core. The falling behaviour of the rocks was investigated in a basin (1 m depth), a fall-tank (6 m depth) and a special facility with up to 20 m depth. The results were statistically analysed and compared with numerical simulations which were developed for these projects. Furthermore, the segregation of the rocks in the resulting rubble mound core was analysed and statistically evaluated. In this paper, the three methods are compared by discussing advantages and disadvantages. Finally, recommendations for designing and dimensioning breakwaters are given to improve the quality of future coastal structures. Keywords: rubble mound breakwater, core construction, grain size segregation. © 2005 WIT Press WIT Transactions on The Built Environment, Vol 79, www.witpress.com, ISSN 1743-3509 (on-line) Maritime Heritage and Modern Ports 459

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Comparison of different dumping methods for breakwater construction

P. Rouault, M. Nehrig, J. Meng & T. Stückrath Department of Hydraulic Engineering, Technical University Berlin, Germany

Abstract

Breakwaters are essential constructions in coastal engineering for the protection of coasts, human lives and infrastructure and are key components in port design. This contribution will focus on rubble mound breakwaters. There are different methods to construct rubble mound breakwaters by dumping rocks of varying rock size distribution. The choice of the construction method mainly depends on the local site conditions, that is the workflow of the construction. In general practice, a homogenous grain-size distribution of the breakwater core is assumed and the workflow of the construction is not considered. In this paper, three different methods of construction are compared, taking into consideration the actual heterogeneous grain-size distributions in the breakwater cores as well as the workflow of the construction. These three methods are:

Land-based construction by dump truck Sea-borne construction by barges Sea-borne construction with fall-pipes by barges

The three methods have been examined intensively, experimentally and theoretically at the Department of Hydraulic Engineering, TU Berlin. The projects were granted by the German Research Foundation DFG. A large number of physical experiments were carried out on different scales with focus on the dumping process and on the resulting grain-size distribution in the structure’s core. The falling behaviour of the rocks was investigated in a basin (1 m depth), a fall-tank (6 m depth) and a special facility with up to 20 m depth. The results were statistically analysed and compared with numerical simulations which were developed for these projects. Furthermore, the segregation of the rocks in the resulting rubble mound core was analysed and statistically evaluated. In this paper, the three methods are compared by discussing advantages and disadvantages. Finally, recommendations for designing and dimensioning breakwaters are given to improve the quality of future coastal structures. Keywords: rubble mound breakwater, core construction, grain size segregation.

© 2005 WIT Press WIT Transactions on The Built Environment, Vol 79, www.witpress.com, ISSN 1743-3509 (on-line)

Maritime Heritage and Modern Ports 459

1 Introduction

Rubble mound breakwaters were constructed for hundreds of years. The design traditionally was based on empirical knowledge. Modern design recommendations are given by the Shore Protection Manual [1] (Figure 1).

Figure 1: Design recommendation of rubble mound breakwaters by Shore Protection Manual.

Due to heavy storms, the breakwaters are often damaged, as seen at the break-waters of Sines or Arzew el Djedid. There are many different kinds of damages and structural failures and repairs are problematical and costly. The manifold reasons for the damages are a design-exceeding wave load or initial design flaws. That is why there is recently much scientific effort in the field of breakwater construction.

Figure 2: Bulk rock dumping by side dumping vessel, split barge and fall-pipe, CERC [2]. Land based dumper, Bruun [3].

The actual non-homogenous composition of the core of rubble mound breakwaters consisting of non-uniform grain size were proved by the laboratory tests. Design and construction practice however assume, that the core has a

© 2005 WIT Press WIT Transactions on The Built Environment, Vol 79, www.witpress.com, ISSN 1743-3509 (on-line)

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homogenous grain size distribution. Segregation is assumed but normally remains undetected until unexpected settlements or even structural failures occur. Research work focused on the construction method of rubble mound structures consisting of non-uniform grain size. The aim was to describe the dumping process to define the distribution of the grain in the resulting structure. The following three dumping methods were investigated (Figure 2):

Land-based construction by dump truck Sea-borne construction by barges Sea-borne construction with fall-pipes by barges

The following questions were investigated and the main results are presented: Does vertical grain size segregation occur during the falling process? Does horizontal grain size segregation occur in the core structure? Do these segregation processes pose a threat to the resulting structure’s

stability, design and settlement? Which are each method’s advantages and disadvantages?

2 Experiments

Most of the experiments were carried out at the hydraulic lab in Berlin with depth up to 6 m. For depths up to 20 m experiments were carried out at a military testing site. The core material was an angular broken greywacke (ρ = 2,66 g/cm3). The rock properties fulfil the standards of CERC [2]. Rocks with diameters from 2 to 180 mm were used. The rocks were coloured according to grain size for better visualisation. All experiments were made with the same grain size distributions. An example is shown in figure 3.

2 10 400

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]

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Figure 3: Example of an examined grain size distribution.

The experiments were documented by digital imaging. Samples were collected to determine segregation tendencies of the falling bulk material. The resulting rubble mound structures were cross-sectioned. Layer-by-layer sieving analysis was carried out to determine the amount of grain size segregation.

2.1 Land-based construction by dump truck

The experiments were conducted in a basin of 1,50 m depth. The core was dumped on an installed slope (Figure 4) by a model truck.

© 2005 WIT Press WIT Transactions on The Built Environment, Vol 79, www.witpress.com, ISSN 1743-3509 (on-line)

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The varied parameters were: Material composition: bulk material with four different size

distributions. Material size with 3 model scales Dumping depth, dumping height, dumping rate.

Figure 5 displays a typical underwater slope with its vertical segregation: the big rocks are at the base of the slope, the fine ones in the upper sections.

Figure 4: Rock Distribution of an underwater dumped core [4].

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Figure 5: Grain size distribution of a typical underwater slope compared with the original grain size distribution of the dumped material [4].

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The slope was separated in 6 horizontal layers of equal volume. To determine the grain size distribution of each layer, the samples were sieved and the fractions weighed. The same experiment was repeated to determine the degree of validity.

2.2 Sea-borne construction by barge

The freely falling bulk rock experiments were documented by video and grain samples were taken from the falling bulks of rocks in different depths and different radial distances from the centre. The observations showed, that the process of bulk rock dumping is clearly distinguishable into 3 phases:

Phase I: initial stage - dropping ‘en bloc’. Phase II: the large block dissolves, the bulk begins to spread. Phase III: all rocks fall individually.

Figure 6: Video capture of bulk rock material in water [5]. Left: release of 100 kg from the surface (phase I). Middle: approaching the probing device at 5 m (beginning phase II). Right: approaching the probing device at 10 m (phase III).

2 10

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layergrading

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Figure 7: Typical result of layer-by-layer sieving analysis [5]: Larger rocks

tend to stay in the top layer.

The falling process is dominated by a distinctive en-bloc movement: in the initial phase the bulk of rocks move as a single large object at high acceleration. This enables the bulk to gain depth significantly with nearly no grain size segregation. The examined rubble mound structures displayed a different picture: the slopes’ top layer and toe section are composed of grain-size distributions significantly more coarse than initially dumped.

© 2005 WIT Press WIT Transactions on The Built Environment, Vol 79, www.witpress.com, ISSN 1743-3509 (on-line)

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2.3 Sea-borne construction with fall-pipe

To determine the effect of grain size segregation using a fall-pipe, experiments were carried out in the falling tanks with a depth up to 6 m in various scales (Figure 8). The falling process was observed by video and examined by self-coded software programs. The resulting rubble mound structures were examined visually, then the grain size distribution of the sections was determined.

Figure 8: Experiments on grain size segregation. Left: Large tank with 6 m depth. Right: After the Falling process the material is completely segregated.

Figure 9: Experiments on moving the pipe and the slope. Left: Movable fall-pipe with stopped grains inside. Right: After moving the pipe, segregated areas remain segregated in the resulting structure.

Using the fall-pipe the grains are stopped after the falling process at the end of the pipe (Figure 9, left). Then the pipe is moved parallel to the pipe’s axis. Thus the grains are dispensed leaving the pipe on a small slope of grain, which has no effect on the grain size segregation (Figure 9, right).

© 2005 WIT Press WIT Transactions on The Built Environment, Vol 79, www.witpress.com, ISSN 1743-3509 (on-line)

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The experiments showed that the resulting structure can only become homogeneous by a continuous input of grain into the fall-pipe. Every starting or stopping of the input flow and even every change of the flow rate will result in inhomogeneous regions in the resulting structure (Figure 8).

3 Comparison of the three dumping methods

The conducted experiments allow a valid comparison and overall evaluation of the investigated construction methods. For this purpose, we focused on the construction process and on the structure’s quality as the two main criteria for a successful engineering task. This can be a decision support to choose the appropriate dumping method depending on local boundary conditions.

Table 1: Evaluation of the three dumping methods concerning the Construction process.

Trucks Barges Fall-pipe Efficiency O +++ ++ Independency from sophisticated infrastructure

+++ ++ +

Temporal and spatial independence from on-site workflow

O +++ ++

Know-how +++ ++ + Independency from weather conditions +++ ++ +

Independency from water depth +++ O O

Table 2: Evaluation of the three dumping methods concerning the quality of the structure.

Trucks Barges Fall-pipe Construction accuracy ++ O +++

Low amount of required reworking effort

++ O +++

Low segregation at the structure bottom

O ++ +++

Low segregation in the structure +++ ++/+++ +++ *

*: This could only be reached by dumping continuously.

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Table 3: Evaluation of the three dumping methods concerning the consequences of the method for the structure.

Trucks Barges Fall-pipe Degree of Reliability without recommendation consideration

O ++ +

Degree of Reliability with recommendation consideration

+++ +++ +++

3.1 Evaluation of the three dumping methods

The three dumping methods are compared in Tables 1–3: the “O”-symbol indicates a disadvantage for the construction process. By “+” up to “+++”an increasing degree of advantage is indicated.

3.2 Recommendations for the design and the construction process

Due to the interpretation of the scientific examinations it is possible to improve every investigated dumping method for enhancing the structures` quality.

Figure 10: Recommendation for limiting the bulk mass when dumping with a barge [5].

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Trucks: For the planification the core segregation must be taken in account. At the base the core is 50% coarser than the originally dumped material. The consideration of this actual grain size distribution is important for the design of the filter between the core and the seabed. Barges: To avoid any vertical segregation, the dumped bulk mass rate can be limited depending on the dumping depth the rock size and shape. Fall-pipe: At the construction site the dumping process must be kept at a continuous flow rate. Any interruption of the process causes a material segregation in the core structure! At the beginning and at the start of a dumping process the material is inevitably segregated. There are two possibilities to avoid damages in the structure: one is to dump this pre-material at another place, the other one is to dump this material into the structure and to take it in account for the design.

References

[1] Shore Protection Manual (2001): Coastal Engineering Research Center. U.S. Government Printing Office, Washington, Draft.

[2] CERC Centre for Civil Engineering Research and Codes (2000): “Manual on the use of Rock in Hydraulic Engineering”, A. A. Balkema, Rotterdam, Brookfield.

[3] Bruun, P. (1985): „Design and construction of mound for breakwaters and coastal protection”, Development in geotechnical engineering, Vol. 37, pp 549-561.

[4] Rouault, P. (2004). “Die Entmischung bei der Schüttung von Wellenbrecherkernen mit Hinterkippern”, Mitteilungen des Instituts für Wasserbau und Wasserwirtschaft der TU Berlin, Nr. 138.

[5] Meng, J. (2004): „Schwarmverhalten von Steinschüttungen in Wasser“ Mitteilungen des Instituts für Wasserbau und Wasserwirtschaft der TU Berlin, Nr. 139.

© 2005 WIT Press WIT Transactions on The Built Environment, Vol 79, www.witpress.com, ISSN 1743-3509 (on-line)

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