Aspects related to design and construction of breakwaters in deep water by Hans F. Burcharth Aalborg University, Denmark

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1 Aspects related to design and construction of breakwaters in deep water by Hans F. Burcharth Aalborg University, Denmark Contents of presentation Introductory characterization of the environment Rubble mound breakwaters Armour placement, reallocation and settlements Armour stability Crane capacity Toe stability Construction roads Rear slope stability Caisson breakwaters Determination of wave loadings Safety of rubble mound and caisson breakwaters New Breakwater at Punto Langosteira, La Coruña 1

2 Introductory characterization of the field Environmental conditions Water depth 20 m Exposed locations facing the ocean giving large and long design waves Wave climates Frequent storms, always some wave disturbance during construction (generally seasonal) Rare (infrequent) storms, generally very little wave disturbance during construction (typical for some tropical zones) The main difficulties are related to the construction and depends on the environmental conditions. The design should minimize the difficulties. 2

3 Rubble Mound Breakwaters Usual specifications for placement of main armour Case 1 Bulky units like cubes placed in two layers 1. Random placement specified as positioning (x, y) in accordance with a defined grid, ± m. 2. Number of units N ± X % within a given area A. 3. Porosity P% ± X % within a given area A. 4. Layer thickness t m and tolerances ± X m within a given area A. 3

4 Comments: ad. 1. Symposium Design and Construction of Deep Water Maritime Works, Gijon, Spain, 2007 Random placement Means random orientation. The term random placement is used by designers only to distringuish from regular (pattern) placement. The degree of random orientation is inherent in the defined set of N, P and t. The accurate position of a block when placed is not known. - only the position at the moment of hook release. Visual checking or (if not possible) advanced sonar measurements are needed if more close control is needed, but generally control of N, P and t should besufficient if A is not defined too large. ad. 2. Number of units, N Generally no problems in fulfilling N. ad. 3. Porosity, P Given N then P depends only on t. ad. 4. Layer thickness t t is always defined in drawings (theoretical layer thickness) but cannot be verified on site unless a method of measuring the layer surface is given. 4

5 Link between porosity P and layer thickness t Porevolume concretevolume / area P = = 1 total volume t Increasing surface roughness and permeability (and settlements) Decreasing run-up and overtopping (and stability) The layer thickness determines the porosity (degree of random orientation) when the number of blocks per area is given. Their tolerances are linked. 5

6 ad. 1-4 The tolerances given in the technical specification should reflect the safety margin of the design. A small safety margin demands smaller tolerances. Design of large structures is based on model tests. The block placement and the related accuracies applied in the model should correspond to the project specifications or be more relaxed in the model. On very exposed locations I recommend to deliberately built-in irregularities like cavities in the models, and base the design on the performance of such models. Regular placement (pattern placed) like a pavement is easier to construct than irregular placement because the first layer of cubes tends to lay on a flat side on the underlayer. The consequence is a more smooth surface which gives more overtopping. On the other hand, the hydraulic stabillity of the armour increases (very high stability can be obtained if the boundaries are intact). 6

7 Case 2 Single layer of complex interlocking armour on steep slopes. Compared to placement specifications for bulky units the specifications are more restrictive with respect to orientation of the units in order to ensure stability. Therefore, I do not recommend such armour in exposed places where visual underwater inspection by divers cannot be performed almost continously during placement of the armour units. 7

8 Settlement of armour layers Settlement caused by wave action cannot be avoided. Contributing to armour settlement can be Compaction of under layers (vertical) Sliding of armour on under layers Sliding of armour blocks relative to each other Deformation of supporting toe The higher and steeper the slope, the larger settlements (SOGREAH limits the height of Accropode armour to 20 rows). The higher the initial porosity, the larger settlements. The smoother the under layer (wide gradation, relative small stone sizes) the larger settlements. 8

9 Settlements generally cause opening (cavities) in the middle to upper part of the slope. dddddd 1:28.5 scale model of proposal for main cross section of Punto Langosteira Port Breakwater, La Coruña (CEDEX 2007). Main armour placed by crane on the slope. Pattern placed on upper berm. 150 t cubes in two layers except 50 t cubes in three layers in six bottom rows. Toe berm of 5 t quarry rock. Armour layer after exposure to design waves. (Hs = 15 m). 9

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11 The major part of settlements should preferably occur in the construction period by occurrence of wave action of some severity (but not damaging) in order to avoid repair by refilling after construction (might be almost impossible due to lack of space in the cavities and due to very large mobilization costs). Armour layers with good self healing ability (generally two-layers) are to be preferred, especially in climates where severe wave actions are so rare that settlement-waves cannot be expected to occur during construction. Settlements cannot be studied quantitatively in models due to severe scale effects. 11

12 Influence of limited crane capacity on toe design 12

13 Reduction of crane capacity by use of high-density armour units in roundheads. Researcher Armour Weight of roundhead armour Weight of trunk armour Jensen Tetrapods 2.3 (1984) Vidal et al. (1991) Cubes Madrigal (1992) Burcharth et al. (1995) Berenguer (1999) Symposium Design and Construction of Deep Water Maritime Works, Gijon, Spain, 2007 Parallelepipeds Accropods Dolos Holowed cubes Antifer Number of displaced cubes in 180º sector % 5 % 1% Normal density cubes =2.40t/m 3, W= 150t Significant wave height Hs [m] High density cubes =2.80t/m 3, W= 180t 13

14 Roundhead design by use of high mass density blocks Block weight in the most critical sector of roundhead must be app. double of block weight in trunk. Double crane capacity needed for placement in roundheads if mass density is not changed. Solution: Example: Increase mass density of blocks placed in the critical sector. Hudson formula p N s H s = = s 1 Dn w ( K cot ) 1/3 Hs = 15 m, T = 20 s, crest level +25 m, slope 1:2 (cotá = 2) D Trunk 150 t cubes, 4x4x4 m, ñ = 2.40 t/m, K = 10.9, N = s D s 300 t cubes, 5x5x5 m, ñ s = 2.40 t/m Roundhead 3, K D = 5.59, N s = t cubes, 4x4x4 m, ñ s = 2.74 t/m, K D = 5.59, N s =

15 ROUNDHEAD ARMOUR STABILITY Normal density, regular placement, waves from NW, water level +4.5 m H s = 14.2 m H s = 13.2 m 15

16 ROUNDHEAD ARMOUR STABILITY High density, regular placement, waves from NW, water level +4.5 m H s = 14.3 m H s = 13.2 m 16

17 Comparison of normal and high density armour stability Random placement Water level +4.5, Waves from NW Number of displaced cubes % 5% 1% Normal density cubes, 154 t High density cubes, 179 t Hs [m] Design wave condition 17

18 ROUNDHEAD ARMOUR STABILITY High density, regular placement, waves from NW, water level

19 New stability formula for cube armoured roundheads (Maciñeira and Burcharth 2004) H s R = e nm cot g D D n 0. 2 % S 0. 4 op S op R nm = radius at SWL in numbers of D n 19

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23 Construction roads (Landbased equipment) Criteria lorries Width sufficient for crane operation and passing dumpers, trucks and Level sufficiently high to avoid damaging overtopping (person, materiel, road surface) during the defined limiting sea states. Sufficient hydraulic performance 23

24 Construction roads Levels 24

25 Design for construction Example: Determination of level and exposure of construction road for land based equipment. SWL Run-up wedge Run-up Internal water table Temporary road +1.5m Beirut Airport breakwater Illustration of run-up on Antifer blocks 25

26 Influence of crest width on rear slope stability Splash down from the large overtopping waves hits slope instead of water surface Rear slope stability a problem if settlement occur Hollowed blocks for rear slope armour 26

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28 Spatial Distribution of Overtopping Formula by Lykke Andersen & Burcharth, 2006 Ratio of overtopping passing travel distance x at splash down level h level : q = exp - ( 0.15 x / cos() h s, ) passing x max level 0p s0p qtotal L 0p where is the angle of incidence x(hlevel=0) H hlevel x(hlevel=h) x 28

29 Temporary construction road with high crest level 29

30 Optimum safety levels in design of breakwaters Main types of breakwaters and typical damage development Damage Damage Hs Hs Design wave conditions and optimum safety levels depend on the damage development 30

31 International standard Organization ISO New standard ISO Actions from waves and currents on coastal structures 7.2 Reliability assessment of structures Structures subject to the actions from waves and currents should be assessed for their reliability at the serviceability and ultimate limit states with due consideration for their economic and social functions, environmental influences, and the consequences of failure. The nature and extents of the uncertainties in Subclause 7.1. should be duly taken into account when assessing the reliability of structures during their design working life. The probability of failure during the design working life should preferably be assessed and confirmed to be less than the minimum value assigned to a specific class of structure, which is to be preset or approved by responsible agencies. The probability of failure may be evaluated by the use of reliability index method or with direct calculation by numerical integration of their probability density functions or Monte Carlo simulations. For a structure that permits a certain degree of deformation at the serviceability and ultimate limit states, the expected amount of deformation should preferably be evaluated. 31

32 Example of safety levels specified in Spanish Recommendations for Maritime Structures ROM 0.0 Economic repercussion index (ERI) (cost of rebuilding and downtime costs) Low economic repercussion ERI < 5 Moderate economic repercussion 5 < ERI < 20 High economic repercussion ERI > 20 Social and environmental repercussion index (SERI) No social and environmental repercussion impact SERI < 5 Low social and environmental repercussion impact 5 < SERI < 20 High social and environmental repercussion impact 20 < SERI < 30 Very high social and environmental repercussion impact SERI > 30 32

33 From ERI is determined service lifetime of the structure ERI < > 20 Service life in years From SERI is determined maximum overall probability of failure within service lifetime, Pf SERI < >30 Serviceability limit state (SLS) Ultimate limit state (ULS)

34 Example a large breakwater in deep water protecting a container port and/or berths for oil tanker would have ERI around 20. This means 50 years service life time. SERI might be low corresponding to 5 < SERI < 20 giving the Pf values SLS 0.10 in 50 years ULS 0.10 in 50 years How does this fit with economical optimization? 34

35 Objective of present study To identify the safety levels related to minimum total costs over the service life. This includes capital costs, maintenance and repair costs, and downtime costs. Capitalized costs (present valu Optimum safety level Total costs Construction costs Maintenenance, repair and economic loss due to downtime etc. Safety of breakwater 35

36 Studied influences on optimum safety levels Real interest rate, inflation included Service lifetime of the breakwater Downtime costs due to malfunction Damage accumulation ISO prescription The ISO-Standard 2394 on Reliability of Structures demands a safety-classification based on the importance of the structure and the consequences in case of malfunction. Also, for design both a serviceability limit state (SLS) and an ultimate limit state (ULS) must be considered, and damage criteria assigned to these limit states. Moreover, uncertainties on all parameters and models must be taken into account. 36

37 Performance (damage) criteria related to limit states Besides SLS and ULS is introduced Repairable Limit State (RLS) defined as the maximum damage level which allows foreseen maintenance and repair methods to be used. Functional classification Tentative performance criteria I Wave transmission SLS: H s, T = m Outer basin Inner basins Jetties Damage to main armour SLS: D = 5 %, RLS: D = 15 % ULS: D = 30 % Sliding distance of caissons SLS: 0.2 m, ULS: 2 m 37

38 Cross sections Shallow water 4Dn Dn relates to main armour 2Dn 1:2 1:1.5 min. 1.5m h 3Dn 3Dn Deep water 1.5Hs 1:2 h 2Dn 3Dn 2.3Dn Dn relates to main armour Only rock and concrete cube armour considered. Crest level determined from criteria of max. transmitted Hs = 0.50 m by overtopping of sea state with return period equal to service life. 38

39 Repair policy and cost of repair and downtime Damage levels S (rock) N od (cubes) Estimated D Repair policy Initial % no repair Serviceability (minor damage, only to armour) Repairable (major damage, armour + filter 1) Ultimate (failure) % repair of armour % repair of armour + filter % repair of armour + filter 1 and 2 39

40 Formulation of cost functions All costs are discounted back to the time when the breakwater is built. T { L 1 CR ( T ) PR ( t) + CR ( T ) PR ( t) + CF ( T ) PF ( t) } t= ( + ) t 1 1 r min C( T ) = CI ( T ) + 2 T Symposium Design and Construction of Deep Water Maritime Works, Gijon, Spain, 2007 where T return period used for deterministic design TL design life time CI(T) initial costs (building costs) CR1(T) cost of repair for minor damage PR1(t) probability of minor damage in year t CR2(T) cost of repair for major damage PR2(t) probability of major damage in year t CF(T) cost of failure including downtime costs PF(t) probability of failure t r real rate of interest 40

41 Optimum safety levels for concrete cube armoured breakwater. 30 m water depth. 50 years and 100 years lifetime. Damage accumulation included. Downtime costs of 200,000 EURO per day in 3 month for damage D > 15%. Lifetime (years) Real Interest Rate (%) Optimum design data for deterministic design Optimized design return period, T (years) H s T (m) Optimum armour unit mass W (t) Freeboard Rc (m) Optimum limit state average number of events within structure lifetime SLS RLS ULS Construction costs for 1 km length (1,000 EURO) Total lifetime costs for 1 km length (1,000 EURO) ,907 86, ,722 81, ,635 78, ,423 93, ,201 84, ,675 79,955 41

42 Case 2.3. Concrete cube armour. 30 m water depth. 50 years and 100 years lifetime. Damage accumulation included. Downtime costs of 200,000 Euro per day in 3 month for damage D > 15 % Total costs in 1,000 Euro year - 2% 50 year - 5% 50 year - 8% 100 year - 2% 100 year - 5% 100 year - 8% Design armour weight in ton 42

43 Conclusions related to rubble mound breakwaters without crown walls. Optimum safety levels correponds to: Approximately one repair of small armour layer damage (D = 5%) in 50 years corresponding SLS repair probability of app (ROM specifies 0.1). This corresponds to the use of the years return period waves in deterministic design! Chances of major damage and collapse will be marginal (ULS: Failure probability < 0.03, where ROM specifies 0.1). Very flat cost minimum. No significant increase in lifetime costs by designing a safer structure. No or marginal influence of downtime costs on optimum safety levels. 43

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45 Economical optimization of Icelandic berm breakwaters Structure lifetime 50 years. Interest rate incl. inflation 5% p.a. Downtime costs in case of failure 18,000 Euro per metre structure Rock mass density 2.70 t/m 3. Wave steepness S op = Case 11. Water depth 11 m. Shallow water waves. Case 12. Water depth 20 m. Deep water waves. 45

46 Cross sections of outer caisson breakwater Caisson on bedding layer bf B br hc Freeboard h = 0. 6 TL c H s h d h' 1:1.5 1:1.5 tf tr Caisson on high mound foundation 46

47 Bulk unit prices for completed caisson structure in Euro/m 3 Structure part Europe Japan Caisson Armour layers Foundation core Repair unit prices Blocks in front of caisson: Europe, 150 Euro/m 3, Japan, 200 Euro/m 3 Mound behind caisson: Europe, 25 Euro/m 3, Japan, 50 Euro/m 3 Limit state performances Limit states Sliding distance (m) Repair Serviceability SLS Repairable RLS Ultimate ULS No Dissipation blocks in front, or mound behind Both 47

48 Table Case B1a. Optimum safety levels for outer breakwater in 25 m water depth. 100 years service lifetime. RLS repair with blocks in front of caisson. 48

49 Lifetime costs, Euro/m h'/h=0.70 h'/h=0.77 h'/h=0.83 h'/h=0.90 h'/h= design return period years Fig Case B1a. Dependence of lifetime costs on relative height of caisson rubble mound foundation and on return period applied in deterministic design. 49

50 Table Case S1a. Optimum safety levels for outer breakwaters in 40 m water depth. 100 years service lifetime RLS repair with blocks in front of caisson. 50

51 Fig Case S1a. Dependence of lifetime costs on relative height of caisson rubble mound foundation and on return period applied in deterministic design. 51

52 Geotechnical failure modes Caisson breakwaters 52

53 Table Case B1b, sand 30o. Optimum safety level for outer caisson breakwater in 25 m water depth. 100 years lifetime. RLS with mound behind caisson. 53

54 case B Lifetime costs, Euro/m h'/h=0.70 h'/h=0.77 h'/h=0.83 h'/h=0.90 h'/h= design return period, years Fig Case B1 b, sand 30o. Dependence of lifetime costs on relative height of caisson rubble mound foundation and on return period applied in deterministic design. 54

55 Table Case S1b, sand 30o. Optimum safety level for outer caisson breakwater in 40 m water depth. 100 years lifetime. RLS with mound behind caisson. 55

56 case S Lifetime costs, Euro/m h'/h=0.70 h'/h=0.77 h'/h=0.83 h'/h=0.90 h'/h= design return period, years Fig Case S1 b, sand 30o. Dependence of lifetime costs on relative height of caisson rubble mound foundation and on return period applied in deterministic design. 56

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58 case B Lifetime costs, Euro/m h'/h=0.70 h'/h=0.77 h'/h=0.83 h'/h=0.90 h'/h= design return period, years 58

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60 case S Lifetime costs, Euro/m h'/h=0.70 h'/h=0.77 h'/h=0.83 h'/h=0.90 h'/h= design return period, years 60

61 Conclusions related to outer caisson breakwaters allowed to slide moderably. Sand seabed, =35º. Wide rear berm. Optimum safety levels for cost optimized designs correspond to the following probabilities. Failure probabilities in 50 years lifetime Water depth Sliding Geotechn. slip failure ROM 0.0 SLS ULS 15 m m m Optimum safety levels seem much more restrictive than recommended in ROM 0.0, and are significantly higher than for conventional rubble mound breakwaters. 61

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63 NUEVAS INSTALACIONES PORTUARIAS EN PUNTA LANGOSTEIRA (A CORUÑA) V JORNADAS DE PROYECTOS Y OBRAS DE LAS AUTORIDADES PORTUARIAS A CORUÑA, 27 DE SEPTIEMBRE DE 2007 Fernando J. Noya Arquero. Subdirector General de Infraestructuras. Autoridad Portuaria de A Coruña. 63

64 ANTECEDENTES: BASES DE DISEÑO: OLEAJE (2/3) TRAMOS RESULTADOS Hs, 140 años MORRO 1A 1B QUIEBRO A 2B 2C 2D

65 TEMPORALES ANTECEDENTES: BASES DE DISEÑO: OLEAJE AÑO FECHA H S (m) Hmax (m) Tp (seg) nov 7,42 13,18 17, ene 7,58 13,54 14, nov 9,61 14,76 13, ene 11,91 18,06 14, nov 8,02 10,69 14, ene 8,76 13,8 15, abr 6,8 10,65 12, ene 9,36 14,65 16, dic 7,81 13,24 15, feb 9,04 13,77 16,7 65

66 Symposium Design and Construction of Deep Water Maritime Works, Gijon, Spain, 2007 PROYECTO: PLANTA Y SECCIONES TIPO. Dique de Abrigo 66

67 SECCIÓN PRINCIPAL DIQUE DE ABRIGO PROYECTO: PLANTA Y SECCIONES TIPO. 67

68 DIQUE DEFINITIVO AGO 2007

69 DIQUE DEFINITIVO AGO 2007

70 DIQUE DEFINITIVO SEP

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