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OCE421

Marine Structure Designs

Lecture #15(Rubble Mount Structures)

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Rubble Mound Structures A wide variety ofrubble mound structures

is built in the costal zone. This includes:

Revetments and seawalls (along the shore)

Groins andjetties (perpendicularto the shore)

Breakwaters (offshore and shore parallel)

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Revetments A facing of stone, concrete, etc., to protect an

embankment, or shore structure, against erosion

by wave action or currents. Vertical structures are classified as either

seawalls orbulkheads, according to their

function, while protective materials laid on

slopes are called revetments.

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Seawalls Seawall - means an upright structure

separating land and water areas,primarily

designed to prevent erosion and other

damage to upland areas due to wave action.

A seawall is generally of heavier or more

massive construction than a bulkhead.

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Quaywalls

QUAY (pronounced KEY)

A stretch of paved bank, or a solid artificiallanding place parallel to the navigable waterway,

for use in loading and unloading vessels. A quaywall is a gravity wall structure having the

dual function of providing shore protection and aberthing face for ships. Its function is similar to a

bulkhead but should be chosen when overallheight requirements or wave environment severityexceed the practical capabilities of typicalbulkhead constructions. (they do not necessarilyretain a soil backfill.)

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Jetties and Offshore Breakwater

BayOcean

offshore

breakwater

jetties

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Typical Cross Section (I)

cover layerunder layer

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Typical Cross Section (II)

quarry stone

tetrapod

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Hudson Equation (1959)

Required weight of armor stone unit (classicalHudson equation)

W =wrH

3

K D (Sr 1)3 cot

W =

wr =

H =

Sr = wr=ww =

ww =

=

K D =

weight inNewtons of an individual armor unit

unit weight (standard surface dry) ofarmor unit inN/m3

design wave height

specific gravity of armor unit

unit weight ofwater

angle ofstructure slope (measured from horizontal)

stability coefficient (SPM, Table 7-8, p. 7-206)

Determine the weight of an armor unit (ofnearly uniform size)

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SPM: Table 7-8

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Stability Coefficient

armor

units

tetrapod

tribar

n

2

2

placement

random

random

structure trunk

KD

structure head

KDbreaking

7.0

9.0

non-

8.0

10.0

breaking

5.0

4.5

3.5

8.37.8

6.0

non-

6.0

5.5

4.0

9.08.5

6.5

cot U

1.5

2.0

3.0

1.52.0

3.0

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Small vs. Large KD

small KD

large KD

less stable more stable

structure head

breaking waverandomplacement

structure trunk

non-breaking waveuniformplacement

cover layer slopes steeper than 1 on 1.5 (i.e. cot U< 1.5)

are not recommended.

W =wrH

3

K D (Sr 1)3 cot

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Graded Riprap Armor Stone

W50 =wrH

3

K RR(Sr 1)3 cot

W50 =

K RR =

the weight of the 50 percent size in the gradation

stability coefficient for angular, graded riprap

(SPM, Table 7-8)

maximum weight = 4 W50minimum weight = 1/8 W50

use of graded riprap: H

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Thickness of the Cover Layer

r = nk

W

wr

1=3

W =wr =

r =

k =

n =

weight in Newtons of an individual armor unit

unit weight (standard surface dry) of armor unit in N/m3

average layer thickness in meters

the layer coefficient

the number of quarry stone or concrete armor units

in thickness comprising the cover layer

(see SPM, Table 7-13, p. 7-234)

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SPM: Table 7-13

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Portion of Table 7-13 (SPM)

arm r u its n placement Layer c ef. (k ) porosity (P )

tetrapod 2 random 1. 4tribar 2 random 1. 2 54

tribar 1 uniform 1.13 47

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Number of Armor Units

Nr

A= nk 1

P

100

wr

W

23

Nr =A =

P =

the required numberof individual armor unitssurface area

average porosity of the cover layer inpercent

(see SPM, Table 7-13)

SPM:

(7-122)

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Derivation for NrEquation

solid volume of an armor unit: V =W

wr

effective volume taken by an armor unit

(including space between units):

V =V

(1 P100)

Nr V = A rtotal effective volume:

NrA

= rV

=

1 P100

rV

= nk

1 P

100

wr

W

23

r = nk W

wr

1=3