433
DESIGN
AND
CONSTRUCfION
OF
RUBBLE MOUND STRUCl'URES: AN INTRODUCl'ION
ORVILLE T.MAGOON and W.F. BAIRDPresident, American Shore
&Beaclt Preservation Association;
P.o.Box 279,Middletown, CA U.SA.and
Principal,
W. F.Baird
&Associates,
38AlItares Drive,Suite 150,Ottawa K2E 7Jl2 CanadaI. Introduetion 1
Il.
Section1:
General Concepts2
lIl.
Section2:
Case Histories8
IV. Section 3: Case Histories--Continued 10
V. Section 4: Nested Units 12
VI. Conclusion 13
I. Introcluction
The design and construction of structures for the coast Iines of the world must consider numerous forces and factors. These include the purpose of the structure, the design waves, the design wave c1imate, the material of which the structure is to be built,
foundation conditions, earthquakes, tsunamis, availability of construction and maintenance equipment, availability offunds for maintenance, availability of laboratory and oceanic equipment and vessels, etc. The focus of this Iecture today however, will primarily include conceptual and practical factors, including those considered in the Hudson formula for design of rubble mound structures against waves. Firstly, perhaps the most difficult question that must be answered in general coastal design is: What are the forces acting on the structure ?
434 ORVILLE T.MAGOONandW.F.BAIRD
ORVILLE T. MAGOON and W.F. BAIRD II. Section 1: General Concepts
As we look at (SLIDE 1) of a tanker heading out of a port, we realize that perhaps the designer did not consider all of the waves and wave farces to which this ship would be subjected. (SLIDE 2) Lets consider the maximum crest elevation of a structure or the deck elevation. Obviously if the waves are somewhat below the level of the deck, the structure might be able to resist the loading, however, once the wave increases a small amount and actually impinges on the deck, forces are greatly increased. (SLIDE 3) Lets just look at some very simple terms about waves. If one thinks of a piling in a shallow sea under very simple wave conditions notice it has a wave length, it has a wave height, there's a trough, a crest and it has a still water level. The direction of motion would be from left to right. If we look at an actual wave record, (SLIDE 4) particularly in little deeper water, we tend to get a very confused pattern. This is a wave rider buoy record taken of Monterey Bay, California, and we notice that, not only are there a lot of variations in the water surface but we may see a certain amount of grouping. The statistics of handling th is type of record have been presented elsewhere in this course, but it is every important that the waves and particularly the maximum waves be weil understood before we get into the design phase. (SLIDE 5) Another way of obtaining wave information is to hindeast or calculate from meteorological data the waves that would have occurred from past storm conditions. Now the problem with all this is that when one compares wave hindcasts done by different investigators the results are greatly different. (SLIDE 6) Shows the wave recurrence intervals offshore of San Francisco by Noble and Dornhelm and we note that if we take some given interval we can get a tremendous variation. Say we take 50 years, we would obtain a wave height of some 25 feet to 60 feet. Since armor stone weight,at least conceptually, varies as the cube of the wave height this is a major concern. This is the prohlem faced by the investigators in design of a structure. I strongly recomment that a conprehensive study of wave elimate and maximum waves be undertaken prior to design stages of rubble mound structures. And although I won't talk about foundations, one must obviously seek good foundations and/or geotechnical assistance.
(SLIDE 7) Lets talk a little about some ofthehistorie structures and concepts. Many years ago solid masonry structures were very important. As some of these structures developed problems, berms were built out in front of these structures using stone. And this is probably one of the beginnings of the rubble mound construction. Another beginning was derived from piles of stone that moved to a stabIe shape under the influence of wave action. Sometimes multiple structures were used. (SLIDE 8) Here we see basically a multiple structure and conceptually this is a possibility. What we are going to talk about most however, is the classical ruhhle mound structure.
DESIGN & CONSTRUCTION OFRUBBLEMOUND STRUCTURES:AN INTRODUCTION 435
(SLIDE 9) This issimply a conceptual slide of wherethe "W"isthe weightof the armor
stone. These are some early suggested cross sections prepared by the Waterways
Experiment Station in Vicksburg, Mississippi. Perhaps these cross sections represent conceptual values tostart your design. Notice that we have a crest elevation
and we have a crest width, of ten dictated bythe kindof construction we are going to be
using. We have a toe-depth and you can see in thiscase, we have a core or perhaps
using the total of quarry, perhaps we have an under layer,a filter-Iayer and armor stone.
If we have over topping (SLIDE 10) you mightwant toextend the armor layer down the
back side. One of the formulas used, (and there are a number of formulas for this) is
the Hudson Formula. Major work was alsodone by Iribarren in Spain, at the Delft, in
Denmark, Canada and Japan. For simplicity it is nice tothink of th is preliminary design
equation. (SLIDE lJ) You really don't want to use this in the final design of a major structure, but it gives a starting point fordesign analysis. And I think we can see the
elements here that are c1assicallyused instructures. These (SLIDE 12) are just a few
of the blocks that are used. 100 ton concrete blocks were used extensively at the Humboldt Jetties, we'lI talk about that later, the trihar patented hy Mr. Rohert Palmer out in Hawaii, the dolos actually I believe was patented hy the Frcnch, however, credit is given for itto the South Africans. A tri-long was an attempt toslip form a unit and,
of course, the tetrapod also patented by the French was commonly used. Now,some of
the basic differences here are that as the structure settles the tetrapod moves like teeth
in a gear and th us relieves stress, but something like a dolos locks together and the
stresses in the unit become very, very important. (SLIDE 13) These were just some
stability coefficient used at a given point in time. Stability coefficients vary from time-to
-time, but you can see that at one time the dolos unit had the highest stability coefficients
and would appear to be very advantageous. Note that consideration was not given to the
stresses in a unit or unit breakage. Attempts have been made (SLIDE 14) to tie units
together, here's an attempt in Japan. If thiswere satisfactory, it could of course, increase
the stability of unit. A little bit about the tetrapod. This isa diagram (SLIDE 15) from
a French tetrapod as patented in Canada. These shapes are all included in the tetrapod
patent. I caution you that when you use shapes, that you have a good patent attorney
review the applicable patents. You notice that the tetrapod as we knowit really appears
on the units on the left. On the right hand sideyou notice that there are units that look
like a dol os unit. Attorneys would probably consider that a dolos unit was patented as
a tetrapod. (SLIDE 16) This was a unit published also that came from France: Le
Dinosaur. Obviously, a unit Iikethis might loek together very weil, however, it would
have a lot of stresses in the unit itself. Now, (SLIDE 17) although we use a formula like
Hudson's formula or some other equations for preliminary design, most major designs
are formulated in a wave tank. This is a wave f1ume at the Waterways Experiment
Station, Coastal Engineering Research Center, and this isa simple look at a section of
a rubble mound after testing. Very large f1umes are available at the Delft in the
Netherlands, at Oregon State in the U.S.A, and in other locations. You have a long
flume, wave generator and at the end of the tank we have a target. In this case we have
arm or stone in the bottom and wehave dolos units on the top, and as we look at this we
note that after testing the units look quite nice. Now the prohlem is that of course, the
movement of the units may not be measured. In the classical tests all that's normally
measured is the fact that the units withstood the wave action and if the strength of the
units is not appropriately considered, the test would not be valid. We also want to note
that in this cross section (SLIDE 18) iswhat I call an egg shell design. We have a large
436
ORVllLE T. MAGOON and W.F.BAIRDarmor layer fails,the entire cross section may quickly follow. Imagine trying to place this
section under water particularly from floating plant. It's just about impossible. When
one comes up with a design, you really have to think of how will it be constructed! We'lI
have more about that a Iittle bit later. For a long time, design work was done based on
using some sart of an optimization calculation (SLIDE 19) where one looked at,
increasing design wave heights increasing the capital cost, but the repair cost would
decrease, and there would be an optimum point calculated at the intersection of these
two curves. That assumes that the concrete armor unit does not break. Those are very,
very, important assumptions and sometimes do not hold true. Hans Burcharth will
discuss this in detail later in this course. In response to that thought, in very early work
in Canada (SLIDE 20),and I just show this conceptually, it wasfound by Timco that if
you had a given mass of dolos that there would be an angle of rotation that would cause failure. But you notice the shape of the curve shows that the smaller the concrete unit,
the greater permissible rotation. Notice that point identified as Sines--here he is talking
about the original Sines breakwater. One of the problems with concrete arm or units is:
if conceptually (SLIDE 21) we assume that the stability coefficient is given for on the top for concrete armor units (as they do not break) and we have some wave height that for which the units would be stabIe (wave height of unit breakage, single prime) and as the
wave heights increase, the units start to break, and when all of the units have broken,
perhaps the stability coefficient is something Iike that of stone, so I say height of unit
breakage double prime. Of course, that is a point of complete catastrophic failure if one
is counting on the full concrete armor stability values. (SLIDE 22) Here's an example
of a dol os unit that broke under its own weight, I believe these are 50 ton units, and you simply let the unit down and at some point it begins to break under its own weight, and although it has been reported otherwise, this looks like a concrete problem. Whether the
armor unit is a cube or a piece of stone, the strength of the armor must be considered.
(SLIDE 23) These are dolos units in South Africa and you can see that they have been
subjected to a fair arnount of rounding. The durability of the concrete is very, very,
important. Curing and termperature control are also very important. You just don't
throw concrete arm or units on the structure and hope that they are going to stay forever: you have to understand what they will do over a period of time. (SLIDE 24) Notice the
broken pieces and the rounding. There has been a little work done on the hypothetical
causes of rounding. Basically wh at this slide is sayingis that for a given distance, a large
piece will go to roundness quicker than a small piece. A discussion about breakwater
stone, I'd like to call to attention the publication that came out in 1992 on the
"Durability of Stone for Rubble Mound Structures" by the Waterways, Port, Coastal and
Ocean Division, Rubble Mound Structures Committee, of the American Society of the
Civil Engineers. This his old slide (SLIDE 26) showsthat ifone starts with a quarry face,
the quarry is blasted and then the stone is removed and in doing that we find th at there
are a host of problems. The durability of the stone is obviously important and you must
have a qualified geologist or a person familiar withstone service record to not only look
at the quarry and evaluate the blast, but examine each stone to be sure that you have
good pieces, for your armor stone. You must dojust as you would with concrete and
each piece has to be sound. One of the interesting things th at was derived from the
"Durability of Stone"workshop was that sometimes quarries mined in the winter had a
lower strength or durability than stone mined from quarries in the summer and I think
that was foundin certain quarries in the Great Lakes and I believe in Texas where stones
DESIGN & CONSTRUCTION OF RUBBLE MOUND STRUCTURES:AN INTRODUCTION 437
would even explode or break apart on the structure, a year or two years af ter they were
placed. There are lots of ways of getting stone to the breakwater, whether you haul
it
by train, (SLIDE 27) or by truck, conveyor, or by barge; the idea is to get this material
as efficiently as possible out to the job site. Great care must be taken in the handling
of stone. (SLIDE 28) This isa split huil barge. When one dumps material in deeper
water the barge opens down the middle and lets the material drop through, being careful to put the barge in the place that you wish to have it. Obviously you can place with
barges such as this (SLIDE 29) old barge. The classic problem of barge placement is
location and getting the material where you want it. You can also place from the cap (SLIDE 30) and when you plan to pIace from the cap you must prepare a working surface for the crane. Customarily you can't do much with a cap width of less than 20 feet because most equipment has a base of about that width. You'lI have to realize when you have a large piece of equipment on the structure you can not get past it with another
piece of equipment, sa the work must be planned to allow operations to proceed
smoothly. Here the crane operator is trying to salvage some stone at Crescent City. This is a very, very difficult question because it is hard to teIl which stone to remove and
which stone to leave. You must have a full-time inspeetor in these instances. You can
also make a temporary cap (SLIDE 31) as we see here. This turntable (SLIDE 32) th at was built to turn the truck around and take it back which saves backing which is very
slow. One of the problems is how you maintain a structure. In this case (SLIDE 33) the
structure was built going from land to sea at a lower crest elevation and it was raised
coming back. The question is,how do you design for maintenance of the structure, and
I think that's a key if you expect maintenance of a structure and most structures do
require maintenance. One must consider the design not only the initial construction but
the maintenance of the structure. (SLlDE 34) Here we see a simple barge placement.
Notice the templates that have been placed on the slopes of your stone. You can't
usually measure every stone, so you simply place it to fit in the template section. Of
course you have to realize that the stone does not fit the neat section directly and you must allow for appropriate construction tolerances in the initial designs. When sounding with a crane, aarmor stone of say 3-5 tons is often used as a weight. The sounding line
is a specially prepared cable affixed to the stone and graduated in length units. This
provides an average elevation of the section. This problem is the result of numerous
construction cost overrun claims. Just another little innovation--a crane was modified (SLIDE 35) to work in shallow water depth by extending the distance between the track and the cab so they could get out into a little deeper water; provide stone from a barge
but have a crane that was land based. Stiff leg equipment is sometimes used in placing
armor material. (SLIDE 36) Here we see a stiff leg derrick beirig used in Maui in the
Hawaiian Islands to place a tribar. Now one of the things to remember is that you
always have to consider stresses and stability in construction equipment. Usually the
contractors are responsible for safety, but ultimately if there's a problem it becomes the responsibility of the engineer and/or the c1ient. Now (SLIDE 37) here's what happens when the crane failed. Of course when the crane goes down you just simply get a pile
of pipe and someone has to call a crane operator's wife and teIl her that her husband
won't be coming home tonight. [t's a very sad thing and you should look carefully at any
type of construct ion equipment that is being used on the job 10ensure that it is being
used in a safe marmer. Here's (SLIDE 38) a track mounted crane that tipped over, of
course construction accidents happen, but the goal of good engineering is to prevent
438 ORVILLE T. MAGOONand W.F.BAIRD
A Iittle bit about some speeific structures, we'lI talk first about the Humboldt Jetties (SLIDE 39) and I'd like to look at a high altitude picture of those jetties. The jetties are located in Northern California, 200 mifes north of San Francisco, the Eel river is in discbarge on the right. This is taken from about 65,000 feet elevation looking into the shore. You'lI notice two parts of Humboldt Bay; a large part to the north and a smaller part to the south, two jetties and extensive stand spits. This (SLIDE 40) is another view' of the spits and the jetty. We see a classical bar, bar formation along the barrier beacb, city of Eureka is on the left. Those of you that will be coming to the Coastal Structures Conference in Eureka in 1994 will be at this site. The view (SLIDE 41) looking in at these jetties, we have a large shoal off shore which basically focuses wave on the entrance, and you see the jetties. south jetty on the right and the north jetty on the left. Anotber view (SLIDE 4Z) of the jetties, here we see (SLIDE 43) the south jetty on the left and the north jetty on the right. Notice the crescent shaped offshore bar. This bar tends to focus wave energy directlyon the head of the structure so that the structure will receive during storms, the maximum wave that the water depth can support from a design standpoint. In this case, the maximum wave will probably occur at high tide and with a water set-up of say 2 feet. That's fortunate in a way, because we can now calculate the maximum wave height. This maximum wave height is the real villain in many design problems. You must understand the maximum values because obviously the maximum is the wave that causes the concrete armor units to move, introduce stress in them, and of course cause breakage. A basic reference on this is the ASCE Rubble Mound Structures Committee's book on "Stresses in Concrete Armor Units". Whife we're thinking about construction, we must think about the effect of tidal currents on construction. At the Humboldt Jetties we have strong tidal current at the jetties and th is (SLIDE 44) is a conceptual model of a tidal prism. Obviously, the larger the bay in the right hand of the slide and the narrower the entrance, the greater the current and the greater the ability to scour through the throat and potentially undermine structures. This is the process for which the jetties are intended to stabilize a channel. (SLIDE 45) This is a very old historie print of the Humboldt Jetties. There was a Professor Hauff in the 188O's,I believe; and he had an interest in the Coriolis force, he somehow feit that all that was needed was to build just one jetty, and here you see the Humboldt Jetty as a single jetty. He thought that in the Northern Hemisphere you only need one jetty--the northern jetty. Of course, nothing could be further from the truth. There you see early outer bar forming. They have built one jetty, the north jetty and the outer har was being formed, the offshore bar was forming. A south jetty was later built as we saw previously. Notice landward of the jetty, there is a little series of contours which was Buhne Point. Lets just look at the slide and look at Buhne Point once the entrance was fixed by the jetties. Before the jetties were built, waves were attenuated on their travel from deep water into the bay and impacted on may different areas of the shore. But once the jetties fixed the entrance, waves attacked th is point and caused major scour. These early photos (SLIDE 46) show faeine mattresses made out of branches and brush. A pier was built and stone was dumped over it, a very simple type of construction. (SLIDE 47 Here you see the train dumping the material on the mattresses, (SLIDE 48) Pile driving In
those days used no safety nets, yet much work was accomplished around the turn of the century in California. Mattresses constructed are built (SLIDE 49) and stone (SLIDE 50) is being dumped down on the mattresses. The man on the left, Mr. Henry Broek, was a man I studied under for many years in the field. Subsequently, (SLIDE 51) larger equipment (SLIDE 5Z)was used (SLIDE 53) but it was still impossible to build from the toe up. Now I consider it very, very, important to remember that there is a great difference in structures built from the toe up or from the top down. Ir one starts from the bottom up and the top down at the same time,
it
is necessary to have jointDESIGN &. CONSTRUcnON OF RUBBLE MOUND STRUCI'URES: AN INTRODUcnON 439
where and the design must consider that joint. Thisjoint isoften a zone of weakness. The best way is from the bottom up. (SLIDE S4) A view of an early tetrahedron. This (SLIDE SS) is a simple concrete form blocks to build a monolific head. Pouring concrete in the monolific is a very difficult job. There tend (SLIDE S6) to be holes and leaks and the wave action pulls the cement out of the concrete and fines unless it sets up very quickly. You may get large cavities in the monolific heads (SLIDE 57). Hundred ton concrete cubes, in this case-regular cubes rather than perhaps the more popular Antifer Cube were case for the early jetty repairs. Considerable attention was given to temperature stresses at the time. It was found necessary to drape the units with canvas cover to reduce thermal cracking. We see here the cubes and they're about to be launched. In this case, the form was built with grease on it and compressed air was blown in the bottom of the farm. The form was tipped up and the blocks were launched. It was found th at when the blocks feil over into water (deep water) the blocks often split apart. If they landed on stone, they just tended to loose a corner. The problem was solved by strapping bal es of hay onto the blocks so that when they feil overboard the hay acted Iike a cushion and that solved the problem of the breakage of the blocks. Seen here (SLIDE 58) is the monolithic head of the Humboldt Jetties before the rehabilitation with the dolos that I'm going to speak about in some length. Notice the remains of some concrete cubes. Most of the hundred ton cubes have been stripped away around the monolith. Just a few blocks were left out there. The problem with a monolith, as is the case for any rigid structure, is that if it becomes undermined and the toe material is eroded, the wave action can get up under the structure and may cause severe damage. As we look (SLIDE 59) at the Humboldt Jetties, here Mr. Broek is demonstrating a crack occurring in a rigid structure, the same jetties that we're looking at. The question is what does one do about the cracks? (SLIDE 60) Here is the crack getting wider. Weil now (SLIDE 61), something is going on in the structure and with a rigid structure it's very hard to fIX this.
This is one of theproblerns of a rigid structure cornpared to a flexible structure in that we must consider how do we maintain these structures as time goes on? Ifyou wait long enough, the structure will start to deteriorate and failure may occur. Back to the head of Humboldt Jetties. (SLIDE 62) ft was decidcd to hcavily reinforce the monolith, sort of Iike a cap on a tooth; a ring levy around the monolith to contain the concrete and for working purposes. Weil, (SLIDE 63) what happens is-this is the way it looked after the monolith was poured. The first storm stripped all the stone and the hundred ton cubes away and we had this large mushroom. H's interesting to note that when the waves came and crashed under the structure, they could in fact, be heard into the town of Eureka-several miles away. One of the great storrns that occurred was the storm of February 1960. (SLIDE 64) Here we see the north and south jetties being attacked by waves. Waves are breaking seaward of the structure, waves that were perhaps forty feet in height. The musbroom shaped cap broke apart into some large chunks; you know it's interesting sometimes when a structure fails it may become stabie. The chunks (SLIDE (5) were actually quite good arm or and they probably would have lasted a long time, however, shoaling increased in the entrance channel an it's a little embarrassing to explain to the press and elected officials, sa
it
was decided to repair the structure. Notice another thing that happens when one has a solid part of a structure that does not fail, there will often be a breach landward of it, or adjacent to it. I don't know the reason for that, but breaches occur next to very stabie areas or adjacent to very stabIe areas. These are some of the wave heights (SLIDE (6) derived by hindeast for the storm of February 1960. We can see that the average maximum significant wave height was something ábout 32 feet.440 ORVILLE T.MAGOON and W.F. BAIRD
Eventually, (SLIDE 67) the structure looked something like this and at that time it was decided to repair it. I worked on the repair and after much consideration the most economical design resulted in the use of the dolos artnor. This structure is subjected to very severe depth limited wave act ion. The problem is how does one survey this; and here (SLIDE 68) is a survey crew out on the hydro survey boat. This is a calm day and you can see that it is not too nice to get near the jetty he ad. Sa we had to use (SLIDE 69, SLIDE 70) Army helicopters and we had a helicopter drop (SLIDE 71) the sounding line into the cracks to determine the depths, One thing about using a helicopter
is
that if you put a load on it, a sudden load; say if the lead line should get stuck or wedged into the cap, it would cause a force onto the helicopter and cause it to veer over on one side, which could cause an accident or a failure of the helicopter. The pilot usually has his best friend there with an ax, and if the line gets stuck they can cut the lead line and the helicopter goes home.To determine the most cost efficient structure a hydraulic model was built (SLIDE 73). Mr. D.D. Davidson was the Chief Investigator and Mr. R.Y. Hudson was the Supervisor on these modeIs. (SLIDE 74) Shows the plan. (SLIDE 75) This is the L shaped flume at the Waterways Experiment Station and (SLIDE 76) shows the close-up of the structure. Mr.D.D. Davidsen on the left is looking at the head of the jetty. This is a close-up (SLIDE 77) of the wave generator. As we look at this aerial view, (SLIDE 78) you can see the jetties are weil clear of the coast sa they get wave act ion from a number of directions. The critical directions are from the southwest and also when the waves are focused on the jetty head from the northwest over the Humboldt bar. You should look now at Buhne Point that I mentioned earl ier. It is gone and it eroded away due to the fact that once you fix an entrance with jetties, the waves tend to now beat in a single direction rather than in a variety of directions. When you focus erosion at one spot it erodes and the spits accrete. You can see the sand going off on the spit to the right.
As
in the prototype, cubes in the model at the jetty head (SLIDE 79) washed away. It is very interesting to note the strong current pattem in Humboldt Jetties. The currents as weil as waves must be considered both in design and construction. These (SLIDE 80) are NASA thermal scanner images of the inlet. Let's look at the lower one first. You see the jetties and you can see that these are taken at slightly different times, but you can also see the warmer water coming out of the harbar mixing with the colder water along the coast. On the top you can see that effect, and you can also see some baat tracks going through.111. Section 2: Case Histories
The plan of restoration for the Humboldt letties (SLIDE 1) th at Mr. Davidson, Mr. Hudson and I came up with used concrete dolos 42 ton units on the head. Landward of the head increased stability was needed and we therefore increased the weight of the dolos by using a heavy aggregate. We have 43 ton dolos on the transition between the head and the tronk we found this to be a very difficult portion of the structure to design. Originally, we considered using barite aggregate as was later used at Sines, Portugal. Obviously, if you use a heavier concrete in the same form, not only is the unit heavier, but of course you have a heavier unit weight. If you look at the Hudson's formula, that tends to increase stability very rapidly. This was the head section that was derived: five horizontal to one vertical slope, 42 ton dolos with 2 layers going down basically to the old materials that were down near the bottom. (SLIDE 2) The filet at the top of the section was to reduce wave action at the cap. In the model test (SLIDE 3) notice there
is
a slight increase of water and turbulence as the wave crest passes the head. I believeDESIGN & CONSTRUCTION OF RUBBLE MOUND STRUCTURES: AN INTRODUCTION 441
that could be the Mach stem effect that Professor Wiegel has described in the literature. We also gel a jet around the back side, which is the jet first descrihed by Robert Palmer in Hawaii. The jet was caused by water moving over and around a conical head. The dolos unit was reinforeed (SLIDE 5, SLIDE 6) and at that time and it was considered unusual to do that, but Mr. Barab and Mr. Shimizu came up with the amount of steel we needed to reinforce the unit. Others will talk about design of reinforcement for concrete arm or in later lectures. We also considered linking (SLIDE 6) the units but that was not
satisfactory. Now it is very important to realize that as you change the unit weight of
concrete, you can increase the stability and this (SLIDE 7) figure shows that using Hudson's formula for a given slope you notice that as the unit weight of concrete
increases, the apparent weight of the dolos unit would decrease. Of course, if you held
the form size the same, you would increase stability. Forming (SLIDE 8) happens to be a fiber reinforeed dolos. The planning and design of the casting yard is always important.
(SLIDE 9) As to how casting is accomplished or how to store units: a large area is
needed and this must be thought out very carefully. Prior to initiating the casting of the
units you must decide how you are going to cure them, which is very important. (SLIDE
10) Shows reinforcement. (SLIDE 11) Here you can see they have been sprayed with
a curing compound. Subsequently, they are picked up with a claw (SLIDE 12) and taken
out to the job site to be placed (SLIDE 13) with a crane (SLIDE 14). An ideal situation
is a land based crane ..A model was built to locate each individual dolos and keep track
of what was going on underwater. Based on this physical model, the coordinates of the
units to be placed that shift were specified, so the operator could put the unit underwater
at the desired location. (SLIDE 15) Here we see (SLIDE 16) the nearly completed
head. Now we see (SLIDE 17) a little wave act ion and most of the wave action is simply
turned into turbulence. Notice the size of the piek-up truck there in the background.
During the construction (SLIDE 18) very simple tests were made of the units. (SLIDE 19) Here we see some testing of reinforeed dolos. Of course the arms just drop off of unreinforced dolos. An experiment was used with fiber concrete (SLIDE 20). Those are
still up there and they appear to be holding weil. (SLIDE 21, SLIDE 22) Here's a
picture (SLIDE 23) taken by my good friend John Corrigan, he was diving out there and noticed that a large arm or stone had been thrown clear over the cap and landed on the
channel side where it stopped. Imagine the force that goes into moving something Iike
that! It is interesting that the Humboldt
Jetties-built
with 42 ton dolos units andsubjected to very large waves have functioned for over 20 years and are still performing
the purpose for which they were intended--the stabilization of the entrance to Humboldt
Bay. Concrete armor units when properly planned, designed and constructed provide a cost effective design. The key element is to adequately consider the stresses in the armor
units.
The next structure I'd like to speak about is Crescent City Harbor. Here we see
(SLIDE 24) a view of Point St. George reef. We can see the little islands, and notice the wakes in back of the islands. Of course, those would be due to two different layers of
water in the ocean. We can also see the wake behind those as the murky water
underneath is brought up to the surface. This is a close-up (SLIDE 25) view of the
harbor itself. The root of the main jetty is on the right and is at 0+00. The first big
turn is at 36+70, and the seaward segment is 1,000 feet long. The area we are going to be looking at is adjacent to the 36+70 and thereabouts where various problems occur. Originally, th is structure had been designed to extend out to Round Rock, which is where you see thewave action off the end of the structure. But the structure could not be held
in that position, 50
it
was turned. Notice where the small white spot is at the end of the442 ORVILLE T. MAGOONand W.F. BAIRD .
one has to remember that there are concentrations of energy at selected spots. Here we see (SLIDE 26) what looks like modest wave act ion with one spot getting a tremendous
attack. The bathemetry is rarely uniform over the bottom. We get extreme
concentrations at locations and that's very important to remember. A1so, as you look at
a structure, say a capped structure (SLIDE 27);you'lI see sometimes a trough on the
harbor side that's simply the result of over-topping. H's a very quick check to determine
that there was material that was placed there before, such as a turnout, and it's now been destroyed or the material is being withdrawn from the seaward side of the structure on
the ocean side and thrown over the cap and then placed on the harbor side. This is the
Crescent City breakwater (SLIDE 28) after the completion of the 1,000 foot extension. Notice the tetrapod units on the right. Then we have the angle section where there are
two layers of tetrapods and a single layer on that large maintenance work JUSt past
36+70. This is just (SLIDE 29) af ter completion. Notice the two layer and one layer
sections and you can see part of the damaged underwater cap from the previous
construction.
Af
ter the storm of February 1960 (the storm that attacked the HumboldtJetties that also attacked the Crescent City breakwater) you can see (SLIDE JO) the
damaged tetrapods. They rolled around, the arms broke off and you can see the broken
arm here. Mr. Weymouth, for wh om I worked for many years, is in this photograph.
Here are (SLIDE 31) two concrete armor units. Both of these armor units moved at
least 100 feet. The one on the left had its cu ring compound hardly worn, it simply
picked up and moved. The one on the right is abraded, and of course, as units move underwater, they abrade and the more they abrade the less effective they get because they begin to loose their shape and their interlocking capability for which they were originally designed. More (SLIDE 32) rounded tetrapods looking out over the damaged section at low tide. An experiment was then tried to place (SLIDE 33) the unreinforced dolos here (and of course you see this section from 36+70 landward with unreinforced dolos). Looking down (SLIDE 34) on a section the X's represent broken units and the
O's represent unbroken units. Tremendous breakage (SLIDE 35, SLIDE 36) of the units
occured and within ten years the section was essentially destroyed. Many broken pieces
were thrown into the old damaged tetrapod section. The Coastal Engineering Research
Center has an excellent program of understanding the prototype stresses in the units at
this location (SLIDE 37, SLIDE 38). The results of that experiment are available in a number of publications.
IV. Section 3: Case Hlstortes-Contlnued
I'd Iike to look at a third example of construction (SLIDE 1), the Noyo breakwater. This is over one hundred miles north of San Francisco on the California coast. Here you have a deep cove with a river entering into the landward end of the cove (SLIDE 2). IC
you'll look across the entrance on a rough day (SLIDE 3),you'lI see a continual stream
of breakers seaward of the entrance. Of course the problem in this is that it makes it
very difficult for boats to traverse the entrance and makes stee ring very difficult. As the waves get to the entrance (SLIDE 4), you get a breaking wave right at the entrance and then you have the effect of the jet (SLIDE 5) of the Noyo River flowing seaward and the
waves coming in. When you have a series of wave groups, waves in the higher part Q~
the group will drive water into the harbor, eventually the water level inside of the harbor builds up to a point that it releases very quickly and flows out the entrance causing
strong currents and sometimes damage to the boats inside theharbor. Imagine yourself
piloting a boat out of the entrance. This (SLIDE 6) is a typical typeof problem that
is
also associated with the Noyojetties; where we have a rigid cap and a flexibie structure.
Whenever you have these rigid caps and a f1exible structure, there is always a problem
DESIGN & CONSTRUCTIONOFRUBBLEMOUND STRUCTURES: AN INTRODUCTION
443
in the structure ifsettlement occurs. The rubblestructure or flcxible structure settles and
the rigid structure can't settle. Then youget a bridge Iikethis that eventually collapses
(SLIDE 7)and failsand drops down something Iike this.
A few words about Sines. Sines is a major port for Portugal (SLIDE 8, SLIDE 9,
SLIDE 10). A description of the Sinesbreakwater isgiven in the ASCE Rubble Mound
Structures Committees publication "Failure of the Sines Breakwater". The breakwater
structure was built using (SLIDE 11, SLiDE 12) unreinforced dolos armor units. There
was a failure, and the structure has now been reconstructed with Antifer Cubes and we'll
look at these two parts of the construction separately. Here isa satellite view. Sines is
in the center left. This is the main breakwater that was to he built with dolos arm or
units (SLIDE 13). Asit was nearing completion, you can see dolos units out along the
far side of the structure. Notice that the depth (SLIDE 14) of the structure is in very
deep water (some 30 to 40 meters) which means that the maximum wave height can be
very, very high and not depth limited. aften H maximum and not H significant begin to
cause damage to a structure. So we must always consider not only what the significant
wave is doing, but what the maximum wave height is also doing on any structure. We
see the beginning layouts (SLIDE 15) of the port which is a very important port in an
indus trial area for the Nation. Asimplified section of the structure and there again we
have a rock fill or the total of the quarry (T.O.T). The T.O.T. is in the center, and we
have the armor and the underlayers. This (SLIDE 16) is a little better view of this
section. We now have a construction sequence (SLIDE 17,SLIDE 18, SLIDE 19, SLIDE
20, SLIDE 21, SLIDE 22, SLIDE 23, SLIDE 24). The casting of the units was done very nicely (SLIDE 25, SLiDE 26, SLIDE 27, SLIDE 28, SLIDE 29, SLIDE 30, SLIDE 31,
SLIDE 32, SLIDE 33). A beautiful casting job. There you can see the casting yard.
This is the casting sequence. However, in a very short period of time (SLIDE 34), a very
high wave action occurred damaging the structure considerably. This is some idea
(SLIDE 35) of the wave storms significant wave height. Of course, H maximum would
be greater than that shown here (SLIDE 36, SLIDE 37). Professor Edge (SLIDE 38)
who is part of the National Science Foundation's team is shown here (SLIDE 39, SLIDE
40, SLIDE 41, SLIDE 42). Dr. Don Treadwell (SLIDE 43). Underwater, we found th at
the unit breakage extended down into deep water, almost to the toe. Basically, as I
understand the result of the damage, we had a flattening of a beach of debris from the
structure SLIDE 44). We have a beach that was built out of pieces and we show the
pieces here. In a model at NRC Canada (SLIDE 45, SLiDE 46),the investigators tried
to replicate this in a model test found a very similar slope could be reproduced if the
units were allowed to break (SLIDE 47). The initial test into the strength of the
concrete armor unit and the stresses (SLIDE 48) were done by the Canadians, directed
by Mr.Joe Ploeg and he tried to replica te the strength arm or unitswith a material that
would break something like the breakage of real concrete. Professor Burcharth will
summarize work on the breakage of the units. Here's sorne of Burcharth's original
recommended tests SLIDE 49). Here the Canadians (SLIDE 50) tried to replicate with
materials in the concrete armor units which is always a very difficult problem. Some
units can break in the model (SLIDE 51, SLIDE 52). (SLIDE 53)shows the rollover test
Burcharth had talked about this test and tried to replica te that rollover test with arm or
units. (SLIDE 54) Shows another test. Wesee Bill Baird and myself in (SLIDE 55) One
of the thoughts was to try a conceptually multiple structure (SLIDE 56).
The idea was to try to find a system where the waves could be reduced seaward by
444 ORVILLE T. MAGOON and W.F.BAIRD
This is a view of the mound using a model of antifer cubes (SLIDE 57) (on the
underwater mound). However, we found that when the waves moved over the mound,
the antifer cubes rocked. So, obviously, the stresses had to be considered. Now the very
early work done in those days looked at cubes very roughly and tried to determine the mount of movement of the cubes and it was feIt that the movement would be such that
they could be damaged and the mound was not stabIe. 1 think you are all familiar
with
the rollover test (SLIDE 58) of the antifer cube. The cube rolls over and can sometimes
break (SLIDE 59). The next really new technology developed by William F. Baird
(SLIDE 60, SLIDE 61) and others in Canada, was to implant a model concrete armor
unit with an internal load cell in the unit in the model. Once those results are obtained, numerical final elements (SLIDE 62, SLIDE 63, SLIDE 64) of the model is made of the unit and then the stresses that one obtained in the model could be used to estimate
stresses throughout the unit and, of course, ultimately determine a stabIe design.
Instrumented dolos unit (SLIDE 65) and some of the early results of loads (SLIDE 66,
SLIDE 67) from the instrumented dolos. From this the design procedure developed and
had taken in account the stresses of the concrete armor units. I would say in the interim,
until this is solved, we have to look conceptually at something like this (SLIDE 68), that
if we take dol os armor units or whatever concrete armor units we take; if we think of
(somewhere way down at the bottom) wave periods in seconds the wave height in feet (it's somewhere down at the bottom) there is no movement, no damage and if we go a
little higher there is some level of movement, but there is not damage. As we get higher
we begin to get some breakage. We get serious breakage and perhaps a failure. Now
this is just a conceptual diagram demonstrating that we must consider the movements of
armor units no matter where those units are placed. We must piek-up the stresses in
those armor units. Sines has now been rebuilt with Antifer Cubes. This wiJl be an
interesting structure to watch in the future.
Some of the other miscellaneous problems we can often get is shoaling in the units
(S_LIDE69). Here we see the abrasion of the tetrapods due 10impact on the gravel of
the tetrapod unit. What are some of the ways to attack this problem? You can reduce
the waves, you can use berrns, you can use multiple structures and you can use exotic
devices.
Lets take the berm breakwater (SLIDE 70, SLIDE 71). There is an ASCE publication
on the berm breakwater. Here we try to build a structure such that the energy is
dissipated in the berm structure. Here we see (SLIDE 72). some berms under
construction (SLIDE 73, SLIDE 74). This is simply a mound (SLIDE 75) at Humboldt
Jetty. I cal1 it the graveyard and it is simply where al1 the materials from the head
washes up. You can kind of teil how long a stone has been there by it's angularity.
Obviously, when it comes out of the quarry it's all angular. The longer it isin the water,
the more rounded it becomes.
-Just a Iittle bit more about quarry operations as we think about them. This is a major
quarry. 'You can notice that the stone can be sorted. Here is the stone and you can see
a latent crack in the structure being marked (SLIDE 76),and of the course, the question
is would you accept or reject a stone Iike this? Here (SLIDE 77) is another latent crack
in the stone, which is of course, cause for rejection.
V. SectIon 4: Nested Units
Some of the possibilities are with nested units (SLIDE 1). We have classic structures
that have been built. Plymouth Breakwater is certainly one. We have special shapes Iike
the tribar, tbe cob, and the SeaBee. The antifer cube has been used extensively. Let's
look (SLIDE 2) a bit closer at a successful breakwater. The Plymouth breakwater was
DESIGN & CONSTRUcnON OF RUBBLE MOUND STRUCTIJRES:AN INTRODUcnON 445
built in the i800's. It was originally designed to be an all stone rubble structure. It was modified to place cut blocks (SLIDE 3) onto the structure. Conceptually, we have a mound and at some point (SLIDE 4) to put blocks over the mound. Here is an aerial view (SLIDE 5) of Plymouth breakwater. Notice how weil the pieces fit together at the round head (SLIDE 6) Now if we were going to build a block structure, one of the paths to success is to fit the blocks together weil and to maintain these blocks or you could have an inspeetor out there looking at the blocks and whenever a hole is found, to fill or fIX it. This structure has had over a century of wave attack. You can see here (SLIDE 7) Mr. Baird and his father looking at the Plymouth Breakwater.
The next example is Nawiliwili Harbor, Hawaii, (SLIDE 8) in this case tribars were used. Here you see (SLIDE 9, SLIDE 10) tribar placement. Eventually, a barricade was placed to prevent the tribars from whipping over the top. The tribars here are ready for placement at the cap. However, when I dove underwater (SLIDE 11, SLIDE 12, SLIDE
13), I
found brok en tribars. We don't know the cause of those broken tribars,'
but
in fact, once you see one tribar broken or displaced it's probably a matter of time before major problems will develop. Here is simply a displaced tribar that got rolled around. Let us now look at the Isle of Jersey (SLIDE 14). Here is a layout of the breakwater of St. Helier (SLIDE 15). Here we see a nested block and in this case, using the cob unit. Here we see another section of nested blocks (SLIDE 16). The most important point is that the seaward toe (SLIDE 17) must be firmly set and if you have a difficult foundation you must build an artificial toe (SLIDE 18) of some type which will contain the blocks at the bottom and on the top. Here we see the toe block being used to contain the blocks, As we look at these blocks (SLIDE 19), you notice that theyare fitted tightly together and you have a very tight looking appearance of those blocks (SLIDE 20). Now what happens if a block cracks? Weil, in this case, these are filled with concrete (SLIDE 21). You hope too many don't crack because of course, then you would have simply asolid structure all the way up to the top which would be quite different from what the designer intended.Also, at the Isle of Jersey, is the SHED unit; the Sheppard Hili Energy Dissipator (SLIDE 22). This block was formed using an inflatable bag inside. The concrete was reinforeed with polypropelene. So as the unit cured and as the concrete set-up, the bag could deflect and therefore reduce forming stresses in the unit. There are the SHEDS being placed on the Isle of Jersey (SLIDE 24).
Another interesting unit is the seabee. Here is the SeaBee placement in Australia (SLIDE 25). The SeaBee is basically a hexagonal structure with a hole in it. The patented unit appears to be quite satisfactory. Here you see them placed (SLIDE 26). Now you have to realize th at a contractor mayor may not place them the way the designer intended. It is always important to get
the
designer of the block involved in the design of the construction of the structure to be sure that the block is placed in the way that it is intended. . Here (SLIDE 27) is a lifting device for the SeaBee unit.VI: Conclusion
In conc1usion, I believe that with good coastal engineering, economical rubble mound structures can be designed, built, and maintainted in exposed coastallocations. However, each site must be considered individually, and additionally, research must be undertaken to fully quantify loadings, stresses and design of concrete armor units to provide stabie designs with appropriate safety factors.
