Bizerte-Zarzouna fishing harbour, Tunisia: Accropode block dropping tests

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BIZERTE-ZARZOUNA

FISHING

HARBOUR

TUNISIA

ACCROPODECR) BLOCK DROPPING TESTS

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1. INTRODUCTION

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The contracting group CAMPENON BERNARD CETRA/ALI MHENI has been

appointed by the Tunisian Ministry of Equipment (Directorate of

Air and Maritime Services) to build the new fishing harbour of

Bizerte at the Zarzouna site, to the east of the existing harbour

of Bizerte.

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The overall layout of the new harbour, designed by SOGREAH-SCET

Tunisia, provided for the construction of two protective

break-waters, the configurations of which were optimised on a mathematical

model. The design provided for a facing constituted by ACCROPODE(R)

blocks of three different sizes (4 m3, 6.3 m3 and 9 m3).

The manufacture of these blocks, which was the subject of a contract

for license concession and technical assistance between SOGREAH and

the contracting group, began on 3rd Mareh, 1983.

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Each set of shells (14 sets in all) pr-oduced. on average one block

per day. It was possible to envisage placing of the protective

blocks on the main breakwater immediately on acceptance by the

Administration of the breakwater core, toe mound and protective

underlayer.

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The first 6.3 m3 blocks were placed on 24 Augustof the western roundhead of the northern breakwater1983, at the level. Once the placing

team had got into its stride, the placing rate reached about 60 blocks

per day.

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With a placing schedule of one day per week, the first phase of

pro-tection of the main breakwater (that is excluding the crest works)

was completed on 25 April 1984.

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The present report gives an account of the dropping tests with 6.3 m3

ACCROPODE(R) blocks, which were performed on 26 April 1984.

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-2-TEST INSTALLATION CRANE

The erane used for these tests was a LIMA 2400. This erawler-type crane, with a eapaeity of about 800 t.m, equipped with a 120 ft lattiee-work jib, had been used to plaee all the ACCROPODE(R) bloeks on the breakwater, and was also subsequently to be used to plaee the quay bloeks.

Dropping of the ACCROPODE(R) bloeks was obtained by disengaging the drum and releasing the brakes. The first tests, earried out with six-line reeving, were affeeted by substantial friction, both through the reeving pulleys and on the erane drum. Moreover, the reeving pulleys had a tendeney to twist, so that the maximum dropping speed was quiekly attained. The reeving was therefore redueed to two lines for the rest of the tests.

The drops obtained with this arrangement proved to be quite satis-faetory, and the erane does not seem to have suffered, whether from the sudden rapid rotation 'of the drum or from the jerks in the jib eaused at the moment of impact of the bloeks.

POINT OF IMPACT

The tests were performed on the breakwater eore.

The part of the strueture on whieh the tests took plaee had been subjeeted to very heavy lorry traffie and erane movements, and was weIl eonsolidated (this impression was eonfirmed by measurements of settiement).

The dropping tests were performed in two stages, the bloek first of all being dropped on the quarry run (0-1000 kg) eonstituting the eore of the breakwater, then on a parallelepiped concrete bloek of dimensions 2.25 x 2.25 x 1.40 m.

SLINGING

The slings used were the same as for plaeing of the bloeks (30 mm diameter and coupled without eyes).

The bloeks were lifted by the upper anvil, the hook being in one of two axes relative to the bloek:

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-3-in one case the impact was effected on the central protuberance

opposite the hook,

in the other case the impact was effected on one of the

protuberances forming the lower anvil.

BLOCKS USED

The tests were carried out with two blocks of 6.3 m3 nominal volume,

complying with the technical specifications for manufacture.

TEST PROCEDURE

When it had been noted that the blocks were subjected to no damage

when dropped on the core of the breakwater, regardless of the

dropping height, testing was continued by dropping the block on the

parallelepiped concrete block described above.

The blocks were dropped from increasing heights, starting again with

the lowest height each time partial breakage of the block occurred.

A graph of dropping time as a function of height was established

(see figure 1), by interpolation from a series of observations,

taking into account the error in use of stopwatch. Assuming the

dropping heights to be exact, the speeds at the moment of impact

were assessed graphically, and it was thus possible to deduce the

equivalent free dropping heights.

_ Figure 1 22 Dropping time (s) I I

DROPPING FUNCTION

L TWO-LINE REEVING

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OBSERVAT IONS

-4-The observations are presented in tabular form hereunder:

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Concrete b10ck Point of impact Estimated height (m)

FIRST 6.3 M3 BLOCK - SIX-LlNE REEVING

Weight 10ss (%) Accumu1ated 10ss (%) Observa tions

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of break-Core water

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2 4 8 0,50 1,00 2,30 8,00 10,00 8,00 12,00 5 % 5 % 5 % 5 % 10 %

The b10ck suffered no damage, but it was noted that the six-1ine reeving had the effect of significant1y slowing down the drop. The dropping speed soon reached its peak. The kinetic energy of the b10ck is

absorbed by deformation of the

quarry run.

The dropping height is progressive1y increased. Breakage

of the central

pro-tuberance occurs

with H = lOm but

the dropping time

is about 4 s.

The b10ck is slung in the other

direc-tion. Breakage of the protuberance sustaining the impact occurs at a dropping height equivalent to that of the preceding test. The breakage area is identica1.

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-7-THE ACCROPODE(R) COMPARED TO THE afHER MAIN ARTIFICIAL BLOCKS

The test procedure used calls for two reservations:

the small number of ACCROPODE(R) blocks subjected to these

tests does not allow precise quantitative information to be

determined,

the procedure does not cover the risk of breakage due to

fatigue, although this is perhaps the most frequent cause of

breakage of blocks, subjected to rocking movements on the

facing.

However, it is possible to make a qualitative comparison of the

approximate results obtained by the present tests with those of

tests performed with other types of artificial block.

CUBIC BLOCKS

Tests were peformed at Sines with cubic blocks of different size (1).

The following table sets out an extract of the results obtained,

with blocks of 9 tf and 27 tf. Refer to document (1) for precise

indication of procedure used.

9 tf 27 tf

V (mis) 5 3

2,4

4,4

3,6

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2 2

c

nf

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6

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3 5 6

n: number of impacts before fissures appear

c

nf: number of shocks before breakage.

TETRAPOD BLOCKS

Numerous dropping tests have been performed during the 30 years of

use of the Tetrapod technique.

The different test installations adopted have the effect of

diver-sifying the results.

The dropping heights generally adopted in order to obtain breakage

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Tests were carried out at Sète -FRANCE- in September 1983 with 8 m3 Tetrapod b10cks (2). Four b10cks were subjected to the tests. Breakage in each case occurred af ter three or four drops through a height of a few tens of centime tres on a non-deformab1e concrete surface. It is to be noted that the qua1ity of the concrete, checked on core samples, was proved to be perfect1y satisfactory for these

bl.ocks ,

DOLOS BL OCKS

Tests were carried out at Sines (3) with 42 tf do10s b10cks.

Breakage occurred with dropping heights of between 0.05 and 0.25 m depending on the nature of impact. Furthermore, it is to be noted that the tests carried out by Burchardt (4) enab1e estimation of the critica1 dropping height (causing breakage) at between 0.09 and 0.16 m for dolosses of 1.5 and 5.4 tf.

It is a1so important to note that static tests have been carried out in the USA with fibre-reinforced concrete (5). By contrast with conventiona1 steel reinforced b10cks, the appearance of fissures is significant1y de1ayed, and it wou1d seem that the use of fibres, whether or not of the meta1 type, brings significant improvements as regards both abrasion and resistance to fatigue. It neverthe1ess appears, despite these improvements, that the do10s presents an excessive fragi1ity due to its shape, and that the risk of breakage on the facing remains (6).

RESULTS AND CONCLUSIONS

Tests of breakage of 4 m3 ACCROPODE(R) b10cks were carried out on the work site in Sète harbour in 1980. These tests, performed with aloader, showed the strength of the b10ck, but the testing method app1ied did not enablé any quantitative information to be derived.

On the other hand, the tests carried out at Bizerte c1early demonstrate the fo110wing characteristics:

The strength of the ACCROPODE(R) b10ck is significant1y greater than that of the other b10cks that are designed to interlink or hook on to each other (Tetrapod, Dolos, Stabit, Dinosaur •••). This is exp1ained by the massiveness of the protuberances and the progressiveness of their connection to the central core. The breakage occurs progressive1y. Weight 10sses are generally 10w by comparison with the unit weight of the b10ck, in marked contrast to what generally hap pens to other types of b10ck. Moreover, the breakage of one protuberance, further increases the strength of the b10ck.

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Thus it appears that the ACCROPODE(R) bloek differs signifieantly

from the other artifieial bloeks used for proteetive armours of

maritime struetures:

the massive shape of the bloek makes it intrinsieally strong;

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the ~ eoeffieient used for preliminary sizing of the bloeks

gives unit weight values that are praetieally equivalent to

those of grooved eubie bloeks, thus giving a high degree of

weight-indueed stability;

the plaeing of the bloeks in a single layer limits the roeking

movements, henee also the risk of breakage due to repeated

shocks;

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the breakage of an ACCROPODE(R) bloek on the faeing, assuming

that sueh breakage were to oeeur despite the substantial safety

margin already demonstrated, would in no way threaten stability

of the faeing, sinee it would not notably deerease the unit

weight of the bloek.

It mayalso be noted that the seale model tests undertaken by

SOGREAH or in numerous other laboratories have shown that even if a

gap is deliberately ereated in the armour (by removing one, two or

even three bloeks), this tends to close up naturally by settlement

of the armour. In the tests earried out to date, there have been

no cases where the underlying rocks (assumed to be eorreetly sized)

are sueked out through the armour and washed away.

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The overall stability of an armour formed with artifieial bloeks

depends on two factors:

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1) Stability of the bloeks under wave attaek

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This stability may be assessed with a fairly high degree of

aeeuraey by laboratory tests. The tests using the ACCROPODE(R)

teehnique, whether general tests or tests for speeifie projects.,

have been suffieiently numerous for the eapaeities of the bloek

in this respect to be eorreetly assessed.

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2) Perennity of the actua1 b10cks

The fragi1ity of the b10cks on the sca1e model is by no means representative of that of the rea1-size blocks. The tests performed to date with a view to tmproving this representative-ness have not yet proved ful1y satisfactory.

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At present, therefore, on1y fu11-sca1e or large-sca1e tests wou1d seem to be capab1e of giving satisfaction.

The tests carried out at Sète and at Bizerte enab1e the

ACCROPODE(R) b10ck to be situated re1ative to the other types of artificial b10ck.

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In conc1usion, the ACCROPODE(R) block seems to be an effective compromise between:

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the solutions using grooved cubic b10cks, which present satisfactory safety provided that they are correct1y sized, but which are re1ative1y disadvantageous from the point of view of cost, especia11y for protection of roundheads,

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_{and the solutions} _{using artificial} _{blocks of sophisticated}

shape, in respect of which the criterion of fragility has hard1y been taken into account if at all, and the behaviour of which in full sca1e app1ications wou1d seem to differ significant1y from the behaviour in sca1e model tests.

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REFEREN::ES

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(1) Manuel A.G. SILVAOn the mechanical strength of cubic armour blocks Coastal structures

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(2) Tetrapod dropping tests at the port of Sète G. MORE and M. DAYRE

SOGREAH-IRIGM report, May 1984 (in French).

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(3) AccidentStabilitydamage and repairsof rubble mound breakwatersto Sines breakwater, Portugal P. COUPRIE

Revue Travaux, April 1982 (in French).

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₍₄₎ _{A design method} _{for impact loaded slender armour units} H.F. BURCHARDT

Bulletin No 18

Aalborg University Central Laboratory for Hydraulics, Hawebygning.

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(5) Use of fibre reinforeed concrete in hydraulic structures in marine environments