Subsidence due to coal mining in the harbours of Ruhrort

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Subsidence due to coal mining in the harbours of Ruhrort

Bumm, H.; National Research Council of Canada. Division of Building

Research

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TECHNICAL TRANSLATION 1615

SUBSIDENCE DUE TO COAL MINING IN

THE HARBOURS OF RUHRORT

BY

H. BUMM

FROM

JAHRBUCH DER HAFENBAUTECHNISCHEN GESELLSCHAFT 30/31: 29 - 38. 1966/68

TRANSLATED BY

ROBERT SERRE

THIS IS THE TWO HUNDRED AND NINTH OF THE SERIES OF TRANSLATIONS PREPARED FOR THE DIVISION OF BUILDING RESEARCH

OTTAWA

1972

NRC

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PREFACE

Ground settlement due to underground activities such

as mining or the pumping of gas, oil or water is now encountered throughout the world. In many cases, especially in developed areas, this settlement causes serious damage and creates

significant economic losses. So far as is known within the Division of Building Research, there is only one major case in which settlement of the ground surface was deliberately induced to assist in solving a serious problem at the surface. This was the lowering by coal mining of most of the Harbour of Duisburg-Ruhrort, on the Rhine, in order to compensate for the reduced water levels in the river consequent upon its improve-ment for navigation during this century. Dr. R. F. Legget, former Director of the Division, was interested in this unusual project and was able to visit the Harbour, by chance on the last day of mining operations. This paper was given to him by the Harbour Director, Dipl.-Ing. Hermann Bumm, by whose permission i t is now included in this series.

The Division is grateful to Mr. Robert SerrA of the

Translations Section, National Research Council, for translating this paper.

Ottawa

November 1972

N. B. Hutcheon Director

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i t Le :

Author:

Reference:

Translator:

NATIONAL RESEARCH COUNCIL OF CANADA TECHNICAL TRANSLATION 1615

Subsidence due to coal mining in the harbours of Ruhrort

(Die Absenkung der Ruhrorter Hafen durch Koh1eabbau)

H. Bumm

Jahrbuch der Hafenbautechnischen Gese11schaft, 30/31: 29-38, 1966/68

Robert Serre, Translations Section, National Science Library

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SUBSIDENCE DUE TO COAL MINING IN THE HARBOURS OF RUHRORT

There have been many technical publications dealing with the subsidence of the harbours of Duisburg-Ruhrort as a result of coal mining. Since these particularly interesting operations were completed in the summer of 1968, this paper will once more summarize the process and consequences of subsidence, with

numerous references to previous publications on the subject(l).

1. Erosion of the Rhine

The causes of Rhine erosion will only be described briefly. The correction of the Lower Rhine during the last century in-volved the provision of a closed navigable waterway by linking several islands. The course of the Rhine was also shortened by about 80 km through the elimination of major bends. As a result the force of the flow increased, causing more erosion. This phenomenon was aggravated when the embankments were built up, since the natural bank erosion changed to bottom erosion. Navigation itself contributes to the drop in the river bed: the propellers of the strong motorboats and tugboats stir up the bottom. Because of the heavy traffic, especially at low water, the bottom is not at rest, and the stirred-up bed load

is continuously carried downstream. This factor is s t i l l at work today. Since the turn of the century, the bed of the Rhine

in the Duisburg area has dropped 2.40 m, and a 1.60 m drop is predicted by the year 2000 (Figure 1). By then the slope of the river bed at the mouth of the harbours will have decreased to the point where excessive erosion will probably disappear. However, i t is too early to tell whether i t will come to a standstill.

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-2. Deepening the Harbours

It was necessary, then, to plan and carry out radical

measures in order to maintain the depth of the navigable water-way in the harbours of Duisburg-Ruhrort. This deepening of harbour docks is not due, as in seaports, to the constant

increase in the size of ships, but is required simply to restore water depth for ships of the same size and draught.

Until now, this subsidence of the Rhine bed has been over-come in the harbours by means of dredging after consolidating the embankments(2). So far, 15 of the 40 km of harbour shore-line have been built up using the common construction method for broken shorelines, i . e . , an anchored sheet-pile wall in the lower section and a stone-paved slope at the higher water levels(3). At present this costs about 3500-4500 DM per linear meter.

Since further reconstruction of the 40 km shoreline would have been quite expensive, the planners considered the fact that there are high-grade coal deposits under part of the harbour docks, the extraction of which would provide some subsidence of the harbour's bed. Until then coal mining had not been authorized for fear of damage to the harbour installations.

However, on the basis of the positive results obtained from coal mining under the Rhine-Herne Canal(4) as well as under a quay wall in the parallel harbour* in Duisburg, where damage to shore installations was kept within reasonable bounds, i t was consider-ed that coal mining could be attemptconsider-ed under the harbour

in-stallations without serious danger.

There was also the fact that the water level in the lower s i l l of the two entrance locks of the Rhine-Herne Canal, which

* "Parallel harbour" denotes a harbour in which the navigable waterway has been widened on one of the banks. (Transl.)

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-are in the immediate vicinity of the harbours, had dropped as a result of the erosion of the Rhine. Here too, the subsidence of these two locks could restore the old water depth. At the same time the Westende Coal Mine, in whose concession the

"Sicherheitspfeiler"* was located, expressed interest in extract-ing the coal in this region, in view of its diminishing coal

supplies. As a result, there were discussions about discontinuing the "Sicherheitspfeiler". Thorough investigations revealed that there were high-grade coal deposits under the most important part of the Ruhrort Harbour docks, and that i t would be possible, through their extraction, to lower the docks as well as the two entrance locks by 1.60-2.20 m, thereby partly offsetting the effects of erosion. In fact the docks and locks, which had in effect risen in respect of the Rhine bed (Figure 2), would once again drop towards the former water levels of the Rhine.

In 1951 an agreement was reached by the three parties, the Hamborner Bergbau AG, the Navigable Waterways Authority of MUnster and the Duisburg-Ruhrorter Hafen AG, whereby the Westende Coal Mine would extract the coal according to a definite plan,

while the Navigable Waterways Authority of MUnster and the Duisburg-Ruhrorter Hafen undertook to share the cost of mining damage

above a set level. The mining company thereby assumed the

important and difficult task of extracting the coal with sufficient care to provide clearly-defined subsidences and keep damage to

the many complex structures at a minimum. The working of a mine of such importance and involving these special security measures had not so far been undertaken. It was also a new and interesting assignment for the Port Authority to maintain the harbours in

commission at all times during the mining subsidences, involving the various structures, viz., 14 km of embankments, four coal tips, 9.5 km of crane rails, 45 km of rail tracks, oil storage tanks with a capacity of 300,000 m3 and their pipe installations, in addition to many buildings, always mindful to reconcile the

various interests of both the mining company and the Port Authority. * The "Sicherheitspfei1er" consists of zones where coal may

not be extracted because subsidences cannot be allowed to develop. (Transl.)

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-3. Coal Extraction Under the Harbours

Allowing for the possibilities and requirements above and below ground, the six harbour docks of Ruhrort - A, Band C as well as the Kaiser, North and South harbours - were to be lowered by 1.60-2.00 m, and the two entrance locks of the Rhine-derne Canal by 2.25 m, through the mining of three coal seams at a depth of 90-500 m. The mining company undertook to carry out the subsidences shown in Figure 3, with a margin of ±5%. The various figures are the result of mining operations.

Lowering the harbour area so as to compensate fully for the 4.00 m erosion of the Rhine would have caused too extensive a subsidence, thereby flooding some of the transport and loading areas even at periods of slight high water. The subsidences which were agreed to are sufficient for the water depth of the harbours and do not substantially affect high-water access to

the area. The remaining difference in level could be compen-sated by sheet-pile walls of lighter construction and reduced dredging. In the harbour area which stretches towards the Rhine and in the Duisburg Harbour area, coal mining was not advocated for financial reasons on account of the location and quality of the coal deposits, of many faults, and so on. Consequently, the traditional method of embankment consolidation and dredging operations would have to be used there.

Since the locations of the coal deposits as well as the ex-tent of the development operations involved in extracting the coal have been thoroughly described in Hansa 1966, No. 17

(the publication of the Hafenbautechnischen Gesellschaft), we will dispense with further particulars here.

4. Mining Processes during Coal Extraction

The mining of coal deposits results in a subsidence trough on the surface over the centre of the zone from which the coal has been removed (Figure 4). Tensile stress occurs at the edges

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of the area so disturbed because the latter's length increases t

while compressive stress builds up directly above the zone from which coal is removed. The tensile stresses cause cracks in

buildings, breaks in conduits and cables and the separation

of rail joints. The compressive stresses are more of a nuisance, for they can cause more extensive damage to buildings as well as the deformation of rails t conduits and pipe installations.

The coal was extracted from three seams in succession. Coal-mining operations extended over the largest area possible in order to achieve relatively even subsidences. The areas of extraction were about 200 m wide and extended in both directions from a point of collaring (Figure 5). For technical reasons

each coal face had to be a certain distance ahead of its neighbour. Extraction at the various coal faces had to proceed at an even

pace in order to reduce as much as possible the occurrence of additional stresses on buildings. Such co-ordination of mining operations certainly complicated things for the mining company. But this requirement had to be assumed for the safety of the surface installations. The zones from which coal had been removed were then partly packed with rock material t whereas

other seams are mined using the so-called caving method, whereby the cavities are allowed to collapse completely. With the caving methodt subsidences would be between 80 and 90 cmt with packing

about 50-60 cm. If everything is running smoothly and no faults are encountered, the mining of a face 200 m wide can proceed at about 2-3 m per day. When operations were at their peakt

subsidences rose to about 1 cm per day. In one particular case subsidences reached 2 cm/day. Surface subsidences were undulatory and corresponded to the progress of the mining operations, there-by altering the gradient of railway tracks and pipe installations.

The choice of the first point of collaring was of special significance for the surface installations. Since compressive stresses build up over its centre (Figure 4), the point of

collaring is chosen where disturbances will cause minimum damage, e.g., under the centre of a dock. The coal face is preceded

by the zone of the tensile stress, followed by the compressive stress over the middle of the extraction area, i.e., tensile

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-stresses are partly neutralized by the subsequent compressive stresses. In 1954 the first point of collaring was set about 100 m west of Entrance Lock I in the outport of the Rhine-Herne Canal (Figure 5). As extraction proceeded towards i t , Lock I

found itself in the zone of tensile stresses. Since the expansion joints of the lock chamber were only 1-2 cm wide, they could

not bear heavy compressive stresses. As expected, the tensile stresses loosened the expansion joints. As the mining operations proceeded under the lock, the compressive stresses were finally brought into play and the expansion joints partly tightened once again. It must be added, however, that this process can only be represented in simplified form, since subsidence involves other forces, e.g., the earth pressure behind the lock chamber whereby the lock walls tend to slant forward. There are also torsional forces, which exert some strain on the mining operations, but they do not constitute a threat to stability.

In the spring of 1957 the first anticipated subsidence of 45 cm was achieved at the lower gates of Lock I. At this time, however, the upper gates of the lock had only subsided 9 cm,

i . e . , the lock had assumed an incline of 36 cm on its longitudinal axis. However, the slope of the 180 cm lock was only 1:500,

and no operational difficulties were encountered. The agreement reached in 1951, stipulating that by 1958 Lock I should have subsided 50 cm, is an indication of the precision with which the mining company conducted its operations, for in the spring of 1957 Lock I had subsided 48 cm. In subsequent operations

the mining company reached its subsidence predictions with similar precision.

Tae successful subsidence of Lock I was followed by subsidences at docks A, Band C. In the 12 years since 1957 subsidences of

1.50 to 2.00 m have been recorded, providing good navigational

advantages, as foreseen. A fault zone has caused special difficulties. It stretches across the docks, and faulting of the seams occurs

over short distances down to a depth of 200 m. Tae extraction of coal from the disturbed zone was at first delayed (Figure 6). As a result, an irregular subsidence developed over the harbour

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-this has been compensated by mining operations in this zone. Extraction is especially difficult in a disturbed zone, seeing that special underground safety measures have to be taken because of the pronounced inclines of the seams.

5. Damage Caused by Mining Operations

Generally speaking, i t can be said that mining damage does arise, but that i t need not necessarily happen. In the light of the foregoing a conclusion could be drawn that some damage would occur. Damage does not take place in a uniform manner, but in places, since the structures are elastic and give up

to a critical point, and are then subject to breakage and so on. However, the extent of the damage can be kept at a minimum if steps are taken in ample time. Thus, preventive measures were taken, as far as was technically possible and justified, to protect the installations, e.g., reinforcements, auxiliary constructions, installation of joints, expanders, etc.

Heavy constructions respond best to subsidences. Thus, tne coal-mixing plant, which consists basically of a reinforced concrete bunker 112 m long and 20 m high with 32 bunker pockets of 240 tons each, responded to a subsidence of 1.80 m witnout undergoing operational difficulties or cracking. Lighter struc-tures such as half-brick boundary walls, light temporary buildings and so on usually suffer heavier damage. Thus, the light supports of the loading bands of the coal-mixing plant became lopsided

and they continued to slope as mining operations progressed, but by correcting the fulcrum supports i t was not difficult to

straighten the steel construction and to avoid operational inter-ruptions (Figure 7).

As had already been observed on the Rhine-Herne Canal, the sheet piling followed the subsidences very satisfactorily,

although the sheet piles involved were of greater length, and the banks, which were all anchored, were subjected to greater stresses. As a safety measure a light crossbeam with short joint spacings was used, the better to follow the movements.

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-In some places the tensile stresses caused many additional stresses in the sheet piles; in some cases t contrary to

expec-tations and previous experiencet the sheet piles cracked over

a short distance (Figure 8). However t this damage was quite easily overcome by using welded fishplates.

The wavelike movementt which the embankment as well as all

structures must undergo as mining operations progress t is shown

in Figure 9. Thus, the structures usually tipped and twisted slightly. A sheet-pile wall 650 m long was built on the north embankment of Dock C when mining operations had begunt and one end had already subsided 80 em. The top edge of the wall was

driven with a slope of 80 cmt so that today i t is almost norizontal,

now that mining operations have ceased. Here t as elsewhere t i t

was shown that the mine survey data concerning the anticipated degree of subsidence were very precise.

Crane tracks may be regarded as among the most vulnerable of the harbour's installations. Any important change which takes place has an unfavourable repercussion on the cranes and tile

loading bridges t in which the hanging supports allow for a l i t t l e

play only. The crane tracks must be kept rigorously straight. On account of the flatness of their foundations, they react very easily to any soil movements. The tracks on the water side and on the land side do not always alter their position in parallel fashion; the position, level and slope of the two rails may vary, so that a very close watch for such happenings must be maintained by continual recourse to measuring. Occasionally, the undercarriage of the cranes had to be altered to make them more elastic. Figure 10 clearly shows to what extent shifting may affect crane platforms of reinforced concrete. In this case the preventive measure consisted of builJing an intermediate structure of I-beams so as to allow movement between the rein-forced concrete slab and the supports. This construction proved useful, since otherwise the reinforced concrete supports would have sheared off.

The effects of mining operations on the tracks were partic-ularly noticeable. The rails compressed and stretchedt shifting

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their position on the ballast bed, formed depressions, and their

gradient was altered. This was especially noticeable in the case

of humps and dumper tracks, where wagons make use of the natural

slope. Service conditions were altered in many places, and could

only be overcome by additional shunting personnel, e.g.,

track-men. If the rails were not adjusted or regulated in due time,

distortions occured (Figure 11). The many switches, turntables

and weigh bridges had also to be kept constantly stress-free

and operating. A loop service duct was subjected to particularly

strong distortions, for i t was not possible to adjust i t before-hand (Figure 12).

Particularly extensive precautions had to be taken with

the storage tank installations. In one fuel depot alone on Dock A

there were 109 tanks with a capacity of 212,000 m3, extensive

filling installations for ship, rail and road transport, and

a network of pipe conduits extending some 60 km. As many preventive

measures as possible were taken, especially in the form of

expanders. In addition, i t was found necessary to organize a

very close supervision by a smail team of mechanics for the

efficient maintenance of the highly complicated systems of conduits

for oil, water, fire-fighting equipment, telephone and control

cables. As a result of changes in length due to tensile and

compressive stresses, i t was necessary, in spite of the use of

expanders, to alter the conduits from time to time.

The most expensive preventive measures had to be taken witn

respect to a bridge built for a city freeway. It was called

the Berlin Bridge ("Berliner Brlicke") and was more than 1800 m

long. The cost was shared between the mining company and the

bridge construction authority of the city of Duisburg. Tue

reinforced concrete box-beam structure required additional

rein-forcement, and the supports had to be installed in such a way

that they could be regulated at will, to compensate the various

subsidences. The bearings for the readjustment presses were

located at the foot of the supports (Figure 13). Upon completion

of the mining operations the concrete work will be removed. The

greatest subsidences are as high as 2.00 m, whereby the bridge's

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the mining operations.

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-During the subsidence period the bridge was measured in detail once a month from the roof of a high-rise building 1 km away which was not affected by the subsidences

and from which the entire bridge could be well observed (Figure 14). It was usually necessary to reset a whole series of piers together; ャセ hours were needed for each pier. The subsidence and alignment of the bridge took place without traffic interruption. The bridge withstood the subsidence without any damage.

As soon as the first subsidences began, i t was obvious that a constant and very careful supervision of the harbour installations would be required. Meetings have therefore been held eacn

Thurs-day for years - some 400 to date - making i t possible to take necessary decisions at short notice and to avoid major damage

to structures as well as breakdowns in operations. Each meeting is attended by 10 to 16 representatives of the mining company, the Federal Railway, and the harbour construction and operations unit . The prearranged agenda is followed by a discussion of

any questions concerning mining operations.

6. Completion of the Subsidence Project

So far, the coal in the area of docks A, Band C has been extracted and the planned subsidences achieved. Following the agreement there s t i l l remained the Ruhr Lock and the Ruhr Dam, as well as parts of the North and South Harbour to be lowered. However, closer underground developments have revealed that the fault extending through the harbour underneath the Ruhr Lock and at the rear end of the North Harbour is causing such strong disturbances that national mining authorities have forbidden further extraction. As a result, the subsidence of these areas as provided for in the agreement is no longer possible. Likewise the subsidence target anticipated for the South Harbour and parts of the North Harbour will not be reached. The harbour authorities will have to lower these harbour areas using traditional methods. Taking into consideration the final subsidence levels, two sections of the embankment in these harbour docks and another in the harbour canal, which is located in the drainage area of the Ruhr Lock,

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have been provided with lighter sheet-pile walls (profile II) in recent years. The depth of driving has been determined from the anticipated subsidences. Since these walls will in future no longer suffice, they will have to be deepened or new sneet-pile walls predriven. However, these can presumably be connected to the old anchorage. These are the only measures which Were taken as a result of incorrect assumptions in this extensive and risky project. In view of the geological conditions, they could not be foreseen (Figure 15).

7. The Actual State of the Subsidence

A comparison of Figure 3, the estimated subsidence, with Figure 15, the actual state of the subsidence, may give the impresslon that the end result is quite different from what was originally planned. however, the extraction target was fully reached in docks A, Band C, and even partly exceeded. In fact, tue final subsidences in the rear section of docks A and B is

2.00 m instead of 1.80 m, as was originally planned. A comparison of Figures 3, 6 and 15 clearly shows that the ridge which had

first remained has now largely disappeared. Between Dock Band the Kaiser Harbour the final level of subsidence has not been reached, but this is not important because this peninsula will soon have to be reconstructed for other reasons, and the Kaiser

Harbour will be largely filled in to provide new leasehold property. Only in the middle of the North Harbour do unfavourable conditions remain, because there the subsidence lines are pressed very

strongly together as a result of the impossibility of mining

the upper North Harbour. Consequently, there is a slope of 1.00 m over a short stretch of this shoreline (Figure 16). The mining company had to carry out and finance the extensive straightening operations on the crane tracks, whereas the harbour authorities are responsible for the additional work on the embankments.

Furthermore, coal extraction will be discontinued in the peripheral areas, for technical reasons. As a result the sub-sidences, which so far have been quite flat on the edge of the mining area, are now strongly concentrated, but this does not

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affect the subsidence of the harbour area.

Thus the mining operations under the harbours ended in the summer of 1968, four years earlier than was planned in the

agree-The Westende Mine had to stop its operations because its ment.

coal supplies ran out. The agreement enabled i t to keep its installations in operation an extra 16 years, for without i t the coal mine would have closed down in 1952.

The coal under the harbours was extracted with such a high degree of skill that the operation was a complete success for the Duisburg-Ruhrorter Hafen AG, except for the few limitations described above. It should again be pointed out that the mine surveyors performed exceptionally well, directing the mining operations and predicting with great precision the subsidences and their consequences, so that by taking preventive measures i t was possible to keep damage within reasonable limits.

It may seem incredible to an outsider that a complex structure such as a harbour, with its extensive and varied installations, could be lowered by 2.00 m, the ground being actually cut from under i t , and yet the experience of many years and the skill of the surveyors have proved that in spite of all assumptions to the contrary i t can be done.

Now that this uniquely interesting project has been completed, all those who took part in i t , working in close co-operation

and harmony and with a ready sense of responsibility, may look upon their successful operations with great satisfaction.

References

1. Bumm, H. Die Vertiefung der Duisburg-Ruhrorter Hafen. Bautechnik, (10): 1952.

- Bumm, H. Die Erosion des Niederrheins und ihr Einf1uss auf die Hafen. Hansa, (46/47): 1952.

- Bumm, H., Schweden, G. and Finke, G. Die Absenkung der Duisburg-Ruhrorter Hafen durch Koh1enabbau. Hansa,

(17): 1966.

2. Schinkel and Grube Sicherung von Pfei1erbauwerken in den Duisburg-Ruhrorter Hafen. Bautechnik, p. 101 ff., 1937.

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po 161 ffo, 1961.

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Stall, Fo J. Hlfen am Rhein-Herne-Kanal unter Einwirkung des Bergbaues. Jahrbo BTG, po 356, 1962/63.

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7. Middle of trough 8. Worked-out seam 9. Subsidence curve 10. Angle of break

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- 22

-,,'L]WGGGGGGGGGGGGGGGG⦅GGGGGGGGGGGGGGBLBGGGGGGGGGGGGセィセG^セGGGセGGG " GG[セG]セG]GGG]GGGGGGGGGGGG]]]]]]]G]Gィ^GGGGGGGGGGG]GG]]]GG

i',

:セN

Fig. 10

Distortion of a crane platform,

with precautionary sl:l..deway

Fig, 11

(25)

Fig. 12

Distortion of a loop service duct above ground

Fig. 13

(26)

RLoセ III セ (,0 45

11

I 4. _ I I -I : , • ! , ' I I . I -J-

セセM⦅Tセ

セN

.--t

.',,-セ

,

illJ:"; -,

, : .' _Gradient aft. er comp . of mlnlng1 . .

: • : : . 0 erat t.ons (shown as a straight:

oI I ! ! !J,I I I I I " I I . .. ...

itョ・IMBNL⦅Nl⦅lセBBBG^BGMMBiMセGMセセNLNMMMMイM⦅N

__._._--'...

;m'29 27 Z52J2fXK18 f7 16zr14 :t3XJI 1f 10 9 l'JJF7 e Y 4 3 III ---,. Pd ILa r

Z942f1:2GiYI.22ZfJ . . - - - . :

: 13800m 13600 13400 13200 13000 12600 13600 12400 13300· 12000 11600 11600 1 セ bi,herigeSer!lvngf/1 his November 1960 4 _ bishengeSenkungen von Hill<196iJhis Mdrz19616 "-,,,bisherige Senlevngen von Mdrz1961bis Juni 1961

2 I11IIItisbc';c SmkurJc, va,J/lni 1,961 hi'OM19615 /}f'!lcriy Sen(u:qf"I'n'olt. 1961hisMarL 11M7 bishmge Smkungen "" Marz'1964/;isJuni 1964

3 1!iffIifbishenge Senleungetl'00Juni1%4his Juf 19fjfi

Fig. 14

Subsidence of the Berliner Bridge

1- Subsidence to Nov. 1960

2. Subsidence from June to Oct. 1961

3. Subsidence from June 1964 to July 1966

4. Subsidence from Nov. 1960 to March 1961

5. Subsidence from Oct. 1961 to March 1964

6. Subsidence from March to June 1961

7 • Subsidence from March to June 1964

!,-"

(27)

-I:'-• - - _..._ \

-: -

\

' I \ . 1 \

\

N +

セセ .L-. ...J o S(;J 11k'" m noc WESTE;"; IlE Fig. 15 I t-) I"n

Levels of subsidence reached in

the harbour of Duisburg-Ruhrort

(28)

I

..

Figure

Updating...

References

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