WESTERN WHARF COLLAPSE

New Zealand Herald, Volume LXI, Issue 18887, 9 December 1924, Page 11

WESTERN WHARF COLLAPSE

COMMISSION'S REPORT. CAUSE OF THE DISASTER. WEIGHT OF BANK AND FILLING. DESIGN DECLARED DEFECTIVE. MR. HAMER'S RESIGNATION ACCEPTED.

The commission of inquiry into the collapse of the Western Wharf on October 28 has come to the conclusion in its report, which was submitted to a special meeting of the Auckland Harbour Board yesterday afternoon, that, in view of all the circumstances, it is clear the designs and plans of the structure were neither good nor sufficient, the design being defective in that it provided a structure and an arrangement of parts of such a nature that stresses of a character which the wharf was unable to bear must develop in it.

The wharf collapsed on account, of the rupture of some of the supporting piles at the back under an outward lateral pressure due to settlement and outward movement in the mass of the rubble mound which surrounded the piles.

In the opinion of the commissioners, want of foresight was displayed in designing and carrying out a structure unable to bear inevitable stresses. Otherwise no negligence had been shown by any officer or employee. It is recommended that the rubble retaining wall and the mound on which it stands be removed as the proper and only permanently effective remedy for disintegrating influences. Similar influences are operating at the Prince's Wharf, Calliope Dock Wharf, and Freeman's Bay and although there does not appear to be immediate danger of disruption at any one of these places, careful lance is urged as a precautionary measure.

' The report was considered in committee by the board and was unanimously approved. It was decided to accept the resignation of the engineer, Mr. W. H. Hamer; to give him six months' leave of absence, and to call applications throughout the world for the position of engineer at a salary of £1250 a year.

[UNABLE TO BEAK STRESS

RUBBLE WALL MOVES OUTWARD.

DISRUPTION OF THE PILES.

STRAIN AT PRINCE'S WHARF.

CONSTANT VIGILANCE URGED.

The chartered civil engineers who held the inquiry, Mr. Cyrus J. R. Williams, engineer to the Lyttelton Harbour Board, and Mr. F. W. Furkeit, undersecretary of Public Works and chief engineer of the Public Works Department, were commissioned to investigate and to answer these question!;:— 1. Were tho designs and plans on which the Western Wharf were, constructed, considering all conditions, good and sufficient ? 2. Were such designs and plans faithfully carried out- in the work ? 3. Did the dredging of tho berth or »ny other dredging contribute to the collapse of the wharf 4. What was the cau?c of the collapse ? 5. Was there any negligence on the part of any officer or employee of the board that contributed to such collapse 6. What works (if any) are recommended to prevent the recurrence or extension of such collapse in respect of the balance of the Western Wharf, and what steps are requisite to make good tho damage ? 7. Whether the causcs which have contributed to this collapse are likely to eppiv to any of the other -wharves of tho board'

Cause of the Collapse. II is noted by the commissioners that after the answer to the question of vrhat caused the collapse of the Western Wharf has been given the three preceding questions' as to construction and dredging substantially answer themseJves. The commissioners state: —

in his evidence that this concrete wall was of similar character to the wall now standing on the tide deflector. Such a wall moving outwards in conformity with the movement of the centre of the mass of rubble mound, would transmit substantial! v the whole of the movement pressure directly on to the back piles, and such a condition would obtain ( throughout the wharf from about 800 ft. 1 outwards. This condition obtained for a short Hi«t.*>" , e under the shed, but little or no back filling had been put tneie. " Tipping of material was being carried i 011 a little beyond the sh< J, and where | this tipped material rested against the I concrete retaining wall the wharf first I gave way, so that it is fair to assume that I the extra outward thrust due to this j material, was the "''ash straw" on an ! already over-strained structure. There ; does not appear to be any accurate record ,of the amount of material tipped here, | but if the tip was at one point 10ft. bej yond the back of the wharf, as stated in ' evidence, and extended for about 100 ft, as 1 shown in a photograph, the amount mrst i have been considerable.

! v;ays, and downhill settlement- is fve- | quentlv not noticed, or, if noticed, no j great importance is attached to it. In I connection with a road, the. fact that. i there is a slight wobble in the alignment j is of very little importance generally, j tho greater part of arty sagging down ! usual]} takes place before the surfacing i is put in position, and surfacing is then ! put on proper alignment. Also, in the. ' case of railways, the formation is done I some considerable time in advance of t.he j plate-laying, and when, under the inI fluence of traffic, the bank sinks and I moves, the track is 'pulled' to proper j alignment. " No importance is attached to the f-ict that there ha:; been a drift, as it were, of the bank downhill, because the remedy if- .so simple. Consequently, this feature does not create a very vivid impression on anyone's mind. But when a structure which, by its nature, cannot submit to deformation is constructed, in a filling on sloping groucd, then this movement becomes important, and may, as in this case, lead !o complete wreck of the structure.

" The wharf collapsed on account, of the rupture of some of the supporting piles fit the back of the wh<iTf under an outward lateral pressure due to settlement; and outward movement in the mass of the rubble mound which surrounded such piles, such outward movement and consequent pressure being intensified by the super-imposed weight of the concrete wall built on the top of the rubble mound, j and by the lateral pressure of the filling j tipped behind such concrete wall. These i influences may have been supplemented by some flaw in the ground, as suggested j by Mr. Hamer. or bv a deposit of soft j mud .on the dredged benches supporting i the I'ubblo mound, as suggested by Mr. i Holderness, but they were sufficient in I themselves to account for all that has hap- ! penedj j

" The fact that a large portion, if not the whole ox the remaining work, was found in a state of distress under file influence of forces to which a braced structure of this kind should never be subjected, enables us to picture without difficulty what actually took place where the collapse occurred.

The question may be raised as to why the rest of the wharf, particularly the P«tof it a few hundred feet inshore, T-j is now full V loaded with filling, old not also collapse: but there are reasons why the portion of it which did -o should be the first to go.

Effects of a Rubble (Mound. " The behaviour a:tid effects of a mound of rubble or other loose materia] 017 sidelong ground may ")e described thus:-— Owing to one side of the mound being longer than the other, the settlement which takes plac* in all disturbed material, particularly when freshly deposited. is greater ia the direction of the long side than in that, of the short side, with the result that tho centre of ma.ss of such a mound moves downwards and toward the long side. This is common experience, t and can be. demonstrated by the laws of mechanics, although the exact direction pnd magnitude "of t.he forces operating in this manner are still a subject of debate and experiment among engineers and mathematicians. " Banks which arc built, on (sloping ground are usually used in connection with the construction of road;; or rail-

■■ Pressure on the Piles, In the inner portion of the wharf the retaining wall behind was constructed of random rubble masonry, this form of construction having been adopted by Mr. • arr 'fir, as stated in his evidence, to allow »or , 'scathing " or movement in the wall to have lull play if there were any en ency to move. .Such movement outarc] and downward has actually taken place, but owing to non-rigidity" of the '•andom rubble wall the lull effect of the pie~sjre from the outward movement has not come on the piles, the bend or swelling between the piles indicating that part o the pressure has expended itself harmess y (jo far as the whuif is, concerned I n merely bulging the wall, as anticipated *'.v ( Mr. Hamer. mu ' t ' , however, be remembered t • en uncer these favourable condis there has been sufficient pressure on ,ff'tu • ,° crack £0n:i0 of them, to spall m then- edges, to crush individual stones, on?*, n ? 0t , h61 ; dama 8 e - But on the cret» Pa , l i , W j lC coll;jse,J a solid con"eta wall had been substituted for the random lubble wall which elsewhere allowed for movement. Mr. liamer stated ■

Outward Movement Inevitable, "It follows from this that even had each separate stone of the rubble mound been placed iu its proper position by a diver, there world still have developed a pressure downwards and outwards p.gainsl the piles around which the mound is built. But, as a matter of actual fact, these stone;; were not deposited in that way. The evidence indicates that the stone form in™ the mound was delivered down a shoot at the back of the wharf. Some of it went straight into the water from this shoot, while some of the stone was placed in position from pontoons. "The evidence indicates that considerable care was taken to pack the stone carefully round the. piles, but if any stone was deposited directly from, the shoot there must have been a considerable weight against the piles from the moment the placing of the stone commenced, even before the mound had risen high enough to generate large forces due fr. changes of shape under settlement. The forces, due to changes of shape under settlement would be accentuated by any settlement of the mas.-> of the mound into the clay bed on which it rested, and would be further accentuated by the, super-imposed weight of the random rubble or concrete wall (as the ease may be) built on the top of the rubble mound, and of the filling placed behind such random rubble or concrete vail.

"That the combined effect of these forces was acting outwards and was very considerable is proved by the fact that the random rubble wall was moved outward a considerable distance, approaching 18i:i., where it was free to move, and has broken the piles where they resisted such movement, and that the concrete wall under "the shed with no filling behind it has moved outwards for a distance of about 1331 n.

"New let us consider the capacity of the whmrf to resist this pressure. Had we 00 info 'illation before us other than that to be g ined from the collapsed wharf, we would .'ave been compelled to fall back on fcheo etical calculations of the resistance to lateral thrust of such a structure as this, and this would have presented some di'jculties on account of the impossibility of accurately assessing the actual forces operating. But wo have the evidence of the damage to the part of the structure left standing and apparently undisturbed landward of bay 30 at 600ft-., which clearly indicates that the wharf is being strained beyond safe limits. Rom the evidence of the portion of the. wharf left standing outside bay 50 at 600 ft., it may safely be inferred that wherever else the piles gave way, they failed at a point immediately under the horizontal walings, and this is where they must fail in the 6vent of any outward movement of the upper part of the wharf, because the solid bracing of the upper part of the wharf would restrict the bending of the piles above the horizontal walings, which would result in a. concentration of the bending movement at the point of connection of the waling to the pile, a point which has been weakened by the cutting and rebuilding of the pile in the process of attaching the walings to it. There is also evidence of disruption at this point on the piles in the inner part of the wharf left apparently undisturbed. Had the upper part of the wharf not been braced, it is conceivable that the natural elasticity of the piles and the structure generally might have allowed of sufficient movement and adjustment, as to avoid actual collapse. "A glance at the structure or at the plans will indicate' that this wharf was not designed to resist any considerable pressure outwards. Land ties of li inches steel were put in at intervals of 40ft., but such ties would only have been suitable to sustain a working load of about 13 tons each, while their method of attachment (if such it can be called) at the outer end was not suitable to withstand even a fraction of this amount. Failure Under Stress.

" It is apparent that these ties have been drawn violently forward, and as the nuts were not anchored to anything which could make a considerable resistance, this is not surprising. In order that land ties should be effective it is essential that they be absolutely rigid (preferably in initial tension), because if it was necessary for the wharf, which they were intended to anchor, to move at all before the ties could take up their load, it appears inevitable that the wharf must be strained before the ties could give any assistance. An examination of the ties now in their original position on the standing part shows that they are affording practically no support to the structure. It appears to us that the aim of these ties should have been to prevent the pressure of such a wall bearing on the wharf, not to hold the wharf against the pressure of such a wall after the same had moved against it. However, as stated above, wo do not consider the ties had any appreciable elect either for or against the collapse. " Th-i actual results in the collapse of part or the wharf and signs of failure under the stresses in the parts still standing, clearly indicate that stresses have been and are still operating on the structure. such as it was not intended to, and could not, withstand. "With regard to the suggestion that the cause of the collapse was movement of the nibble mound due to a flaw or ' greasy back ' in the clay under such mound, the evidence of the pile-driver, Mr. Loch, who drove the piles at this place, of the dredgemaster, Mr. Brebner, who carried out the dredging of the benches under rubble mound, and Mr. Windsor, who carried out borings, do not indicate the likelihood of any such condition.

No Accumulation of Silt. " A further suggestion has been made that the interval of time which elapsed between the dredging of the benches at this place and the deposition of the rubble mound, allowed such an amount of silt to accumulate on the dredged benches, that the stone of the rubble mound never reached the hard ground of the benches, but was substantially 'afloat.' " We are of opinion that with a depth of stone of from 12ft. to 20ft.. gradually tipped in or placed, any soft accumulation of silt must have been displaced by the stones, and under these conditions the stones would gradually be forced down by the weight above until they reached the hard benches made by the dredge. This

opinion is entirely in accordance with the evidence of the board's engineer, who said ' the stone beds itself firmly and hard into the surface,' and in another place, ' the stone would dog-tooth into the floor and become a solid mas?." "An _ examination made under water in the diving dress by Mr. Vickerman, indicates that the rubble mound generally in the immediate vicinity of the first collapse, is spread out, and is now much flatter than when deposited. This is as might be expected, as when the pile or piles broke a considerable amount of stone resting against the back of the piles would run down and carry with it other stones so that the ultimate position of the stones would have a much flatter slope than the natural angle of repose of the material. _ _ That is to say, that under such conditions there would be a momentary rush of stone downwards. The evidence of the other diver indicates that this condition only obtained in the vicinity of the piles and that elsewhere the change was not so apparent." Design and Construction.

As to the question concerning the earrv- ! ing out of the work, the answer of the J commissioners is that all the evidence I which came before them, fortified by their own observations, indicated that the works were faithfully carried out. They note that "the pile-driving records accord with the borings, and indicate that every bearing pile was driven into the solid bottom or 'rock.' The appearance of the disintegrated structure, as shattered by the collapse and subsequent disruption by blasting, indicates that the concrete was first-class, and that the reinforcement was placed as intended by the designer. The records of soundings after dredging indicate that the dredging erf the benches and the 33ft. trench along the front of the wharf were carried out as closely in accord with the design as the limitations of dredging will allow." In reply to the question whether or not the designs and plans, considering all the circumstances, were good and sufficient, the commissioners give the following answer:—"ln view of all that has been stated above, it is clear that the answer to this question must be in the negative. The design was defective in that it provided a structure and an arrangement of parts of such a nature that stresses must be developed in it, stresses which a structure of such a character was unable to bear." The commissioners' answer to the third question, "Did the, dredging of the berth, or any other dredging, contribute to the collapse of the wharf?" is "No." They explain that because of the precaution taken by the engineer in dredging a 33ft. trench well inside and outside of the outer row of piles, no subsequent dredging down to this depth could have any effect, and as a matter of fact no further dredging has been carried down to this depth. "In view of what has gone before in this report." state the commissioners, in dealing with the question of possible ; negligence as a contributing cause of the collapse, "it is evident we are of opinion that want of foresight was displayed in designing and carrying out a structure unable to bear the stresses which must inevitably come upon it, otherwise no negligence has been shown by any officer or employee of the board." Wall Must be Removed.

Referring to necessary works for preventing a recurrence of extension of collapse in respect of the balance of the Western Wharf, and the steps requisite to make good the. damage, the commissioners state: " In preceding paragraphs wo hive shown that the influences which caused the collapse of the outer part of the Western Wharf, are operating throughout the inner part, and that the wharf has indications that it is disintegrating under the stresses now upon it. The proper and only permanently effective remedy for this state of affairs is to remove the stress and its resultant strain. This can only be done by taking out the filling behind the random rubble or concrete retaining wall, removing the wall itself, and taking away the rubble mound on which that wall stands. \ " The removal of these will take all actual stress from the wharf, when the small cracks in the piles will probably close owing to the elasticity of the material in the structure, and such portions of the concrete as remain visibly damI aged can be cut out and made good, as 1 it was before the rubble mound was de j posited under it. " With regard to the portion of the ,wharf between bays 29 and the existing end, it may be found necessary to pull

back the wharf into the original line by means of land ties, which ■ should first havo their ends properly enclosed in adequate masses of concrete abutting against the wall on the top of the tide deflector. After the filling, the wall, and the rubble mound have been removed, and the wharf pulled back, if necessary, into its original position, . any damaged concrete should be cut out and made good, after which this part of the wharf will be quite suitable for use and may be extended without danger of damage from the former causes. The Danger of Blasting.

proved subject to the reservation that the board should accept {till responsibility for 'the strength and sufficiency of the structure. Tlw responsibility was accepted by the board on January 23. 1910. The department's modified approval ; pave no indication as to what aspects of 'the design made the marine engineer hesitate in his approval. "We consider it unfortunate," add the commissioners, "that the marine engineer did not, more clearly indicate to the board the reasons for his evident uneasiness. Sub-section A of Section 150 of the Harbours Act, 1908, indicates an intention by the Legislature that the whole question of the stability of harbour works and the methods of their construction should be subject to Government approval." Process of the Collapse.

" With regard to the wrecked portion we consider that, the whole of the top, the braces and walings, and the distorted portions of the piles, should be blasted away and removed, so that the old retaining wall and the materials of the rubble mound may be dredged away by a grab or other appliance, after which any | extension to the wharf may be carried • out.

" But on no account should any blasting, or other work liable to create shocks, bo carried out until the whole of the standing part of the Western Wharf is made safe, as provided for herein. '* If the board requires the use of the area between the back of the wharf and the tide deflector wall, as we presume it will, we recommend that the area be either covered with a reinforced concrete wharf, as suggested by Mr. Holderness in his sketch design for wharf at the Eastern breakwater or tide deflector, or that any filling be held back by a gravity wall founded on piles driven at a slight rake to provide against forward movement and furnished with adequate land ties. Any new extension of the present wharf toward the tide deflector would, of course, be made monolithic with the present wharf. » Strain on the Prince's Wharf.

In reply to the final question on the order of reference, "Whether the causes which contributed to the Western Wharf collapse are likely to apply to any of the other wharves of the board," the commissioners submit the following report :—

" In this connection we have to say I that the influences which led to the col- 1 lapse of the Western Wharf are operat- j ing at all the board's wharves where the ; filling behind such wharves is held back j by a retaining wall founded on a rubble • mound on sidelong ground. We have i carefully examined all these wharves and j in most cases have found indications of j strain in the form of cracked piles, braces, | or walings, notably at Prince's Whart, at Calliope Dock, and at Freeman's Bay. At nono of these places does there appear to be immediate danger of disruption, but we consider that all these places should be carefully inspected from time to time, and in the event of any considerable development of the existing damages or any new development, steps should be taken to relieve the pressure from behind. "At a point on the western side, of the reclamation under the Prince's Wharf the retaining wall holding back the filling and standing on a rubble inound, has moved forward and is pressing so severely on one of the piles as to have broken it and cracked the brace where it joined such pile. Such evidence as we have indicates that the movement of the wall has been small, a few inches only, but its effects are very marked. The front or northern wall of this reclamation shows a crack some inches wide, and this crack may be considered as the measure of the advance of the western wall. The amount of wharf mass, piles, braces, walings, beams and deck, opposite the point where the b'ick pressure is on the pile, is very considerable, and it is quite possible that it may be able to resist the outward pressure, without the damage extending. But we consider that in such an important work as the new Prince's Wharf, nothing should be left to chance, and that the piles should be relieved of transverse pressure. "On the north wall and the east wall under the Prince's Wharf approaches there are other similar cracks, some of considerable magnitude, indicating some movement, of the walls, but so far as can bo seen these movements have not yet brought about any indication of strains in the adjacent parts of the wharf. The wharves of the board generally other than these above referred to, are jetty wharves, and cannot be affected by the causes which have brought about the collapse of the Western Wharf.'' The commissioners note that the design of the Western Wharf was submitted to the Marine Department for approval in October, 1918, and that plans were ap-

The commissioners discuss the process of the collapse of the wharf, and note that when the collapsed portion finally came down, the table-top swung outwards until the outer end was about 30ft. outside the original line. The part still standing, although it sprang back partly when the wrecked portion was cut away, is still some 7jin. outside the original position at its outer end

On examining the remaining portion of j the wharf the commissioners found that ! the retaining wall of rough rubble had bulged :n shifting from the position in which it had originally been placed. The balance of this wall appeared to be in imminent danger of collapsing. Observafions indicated that the random rubble I wall behind the wharf had settled down- ! ward something more, perhaps much more than 12 inches, and has moved outwards about IB inches, where the back piles did not resist such movement. Adverse Conclusions Regretted.

" We regret," state the commissioners, in concluding their report, " that certain of our conclusions appear to reflect ad versely upon some of the board's officers, but these conclusions have been irresistibly forced upon us, and we would impress the board when considering these findings to bear in rnind that no great engineering works, extending, as these have done, over nearly a quarter of a century, have ever been constructed without something having been done which the passage of time has indicated should not have been done."

SEVER POINTS EXPLAINED

QUESTIONS BY THE BOARD.

At the conclusion of the board's meeting, tho chairman stated that din-ing the afternoon the commissioners attended the me .'ting, and in reply to questions by board members, made the following statements: — That the Marine Department should have been more explicit, in its objections to the plans of Western Wharf. That the alterations made in the original design of the wharf were an improvement.

That the defect shown in tho stone bank at Prince's Wharf did not in any way affect the stability of the wharf. That their report in regard to defects in stone walls did not affect King's Queen's, Northern or Central Wharves.

That the design of Western Wharf was sufficient to carry the loads it was intended to bear.

That the bracing of a wharf with a stone bank on sidelong ground was not a universal practice, and was, in their opinion, defective. That the disaster might have been averted had the structure been watched and steps taken to relieve the pressure. That they did not state in their report that all retaining walls were built on sidelong ground. That there was no immediate danger of collapse of the Slieerlegs and other wharves built on embankments, but they would need to be watched careful!v.

Mr. W. H. Hamer, engineer to the board, also attended the meeting, and showed plans of important wharves in America and Sydney with rubble mounds as a brace, similar to the Western Wharf. He also stated, said Mr. Mackenzie, that the whole of the Quay Street frontage on the harbour is built of solid masonry and stone, and is in no case built on a sidelong slope, as assumed by the commissioners. Further, that the Sheerlegs Wharf is also built on a solid foundation, a.nd not on a "sidelong slope, as assumed by the commissioners. In reply to a question, Mr. Hamer said he had no statement to make in reply to the leport of the commission.