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Foundation Performance of a 100-year-old Bridge1

R. F. LEGGET 531 Ecllo Drive, Ottn,vn, Cntzudrr

AND

F . L . PECKOVER Geotecl~t~ical Services, Cotlnclintl Natiotinl Raihtviys, Box 8100, Motlrrecil 101, Q~reOec

Received March 5, 1973

Accepted April 26, 1973

The Miramichi bridges in New Brunswick, each of six 200-ft (61-m) spans, were built as part of the Intercolonial Railway by Sir Sandford Fleming in 1871-75. When construction of the Northwest Bridge started, it was found that early borings were incorrect and the layer of gravel and sand on which the piers were to rest was underlain by deposits of silty sand and clay-silt. The caissons of the piers were therefore enlarged in order to spread the load better. On observing settlements during construction, Fleming stopped the work and preloaded each pier in excess of the design load. He resumed construction when he observed that settlements had stopped.

The level of the bridge piers has been measured periodically since 1914. Recent borings have been made, giving a basis for analysis of the bridge performance. Calculated primary settlements are compared with those measured during construction. Surveyed pier movements are adjusted to reduce errors, and compared with calculated secondary settlements. It is deduced that the piers have settled at a rate equal to or less than that indicated by the theory of secondary compression.

The bridge is continuing to perform in a predictable and safe manner, providing an outstanding example of early Canadian engineering.

Les ponts de la Miramichi au Nouveau Brunswick, chacun de six travtes de 200 pi., ont Cte construits en 1871-75 par Sir Sandford Fleming, faisant partie du Chemin de fer Intercolonial. Au debut des travaux de construction du pont nord-ouest, on a decouvert que les forages preliminaires Ctaient inexacts et qu'en realit6 des dCp6ts de sable silteux et d'argile-silt se trouvaient au dessous de la couche de gravier et de sable sur laquelle devait reposer les piliers. Pour compenser. les caissons de fondation des piliers furent elargis afin de repartir d'avantage la charge. Observant des tassements, Fleminz devait suspendre les travaux et appliquer une surcharge depassant la charge de calcul sur chaque pilier. Les travaux devaient reprendre lorsqu'on observa 1'Ctat stable des piliers.

Le niveau des piliers du pont a CtC mesurt periodiquement depuis 1914. Des forages ont ete effect~lks recemment permettant ainsi d'analyser la performance du pont. Les tassements primaires calc~lles sont compares ceus mesurCs durant les travaux de construction. Les etudes des mouvements des piliers sont ajustCes afin de reduire les erreurs et sont comparkes aux tassements secondaires calculCs. Ceci demontre que les piliers ont subi un tassement d'un degrC de vitesse egale, ou moindre que, celui indique par la thColie de compression secondaire.

La performance du pont se continue de f a ~ o n prkvisible et s i re , fournissant un exemple marquant du genie canadien de I'Cpoque.

A Canadian single track main line railway bridge, opened for use on 26 August 1875, has performed satisfactorily for almost a century, despite serious doubts raised about its foundations during its construction which started almost exactly 100 years ago. Preliminary studies included some pioneer penetration tests on the underlying clay. During construction, preloading of the piers was carried out to accelerate anticipated settlement. Correlation of 'the rec-

'Presented at the 25th Canadian Geotechnical Con- ference, Ottawa, Canada. December 7-8, 1972.

ords of levels on the piers taken at that time and since then with calculated primary and secondary consolidation settlement is discussed. This paper contains a summary account of the construction and design of the foundations of this important structure by one of Canada's most eminent early engineers, Mr. (later Sir) Sandford Fleming.

I t seems desirable to thc authors that they should, at the outset of the paper, acknowledge their appreciation of the inadequacy of the geotcchnical information which they have been able to assemble for the Miramichi Bridge. The

Can. Gcotcch. . I . , 10. jO!4 (1973)

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LEGGET A N D PECKOVER: FOUNDATION PERFORMANCE 505

FIG. 1. Location of Miramichi Bridges, from Fleming 1876. I

records do not compare, for example, with the unbroken sixty-year settlement observations at the Emprcss Hotel, Victoria, reported upon by Crawford and Sutherland (1971 ). The authors vcnture to think, however, that the fact that it has been possible to prepare even a partial record of the performance of a major Canadian bridge over a period of 100 years will compen- sate for the gaps in the record now presented. The authors present their considered evalua- tion of thc assembled information while regret- ting thc impossibility of more rigorous analysis.

The Intercolonial The bridge is one of the two six-span bridges

which now carry the main Montreal to Halifax linc of Canadian National Railways over the northwest and southwest branches of the Miramichi River, New Brunswick, about 2 milcs upstrcam of the junction of the two branches and 3 milcs from Newcastle Station on the west bank of the combined river (see Fig. 1 ) . Thc two bridgcs are similar in overall design but that over the southwest branch gave risc to no unusual problems in design, con-

struction, or maintenance. Attention will be dirccted to thc bridge over the northwest arm (Fig. 2 ) but since both were constructed at the same time, and each has six 2 0 0 4 (61-m) spans, many general comments apply to both.

The Halifax to Montreal line of the C.N.R. was built as 'The Intercolonial Railway', a name still used by sonre who live along its route. It was thc first grcat pionecr railway of Canada, pcnetrating difficult underdeveloped country for more than half of its total length. Connection by rail of the Maritime Provinces with Montreal was suggested as carly as 1832. A numbcr of projects were discussed in the following 25 years, one almost approaching the construction stage, but all these attempts proved abortive. The Grand Trunk Railway extcnded its lincs eastward as far as Rivikre du Loup. Thc Nova Scotia Railway built its line from Halifax to Truro but between Truro and Rivikrc du Loup there still remaincd 500 miles of relatively unexplored country to be crossed.

The vital need for this physical transporta- tion link bctween the separated provinces be- came evident as the possibility of federation

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506 CAN. GEOTECH. J . VOL. 10, 1973

FIG. 2. The Northwest Miramichi Bridge.

approached a reality. Sandford Fleming was therefore appointed in 1862 as Engineer for the pros~ective railway by the Imperial Govern- ment and that of the United Province of Canada. He initiated preliminary surveys. Con- struction of the railway was promised through clause 145 of the British North America Act. It now seems entircly probable that Confedera- tion would not then have been achieved had this assurance not been given. Four Railway Commissioners were appointed in 1868 but Sandford Fleming's appointment as Chief En- gineer was independently confirmed. Argu- ments soon started, especially between Fleming and C. J. Brydges, one of the Commissioners who was also the general manager of the Grand Trunk Railway.

The Commissioners had their way in forcing Fleming to award lump sum contracts instead of the more usual type of civil engineering con- tract based on unit prices, with what doleful results he relates in the book that he produced as his final report to the Minister of Public Works (Fleming 1876). On the other hand, Fleming had his way in having all bridges and other structures constructed of masonry and iron instead of in timber, as the Commissioners wished as a measure of economy. C.N.R. engi-

neers of recent years have testified to the ex- cellence of the original masonry work all of which is still in service."

Major civil engineering works on the line consisted of a number of short tunnels, some extensive drainage works including tunnels beneath the track to serve as culverts, and a variety of bridges. That over the Restigouche River at Matapedia, and the twin bridges over the Miramichi, would rank as important structures even today. When built a century ago, they were truly outstanding examples of ad- vanced bridge design and construction.

The Miramichi Bridges The sites selected for the Miramichi crossings

were 1350 and 1600 ft (41 1 and 487 m) wide, respectively, for the northwest and southwest branches. The initial surveys and borings suggested that bedrock would be reached across both sites between 45 and 50 ft (13.7 and 15.2 m) below H.W.0.S.T.3 with a tidal range

'Condon, F. H., 1939, personal communication to Legget, R. F.

H.W.0.S.T.-High Water on Spring Tides, but since some readers may not be familiar with Maritime ter- minology, H.W.L. will be used hereafter to denote high water level at the bridge.

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LEGGET AND PECKOVER: FOI JNDATION PERFORMANCE 507

of 10 ft (3 m) , the depth of water ranging generally between 15 and 33 ft (4.6 and 10.0 m) . The original borings showed a bed of silt ovcrlying the assumed bedrock. Designs were accordingly prepared using standard iron trusses of 200 ft (61 m) span, with five for the northwest bridge and six for the southwest bridge, the effective widths of waterway between the masonry abutments being thus, respectively, 1000 and 1200 ft (305 and 365 m ) . The bridge piers were all to be founded upon open timber caissons, floated into place and filled with tremie concrete to the appropriate level.

When construction work started in 1869 it was quickly found that the material thought to be bedrock was actually gravel and sand, beneath which was found 'silt' in the northwest branch to a depth of 112 ft (34.0 m) and 'clay' in the southwest branch to a depth of 90 ft (27.4 m) , both being average figures. In his own account, Fleming quite frankly admits that the initial borings were made with "only such boring implements . . . as could be extem- porized in the neighbourhood by a country blacksmith".

He goes on to explain that "the operator saw sandstone rock cropping out on the river- bank; and he naturally, but as it afterwards proved incorrectly, inferred that he had struck a continuation of the rock formation underlying the river" when he encountered the hard material.

"During the winter of 1870-71" Fleming goes on to relate, "more perfect boring implements were employed in testing the nature of the river bottom before building operations were cornmcnced. It was then that the true nature of the river bed was discovered" and designs were accordingly reviewed. Even with the still imperfect equipment then available, Fleming was able to deduce the surprisingly different characteristics of the river bed material beneath the gravel and sand stratum in the two branches. This difference is itself of much interest, due at least in part to the significant differences between the geology of the two catchment areas. No changes were needed in the pier designs for the southwest bridge, construction of which proceeded as planned and

~ was completed without any serious difficulties. The second set of borings, taken on center

line and 50 ft (15.2 m) on either side of each pier, showed that the material beneath the gravel and sand in the northwest arm was of a decidedly weaker character. Fleming therefore decided to increase the number of spans to six which would permit both abutments to be founded on rock, moving the south one closer to the natural rivcr bank. This involved an extra payment to the contractor which Fleming officially recommended on 13 January 1872. The Commissioners did not support his recom- mendation but insisted on an independent opinion bcing obtained. Engineers S. Keefer and C. S. Gzowski prepared a report after care- ful study, advising the use of timber piles to support the piers. Fleming disagreed and carried out what would today be called deep penetration tests in the weak material in order to support his view (Peckover and Legget 1973).

He won the support of thc Chairman of the Commissioners and so proceeded with his re- vised design, still using open timber caissons although of increased base area to reduce the unit load on the gravel and so upon the underlying clay (Legget 1972). He also introduced a fill of quarry rubble stone with alternate layers of concrete for the lower part of each caisson, in place of the weak material in the upper part of the river bed. Concrete was then ~ised above the level of the river bed up to low watcr level, and thereaftcr solid masonry.

Indicative of the speed of construction achieved, with abundant manpower, was the start of building the timber caisson for one pier on 19 June 1872; it had been completed, floated out, and sunk in position by 6 July; and by 26 July concrete had been brought up to the required height for masonry to proceed. Exca- vation was carried out by buckets and dredge pumps operated by fixed 'dredging towers' at each pier. Divers were used as necessary, as for the removal of buried logs, many of which were encountered. Concrete was made from "broken stone, coarse river sand, and the celebrated English Portland Cement". Stone for masonry came from two quarries, one 4 miles upstream on the Northwest Miramichi and the other 17 miles away at the mouth of the River Bartibogue. Messrs. Brown, Brooks, and Ryan were the contractors for both bridges. Mr. W. B. Smellic was Resident Engineer for both, under Mr. A. L. Light, the District Engineer.

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PIERS AND N 5 ABUTMENTS

DISTANCE FT

FIG. 3. Cross section showing foundation conditions and 1961 test borings, N.W. Miramichi Bridge.

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L E G G E T AND P E C K O V E R f:OUNl)r\TION PERFORMANCE 509

The first train passed over the completed bridges on 26 August 1875 and they have been in use contin~iously since that day.

Piers for the N.W. Bridge The difficulties that Sandford Fleming had

to contend with in redesigning and constructing the piers for the N.W. Bridge and the degree of his success can best be judged by combining a detailed account of his approach with an analysis based on the results of a recent study made by Canadian National Railways. The purpose of the latter was to determine the con- dition of the masonry in the piers and to predict future performance of the bridge.

The river bed profile which Fleming records from his second set of construction borings is shown in Table 1, compared with the findings of C.N.R. borings made in 1961. Figure 3 shows a section across the river bed, interpreted from both the 1961 borings and Fleming's findings.

The most obvious difference between the 1871 and 1961 borings is Fleming's identifica- tion of the thick deposits of fine sand and underlying marine clay and silt as 'silt', an understandable error with the wash boring techniques of the day. Although the term 'clay' is used in his reports, it is reserved for material such as tough clay till with s~tbstantial cohesion which probably still appeared as chunks in the wash water of the borings. All material which would have appeared in suspension in the wash water, with grains too fine to be seen, appears to have been called 'silt'.

In dredging the sand and mud layer above the gravcl, Fleming reported that pieces of dc-

TABLE 1. Comparison of river bed profile from 1871 and 1961 borings

.- -. .-

1871 boring~ 1961 boring~

Sand and mud, organic matter, logs

Gravel

Silt

Clay, overlain by gravel on ~ 0 ~ 1 t h side

Sandstone bedrock

-

16 ft Fine sand, pieces 16 ft of wood

9 ft Gravel and sand 3-18 ft Fine sand (a lens-

40f t { shaped deposit) 0-44 ft Silty clay, thicker

near banks 17-41 ft 10 ft Gravel and sand, 0-7 ft

missing on north side

Sandstone bedrock

cayed wood frequently jammed his pumps, and he indeed had difficulty in removing two birch logs 16 and 20 in. (40 and 50 cm) in diameter from within the caisson of Pier 1, 40 ft ( 12 m) below H.W.L. Pieces of wood were also frequently found in the 1961 borings. This upper stratum has clearly been subject to removal by river scour and redeposition at times during its history. No organic material was found in the lower layers.

Foundation Soil Conditions Fleming describes in detail his procedures

and observations during pier construction. A review of the engineering properties of the foundation soils will assist in appraising what he rclates; the 1961 borings provide this information. Five holes were put down to bedrock, one near each pier. Borings were also made down through the entire depth of Piers 1, 3, and 5 in order to determine thc coildition of the masonry and to establish on what the piers were resting. The location of all borings is shown in Fig. 3.

The borings through Piers 3 and 5 showed them to be founded on the lower part of the layer of fine sand instead of on the gravel as Fleming had intended. Sand to a depth of 3 and 6 ft (0.9 and 1.8 m), respectively, was identified under the base of the piers before the borings were stopped. In the base of Pier 1 the boring passed through a mixture of fine sand and gravel-sized picccs of sandstone between El. -19 and -32 before stopping. It is thought that this deposit may be a layer of the quarry rubble stone used to fill the caissons, now in a state of disintegration due to long submergence. The rubble was interlaid with concrete up to river bed level. The 1961 soil borings were made from a scow, samples being recovered with a 2-in. (5.1-cm) spoon, 2-in. (5.1-cm) thin walled tube, or 3-in. (7.6-cm) piston sampler as appropriatc. The latter sampler was used generally in the clay-silt material. General soil test properties are shown in Fig. 4, representing a boring log adjacent to Pier 1. Descriptioils and average properties of the various soil deposits are given in Table 2.

Clay-Silt Materia! The layer of clay and silt is that mainly

influencing the bridge behavior. Although this

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C A N . GEOTECH. J . VOL. 10, 1973

TABLE 2. Properties of soils and rock as determined in 1961 - - - . - - -

-

Deposit Average properties

SAND, fine to medium, little silt, pieces of wood grey, very loose to loose

GRAVEL and SAND, grey, dense

SAND, fine, little silt, uniform, grey, medium dense

CLAY and SILT, some sand, layered, grey, firm, sensitive

GRAVEL and SAND, grey, dense

SANDSTONE

Clay and silt sizes 12% Water content 32% Standard penetration test

5 blows/ft

Variable grain size Standard penetration test

24 blows/ft

Silt and clay sizes 12% Water content 28% Standard penetration test

15 blows/ft

Sand (and gravel) sizes 21%

Clay and silt sizes 79% Water content 30% Liquid limit 31 Plastic limit 22 Wet unit weight 121 Ib/ft3 Void ratio 0.8 Undrained shear strength

740 Ib/ft2

Standard penetration test 38 blows/ft

Fine to coarse grained, brown, weathered, weakly cemented

was recognizcd at the time of the 1961 study, many of the sample test records have been mis- laid unfortunately, since that time, and so were not available for the prcparation of this paper.

The clay-silt is not a homogeneous material. Its percentage of silt content is not known but the silt occurs mainly in layers and is indicated by the average low plasticity index and relatively high unit weight and low void ratio of the deposit. A typical pressure - void ratio curve for the material is shown in Fig. 5. From tests on samples from various depths in borings adjacent to Piers 1 and 5, values of functions for settlement analyses have been chosen as follows, keeping in mind the anisotropic nature of the material.

Compression Index Cc Test values ranged from 0.13 to 0.30. Since

the low values appear to be due to sample disturbance, a value of 0.3 is used in analyses, consistent with water content and liquid limit relationships.

CoefJicierzt o f Recompression Cr A value of 0.05 is used from consolidatioil

test results.

Coeficient of Consolidation c,; Average value from laboratory tests is 0.4 ft2

(0.04 m" per day. It is thought that field values will be higher than this since the laboratory c, is reduced by sample disturbance; in particular, three-dimensional drainage and mul- tiple drainage layers are present in field conditions but not in a laboratory sample. For these reasons a value of 1.0 ft2 (0.09 m2) per day has been used in analyses.

Preconsolidation Pressure p,. Average values from consolidation curves

range from 1.2 to 1.7 t/ft2 (1.2 to 1.6 kg/cm". These may be considered low if samples are disturbed, or high if samples have been subject to the influence of pier loads nearby. The relationship of the values to effective stresses is discussed below.

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LEGGET AND PECKOVER FOUND4TION PERFORMANCE 51 1

SAND 100 O/o

UPPER PART, GREY,

VERY LOOSE TO SAND 100 '10

GRAVEL 15, SAND 40,

M E D I U M D E N S E

C L A Y AN[? SILT,

LAYERED, F I R M ,

SENSITIVE, GREY 21 * 134p.d. c.735ps.f.

FIG. 4. Log of boring adjacent to Pier 1 and test results, 1961.

Coefficient o f Secordary Conzpressiorl C , Value varies as a function of stress level:

values used are 0.004 at Piers 2 and 3, 0.005 at Pier 4, and 0.008 at Piers 1 and 5.

Loading Conditions Load distribution under the piers is estimated

as the basis for calculating settlements in each case. Pier 1 is selected as an example for the procedure since it was built first, Fleming describes what he did there in particular detail, and it has undergone appreciable settlement since that time.

Many assumptions are required in analysis, since as-buiIt drawings are naturally not avail-

able. Dimensions of Pier 1 are shown in Fig. 6 (a) . The pier and its foundation conditions are shown in (b) . The original natural effective stress yo at mid-depth of the clay-silt layer, 40 ft (12.2 m) below the river bed, is 2400 Ib/ft2 (11 700 kg/m2), assuming an average submerged weight of soil as 60 1b/ft3 (960 kg/m3). If it is conservatively assumed that no load is distributed by friction at the sides of the caisson, and that pier and riprap are circular loads, their weight will increase stresses at mid- depth of the Iayer by 1600 lb/ft2 (7800 kg/m2) due to the pier as shown in Fig. 6(c) and an additional 300 lb/ft2 (1460 kg/m2) due to the riprap, giving a total increase of

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CAN. GEOTECH. J . VOL. 10, 1973

1 TEST BORING NW-2 SAMPLE No.9

I I 10

PRESSURE TONS/ FT2

FIG. 5. Typical pressure - void ratio curve for clay-silt material.

CLAY - SILT

?COO 4 0 0 0

S O I L PRESSURE Ib per sq f t

FIG. 6. Foundation conditions at Pier 1; ( a ) out l~nes of pier; (b) foundation conditions; (c) increase in soil pressure due to pier; and (d ) effective stresses and preconsolidation pressures.

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LEGGET AND PECKOVER: FOUNDATION PERFORMANCE

FIG. 7 . Measured and calculated primary settlements vs. load at Pier 1 during construction period.

E o W

0

1900 lb/ft"(9270 kg/mq. This change in loading conditions, together with preconsolidation pressures found in laboratory tests, is plotted against dcpth in Fig. 6 (d) . It is apparent that application of the pier load will cause stresses to exceed p,. appreciably only in the upper part of the clay-silt layer. Most of the consolidation will have taken place there.

The factor of safety against general shear failure of the pier foundation is calculated to be nearly four. As the creep strength of the clay-silt is unlikely to be exceeded by this load, the possibility of pier movement due to creep failure is not considered.

2 5 0 0 -

- cn

!5

Settlements During Construction A review of the construction of Pier 1 will

show the manner in which the loads were applied and the settlements took place. A load - settlement time plot is shown in Fig. 7. Con- crete was placed from river bed level to El. -15 within the caisson during June and July

-

a ? -15 I PRELOAD * 2 0 0 0 - Z - ' 0

$ 2 6 2 ; REMOVED -

LL - w J \ MASONRY 1500-,: V , W O COMPLETED-

%I- z p " 7 0 0 DAYS

1873. After some dificulties in dewatering the caisson to place concrctc in its upper part, the work was deferred until the following spring. Concrete was then completed and masonry work started on the pier in May 1874. By the end of June it was found that thc caisson had settled 6 in. ( IS cm) from its original position. Fleming instituted regular measurements that showed gradual settlement was taking place. When the masonry was nearly to full height in July i t was stopped, settlement at that time totalling 10.5 in. (26.7 cm).

Fleming judged that this settlement was due to consolidation of the stone filling in the caisson under the weight of the pier. The idea of foundation consolidation settlement was not then understood. By his penetration tests he had already satisfied himself that the gravel layer would support the pier without failure. With his usual initiative, he therefore set about solving the problem-by building a platform around the masonry and placing a load "greater than, on completion of the bridge, it would be

00

0 1000- v,s - a S

TIME DAYS -

100 400 5 0 0 6 0 I

- C Y .25 - I-

. 5 0 W I y .75- I- I- w 1.0 V)

I

- -

MEASURED (FLEMING+ - -

CALCULATED

b, .\ - \ -

&-----

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514 CAN. GEOTECH. J . VOL. 10, 1973

required to carry, and thus by direct weight forcing the whole structure to a permanent bearing". Stones and rails to a weight of 450 t (4.6 x 10" kg) were placed. The calculated weight of the finished iron trusses and bridge deck is 400 t (4.1 x 10" kg).

A further settlement of 2.5 in. (6.4 cm) was measured under this load up to the beginning of October. The load remained in place until the following spring when another 100 t (1.0 x 10" kg) were added. Fleming says that "no further settlement was perceptible" after Oc- tober. He closely examined the caisson and reported no disturbance to thc piling and no yielding of the supporting gravel stratum. With this confirmation of his opinion of the cause of the settlement, he had the load removed, the structure complcted, falscwork and sheeting removed, and riprap placcd.

The typical sequence of loading for all other piers was caisson, masonry, and preload in quick succession, followed after a few months by the addition of riprap. Removal of preload and substitution of truss and deck loads are not detailed by Fleming and are neglected in settlement analyses as they are believed to have occurred over a relatively short time during the pcriod of secondary scttlcment.

From present knowlcdge, the settlements observed during construction were made up of an immediate settlement due to the elastic compression of the soil under undrained or no volume change conditions and primary consolidation due to the dissipation of pore pressures. The immediate settlement was naturally small in comparison with the primary settlement.

Immediate settlement can be calculated from the equation (Leonards 1962) :

where ~ p = uniform pressure on surface of loaded area (2500 Ib/ft2 (1 2 200 kg/m" )-see Fig. 6 (c) ,

b = width of loaded area (32 f t (9.7 m ) ) ,

p = Poisson's ratio (0.5), I, = influence value, depending on foun-

dation shape (0.8),

E = modulus of elasticity for gravel- sand 'and clay-silt layers (2800 Ib/in.2 ( 195 kg/cm2) ) . This value is interpolated with judgment from average values given by Leonards.

Immediate settlement AH^ is thus calculated as 0.12 ft (3.7 cm) for Pier 1.

Primary settlement is found from conven- tional formulae:

C H AH-' log' l - 1 - 1 - e ~ po

for consolidation up to the preconsolidation pressure, and

COH* Pa A H z = - log - 1 + eo PC

for consolidation beyond p,. where H, = thickness of compressible layer

(25 ft (7.6 m ) ), and pa = total stress at mid-depth of layer

(4500 Ib/ft2 (21 960 kg/m2) ). Values calculated for Pier 1 are AH1 = 0.09 and AH^ = 0.63 ft (0.03 and 0.19 m) . Total settlement during construction is therefore calculated as:

If the preconsolidation load is not considered, primary settlement will be greater than calculated. Using c,, the time to 90% consolidation is calculated as 186 days.

The approximate primary settlement curve is plotted in Fig. 7. It is seen that measured settlements are roughly 50% greater than calculated. The difference may be due in part to the influence of the stone filling within the caisson which under load would have penetrated into the zone of soil at its base which was loosened by excavation. In addition, the value of compression index may in reality be higher than that assumed, depending on the degree of disturbance of the samples tested.

The construction of the other piers was similar to that of Pier 1 with minor variations. All were preloaded when settlement was detected. In each case the preload applied was more than the finished weight of the structure. All settled monolithically and vertically, and their settlement was observed to stop within a few weeks or months at the most.

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CAN. GEOTECH. J . VOL. 10, 1973

PlER N 5 4 3 2 I S

C A L C U L A T E D

1.5 k I I I I FIG. 9. Measured vs. calculated primary settlements for all piers.

TIME YEARS 94 9 8

- 2 - -

- 3 - -

5 - -

(A) SURVEY DURING (0) 1914-53 SURVEYS (C) 1961 - 6 8 SURVEYS CONSTRUCTION

1874 LEGEND - S-ABUTMENT

&--A PlER I

NOTE DIFFERENT MARKERS USED FOR EACH - GROUP OF SURVEYS

FIG. 10. Results of settlement surveys on Pier 1 and south abutment made in: (a) 1874; (b) 1914-53; and (c) 1961-68.

From Fleming's text the details of construction and preloading can be plotted as in Fig. 8. In Fig. 8 (a) the time duration of the masonry construction and preloading as well as 90% primary consolidation are shown. In Fig. 8 (b) the time rate of measured settlements is plotted for each pier. For Piers 1, 2, and 3 the surcharge loads were left on throughout a winter. It is reported that settlement was not observed

after the last readings shown at about 130 to 150 days. Piers 4 and 5 were built last, which probably explains why they were surcharged only briefly as time was pressing for completion of the bridge.

From Fig. 8 (a) it is seen that the preloading time exceeded the approximate duration of primary settlement for Piers 1, 2, and 3. The effect of the reduced prcloading time for Piers

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PIERS N 5 4 3 2 I S

- V - CALCULATEDSECONDAR

S E T T L E M E N T

FIG. 1 1 . Adjusted levels of piers vs. calculated secondary settlements.

4 and 5 is shown most clearly in Fig. 9 where measured and calculated primary settlements are compared for each pier. The abutments are known to be founded on rock. It seems evident that if preloading on Piers 4 and 5 had been continued, the same order of relationship with calculated values would have been reached. This figure shows another point worthy of note. The general pattern of pier movement is accounted for by the distribution of soil deposits across the river, as shown in Fig. 1. Piers 1 and 5 rest most directly on the layer of clay-silt. At the other piers the load is spread by the deposit of silty sand before reaching the clay- silt. Under these conditions it would be ex- pected that Piers 1 and 5 would settle most, and the observations confirm this.

Settlements Since Construction There is no evidence that movements of the

piers were of concern for many years after completion of the bridge. Eventually in 1914, some 41 years after construction was started, a check on pier movements was made. Survey marks were installed and level surveys made in 1914, 1919, 1950, and 1953. In the latter year the survey marks were unfortunately dis- placed when the bridge masonry was grouted

and repointed. Surveys were resumed with new marks in 1961 and were made almost yearly to 1968. Although sight distances between piers are 200 f t (61 m) , all these surveys were made with a standard level and rod, recording to the nearest 0.001 ft (0.0003 m) . Bench marks used for the first survey are not known but for the latter, steel rails were driven in the approach embankments to the bridge.

Taking Pier 1 as an example again, Fig. 10 is plotted showing pier settlement during the course of each survey, as well as during Flem- ing's original measurements. Interpretation of all the surveys is difficult as the relationship between bench marks is not known. The pattern is similar for the other piers.

There are unknown errors in all the surveys. An indication of these is shown by the plot in Fig. 10 of the elevation of the south abutment relative to Pier 1 for the 19 14-53 and 1961-68 surveys. This abutment is founded on rock and should therefore be at a constant elevation. The magnitude of survey errors in the 1961-68 surveys can readily be seen in the varying difference in elevation between the south abutment and Pier 1. These surveys do not warrant any accurate conclusions as to the recent settlement of the pier. The apparent downward

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518 CAN. GEOTECH. J . VOL 10, 1973

trend in the elevation of the abutment on rock, A comparison of these findings with the however, raises suspicion that the driven rail calculated secondary compression for all piers bench mark may have been subject to upward will be of interest. The following formula (U.S. heave by frost action. Department of Navy 1963) is used for calcula-

The 191 4-53 group of surveys shows a more tion: definite trend of pier settlement while the elevation of the abutment is relatively constant. If AH, = C,H, log&

plotted against the log of time, these surveys t ,

could be taken to mean that secondary com- Where t,,, = time over which A H , takes place pression was proceeding, 79 years after con- (1968-1875 = 93 years), and struction, at the rate of nearly 0.007 f t (0.002 t, = time of 90% primary consolida- m) per year, or about six times the theoretical tion (186 days;. rate. If this trend continued to the present time, Using values of C , varying with the stress secondary compression since construction level in the clay-silt layer as mentioned previ- would amount to about 2.0 ft (0.6 m). ously, the secondary compression for each pier

There are two practical points to cast doubt is obtained and plotted below the grade line on such a conclusion. Firstly, the substantial between abutments in Fig. 11, assuming that granular (silt) content and restraining effect most of the primary consolidation was complete of the silt layers in the clay-silt deposit make when the piers were finished. it unlikely that secondary com~~ression is still The plot shows reasonable agreement with SO active. Secondly, there is no l~hysical indica- the adjusted survey levels for Piers 1, 4, and tion that the bridge piers have settled this 5. Piers 4 and 5 show no sign of excess settle- amount. AS the present capstoile of Pier 1 is ment due to the unfinished primary consolida- now about 0.4 ft (0.1 m) below the south tion at the time the piers were finished, as noted abutment, it would be necessary to assume that in Fig. 8. Piers 2 and 3 are relatively higher in the masonry at the top of the pier has been elevation. This suggests the faint possibility raised at some time. There is no sign of this that Fleming, although observing no more and it is not known to have happened. settlement during the preloading time at his

In past interpretations of the surveys, Fkm- disposal for these piers, perhaps sensed that ing's assurance that both abutments are there would be some continuing movement and founded on rock has been apparently ignored built the bridge higher toward the center. He in favor of the installed bench marks. If it is does not mention such an intention. assumed that the abutments have not settled- Although individual judgments on the in- and this seems reasonable-then the results of terpretation of the survey data will vary, it is each survey can be adjusted by taking the the authors' conclusion that the weight of evi- averaged surveyed elevation of the abutment dence indicates that secondary compression of as fixed. Individual survey profiles of pier levels the piers is generally proceeding at a rate close may then be matched to these assumed abut- to or less than that given by theory, rather than ment levels by a linear distribution of differ- at a much faster rate as one interpretation of ences in level. The results are plotted for ap- the data would indicate. praisal in Fig. 11. The abutments were not Regular annual surveys have been started built at the same level since the bridge was on to measure pier movements in all directions, a slight grade. using the most accurate available methods, to

This plot should show the net change in check thesefindings. elevation of piers relative to abutments over the whole period 1914 to 1968. With the ex- ception of the 1914 survey levels for Piers 1, Acknowledgments 4, and 5 , it is indicated that the levels of each The paper has been prepared with the valued pier have only changed relative to the abut- assistance of Messrs. M. N. Mahendran and ments by amounts which are within the ex- W. E. Jubien of the C.N.R. geotechnical staff. pected survey errors. It is published with the interest and approval of

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LEGGET AND PECKOVER: FOUNDATION PERFOR1,lANCE 519

Mr. R. J. Hansen, former Chief Engineer and Mr. R. L. Gray, present Chief Engineer, Cana- dian National Railways, Montreal.

CRAWFORD, C. B., and SUTHERLAND, J. G. 1971. The Empress Hotel, Victoria, British Columbia. Sixty-five years of foundation settlements. Can. Geotech. J. 8, pp. 77-93.

FLEM~NG, S. 1876. The Intercolonial-a historical sketch. Dawson Brothers, Montreal, Quebec. pp. 200-202.

LEGGET, R. F. 1972. Intercolonial Bridge-A monument to Fleming's fine judgment. Can. Consult. Eng. April. pp. 56-57.

LEONARDS. G. A. (Editor.). 1962. Foundation engineering. McGraw-Hill. Toronto, Ont. pp. 556, 789.

PECKOVER, F. L.. and LEGGET, R. F. 1973. Canadian soil penetration tests of 1872. Can. Geotech. J. 10(3), pp. 000-000.

U.S. Department of Navy. 1963. Navdocks design manual DM-7. Soil mechanics, foundations and earth structures. Washington, D.C.

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