Flat Top and Special Tunnel Construction

From Engineering Heritage New South Wales

City of Sydney Underground Railway.


By Albert Henry Dix Humphries.
Associate Member.

To read more of the career of Albert Humphries follow this link.

Albert Henry Dix Humphries.

    Summary. -The construction of the City of Sydney Underground Railway has been described generally in papers previously published in "The Transactions of the Institution," and in THE JOURNAL.

    This paper describes, specially, the construction between Goulburn and Druitt streets on the western side of the city.

    The paper is divided into two parts; the first describes the details of tunnel construction between Goulburn and Bathurst streets, including underpinning of buildings, etc., and the second part, to be submitted later, describes the construction of the Town Hall Station.[1]


    The portion of the City Railway covered by this paper is shown in Fig. 1, which is a plan and section of the work from Goulburn to Druitt-street. Starting from Goulburn-street, on the right, where the City Railway enters underground, there are four tunnels side by side. From East to West these tunnels are designated- City Inner, Up Shore, City Outer and Down Shore, respectively. The City Inner and the City Outer are the return tracks, via Circular Quay, of those now in use on the eastern side of the city to St. James Station; and the Up and Down Shore are the through tracks from Central Station to the North Shore, via the Sydney Harbour Bridge.

    The route follows a reverse curve of generally from 660 to 850 feet radius with compounds and transitions. At the Goulburn-street end, owing to the tracks converging, a limit of 500· feet radius is reached, in one place, for a short distance. On reaching the eastern side of George-street, the four tunnels pass over two additional tunnels - designated the Down West and Up West, which have been provided for a future connection between the Eastern and Western Suburbs via Town Hall Station, Bridge-street and St. James Station. The six tunnels then continue to Town Hall at varying levels and alignments.

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    At the Goulburn street end, the rail level of the four tunnels is each about 21 feet below the surface, and the tunnels fall at a grade of about 1 in 33 to Liverpool-street, where, after a vertical ease, they rise at about the same rate to Town Hall Station, with the exception of the City Outer runnel which, after passing over the Down and Up West tunnels, grades down to the same level as these lower tunnels at Town Hall Station, the latter tunnels having risen in the meantime.

    Thus, at Town Hall Station, there are two levels, each having three tunnels; those on the high level from East to West being the City Inner, Up Shore and Down Shore, and on the low level, from East to West, the Down West, Up West and City Outer. The rail levels of the upper tracks are each about 35 feet below the surface, and the lower tracks about 57 feet below the surface. The distance from the Goulburn-street face to the southern end of Town Hall Station is about 26 chains.


    The ground through which the tunnels pass consists of clay with ironstone bands and soft indurated shale, down to~ generally, a few feet below the level of the roof of the tunnels, and from thence downward of sandstone with varying bands of soft to hard shale; the sandstone being generally fairly hard, and in some cases very hard.

    In addition to the usual fairly horizontal bedding of the sandstone and shale, vertical planes of cleavage, running in approximately a north-easterly direction,. and dipping slightly north-west, occur at frequent intervals, sometimes only a few feet apart. Somewhat less frequently other planes of cleavage, approximately at right angles to the former, were encountered. The fractures were generally very clean and free, and necessitated great care in the progress of the work.

    The cover, from the surface to the top of construction, varied from 3 feet at Goulburn to 13 feet at Bathurst-street. This being insufficient to distribute the loading from buildings, arch top construction was practically confined to under Goulburn, Castlereagh and George streets. The balance of the work, including that under Pitt and Liverpool streets, consisted of flat top construction, with the exception that under the Central Police Court, arch top construction was used with provision for flat top if necessary to carry future heavier buildings. Arch top was not used under Pitt and Liverpool streets as the change of type would introduce unnecessary complications.

    Owing to the angle of construction to the building alignments, the changes from arch to flat top construction were effected by right angle steps in each tunnel, keeping the arch top outside the influence of building loads. The grade in the flat top sections was obtained by a series of steps in the roof of the tunnels. The arch top tunnels were built parallel to the rail level.


    On the Eastern side of Pitt-street, under the footpath fronting Snow's and Supply Stores' buildings, was situated a telephone cable tunnel, 5 feet wide and 6 feet high with semi-circular top. The floor of this tunnel came below the top of construction and had. to be raised. The tunnel carried a large number of cables, including main trunk lines, and the size could not be restricted. To overcome this, it was necessary to cut off the semicircular top, build up the sides and provide a flat top at about 18 inches below footpath level; incidentally one of the sides could not be made vertical, owing to a water main close to the kerb line, and had to be built sloping inwards to the tunnel.

    To carry out this alteration and also to provide working headroom during construction of the railway tunnels, it was necessary, first, to lower all the cables to the floor and box them in to prevent damage, then to excavate from the surface, cut off the top and rebuild the sides and roof, then to lift all the cables close to the roof and provide a temporary timber floor so that the telephone mechanics could have free access. The floor and part of the old sidewalls were then cut away in sections as the railway tunnels were being constructed, and were finally rebuilt at the higher level with easy ramp at each end, the cables being then replaced to their proper spacing on the sidewalls.

    A similar telephone tunnel ran across the railway tunnels on the southern side of Liverpool-street, but in this case, as it contained fewer cables and the railway encroachment was not so great, it was not necessary to' rebuild the roof.


    The general methods of arch construction used on the City Railway have been described previously. In the section now being dealt with, the number and disposition of adjoining tunnels-also their proximity to the surface necessitated different methods being adopted.

        Figure 2 shows the commencement of the tunnels looking north, after excavating up to Goulburn-street. On the right are the twin tunnels now in use for traffic on the eastern side of the City. On the left are the five headings for side walls of the four tunnels leading to the western side. The tunnel walls were built in these headings, the outer walls to arch springing level and the intermediate walls to 2 feet 3 inches above springing, as seen in the two completed tunnels on the right. The arch tops were then driven under Goulburn-street and Castlereagh-street, the latter being seen on the left, using curved rails resting on the walls to support the street surface, which also carries tram lines. No restriction to traffic was caused during construction although the cover was very limited. The forms for the concrete arch were supported by curved tee irons suspended from the rails, and, after concreting, the dumplings left in the tunnels were removed.

Fig. 2.- Commencement of Tunnels under Goulburn Street.

    This type of construction necessitated the four arches being driven simultaneously as the curved rails butted against each other on the walls ; the overall width of the tunnels being 75 feet, only a short length could be concreted at one time owing to the quantity required.

    Flat top construction commenced immediately on the northern. side of Goulburn-street and continued under Mark Foy's building, shown in the centre of Fig. 2, and recommenced under the Masonic Hall and Wills' Buildings, the two highest buildings on the left. These variations in construction of arch and flat top in adjoining tunnels with either steel columns or brick walls between them, and also the fact that one of the outer walls had to be taken to a higher level to carry a sewer diversion, necessitated the curved rails for driving the arch top being used in varying lengths and combinations. To overcome this the type of curved rail used was later modified and a single type adopted suitable for all conditions to be met with. Figure 3 shows this type of construction, the curved rail being made in one piece. The tunnel walls, including the curve of the arch and rail seating, are built up to a height above springing level, depending on the thickness of the arch required; for an arch 2 feet thick this height is 5 feet 9 inches, as shown. In building the walls, profiles were set up giving the curve of the arch and the angle of the skewback. The courses, in the case of brick walls, were set over to conform with these, the actual seatings for the curved rails being the only part accurately faced. This obviated the necessity of building cur courses in the walls where an adjoining flat top tunnel required level course and the arch tunnel was on a grade.

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    The rails were usually spaced at about three feet centres, and braced together by means of tie-bolts through holes provided in the web of the rail. These rails can be set up in one piece, without the necessity for a joint and fish-plate at the crown, as previously used with the longer rails. To assist in the placing, the feet of the rails were frequently pulled closer together by means of a tie-rod with turn buckle, and then released when in place, making a very tight fit on the walls. The usual bridge blocks and lathing were driven over the top of the rails to support the ground, all voids on the top of these being filled in with cement mortar and stone packing. The forms for the arch were suspended from the rails as previously described. Examples, where this type of construction has been used, will be given when following the progress of the work.

Fig. 4.- Flat Top Construction on Steel Columns.


    Figure 4 shows the type of flat top steel construction under properties on the north-eastern corner of Goulburn and Castlereagh streets, and partly under Mark Foy's building. This is the Up Shore tunnel and shows the commencement of arch construction in the background under Castlereagh-street. Through the steel columns on the right is the City Inner tunnel, and to the left the City Outer and Down Shore tunnels. On the right are the concrete racks and troughing for electric cables, etc. The wide spacing between the columns on the left is under the northern footpath of Goulburn-street, where a section of the roof beams was placed on an angle, and between them are carried 24 in. and 12 in. water mains set in the roof concrete. Above these were replaced other underground services which could just be accommodated under the footpath level.

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    The wall columns and longitudinal cap beams of this section of flat top follow a series of chord lines round the curve, and were erected in headings without disturbing the surface, in order not to occupy Mark Foy's premises longer than necessary, or to restrict the traffic in Castlereagh-street under which the steps of the flat top project; the roof beams and concrete were placed by cut and cover, in short sections at a time.


    Between Castlereagh and Pin streets the heaviest work was encountered, the tunnels passing under the Masonic Hall and Wills' Buildings fronting Castlereagh-street, and Woolworth's, Supply Stores and Snow's fronting Pitt-street. In Fig. 2 can be seen the additional stories built on the Masonic Hall in 1923.

    Figure 5 is a foundation plan of this building and arrangement of the tunnels showing the arch top construction under Castlereagh-street stepping into the flat top under the building. The top of tunnel construction was practically level with the basement floor, situated at the lower right corner; the foundations of this portion of the building being of concrete a few feet lower on to a fairly hard sandstone. The foundations of the remainder of the building were about 8 feet above the tunnel roof, and 3 feet below the surface, and consist of sandstone blocks set in sand on clay and ironstone formation.

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    Figure 6 is a foundation plan of half of Wills' Building, formerly the property of the Australian Tobacco Company; the other half, not detailed, is on the left, the Masonic Hall being on the right. It was resumed by the Railway department and used for offices, etc., during the progress of the work. A basement, reserved for construction requirements, covered the whole of the area, with a floor level averaging 8 feet above the top of the tunnels. The foundations averaged about 5 feet above the tunnels on a clay and ironstone formation.

    The Supply Stores' building was not provided with a basement, and the ground floor, which could not be used during construction, varied from 4 feet to 7 feet, and the foundations from 2 feet to 4 feet above the top of the tunnels.

    The basement of Snow's building was about the same level as the top of the tunnels, the foundations being about 2 feet below. The same clay formation, with thin ironstone bands of '1arying thickness continued under both these buildings, the sandstone being a few feet down. The wall loading of these buildings ran from 5 up to 22 tons per foot run and the column loading from 50 up to 250 tons. The total building load on the section of the work between Castlereagh and Pitt streets was about 18,000 tons, or 0.85 tons per square foot over the whole area of the tunnels, exclusive of ground overburden. All the buildings were in occupation during the progress of the work.
         The construction consists of four flat top tunnels, side by side at the same level, each 14 feet II inches wide, with walls 3 feet 1 inch thick; the clearance height above rail level being 17 feet 3 inches, making the overall size of construction 75 feet wide and 23 feet high.

    The roof consists of 34 in. x 12 in. x 195 lb., broad-flanged beams, each 20 feet 6 inches long and weighing about 1 ton 16 cwts. They were placed radially to the curve of the tunnels and spaced at 2 ft. 1 in. centres on the centre line of the inner curved track. They are set on steel templets 2_ feet 6 inches wide and 1 inch thick, bedded on top of the walls. When headroom was limited, owing to sewers and other obstructions, 28 in. x 12 in. broad-flanged beams, plated to equivalent strength, were used.

    The beams lapped each other side by side nearly the full width of the division walls, the space between the flanges varying from ½ inch to 1½ inches wide. To obtain the grade, the roof was stepped 3¾ inches every fourth or fifth beam, or less frequently when required.

    The tunnels are designed to carry a loading of 5 tons per square foot placed in any position inside building alignments. Between Castlereagh and Bathurst streets, over 2,020 beams were required for flat top construction, including a few of lighter section under Pitt and -Liverpool streets, where the beam spacing was wider.

    The carrying out of this construction under the buildings was a difficult undertaking. To place the roof beams into position from the surface would necessitate a large amount of additional excavation, and cause serious interference with underground services, as well as dislocation of street traffic and business operations. Under these conditions it was decided to erect the beams from underneath, which would also afford uniformity of method.

    Access was made to the work by sinking a shaft over the Down Shore tunnel in an open space at the rear of Wills' building, and a second shaft over the City Inner and Up Shore tunnels on resumed land north of this building at the rear of the Supply Stores. These shafts were each 10 feet wide and placed at right angles to the tunnels. The City Outer tunnel could not be reached from the open in this locality, and drives were made from each of the shafts mentioned extending across the four tunnels at the bottom level.

    Access was also made from a shaft on a vacant block between the Supply Stores and Snow's, which permitted a number of beams being placed under the buildings from the surface.

    From these shafts, and also from the Goulburn-street end where the arch tunnels had been previously constructed up to the western side of Castlereagh-street, bottom headings for the five tunnel walls were driven, about 7 feet wide and 8 feet high, providing a working space of about 3 feet wide on one side of the wall and one foot clearance on the other. These left about an 11 feet wide dumpling between, except in the case of one tunnel, where the working space had necessarily to be made on both sides, the dumpling being about 9 feet wide. The side on which the working space was provided was a matter of careful choice as a number of conditions had to be met, including setting out requirements, erection of roof beams and junction with arch tunnels, etc. These headings had sufficient hard rock cover to permit blasting being carried out without damage to buildings and also facilitated the setting out. The low-level cross drives from the shafts enabled the latter to be checked. To have driven the wall headings at the full height in the first instance would leave the buildings standing on very tall dumplings with greater risk from blasting, and shrinkage by the upper clay formation drying out.

    The concrete footings were placed in the headings and the brick walls carried up as high as possible. This ensured less risk of shrinkage from green work when succeeding operations were being carried out, and greatly expedited construction.

    When these operations were sufficiently advanced, the basement of the Masonic Hall was partitioned off clear of the work, and the completion of the tunnels and underpinning of the buildings commenced. The wall headings were first completed to the required height and long enough to accommodate one beam; any portions of the foundations below this being cut off. The brick tunnel walls were then built up and the beam bearing plates set.


    In order to place the roof beams from below, it was necessary to cut down the dumpling left between the headings to as steep a batter as possible, as, owing to the length and size of the beams, they had to be erected on an angle as nearly as possible at right angles to the tunnel walls. It was also necessary to make the top of the crosscut at least 5 feet 3 inches above the top of the walls to clear the top corner of the beam during erection.

    Figure 7 shows a beam being erected. Two sets of ¾ inch wire rope tackle were used, as seen, running along each wall. These led back to two winches, each capable of lifting two tons off the barrel. The winches are secured to the floor, but are not in view in this illustration. The tackles lead up over pulley blocks, secured to timbers placed in the top of the cross drive, and back to the clamps seen attached to the beam.

    During the first stage of erection the right-hand tackle was secured to the top end of the beam until the latter was resting on the wall. The clamp was then slid down to the position shown on the left, to keep that end of the beam off the wall while the left tackle was hoisting. The beams being 5 feet 6 inches longer than the width of the tunnels, made one end project at least 2 feet 6 inches beyond the wall before it could be brought up level and slid back to take its bearing on the other wall.

    In the case of a single flat top tunnel, the top cross-cut was made that much longer on one side, but in the case of more than one tunnel being constructed simultaneously, as was generally done, the top cross-cut was taken right through down to a level with the tops of the walls, and the beams slid through it into place. This enabled the dumplings to be left in the other tunnels, forming a buttress to support any loads that may be just in advance of construction.

Fig 7.- Erecting Beams on Flat Top Tunnels.

    In the tunnel shown, the beams were brought in by a flat-top, end-tipping, petrol lorry from the Goulburn-street entrance; where the work was carried out from a shaft, the beams were lowered by a 3-ton steam or electric travelling crane, on to two flat-top trucks, 2 feet gauge, one at each end and pushed up to the face. The front end was lifted by one hoisting tackle, the back end remaining on the other truck until the beam was nearly up when it was picked up by the other tackle.

    The tunnel in which the beams were erected was termed the hoisting tunnel, the most suitable one being chosen, and changed when necessary owing to any bad ground, loading, or obstruction being encountered. The illustration of erecting a beam was taken where the work met from both ends under Wills' building, only the last two beams remaining to be placed. The beams erected from the other direction are visible on the top. They were not hoisted in this tunnel, which is the Down Shore looking north, but in the City Inner tunnel on the opposite side and slid across to this position.

Fig 8.- Erecting Beams (Top View) and Underpinning on Flat Top Construction.

    If the necessary headroom for hoisting 34 in. x 12 in. beams could not be obtained, either 28 in. x 12 in. or 24 in. x 12 in. broad flanged beams, plated to equivalent strength, were used as previously mentioned, or else the top right, and sometimes the top left corners of the 34 in. beams were cut off at an angle. This, as will be readily seen, reduced the required head room considerably, and did not reduce the bearing value of the beam on the wall. When these expedients failed, a few courses were left off the top of the wall until the beams were erected, when they were built up and the bearing plate set under the beam. In this condition, only the actual height of the beam above the wall was needed for clearance.

    Figure 8 shows, from the top, the last beam being erected. The lap of the beams on the wall can be seen, also the underpinning, as well as grillages that were placed under column footings and other derails that will be described later.

Fig. 9.- Underpinning Foundations on Flat Top Construction.

    This view was a cross cur under Wills' building at a point that had been previously selected carefully, where the construction from both ends met. The cross-cut here was twice as wide as usual, taking two beams on each tunnel.

    Figure 9 is the same cross-cut looking in the opposite direction. A column base of brick can be seen on the top left with temporary timber struts under it; others to the right and in the background. The overhanging ends of grillages can also be seen.

    The time taken to erect and place four beams, one on each tunnel, was usually about 80 minutes, although this operation has been done, under favourable conditions, in 65 minutes.

    The time taken to advance the four tunnels to take one set of beams, or a length of about 2 feet 2 inches, was usually about one week. This included excavating for the five walls, building them up, cross driving on the top for the beams, temporary underpinning foundations, cutting down the face in the hoisting tunnel, erecting the beams, concreting between them, brick underpinning to ground and foundations, waterproofing, backfilling and other incidentals, working the various classes of labour in rotation and shifts as required.


    Under the Masonic Hall, access was obtained to the top of the tunnels for underpinning, etc., by means of small shafts in the footpath close to the building. From these, through openings left in the wall underpinning at suitable places, small drives radiated to the various working places. As the work progressed, these were filled up and the underpinning made good. Under Wills' building, where the whole basement was available, holes were cut in the floor as required.

    In the plan of Wills' building, Fig. 6, the method of underpinning column bases, etc., is illustrated. Where continuous foundation walls, or isolated piers and columns bases located over the dumplings between the tunnel walls, were encountered, vertical and sometimes sloping timber struts, about 10 in. x 10 in., were placed under them down to hard formation, and tightened up by means of folding wedges on sills. These were so placed as not to foul the roof beams, as seen on the right in Fig. 9. When wall piers or isolated columns occurred over, or close to, the tunnel walls, this method could not be adopted, and grillages were placed under them to span the gap during cutting for, and building, the tunnel walls.

    In Fig. 6, a row of four of these grillages is seen, the fifth, over the centre of the nearest (Down Shore) tunnel, was rendered necessary by reason that in this case there was no other choice for a hoisting tunnel, consequently this dumpling had to be cut away to erect the beams. Advancing from the right, the tunnel work was carried up to as close to the columns as was deemed to be safe. Small drives, shown edged round, were then driven ahead between the columns and continued far enough behind them to house all the grillage beams required, as these, generally, could not be put in later. The grillage beams were usually 10 in. x 8 in. rolled steel joists cut to suitable lengths, though sometimes, where headroom was limited, 100 lb. rails were used. A temporary cribwork of seasoned 12 in. x 12 in. ironbark timbers, well bedded, was built behind each column. The grillage beams -were then placed, one at a time, under the columns and well wedged and grouted up; one end bearing on the cribwork and the other on the permanent brickwork built up on the last beams erected.

    The span of the grillage beams was relieved, if necessary, by means of vertical struts under them. The tunnels were then advanced as usual until the timber cribwork was reached, when it was removed.

    Other grillage beams, also shown in Fig. 6, are use under the corner of column footings close to the tunnel walls when full grillages were not considered necessary; the remainder of the footing being supported on struts as previously described. Some of these cases are shown on the right in Fig. 8, and on the left in Fig. 9. These were put in advancing from the left in Fig. 6. The cross drive shown in these illustrations, where the work from both ends met, is on a line between the two rows of five columns in Fig. 6, from which the grillage beams referred to may be identified.

Fig.10.- Shaft between Supply Stores and Snow’s Building.

    Columns and wall piers close to, or partly on, the outside tunnel walls were completely underpinned to below the tunnel top on to rock. Examples of these cases may be seen on both outer tunnel walls in Fig. 6.

    The work was carried out similarly under the Supply Stores. Where the headroom was very limited, the underside on the ground floor had to be exposed in places, and difficulty was experienced in maintaining the sanitary and storm water pipes, etc. This, however, was generally a matter of concern.

    As mentioned, the space between the Supply Stores and Snow's building, was vacant, and was used for a shaft to obtain access under both these buildings, also for driving. under Pitt-street.

    Figure 10 shows this area, with the Supply Stores northern wall on the left, and Snow's southern wall on the right, and a show window which fronts Pitt-street in the centre. In the background is part of the Hub building on the opposite side of Pitt-street; the tunnels pass just under and to the right of this corner.

    Part of the tunnel flat top is visible in course of construction, and on the left may be seen buttresses of rock left between the tunnel walls to· support the Supply Stores' foundations; similar buttresses being left on Snow's side.

    The Pitt-street frontage of Snow's building is shown in Fig. 11, with the corner of the Hub building, just mentioned, on the right ; the Supply Stores' northern wall, in shadow, also on the right, and Mark Foy's new building in Castlereagh-street in the background.

Fig. 11.- Snow’s Building, Fronting Pitt Street.

    In the foreground is the south-west corner of Pitt and Liverpool streets where the land was resumed, and the buildings removed. This was used as a working area for driving under Pitt and Liverpool streets. The top of the wall headings under Pitt-street can be seen showing the limited cover.

    The outside wall of the eastern or City Inner tunnel passes under the last S in Snows, on the face of the building, and under W on the water tower, while the outside wall of the western or Down Shore tunnel passes under the Hub corner.

    Figure 12 is the basement plan of Snow's building showing three of the tunnel walls with the roof beams omitted, the top of the tunnels being practically level with the floor. The telephone tunnel under the footpath, previously mentioned, is also shown. The basement was partitioned off clear of the work and all sewer pipes in the area diverted.

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    A Roots blower, for the pneumatic cash system, was located in the right lower corner, and was moved temporarily to, the left side, being replaced permanently on the tunnel top to the right in a more compact arrangement after the work had sufficiently advanced from that direction. It will be seen that one of the tunnel walls passes under Snow's southern wall at a long angle. To bridge the gap while the tunnel wall was being built under the foundation, long heavy ironbark timbers were placed on each side of the wall. These were carried on the finished flat top, to the right, and by blocking on the rock to the left, they supported " needles " placed through the wall as the work advanced.

    The tank pier, which came over the outside wall, is to the right of the centre. This was shored up with 12 in. x 12 in. ironbark struts on a sill set to the correct angle on concrete and a cap in a seating cut in the face of the pier.

    Folding wedges were placed between the struts and the sill for tightening up and releasing, and also on each side of the pier face to confine the brickwork. The foundations of this pier and the building generally were on clay with ironstone bands, the sandstone being about 3 feet lower. In the centre is a plated 10 in. x 8 in. steel stanchion supporting all the floors of the building. The base came below the tunnel top and had to be cut off. Shelf brackets were bolted to the web of the column on each side with two 20 in. x 7½ in. rolled steel joists placed under them, tightly clamped together. The right end of these rested on the finished tunnel and the left on blocking set on the rock. The foot of the column was then burnt off and a new base of angles and gussets riveted on, a heavy steel billet being placed between it and the tunnel roof.

    Only a portion of the tunnel roof beams could be brought under the building from the open area. The balance, including all those under the front wall and part of the southern wall, had to be hoisted up from underneath in the City Outer tunnel under Pitt-street and threaded across under the cable tunnel into position. Owing to the limited clearance under the cable tunnel, some of these beams were 24 in. x 12 in., plated.

    The flat top tunnels under Pitt-street were driven in practically the same manner as under the buildings, excepting that here the wall headings were taken through at the full height required in the first instance, and the walls built up. Owing to the soft ground extending much lower down and several large sewers under Pitt-street having to be cut away, the dumpling between the wall headings in the hoisting tunnel was not stable· enough to stand up the full height and support the roadway during the erection of the roof beams. It was, therefore, cut down in sections to the harder portion, the ground above being carried on long cap timbers at right angles to the tunnel, with several vertical legs under each, resting on sills on the remainder of the dumpling. The roof beams were then erected in the usual way and threaded across to the adjoining tunnels, when, after underpinning off them, the timber caps were moved ahead for the next section.

    From the working area at the south-western corner of Liverpool and Pitt streets, shown in Fig. 11II, the next buildings to be dealt with were, 103A, 105 and 107 Liverpool-street. These had basements at the level of the tunnel roof, and are not shown, being further to the left. Here, the work was carried out in a similar manner to that under the Masonic Hall. The sewer system drainage from the buildings had to be kept in service, but diverted and carried along the outer wall as the work progressed.


    The flat top tunnels under Liverpool- street and the remainder to the Town Hall Station, including the low-level tunnels under George-street, were, with some short exceptions,. carried out by a different system which was adopted where no building foundations had to be underpinned directly on the tunnel roof, and the available overhead working space was not so restricted.

    The wall headings were driven first, generally to the full height without a bottom heading. In these cases, a top heading was driven in the softer ground in advance of the full section. The headings were timbered with sets about 4 feet 6 inches to 5 feet 0 inches centres, consisting of 10 in. x 6 in. hardwood caps to carry the roof lathing, set on 10 in. x 6 in. legs. Under the legs were placed 10 in. x 6 in. cross pieces hitched into the ground on each side of the heading. The cross pieces were placed below the level of the top of the wall so as not to interfere with the setting of the bearing plates. If necessary, as the heading was deepened, additional legs and cross pieces were placed until hard ground was reached. In some instances, as under Liverpool-street, this timbering had to be carried to the bottom and the sides lathed behind the legs, in places, to retain the ground. As mentioned previously,, a clear height of 5 feet 3 inches above the walls was needed to erect the beams from underneath. In order to give suitable headroom for working, etc., the underside of the top caps were placed from 6 feet to 6 feet 6 inches above the finished height of the walls. This made a total height from foundation level of 26 feet 3 inches, leaving tall dumplings between the headings. The cross-pieces formed very convenient scaffolding for building the walls, which were built round them and taken out later as the dumplings were removed. In the case of the outer walls, where one end of the timber could not be released, as soon as the brickwork was built up to support them, a saw cut was run through close to the outside leg, so that they remained acting as bearers and struts until they were no longer required, when they could be readily withdrawn.

    On completion of a length of headings, and after building the walls, driving of the flat top was commenced. The system used is shown in Fig. 13, which is a view of the City Inner tunnel starting from a shaft which served the four tunnels on resumed land on the northern side of Liverpool-street in front of the Central Police Court. The steel frames shown, are bolted together in the centre, and the projecting end, seen on the left, takes a bearing on the stool seated on the tunnel wall. The blocking piece between the stool and the top member can be taken out to reduce the height when the headroom is limited. The frames, after being placed, are tightened up to the load by means of the jackscrews seen on the left. These screws are 2¼ inches diameter, 12 inches long with ½ inch pitch, and can be screwed right up or down through the holes in the frame angles. A loose bearing plate with guide recesses for the ends of the screws, rests on the stool, and a similar plate, with holes for the screws to pass through, is placed between the bottom nut and the lower angle of the frame. The locking nut on the top fixes the screws tightly to prevent lateral rocking movements.

    The original design of the frame was 2½ in. x 2½ in. x ½ in. angles, and the total weight of each half about 10 cwts. As these angles were not procurable in sufficient quantities, 3½ in. x 2½ in. x ½ in. angles were substituted in some cases, making the total weight of each half 13 cwts., including the stools. For the projecting seating 5 in. x 3½ in. x ½ in. angles were used. The space between the angles in the top member is for threading the roof lathing through on to the next frame. These had naturally to be driven at as small an angle as possible. Bolted to the stools, a bearing plate projects down the face of the wall to take the thrust from the frame; packing pieces of sufficient thickness being placed between, to put a small camber upward in the assembled halves. Provision was made in the gussets at the lower ends of the frames to place a r¼ in. diameter tie-rod across the tunnel, with a jointed turnbuckle, to reduce the side thrust on the top of the walls if necessary. The butt joint at the centre consists of plates bent at right angles and bolted together as seen in Fig. 13. Continuous splice plates were not adopted owing to possible difficulty in matching the holes, and the plates did not require to be taken off during removal and re-erection of the frames. They also avoided the necessity of accurately squaring the ends of the frame angles, and were better suited to the conditions of loading, etc.

Fig. 13.- Steel Frames in Position, Driving Flat Top Tunnels.

    The frames were erected between every second beam, the location marks being previously set out on the walls. As the top of the walls were stepped at intervals to obtain the grade and the spacing varied to suit the curvature of the tunnels, the average distance between frame centres was about 4 feet 2½ inches. The sets of frames were connected by means of tie rods for lateral stability These may be seen at the top and bottom on the right of Fig. 13, and below the connecting plates in the centre Two holes, close together, were provided in the gusset plates for this purpose, one each for the rods going in either direction. The tie-rods across the tunnel are not seen in this view.

    In the case of beams having been already erected in the adjoining tunnel, as seen to the right in Fig. 13, they, naturally, owing to the lapping on the walls, occupied the position required for the frame stools. The frames were, therefore, carried on the ends of the beams placed, the stools having been made the same height. The stools were also made with a 1-inch space through them, excepting the top plate, so that in the event of the top corner of the beam having been cut off, owing to the limited headroom for erection as previously mentioned, the stools could be slipped over the web of the beam with the base resting on the lower flanges. Small plates for connecting the two halves were provided and bolted on the back as seen to the left in Fig. 13. The strap down the wall performed the same function on the front.

    On starting to drive, a frame was set up in the shaft close to the side timbering, but not necessarily on the correct centre. Excavation of the dumpling then commenced; the roof lathing being driven over the top of the frame. To keep these tightly up to the work, a "kicking beam " was placed further out in the shaft to keep the back ends of the lathing down during driving until a second support could be placed under the leading ends. Sometimes one of the beams was placed in the shaft instead of a frame, and the lathing driven over blocking placed on this at the required height.

SHB Humphries Photo 14.jpg

    To avoid using long lathing and consequent waste, also to hold up the ground more securely, a temporary leading set was used in advance. This was of timber and consisted of a cap with sloping struts on to the comer of the walls as illustrated in Fig. 14. - The lathes were driven over this sufficiently far to be caught by the next frame; this frame having been placed and tightened up; the leading set was moved ahead. This set was also connected by tie-rods back to the last frame. This view shows the timbering in the tops of the drives, which was also carried lower down when necessary. The removal of the dumpling necessitated taking out the inner legs under the cap. Prior to commencement of driving, long 10 in. x 6 in. timbers were placed under them, as shown on the left, being supported on legs off the cross pieces or on to the top of the wall. A similar timber, not shown in this view, was also placed in the other heading. As driving proceeded, these longitudinal timbers were caught up by the frames, as seen in Fig. 13. Two lengths were used in each heading with the ends overlapping, each one being moved ahead as required.

    The number of frames needed for driving depended upon the nature of the ground. In bad ground, the toe of the dumpling when taken out had to be further back from the face, and usually 5 sets were sufficient, but if spare frames were available at the time, more were used.

    Figure 15 shows eight in position, four being in advance of the last beam erected, the other four between the beams still supporting the ground pending the completion of underpinning.

    This view was taken in the Down West tunnel on the east side of George-street, the floor level being about 60 feet below the surface, there being about 30 feet of cover over the frames to the buildings above. The sandstone here was frequently very hard but jointy, with bands of shale at intervals, and had to be heavily timbered in the wall headings. Some of the holes left in the walls by these timbers may be seen, with two timbers on the right still projecting, in this case from an outside wall. In the top background can be seen the heading timbers in advance of the frames. The jointy nature of the ground is evident, the cleavage planes being clearly seen. The loose filling along the walls came from the excavation above.

Fig. 15.- Driving Flat Top Tunnels using Steel Frames.

    The beams were erected in a similar manner to that described previously', and as the work proceeded more quickly, two skip roads for removal of the spoil were necessary on the tunnel floor. The hoisting winches were either erected on stages carried on bearers across the tunnel, or else the ropes were led back along the walls on top of the beams to the winches fixed on platforms in the shaft.

    As soon as a pair of beams was erected between the frames, precast concrete form slabs were placed between them resting on the lower flanges and protecting cover slabs and waterproofing along the top flanges. Particulars of these will be given later. Brick underpinning, 9 inches thick, was built on the top of the beams to support the ground above and also the heading caps. The frames were then lowered by the winches and re-erected in advance. The brick underpinning was not made continuous along the beam, an opening about 4 feet 6 inches wide and 3 feet high being left in the original wall heading, forming passageways. In these, skip-roads were placed for bringing in the concrete for the beam filling, bricks, and other materials required. A small opening was also left over the centre of each beam through which the steel frames could be separated for lowering.

    After concreting and completing the waterproofing between the beams, the spaces between the underpinning walls and the centre holes were packed tightly with stone filling, and finally the passageways were treated similarly.

    Under this system, after the walls had been built, up to 12 beams per week have been placed in one tunnel, including driving of the flat-top, removal of dumpling, underpinning, waterproofing, concreting and back-filling; working two shifts. During the progress of the work the frames were subjected to severe conditions, and they proved to be very rigid. Under Liverpool-street, with a cover of about 10 feet, they carried a load of from 40 to 50 tons each, not including the traffic in the street, which was not restricted. In the low-level tunnels under George-street and other places where the formation was harder and blasting had to be done, the shots were fired close up to them in the face without causing damage.

SHB Humphries Photo 16.jpg


    Between the two portions of flat-top construction just referred to, short sections of arch construction in each of the four tunnels were driven under the Central Police Court on the northern· side of Liverpool-street.

    Figure 16 is a cross-section under the building showing the foundations above. The wall headings were driven right through from both ends to the full height, as outlined, and were timbered on the top and sides where necessary, as in the case of flat-top construction. The walls were then built hard up to the roof of the drives, the skewback for the arch being built at the same time, as previously described. The width between the skewbacks on top is the same as the walls below. In the event of rebuilding operations, this would later admit of the walls being carried up and beams placed across, as required, relieving the arches of the weight. It can be seen that unless the tunnel flat-top had been previously constructed entirely within the limits of the building area, these beams would project beyond into other property. This, of course, applies generally to arch construction at shallow depths under properties.

SHB Humphries Photo 17.jpg

    After completing the flat-top on each end, the arch top tunnels were driven, using the curved rails as in Fig. 3, and leaving the dumplings at a convenient height for concreting the arches. The curved 'rails were spaced about 3 feet centres, excepting that where the heavier footings of the building occurred overhead they were spaced more closely, sometimes only I foot apart, where the ground was particularly soft.

    It will be seen from the test bore shown in Figure 16 that the building footings are on medium hard shale, under which is soft sandstone at about tunnel top level.


    Returning to the flat-top construction on the northern side of the Central Police Court towards Town Hall Station, an area was resumed fronting George-street and shafts sunk from which the runnels were driven. At this point the low-level Up and Down West tunnels commenced, one of which is shown in Fig. 15.

SHB Humphries Photo 18.jpg

    Figure 17 is a cross section, a short distance north of this, at about the centre of the new Plaza Theatre, the low-level tunnels having worked over from the left side.

    These two tunnels were constructed first and then the top ones which cross them at a long angle. The Down Shore tunnel on the left has just changed to arch section, and the City Outer tunnel next to it is beginning to fall, the change to arch-top being just beyond this section. In part of the roof of the Up West tunnel under this, 24 in. x 12 in. plated broad flanged beams were used as shown, instead of the usual 34 in. x 12 in., on account of the diminishing headroom between the tunnels.

    Of the four top tunnels, the City Outer tunnel had necessarily to be driven first, then the two higher flat-top tunnels on the right, and finally the Down Shore arch on the left. The latter was done in a similar manner to those under the Central Police Court, excepting that the headings were driven in short lengths, and the lining completed as these progressed.

    Figure 18 is a plan of the front portion of the Plaza Theatre, five stories high, which is carried on the roof of the top tunnels. The walls of the low-level tunnels are shown in dotted lines. This building displaced Hoyts' old theatre during tunnelling operations.

    Work on the top tunnels had advanced from the right, partly under the old theatre, when the proprietors decided on re-building operations and were in a hurry to complete the work. This necessitated a re-arrangement of tunnel programme and alterations of method.

    As soon as the front portion of the old building was demolished, a shaft, about 12 feet wide, was sunk from the surface across the City Inner and Up Shore tunnels. Parts of these tunnels were then built in the shaft to take the centre cross-wall of the building and the front wall column in line with it; the three flat-top tunnels being meanwhile continued from the right to connect with this, while the rear portion of the building, and also the front portion to the left, were being constructed. In building the latter portion, special provision was made in the foundations to facilitate tunnelling operations.

    A continuous reinforced concrete footing was provided, under the front row of columns, carrying the superstructure, also under the wall behind it and the connecting corner to the left, to avoid underpinning isolated and varying loading as the building progressed.

    The depth of these footings below the surface was restricted by the minimum height necessary above the tunnel top for erecting the beams. A basement was also required behind the second wall as deep as possible, and between the first and second walls over part of the area, but not quite so deep. The bottom of the front wall footing was therefore kept at a higher level permitting the beams to be hoisted in the Up Shore tunnel underneath it and then threaded across under the second wall over the City Inner tunnel, with just enough room to clear the beams. The basement floor beyond was put into the same level and also reinforced. As the excavation underneath was taken out and the tunnel built these foundations were underpinned to the tunnel top.

    The flat-top on the Up Shore tunnel had been arranged to change to arch-top immediately the building line had been cleared, but in order to place the beams on the City Inner tunnel under the new conditions imposed, it was necessary to extend the Up Shore flat-top to opposite the end of the building.

    The usual passage-way left over the tunnel walls for access had to be dispensed with on Nos. 1 and 2 walls, counting from the top, but was retained on No. 3 wall leading back to the shaft some distance on the right, openings being left in the foundations of the building at the bottom right corner to pass through. The theatre being completed and in use before tunnelling operations were finished, no other means of access to the top of the tunnels was possible.

    Provision had also to be made for the sewer service pipes leading from the basement, along the inside of the wall on the right, into the street. This had to be syphoned across the passage-way referred to during the period the latter was in use. Figure 19 shows in the foreground the resumed area for construction purposes at the rear of the Central Police Court (the low wall on the right) and fronting George-street on the left. The shaft in the immediate foreground in front of the two travelling cranes serves the Up Shore, City Outer and Down Shore tunnels. Behind the crane, on the right, is the shaft for the City Inner tunnel and in the lower left corner can be seen, near the man standing on the gangway, the corner of the shaft for the Down and Up West low-level tunnels, which are here just clear of the higher tunnels and partly under George-street.

Fig. 19.- View along Route of Tunnels, Showing Plaza Theatre and George Street.

    The low-level tunnels swing to the right from this point beginning immediately to pass under the higher tunnels which bear to the left. The right-hand outer wall of the lower tunnels passes beyond that of the top tunnels before leaving the Plaza Theatre boundary and finally swings back again until it is immediately under it at Town Hall Station.

    In the centre of this illustration is the Plaza Theatre in course of erection. The rear portion has the roof partially constructed; note one of the trusses; these are of timber with a span of about 96 feet. At the time this view was taken, tunnelling at the higher level was in progress under the nearer front portion, and the farthest front portion, under which the special foundations were placed, is partly built up to the first floor. The cross section shown in Fig. 17 is just before this part, being on the right of the centre cross wall in Fig. 18. The outer wall of the City Inner tunnel passes close to the nearer corner in Fig. 19 where the brickwork is toothed, and under the centre of the dark opening beyond.

    The flat-top construction on the City Inner tunnel continues some distance under the buildings past the Plaza Theatre, until both walls clear the building line of George-street, when arch-construction commenced and continued to Town Hall Station.

Fig. 20.- View in City Outer Tunnel under Plaza Theatre.

    As no basement existed under these buildings, sufficient height was available for erecting the beams without baring the foundations, the ground under them only being underpinned.

    In the background of Fig. 19 on the opposite side of George-street, is seen the front of the Regent Picture Theatre, and adjoining it, beyond, the Bank of New South Wales, on the comer of George and Bathurst streets. The left outer wall of the Down Shore tunnel passes under the front of the Regent Theatre at about the centre of the building. The tunnel itself, with flat-top construction, is partly under the bank, where the work was carried out in a similar manner to that under the Masonic Hall. Advantage was taken during the building of the Regent Theatre to excavate for and build the tunnel wall from the surface, leaving the tunnel, which is of arch-construction, in front of the building, to be driven later.

    The Town Hall clock tower is seen beyond the Regent Theatre, with the Queen Victoria Markets dome in the background. On the right side of George-street, in the background, is the tower of Murdoch's new building, built on top of the low-level Down and Up West tunnels, which at present continue past Town Hall Station up to this point.

    Figure 20 is a view in the City Outer flat-top tunnel under the Plaza Theatre, showing the commencement of arch-construction. The cross-section in Fig. 17 is located near the man on the elevated staging. The curved tee iron centres, previously mentioned, for supporting the lagging when concreting the arch, can be seen in position; also the brick skew back built with the wall to carry the curved rails when excavating the top. One of the profiles used for building the skewback is seen down on the timbers along the left wall. The man on the stage is driving an air winch for hauling material skips up the ramp, also for hauling spoil buckets and materials on the lower track to the left. The ramp ends at the top on a length of staging carried across the tunnel walls, and is moved forward at intervals as the work progresses. On the right, near the roof, are the down-pipes for draining water off the flattop, which is here at a lower level than the adjoining tunnels, as will be seen in the cross section, Fig. 17, this tunnel being now on a falling grade. In the left wall is shown a manhole leading to the higher Down Shore tunnel on that side. A back will be built in this later to form a refuge in the latter tunnel. A similar refuge to serve the tunnel itself is shown on the right wall. The nearest end of the splayed corner at the top left side marks the end of the fiat-top in the adjoining tunnel, and the commencement of the arch section.

    This splay is concreted with the roof filling and is placed on the outside walls of flat-top construction or adjoining arch-top tunnels. It serves the purpose of assisting the outside walls as retaining walls in case the flat-top may be unloaded at any time, and the ground behind remaining loaded. It also assists in taking the thrust from an adjoining arch, as in some cases this comes well down the wall where the tunnels are in different grades, and also forms an effective seal from water seeping along the bed plate level.

    Figure 21 is a view of the adjoining Down Shore tunnel at the commencement of the arch section just referred to. The corbelling over for the arch seating may be seen on the top of the left wall. The wall heading is here completed for the required distance in advance, and the brickwork is visible at the far end on the bottom in course of erection. The wall is started at this end and worked out so that the materials may be brought close up on the skip road.

Fig. 21.- View in Down Shore Tunnel commencing to pass under George Street.
SHB Humphries Photo 22.jpg

    The heading timbers can be seen, also the face timbering on the end of the dumpling, which had here to be cut almost vertically in order to place the last beam on the flat-top.

    Figure 22 is a cross-section just beyond where the right-hand wall of the top tunnels clear the eastern building line of George-street. The two lowest tunnels are still flat-top construction and continue so for a short distance beyond this section, where they also change to arch construction after passing outside the building line.

    The City Outer tunnel is dropping lower, note the high walls necessary, still having a common alignment with the higher tunnels. This tunnel soon begins to pass under the Down Shore tunnel, on the top left, until it is almost completely under it at Town Hall Station. The other top tunnel wall also separates from it shortly after this point, as will be indicated later.


    The end portion of the flat-top construction, about 66 feet in length, on each of the two low-level tunnels, was driven by the pitched-roof-timber method, as shown. The Down West tunnel on the right was driven first from the Town Hall end. The pitched-roof-timbers were 12 in. x 12 in. Oregon, spaced about 3 feet centres, with· a span of about 24 feet and a rise of 6 feet. The full section was taken out beneath them wide enough to clear the centre wall. The side and centre walls were then built and the roof beams erected. In erecting the roof beams, there was not sufficient room over and outside either wall to lift them at an angle vertically square to the tunnel, as previously described, they therefore had to be lifted from the centre at an angle horizontally to the tunnel. This occupied a length along the tunnel of about 16 feet, consequently a number of the beams had to be crowded together and later spaced out. The top of the walls having been built in steps, on account of the grade, the beams had to be lifted down these and then back again. The ground in this locality, though hard below the timber seatings, was very jointy and careful attention had to be given to the hitches cut for the ends of the roof timbers. The apex of the roof ran very close up to the shale and the timbers were fairly heavily loaded before they could be supported.

    The concrete filling between the beams could not be done within 3 feet of the centre wall, as this space was later needed for placing the beams in the adjoining tunnel, these having each to be erected in the correct position.

    After concreting and waterproofing the remaining portion of the roof, the outside wall of the top tunnel was set out. This could be only done approximately, as that section of the wall was in the middle of a transition curve and the tangent points were not available to do the work accurately. The line was therefore kept back a few inches to admit of the wall being faced up later. A brick wall, toothed for the face, was then built up to the roof timbers, the space behind being concreted round the timbers as tightly as possible as the latter could not be drawn. The other half of the roof timbers was then supported by struts as shown.

    In driving the adjoining tunnel, consideration had to be given to the erection of the roof beams, the roof timbers having to be carried sufficiently far back on the tunnel already constructed to provide sufficient head room and end clearance, thereby increasing the span. A seating for this end of the timbers had been provided, as shown, when concreting the roof of the first tunnel, and they had to be spaced over the beams already placed, which were about 2 ft. 1 in. centres.

    When driving this tunnel, the projecting ends of the timbers from the first tunnel had to be cut off and the balance carried on the new sets. The full section of the tunnel was then excavated, and when this was done an area of ground 30 feet wide and 66 feet long was supported by the roof timbers, causing heavy loading before additional strutting could be provided. The remaining wall being built, the roof beams erected, concreted and waterproofed, this strutting was placed as shown.

Fig. 23.- Driving Flat Top Tunnels by Pitched Timber Construction.

    Figure 23 is a view of the first, or Down West tunnel, after the pitched timbers had been strutted to the concrete, as seen on the right. This concrete is not the face wall of the upper tunnel, mentioned above, that being still more to the right and not in view. The concrete seen is filling for the upper track roadbed with the step provided for butting the timbers from the adjoining tunnel. Some of these timbers can be seen in the background with vertical struts off the ends of the beams to assist in taking the load as the ends of the timbers in the first tunnel are cut off.

    The portion of the roof beams not concreted is seen, also where they step up, and the jointy nature of the ground under the ends of the timbers on the left is evident.

    Figure 24 is a view of the second, or Up West tunnel before the roof beams are erected. The ends of the beams in the first tunnel are visible on the right, with the struts just mentioned on top; the other beams having to be placed between them. The end timber of the first tunnel can be seen butting on the rock, the jointy nature of which is visible.

    The remaining portion of these two lower tunnels, which shortly separate, is of arch construction and was driven from the Town Hall end in full section, with crown bars strutted by legs on to horseheads supporting the roof in the usual manner.' Four of the crown bars can be seen over the finished concrete arch, the centre two having been cut off.

    Two walls of the upper tunnels are located approximately over the centre of the lower tunnels, as may be seen in Fig. 22, and great care had to be exercised in driving the headings through the ground supported by the pitched timbers. This was done in short sections, the apex of the timbers being cut out to provide sufficient width, and struts placed between them, round which the brick walls were built.

Fig. 24.- Driving Flat Top Tunnels by Pitched Timber Construction.
Figure 25 is a view taken in the City Inner tunnel showing the walls with the short heading open on the right, and the brickwork being extended on the left; the wide side of the heading is behind this wall.

    At the bottom are seen portions of the pitched timbers supporting the dumpling above. In front of, and behind the man on the right, is visible the timber packing placed originally on top of the pitched timbers in voids caused by the bad ground. Above this may be seen the smooth faces of the joints in the rock and shale where the adjoining portion had been taken away on driving the heading; note the strut against it. Also visible are the breast timbers placed across the face of the dumpling and dowelled on to the tunnel walls to prevent the ground from coming forward.

    Reverting to the cross section, Fig. 22, the two low level tunnels on the right separate soon after they change to arch construction just beyond this point. The two high level tunnels on the right also separate at about the same place, the driving of the latter being continued from this end, and also from the Town Hall end and by means of the wall heading and curved rail system as described (after the bottom tunnels had been completed). The top tunnel, on the left, was also driven from both ends by the same method, excepting the length of flat-top construction under the Bank of New South Wales.

    The remaining tunnel, City Outer, was previously driven from its low-level position at the Town Hall by means of full section and crown bar method until, on rising, it came as close to the low-level tunnels as was deemed to be safe; the remaining portion being completed later from this end in advance of the other top tunnels by the curved rail method.

    Figure 26 shows the arrangement of the tunnels under the Bank of New South Wales, just before they reach Town Hall Station, when the relationship is slightly different, the top left runnel being more completely over the lower. The front wall of the Bank of New South Wales is seen on the top right, the extreme position of this wall being over about the centre of the tunnel.

    It will be seen that the arrangement of the tunnels, involving the walls leaving each other and passing over the crown of lower tunnels at long angles, caused varying and unbalanced loading over the latter during construction, and great care had to be taken to prevent damage. It was partly for this reason that the curved rail system was adopted for driving the higher tunnels.


    The underground seepage along the line of the tunnels was a matter for careful attention. The level of the greatest amount of seepage was generally below the top of the tunnels, where the ground began to harden.

SHB Humphries Photo 26.jpg

    The outer walls, acting as training walls, would impound and conduct the water along them, causing possible damage to adjoining basements. The water on top of the 75 feet width of fiat-top would also travel down the grade, causing the same trouble.

    Provision was made to avoid this as much as possible, Fig. 27 showing the system adopted for the flat-top tunnels. The centre of the top cover was made higher than the sides to throw the water off in both directions.

    As underpinning on the beams had generally to be done before they could be concreted, precast concrete cover slabs, 2 feet long and 12 inches wide and 2 inches thick, were used, and first bedded on top of the beams, one layer in the outer and two on the inner tunnels. On top of these was placed bituminous waterproof sheeting about ¼ in. thick and 2 feet wide. Another layer of slabs was placed on this and then the brick underpinning commenced. The slabs prevented possible damp conducting brickwork coming into contact with the steel. When the concrete between the beams was completed similar waterproof sheeting 12 in. wide was placed on top, lapping under the sheet previously placed on the beams, and sealed with waterproof compound. The top was then covered with cement mortar slightly dished and falling to the sides of the tunnels. The sheet waterproofing was carried over the ends of the beams into a channel left along the wall below bedplate level, as shown in the illustration.

    Similar concrete slabs to those on the top of the beams were also placed across the lower flanges as forms for the concrete filling. These also provided a ready walkway between the beams for underpinning, etc., taking full advantage of the available headroom.

    Down pipes, as shown, were placed when required in the intermediate tunnel walls leading to the track drainage system to drain off and relieve the water pressure above the roof.

    Vertical and horizontal chases, as indicated, were left in the brickwork behind the outer walls, serving the same purpose as the downpipes, the brickwork being otherwise built hard against the ground.

    Under Liverpool-street, at the bottom of the tunnel ease, a pumping chamber was built outside the eastern wall, with a large drainage well. Into this the water from each of the tunnel track drains was led through a trapped silt pit, placed in an accessible position for cleaning. An automatic electric pump set is provided in the pumping chamber, discharging up a shaft into a stormwater pipe under street level.


    The setting out of the various classes of work in this section was a very important item. The checking of the permanent traverse and levels of station points on; the surface, transferring of base lines down the shafts, setting out the various circular and transition curves in the numerous headings in progress at one time, setting rail levels and the steps on top of the walls, to the corresponding values across the tunnels, in the separate headings; locating fixings in the walls and roof for overhead wiring attachments, and numerous other requirements, including progress measurements of the various classes of work, called for a large amount of skill and care and was carried out in a very satisfactory manner by the assistant engineers.

SHB Humphries Photo 27.jpg


    In concluding the first part of this paper, the author desires to thank Mr. R. L. Ranken, M.I.E.Aust., Chief Civil Engineer, New South Wales Government Railways, for permission to use material for the paper, also Mr. B. McIntyre, B.E., A.M.I.E.Aust., for his assistance in its preparation.

  1. The second paper was never published and is not known to exist.
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