Methods of Tunnelling on the City Railway.

From Engineering Heritage New South Wales


As used on the City of Sydney Underground Railway.

By Keith Aird Fraser.

This paper was published in Transactions of the Institution by The Institution of Engineers Australia in 1930.

Keith Aird Fraser was one of two Resident Engineers on the project for its whole ten year duration from 1922 to 1932.

    Summary.-The construction of the City of Sydney Underground Railway has now been in progress for eight (8) years. During this period a very large amount of tunnel work both in single and double track, has been completed. This paper describes briefly:(a) The present position in the construction; (b) Nature of strata penetrated; (c) Methods of tunnel construction; (d) Timbering of tunnels; (e) Ventilation of tunnels; and (f) Concreting of tunnels. No account has been given of the actual blasting out and removal of the spoil, because in these particulars one tunnel does not vary greatly from another. The temporary and permanent supporting of the overburden are the main factors in any tunnel, and , to deal successfully with these two items, a variety of methods has been adopted.

PRESENT POSITION IN THE CONSTRUCTION OF THE CITY RAILWAY.

    The City Railway, as it now stands, consists of a completed portion of about half a mile of under-ground railway, including two stations, and a quarter-mile above ground. There are about one and three-quarter route miles of underground railway, including two stations, in course of construction.

    The completed portion, which includes a part of Central Station, in the open air, and underground stations, at Museum and St. James, was completed and opened for traffic in December, 1926. By means of turn-back arrangements and cross-overs at Central Station and St. James, this length of line now carries as many as 40 trains per hour during the business rush periods.

    The tunnel work on this side of the city did not present many great difficulties as the route for the most part lay through public parks and under the roadways, the worst portion being that under Messrs. Mark Foy's building near Goulburn Street, where the railway comes into the open air.

    On the western side of the city where-the work is now in progress, a considerable amount of tunnelling has been completed.

    Commencing at Goulburn Street, four steel-top tunnels have been driven and partially completed as far as Hoyt's Picture Show in George Street. The part of this section still under construction includes the underpinning of the Supply Stores and Wills' Tobacco Factory, the latter now being used for Departmental Offices.

    From Hoyt's Picture Show towards Town Hall Station two low-level tunnels for the Western Suburbs, and one low-level city tunnel have been excavated and lined. The Western Suburbs tunnels are of steel-top construction to carry high-level tunnels superimposed, and the city tunnel is of arch construction.

    North from Town Hall Station, two Western Suburbs tunnels have been completed sufficiently far to clear the high-level tracks. They are of steel-top construction to carry the high-level tracks and also buildings. The city outer tunnel (a low-level track) has been completed to Wynyard Station, as has also the Down Shore tunnel which is superimposed on it. The City Inner Tunnel is also complete to Wynyard Station, and the Up Shore tunnel excavation is complete, but the lining has only been finished from Market Street to Wynyard Station.

Just south from Wynyard the two Shore Local tracks have been completed by cut-and-cover methods to Barrack Lane sufficiently far to give over-run for trains which will terminate on these tracks at Wynyard Station.

North of Wynyard, the four Shore tunnels have been completed to the open-air just north of Grosvenor Street.

The City Inner and Outer tunnels converge and become a double track tunnel about four chains north of Wynyard and this has been completed to a point about 17 chains north of Wynyard Station. Further work, which will be commenced shortly, will be necessary on these tunnels in order that -cross-over arrangements similar to those at St. James can be put in to work Wynyard Station prior to the complete circuit including Quay Station being in operation.

Further tunnelling and cut-and-cover work has been completed, and is in progress on the eastern side of the city between Macquarie Street North and St. James Station.

This includes the two City tracks and a portion of the Western Suburbs double tunnel over which the City Inner track has to be constructed.

NATURE OF STRATA PENETRATED.

In most cases the nature of the ground to be driven through, to6ether with the depth of the overlay, determines the method to be employed in the construction of tunnels. This held good to a certain extent on the City Railway but for the most part the ground was good and the method adopted depended upon other circumstances. On the eastern side of the city, the bulk of the tunnelling between Goulburn Street and St. James Station was through ironstone clay and shale with the sandstone mainly below the middle section of the tunnel.

SHB Fraser Paper Photo 1.jpg


The ironstone clay was easily worked but had to be closely timbered. The shale was not particularly hard and lay in blocks' cut in irregular directions by narrow clay seams. This clay when wet was most treacherous and readily allowed the shale to become dislodged and become a load on the tunnel timbers. The tunnels through this strata had to be heavily timbered, and as the lower half of the tunnel was usually of hard sandstone, which had to be blasted, constant care was necessary to avoid dislodging the timber in the top with shots fired in the bottom.

From St. James Station northerly, the tunnels are in sandstone, the bedding of which is peculiar and necessitated timbering in the roof and, in places, of the sides also.

This sandstone is divided up by beds or faults which lie approximately 25 to 30° off the perpendicular and all appear to be bearing in a north-easterly direction. These faults sometimes occur from 6 in. to a foot apart, and then in varying distances up to a chain or more.

Such faults were encountered in all parts of the city where the construction was in sandstone, the sole exception being the double-track tunnel north of Wynyard, which lies through a fine-grained yellow block stone in horizontal strata.

The City Railway has been located for the most part fairly close to the ground surface in order to give easy access to the stations. The shallowest depth to the top of the tunnel is about 3 ft. 6 in. in York Street, and the greatest, 57 ft. in Macquarie Street near the Mitchell Library

This fact caused a considerable amount of interference with various public services, such as water, electric mains, sewers, etc.

Water, gas and electric power mains were fairly easily dealt with as no account had to be taken with regard to grade in their diversion.

The sewers were difficult and very extensive diversions had to be made, particularly in York Street and George Street near the Town Hall.

A special low-level line of sewer of over a quarter of a mile in length had to be constructed from York Street to the low-level pumping station in Sussex Street and connections to this had to be made from Wynyard Lane, Barrack Street and King Street.

A special line of sewer had to be constructed along George Street from Bathurst Street to Druitt Street and carried across the Railway between the Upper and Lower tracks. The telephone tunnel carrying telephone lines to the Eastern Suburbs had to be lowered for a length of 150 ft. in Liverpool Street necessitating the re-laying and re-connecting of some 30,000 telephone wires. The re-location of all these services was, of course, carried out by the Departments concerned.

METHODS OF TUNNEL CONSTRUCTION.

The method of constructing the tunnels on the City Railway, particularly on the western side, have been many and varied.

On this side, there is the case of superimposed tunnels, i.e., one tunnel directly on top of another for a length of a quarter of a mile, also one tunnel crossing skew-wise over the top of another. There are three tunnels running side by side on the same level with a 3 ft. wall between each. Further on, this becomes four tunnels side by side with 3 ft. walls and building loads on top of them; also there is the case of two single-track tunnels converging into one double-track tunnel.

The above cases have been the determining factors in the methods of construction adopted.

The standard width for single-track tunnel is 15 ft with a height of 17 ft. 3 in. from rail level. These are finished sizes. Standard double-track width is 27 ft. and where two tracks diverge this steps up to 31 ft. and then to 35 ft. where the tracks gain sufficient clearance for two 15 ft. tunnels with a 5 ft. wall between.

For ordinary single-track arch tunnel, the method used has been that which has been generally used in all tunnels constructed on the New South Wales Railways. It is known as the top heading and bench method. Fig. 1shows a single-track tunnel in course of construction by this method.


Figure 1. Single Track Tunnel without timber.



The top heading is more or less a pilot tunnel, usually 6 ft. high by 4 ft. wide, the excavation of which is kept ahead of the opening up of the full section of the tunnel and which provides the basis for this opening out. The muck excavated from the heading has to be loaded into skips and pushed by hand to where it can be tipped into trucks or kibbles at the working face. Consequently, it is not desirable to have too long a heading, the usual length being about 33 ft. from the working face of the tunnel.

In this heading the two leading crown bars are placed about 3 ft. 6 in. centres and lathed over tight to the rock. If the ground is very bad, square sets can be set up in the heading and the crown bars run in under the caps when sufficient length has been taken out. The crown bars used were all ironbark sticks, 22 ft. long and approximately 8 in. diameter at the small end and the laths 4 in. thick split hardwood.

From the heading, the arch of the tunnel is widened out on both sides, additional crown bars being put in at approximately 3 ft. 6 in. centres and lathed over. The crown bars are all temporarily supported on legs from the ground.

At the same time as the heading and arch are being excavated, the lower portion of the tunnel is being pushed forward. Benches are formed by the beds of material driven through and this material is blasted down from bench to bench until the bottom is reached.

Horizontal cross-timbers called horse heads, which are usually ironbark sticks 20 ft. or over in length and 14 in. diameter at the small end, are put in at the proper time, the crownbars being legged on to them. These horseheads are usually supported in hitches cut in the side of the tunnel to a depth from 18 in. to 30 in. In places where the ground is not sufficiently good to support the horse heads, temporary legs are placed under them on to the nearest bench and these in turn are replaced by permanent legs when the tunnel has been excavated to its full depth.

The normal spacing for the horse heads is 16 ft. 6 in. centres, but in cases where the overburden has been heavy this has been reduced to 8 ft. 3 in.

A very necessary point in this class of timbering is the provision of timber stretchers between the bars and between the lowest bar and the ground. These are usually cut out of 4 in. split laths. They are cupped at either end to fit on the round of the crown bar and are made a driving fit between bars. Three or four are spaced equally along the length of the bar and between the bottom bar and the ground, thus forming the timbering into an arch which makes the timbering self-supporting in case a leg becomes dislodged by blasting. These timber stretchers can be seen at "A" in Fig. 5.

This form of timbering is also suitable for double line tunnels, the only difference being that a greater number of crown bars is required, and if the ground above is at all heavy the horse heads are sure to need legs under them to support them over the greater span.

In order to obviate the use of long horse heads, an alteration was made in this method in certain double-line tunnels. In these cases, the usual excavation methods were used down to the springing line of the tunnel arch. At this level the bench was left intact with the exception of a gullet about 12 ft. wide in the centre of the tunnel which was excavated to formation level. On this bench and spanning across the gullet short horse heads were placed. These were sufficiently long to carry legs from about six crown bars only. Any additional crown bars were legged directly off the ground. On either side of the tunnel and on a line 6 in. inside the face-line of the tunnel, jackhammer holes were bored at about l} in. centres and 6 ft. to 8 ft. deep.

The concrete arch was poured with the natural rock as a skewback, the holes being covered over to prevent the concrete from going down. After a section of, say, 4 or 5 chains had been arched over, the sides were taken out, the rock breaking off on the line of the close bored holes having a thickness of 6 in. to be faced up with concrete as a sidewall. An illustration· of this method can be seen in Fig. 2, where the arch concrete is shown resting on the rock which has been blasted off to form the wall line by the close bored holes at "A."

Figure 2. Breaking from Double Track Tunnel into Single Track Tunnel north of St James Station.
Figure 3. Single Trach Tunnels Converging.



The main objection to this method of excavation was that it necessitated the use of very long legs from the horse heads to the crown bars and as the tunnels were all in rock there was always a possibility of these legs being knocked out by the blasting. This was avoided to a certain extent by driving the bottom gullet on under the top heading and breaking the rock down into it and also by drilling flat holes instead of vertical holes for blasting down the bench.

Notwithstanding these precautions, the legs were knocked out occasionally, but the crown bars were held up by the stretchers and did not become misplaced.

An entirely different method of construction was used in twin single-track tunnels between Liverpool Street and Goulburn Street, under Park Street and also near the Central Police Court.

In these places the ground pierced was a clay with ironstone bands in the top, a few feet of blue shale below, and then sandstone in the bottom. The ground was not suitable for the construction of a double-track tunnel, nor was there sufficient head room to the surface to allow of it.

The track centres were 17 ft. apart, which allowed of two 15 ft. tunnels with a 2 ft. wall between.

The method of construction adopted was as follows:- First of all three headings were driven on the line of the three walls of the tunnels. The outside two were 10 ft. 6 in. high from rail level by 4 ft. wide and the centre one was the same width but 2 ft. 3 in. higher. These headings were timbered with square sets using 8 in. x. 5 in. sleepers for caps and 8 in. x 5 in. hardwood legs, the sets being at about 4 ft. 6 in. centres. They were close lathed with 1½ in. hardwood laths on the top and also down the sides to the rock level. They were driven in first of all for a length of 100 ft. Bricklayers were brought in and, starting at the extreme ends of the headings, they built the three walls of the tunnels. The sidewalls were made 2 ft. 8 in. thick and finished at the springing line of the tunnel arches, viz.:-9 ft. 9 in. above rail level. The centre wall was 2 ft. thick and was carried up to 2 ft. 3 in. above springing level on the radius of the arch, thus giving an over-hang of 3 in. on each side from the wall line. When this brickwork had been finished, the excavation of the arched portion of the tunnels was commenced. The two tunnels were excavated simultaneously and in order to carry the overburden curved steel rails were used. These were 80 lb. rails and were curved to the outside radius of the tunnel allowing for 18 in. thickness of concrete.


Figure 4. Double Track Tunnel Face, Museum Station excavation.
Figure 5. SDouble Trach Tunnel Timbering



There were 2 pieces of rail to each tunnel, or four pieces in the set. (See Fig. 6). · The ends which were to rest on the sidewalls were cleated on to 12 in. x 12 in. ½in. plate, while the ends which landed on the centre wall were made straight for the first 12 in. of their length so they could be bolted together back to back and were cleated to 12 in. x 6 in. x ½ in. plates. '

The first set was stood up on the end of the brick walls a few inches clear of the muck face, the sections being joined in the centre with fish plates and fish bolts and bolted together on the centre wall. The arch was then excavated for a distance of 3 ft. ahead, the excavation being taken out slightly bigger than the radius of the rails. A second rail set was stood up at 3 ft. centres from the first and the intervening spaces lathed over with 1½ in. sawn hardwood laths; 4 spacers of I in. black iron pipe with ¾ in. · tie bolts were put in between the sets at equal distances, holes having been provided in the webs of the rails. The rails were pulled up tight against the spacers with nuts on the ¾ in. tie rods.

The next set and all succeeding sets were put in at 3 ft. centres in a similar manner and lathed over. Any voids behind the laths were carefully packed with stone. Where the ground was not sufficiently good to stand up without support over a 3 ft. span, bridging pieces were put in and blocked up I½ in. off the rail sets. The back run of laths was driven over the top of the bridging pieces and as the ground was excavated ahead, laths were threaded under the bridge pieces and driven ahead, the back end of these laths being wedged down with blocks under the laths in the set behind. The bridge blocks were knocked out as the laths were put in. The laths and cap pieces in what had been the wall heading were recovered and taken out as the driving of the arch progressed. The excavated material in the arched part of these tunnels was mainly ironstone clay, and was taken out with air-driven pick machines.

After the arches had been excavated there remained a dumpling of rock and shale about 11 ft. high by 9 ft. wide. This was. shot down and filled on the floor level of the tunnel.

As soon as the excavation had been completed up to the end of the brick walls, the excavation of the side and centre heading was recommenced and a further section of 100 ft. was taken out. The brick walls were then built in this section and the remaining excavation taken out as before. The only alteration made in this system was when the driving of the arches was done under the south-eastern wall of Mark Foy's building in Elizabeth Street. The foundations at this point were known to be very close to the side of the arch and in order to take the extra loading the spacing of the rail sets was reduced from 3 ft. to 2 ft. centres for about 10 ft. before reaching the building and was then again reduced to I ft. 2 in. for 6 ft. when directly under the corner column. The packing of any voids behind the laths was done with concrete instead of stone. The spacing was again opened out after passing the building.

Another departure from ordinary- methods was adopted in several instances on the western side of the city.

The necessity for some new scheme of timbering became apparent when a tunnel for the Western Suburbs Loop had to be driven under the site of Messrs. Murdoch's new building and the Commonwealth Hotel in George Street.

In order to carry building loads above, the roof of this tunnel was to be composed of 34 in. x 12 in. broad flanged beams encased in concrete. This necessitated the full width of the tunnel being clear of any timber to allow of the raising of the girders and also the provision of a space above the girders for the placing of the concrete and waterproofing.

The ground was fairly good, being hard rock in the bottom, and medium hard to soft in the top. It was quite dry and would obviously impose no great load on the timber provided that no movement was allowed to take place.


Figure 6. Curved Rails as used in Twin Tunnels.
Figure 7. Hip Roof Timbering under Murdoch's Limited.



It was finally decided to go in for a hip roof style of timbering. The excavation was taken out in the form of an inverted V, the lower ends being about 15 in. above what would be the top of the girders and the rise was made about 1 in 4 to the peak. (See Fig. 7).

Oregon timbers, 12 in. x 12 in., were cut with the correct bevel at one end and square at the other. Hitches were cut in the rock for the lower ends to rest in, the bottom of the hitches being roughly square to the line of the timbers. A pair of 12 in. x 12 in. Oregons was then cut to the proper length, an allowance being made for packing behind the square end in the hitch. They were lifted into position by hand and butted together at their bevelled ends. Packing pieces and wedges were placed behind the square ends in the hitches and driven home until the butted joint was perfectly tight. Wedges and packing pieces were put in here and there between the top of the timbers and the rock to prevent the timber from rising while being tightened up in the hitches. These sets were put in in most cases at 4 ft. centres and were lathed over with 4 in. split hardwood laths.

In the case of single-line tunnels, the broad flanged beams were 20 ft. 6 in. long and so the span for the timber bents was made 22 ft.

At the north end of the Town Hall Station, a length of 80 ft. was tunnelled under Druitt Street on three tracks, and this same system of timbering was adopted. The span in this case was 28 ft. 6 in., and on that account the bents were put in at 3 ft. instead of 4 ft. centres.

The view shown in Fig. 8 was taken at this point. A section of the girders has already been erected at the back end of the tunnel and temporary supports carried from these up to 12 in. x 12 in. Oregons in order to take additional loading imposed upon them through excavation having been taken out on a higher level.


Figure 8. Timbering, north end, Town Hall Station.
Figure 9. Layut of Tunnels, north of Wynyard Station.



This system of timbering was most satisfactory for the locations in which it was used. Space for handling such big beams as the 34 in. x 12 in. x 20 ft. 6 in. was very necessary and the ordinary tunnelling methods could not be applied. The 12 in. x 12 in. Oregons were used for hoisting the broad-flanged beams up into position and, when they were in position, there was still ample room above for the placing of the concrete and waterproofing.

In the majority of cases where this system was used, the space above the girders was not re-filled as it became part of another tunnel on the higher level. This particularly applied to Town Hall Station where three additional tunnels are directly superimposed on the lower level tracks. Under Messrs. Murdoch's building a high level tunnel cuts skewwise across the top of the low level track and a portion of the sidewall of this tunnel was built in the space above the roof beams, the remaining space being filled with light brick piers and rubble packing.

The Oregon timbers and laths were taken out one by one, and the brick piers and rubble being put in in their place.

In connection with the construction referred to under Messrs. Murdoch's building, the following may be of interest:-

In November, 1926, Messrs. Murdoch's Ltd., purchased, and decided to pull down, several old shops fronting George Street and adjoining Messrs. Gowing Bros. Ltd., and the Commonwealth Hotel, and to erect a new building of 150 ft. in height on the site.

The location of the Western Suburbs loop passed under the front of this building and the wall of the City Inner tunnel was along the George Street building line. It was decided to obtain access to the site from Messrs. Murdoch's Ltd. for sufficiently long to enable this portion of the tunnels to be built before the building was erected. The cost and inconvenience of underpinning this high building would thus be avoided.

A site for a shaft 24 ft. long x 12 ft. wide was granted for a period of 4 months, and a shaft of this size was sunk to the formation of the lower tunnel.

From this shaft, tunnels were driven north and south to the boundaries of Messrs. Murdoch's property, the methods used being as described. As soon as the concrete sidewalls and beam top had been erected and concreted, a heading was driven for the sidewall of the tunnel above, also to the boundaries of the property. This sidewall was then built-in brickwork and, on this brick wall, foundations were provided for the front line columns of this building.

The brick wall was, in places, 15 ft. thick at the base to provide for the column loading which amounted to 750 tons on the corner columns, and up to 450 tons on the remainder. Foundations were provided for all columns in the first two rows in the building.

In addition to the above, a small drive was run for about 150 ft. north to provide for a low-level sewer which will eventually extend from Park Street to Market Street. The 16 in. earthenware pipes were laid in this portion and the drive back-filled.

All this work was completed in 3 months and the site handed back to Messrs. Murdoch's Ltd.

By this time the owners of the Commonwealth Hotel had decided to re-build on their site on the corner of George Street and Park Street, and, as soon as their building had been dismantled, permission was sought to complete the tunnels under this area also. This was granted, and the construction was carried out from a shaft 12 ft x 9 ft. just inside their northern boundary.

This section completed the building of the Western Suburbs tunnel and the City Inner sidewall up from the boundary of Lowe's Ltd. to the northern end of the Town Hall Station.

All the work was covered up and it was not until the middle of 1929 that the City Inner tunnel, being driven from Market Street, was completed up to the piece of wall that was finished in 1928.

A second Western Suburbs tunnel was also constructed at this point in a rather unusual manner.

Just in front of Lowe's building, the City Inner high level tunnel reached a point where the Western Suburbs tunnel crossed below it on a long skew.

The driving of the upper tunnel was stopped, and a shaft about 8 ft. x 6 ft. was sunk from the floor of the tunnel down to the formation level of the lower tunnel, a depth of 23 ft. Also, an adit was driven in from the top formation level on the level of the roof of the lower tunnel. From this adit, the excavation of the top portion of the lower tunnel was commenced, using the hip roof style of timbering. The lower part of the tunnel was taken out down to formation level and connected up to the shaft. A short poppet head was erected in the top tunnel over the shaft and an electric winch_ installed. By this means all the spoil excavated was lifted up in kibbles and run out along the City Inner track to Market Street. Also all the timbers, and afterwards the broad-flanged beams, were lowered into the bottom tunnel by the winch.

This lower tunnel was driven and completed up to the northern end of Town Hall Station before the driving of the upper tunnel was re-commenced.

Just south of Market Street, the Down Shore high level tunnel passes un<l er the corner column and two adjacent piers of the Queen Victoria Buildings. The load on the corner column was estimated at 400 tons, and on the piers at 370 tons each.

The concrete bases were known to be well spread and were expected to be on good hard rock. These loads were to be carried on 28 in. x 12 in. broad-flanged beams, at 2 ft. centres, which were to form the roof of the tunnel. It was decided to build the sidewalls of the tunnel with brickwork.

The ordinary single track arched tunnel was excavated and concreted up to the building line and stopped. Two headings for the two walls were then commenced. The eastern one, which was clear of the building, was made 9 ft. wide, and the western one, which passed under the columns, was 7 ft. wide. The headings were in good rock but were timbered with 12 in. x 6 in. Oregon caps on 12 in. x 6 in. Oregon legs. The western heading was made only 12 in. above the top of the girders and the eastern heading was 2 ft. 6 in. above this to give drift for hoisting the girders into position. The Oregon sets in the eastern heading were placed at 3 ft . centres and lathed over: with 1½ in. hardwood laths. The legs were made 7 ft. long and were let ½ in. into the ends of the cap pieces and were splayed 6 in. at the bottom where they rested on a ledge of good hard rock.

On driving the western heading, it was found that 2 ft. of the concrete foundation of the column had to be cut away to give clearance for the caps. This heading was also timbered with 12 in. x. 6 in. Oregon caps on 12 in. x 6 in. splayed legs. But in this case the caps were put in side by side so that the roof was actually dose timbered with 12 in. x 6 in. Oregon. The legs were splayed 6 in. in their length of 7 ft. and rested on the rock.

The concrete in the foundations was very hard, and was cut away with compressed air pick machines, while a water jet played on them to lessen the dust. Portion of a trachyte base stone of one of the cast-iron columns supporting the first floor system had also to be cut away so that it will be seen that the tunnel roof was running very close to the actual structure of the building./

The top 7 ft. of these headings was excavated first, then a line of jackhammer holes was close bored 6 in. inside the bottom of the legs. This was done so that when the bottom portion of the excavation was being taken out, the rock would break off on this line and would not break away under the legs and allow them to become loose.

In order to avoid having more than one column load on the timber at a time, these wall headings were driven to; a point past the first column and not quite up to the second column of the building. Bricklayers were then started, and the brick sidewalls erected, and the bed plates for the girders put in.

Sufficient excavation was then taken out between the two walls to give clearance for the first girder to be hoisted into position. Two timber sets in the eastern heading, which would foul the girder, were also removed. The girder was pulled up with tackles, one end being pulled up first into the high heading on the eastern side, the other end then being lifted up and launched into position on the western wall. As soon as the girder was in position, brick packing was laid on the top flange and wedged up tight on the concrete foundation and the rock. A further 2 ft. of excavation was then taken out ahead of the first girder and this and succeeding girders were put in in a similar manner.

The space between the girders was filled with concrete, and packed hard up to the foundation and the rock.

When the girders were erected up to the end of the first length of brickwork, the driving of the headings was re-started and carried under the second column and up to. the third column. The brick walls were built and the girders erected in a similar manner as that described above. The third column was similarly treated, after which the tunnel construction was clear of the building.

The second column was actually the most difficult to deal with as the base was only 6 ft. square and the centre line of the column practically coincided with the centre line of the heading so that the heading timbers covered the whole area of the column base. The load was satisfactorily carried on the 12 in. x 6 in. Oregon caps, and no movement of the column took place.

Much more intricate underpinning of buildings than that described above has been done between Goulburn Street and Liverpool Street but a description of the methods employed would provide sufficient material for a separate paper.

VENTILATION OF TUNNELS.

The efficient ventilation of a tunnel is a matter of considerable importance during the construction period. It is not only a necessity from the point of view of safe-· guarding the health of the employee, but represents a considerable monetary saving to the employer.

By the installation of an efficient system, the workmen are able to return to work within a short time after blasting has taken place, and they are enabled to maintain a higher output of work in an atmosphere which is not vitiated by the presence of a large gang of men and horses in so confined an area.

Also it has been laid down by an award of the court, that, where the atmosphere contains more than 200 particles of silica dust per cubic centimetre, as measured by an instrument called "Owen's dust collector," the employees concerned shall not be permitted to work for more than 40 hours in any one week. This is a somewhat high standard and it is practically impossible to detect so small a quantity as 200 particles by eye. On a reading of 300 particles, the air only appears to be slightly foggy and would be considered quite clear by most observers.

Silicosis is a fatal disease and it is only right that the standard laid down should be such as to render remote the possibility of the employees being stricken by it, and also that the fear of the disease should not be present in the minds of the men.

The readings are taken in different parts of the tunnels daily, and for the most part at the time when the men return to work after firing has taken place. This is the time when the greatest amount of dust would be present in the atmosphere. The day following the taking of the readings, a copy of the results read is posted on a notice board for perusal by the gang concerned.

In the event of several readings being above 200 particles, the gang, through their Union, has the right to apply to the . Medical Officer of Industrial Hygiene for a reduction of hours, which is usually granted The City Railway is in a fortunate position for ventilation and the state of the tunnels at all times is very nearly perfect. In fact, during the past two years, out of some thousands of readings taken not more than 0.1 per cent. have been over the 200 mark.

The shallow depth of the bulk of the tunnels has been the greatest factor in simplifying the ventilation problems

When the first tunnel was started at the northern end of Museum Station, a scheme for clearing the smoke and dust rapidly out of the tunnels was sought.

As the smoke was usually thickest in the top of the tunnel, it was decided to try the effect of boring holes from the surface to the tunnel roof and to see if any smoke would rise through these holes. A boring machine and an outfit of 3 in. drills were obtained and holes drilled to the tunnel. Up-draft cowls were placed on top of these, but, as the wind directions varied so much owing to the proximity of buildings, these cowls very often acted in the wrong direction, and other more positive methods were sought.

A couple of exhaust fans was discovered which had been used for exhausting sawdust from circular saws, but these were not very efficient.

The next type tried was a couple of 15 in. Sirocco fans, and in order to obtain greater efficiency from these, 6 in. boring bits were made, and all holes put down of this diameter. The Sirocco type of fan. appeared to be the right appliance for the job and has been adopted throughout the remainder of the tunnels.

As a rule. 20 in. fans are used. These are belt-driven from a 15 h.p., 600 volt, d.c. motor, mounted on a hardwood frame.

As a 6 in. diameter hole was the largest which could be drilled with the machine available, the fans were throttled down to a certain extent. However, at 2,500 revolutions, each fan exhausts approximately 4,500 cubic feet of air per minute. Three of these fans will give a complete change of air in a chain of tunnel in less than 2 minutes.

The system adopted has been to drill the holes on the tunnel centre line to the required depth and to keep the drilling ahead of the driving of the tunnel.

It was found that drilling the holes after the tunnel was excavated was liable to loosen large pieces of rock from their beds and possibly to have them fall into the tunnel.

These holes are put down 16 ft. 6 in. apart, and fans are kept on the three leading holes as the driving advanced. They thus cover a space of three-quarters of a chain and this is sufficient to take in the heading and the remainder of the working area. The foul air and dust are exhausted continuously and a current of cool clean air is drawn in from the mouth of the tunnel or shaft.

Where firing is being done, the smoke from the fuses is out of the tunnel before the shots go off and the tunnel is practically clear within 10 minutes after firing a round of, say, forty shots.

The fans are housed in small galvanised iron sheds about•' 10 ft. long by 6 ft. wide. These houses are fitted with lights all round and a pole for carrying the power wires from the nearest permanent pole. The houses are built on skids and when a move has to be made the wires are disconnected off a plug on the permanent pole, a lorry is hooked on to the house, and it and the fan and motor inside are pulled along to the next hole and the wires reconnected. The whole operation of shifting a blower only takes a few minutes.

The only places where this system cannot be used is, of course, where the tunnels pass under buildings or tramlines.

In these cases, a couple of exhaust fans are left on the last holes possible and a blower is installed inside the tunnel as near to the working face as practicable.

Usually a 6 in. diameter pipe is carried along the top of the tunnel from one of the exhaust fans, and this can be brought right up into the heading to keep the heading clear. The blower in the tunnel is only run while blasting is taking place and this has the effect of raising the smoke and dust to the top of the tunnel and causing it to travel back to the exhaust fans.

Pipes of 9 in. diameter are usually connected to the blower and brought as near to the working face as the blasting will permit. As soon as all the shots have gone off, one end of a length of canvas hose of 9 in. diameter is slipped over the end of the pipe line and the other end carried up into the heading. Fresh air is thus pumped right against the tunnel face and the smoke and foul air are forced back to where the exhaust fans can deal with them.

It has been found that a blower working by itself is not satisfactory. It will force the smoke and dust back from the working face temporarily, but will not remove it from the tunnel. The result is that, though the working face may be clear, a large body of smoke will be hanging in the air somewhere in the length of the tunnel and, by degrees, will drift back again to the working face. Also the blower cannot be run whilst the men are working as the cool air striking on their backs causes rheumatism.

CONCRETING OF TUNNELS.

The whole of the arched tunnels on the City Railway has been lined with concrete all round, including the bottom beneath the roadbed.

This lining is put in in four stages-first, the footings of the walls are put in to a height of 3 in. below rail level - second, the sidewalls are carried up to a height 18 in. above the springing line of the arch, or 11 ft. 3 in. above rail level - third, the arches are poured, and, lastly, the roadbed foundations.

The footings are poured in varying lengths as required, but the walls and· arches are always poured in lengths of 16 ft. 6 in. The pouring of the roadbed foundations is not confined to any exact length.

Single track arches are semi-circular and of 7 ft. 6 in. radius. Double track and special arches are three centred, the radius from the springing line being the same in all cases, viz ., 7 ft. 6 in. This means that the same sidewall forms may be used for any diameter of tunnel.

Refuges for maintenance gangs are placed in the tunnel walls on one side at 66 ft. centres. These refuges are 4 ft. wide, 2 ft. 3 in. deep and 7 ft. 6 in. high, and the floor of the refuge is 3 in. below rail level.

In the opposite wall, at 132 ft. intervals, there are placed cabins for signal and interlocking gear. These are 6 ft. wide, 3 ft. 3 in. deep and 9 ft. 6 in. high, the floor being 13 in. below rail level. These cabins and refuges all have segmental arch tops.


Figure 10. Double Track Tunnel diverging into Twin Tunnel.
Figure 11. Concrete Mixing and Boring in Macquarie Street.



Right throughout the progress of the work, the concrete lining has been carried on simultaneously with the driving and has been kept up as close to the working face as practicable

The use of the holes bored from the surface has enabled the concreting to be carried on without any interference with the excavating gangs.


Figure 12. Double Track Tunnel Concrete.
Figure 13. Single Trach Tunnels being poured.



The excavating gangs take out the footings to a depth of about 2 ft. to 2 ft. 6 in. below rail level and also take out the excavation for the refuges and signal cabins where required. Carpenters come in and set up the timber for the footings. This usually consists of 12 in. x 3 in. planks on edge and strutted off the rails of the skip road. Chutes are hung up under the most convenient hole in the roof of the tunnel, and the concrete is poured down from a mixer in the street or park above.

At a point 11 in. below the top of the footing, the concrete is allowed to project out 12 in. into the tunnel. This forms a sort of step and provides a foundation for the sidewall forms and later for the arch form trestles to rest upon and also to be rolled along on when shifting from one set up to the next.

After the footing forms have been· shifted, the sidewall forms are stood up. These are timber forms, 11 ft. 9 in. high and 17 or 18 ft. in length. They are built out of 8 in. x 6 in. uprights at 3 ft. 6 in. centres, tenoned into a 12 in. x 6 in. sill, the whole of the timber being Oregon. The laggings are 9 in. x 3 in. Oregon nailed to the uprights. Top, bottom and centre waling pieces of 6 in. x 4 in. are bolted on the outside of the uprights. As the springing level of the tunnel arch occurs 18 in. below the top of the sidewalls, the 7 ft. 6 in. radius is cut out of the uprights. This piece is lagged with 4 in. x 3 in., with the edges planed off to conform to that curve. An illustration of a sidewall form may be seen at "A" in Fig. 13.

In some cases these forms have been covered with 22-gauge galvanised iron, and have given satisfactory results.

The sidewall forms are erected in pairs one on either side of the tunnel and strutted apart with 6 in. x 6 in. Oregon timbers, one in the centre and one on top of each upright. This allows clearance for the buckets carrying excavated material to pass underneath.

The bottom of the sidewall form laps 3 in. below the top of the footing to make a tight joint.


The bottom -of each post is strutted from the rail of the skip road, and blocking pieces are put in between the rails, in addition to the sleepers, to keep them from coming together.

The struts are tightened up with wedges between their ends and the formwork and temporary toms are put in between the forms and the tunnel sides to take the thrust. These toms are taken out as the concrete is filled behind the formwork. The forms are wedged up to the correct height with large hardwood wedges which are placed between the bottom of the form and a hardwood plank laid on the concrete step in front of the footing.

The bulkhead on the ends of the forms is made up of 1 in. Oregon, nailed on to the end post, and held in position by a 9 in. x 3 in. plank, stood on end. Jackhammer holes are bored in the rock of the tunnel side, and pieces of 1½ in. diameter galvanised iron pipe, about 2 ft. 6 in. long, put in the holes. There are four holes in the height of the sidewall, and wedges and blocking pieces are driven in between the pipes and the 9 in. x 3 in. plank. In addition to these, an inclined strut of 6 in. x 6 in. is often put in against the 9 in. x 3 in. plank and hitched into the concrete footing or the rock bottom of the tunnel at the other end

The concrete is poured behind these forms from the holes in the tunnel roof and is picked and rammed into position by concrete packers.

Separate forms are made for the refuges and signal cabins and these are put into position and held in place with struts off the rock prior to the sidewall forms being placed in front of them. All temporary struts and toms are knocked out by the concrete packers as the concrete rises behind the forms.

The minimum time allowed for the concrete sidewalls to set is 24 hours.

To shift the sidewalls forms, the slack blocks or wedges behind the struts and the wedges under the forms are loosened and the forms are pulled in towards each other with rope twitches or union screws on wire ropes fastened to an upright at either end. When the forms come clear of the concrete, they are held clear by a light batten nailed to either end.

The 6 in. x 6 in. struts are not taken down but are left resting on the top and centre walings. Short pieces of the forms are lowered on to these by further slackening the wedges. Hardwood planks are laid on the concrete ahead, and the forms arc rolled ahead by men with crowbars and levers or light tackles.

The form is lapped 6 in. on the concrete previously poured to give a tight joint.

Before being set up, the laggings are well scraped and cleaned of any concrete which may be adhering to them. They are then set up again ready for the next pour. The thickness of the concrete sidewalls ranges from 6 in. to over 2 ft., according to the nature of the ground and the construction.

After a couple of lengths, say 33 ft., of sidewalls have been concreted, the arch forms are brought in. In the case of single-track tunnels, these consist of ribs built up of 1½ in. Oregon, laminated in 3 pieces and cut to the radius. The ribs are bolted together with the joints of the middle laminations staggered, the outside joints being covered with ½ in. steel cover plates bolted on.

The tie beam between the ends of the ribs consists of two 12 in. x 4 in. Oregons on edge, the laminated rib coming in between them and held with bolts.

At the third points of the tie beam, two 8 in. x 8 in. posts, checked out to fit between the two pieces and bolted through, are carried up to support the rib. They are cut away on top to give bearing to each of the three laminations.

The ribs are made sufficiently long to give a 3 in. lap on the concrete already poured with the sidewalls.

Five of these ribs equally spaced go to a r6 ft. 6 in. set. They are braced and held together longitudinally with 9 in. x 1½ in. hardwood. Fig. 14 shows a set of these ribs being set up ready for concreting.

To support them in position, timber trestles are built. These are built about I 7 ft. long with 9 in. x 6 in. hardwood caps and sills and legs of either round hardwood posts of 6 in. x 6 in. Oregon. These trestles are built so as to leave a space of 9 in. between the top of the trestle and the arch rib when in position. In this space is placed a 9 in. x 3 in. plank under the ribs, and between the plank and cap of the trestle are the wedges for lowering and raising the ribs.

The laggings, which are of 4 in. x 3 in. Oregon dressed all round, are not fixed to the ribs. They are laid in position and are tightened up with a key lagging in the centre. To stop them from rising, combing pieces are made in lengths of about 3 ft. 6 in. to 4 ft , cut on the radius. These are placed on top of the laggings at the ends and are held down with toms from the roof of the tunnel. The bulkheads on the ends of the arch are also fixed to these combing pieces.

The pouring of the arches is also done from the surface, and it is in this connection that the actual position of the ventilating holes was fixed. It was found that, when pouring the concrete into the arch forms, it was always better to have the hole at the back end of the arch, so that the packers with long rakes could draw the concrete towards the front, rather than to have the hole at the front end and to try and push the concrete towards the back end.

When once the front end had been filled up, by pouring down a few more batches, the back end filled itself. Consequently, the holes on the surface are always set out so that they will come about 3 ft. from the back end of a 16 ft. 6 in. arch.

When placing the timber in a tunnel, consideration also has to be given as to which end the concreting will be done first. The crown bars are, as previously stated, 22 ft. long. With 16 ft. 6 in. between horse heads, this would leave an equal lap of 2 ft. 9 in. at each end. This lap is not made equal, however, but is usually about 3 ft. 3 in. at one end and 2 ft. 3 in. at the other, the long end being in the direction in which the concrete arch is approaching. This overlap of crownbars is thus well caught up in the concrete arch which then forms its support so that the horse head and legs can be taken out leaving a clear space for the arch form to be moved ahead into the next bay ready for concreting again.

The first arch poured in a tunnel has to be set up between the horse heads which are at 16 ft. 6 in. centres. This means that the spacing of the ribs has to be reduced and length of arch poured has to be only about 15 ft.

As soon as the first length is concreted, thereby catching up the ends of the crownbars in the set ahead, the first horse head can be removed and the ribs opened out to give a full arch of 16 ft. 6 in. in length on the next and subsequent moves.


Figure 14. Single Track Tunnel Concrete Centring.
Figure 15. Double Track Tunnel Centring.



The last rib in the arch form is always set so as to come under the end of the preceding pour. This is done so that the ends of the laggings can be wedged up tight to the concrete with small wedges driven between the laggings and the rib. Each lagging is wedged up and a tight joint is thus formed.

Owing to the fact that the arch ribs finish at 1 ft. 3 in. above springing level, the moment that they are lowered a few inches, they come clear of concrete already poured and give ample room for removal from one set up to the next.

After each pour the laggings are lowered one by one to the· ground and are thoroughly cleaned all round with scrapers before being again used.

The thickness of single-track tunnel arches is usually about 1ft. 6 in. below the timber. The concrete is, however, well grouted in and packed around the timber so that the resultant thickness of the arch is more nearly 2 ft. 6 in.

The aggregate used for the concreting of the footings, sidewalls and arches is usually 1½ in. blue metal and Nepean River sand. The mix is in most cases 1:3:6, though this is reduced in places where bad ground or heavy loads are encountered to 1: 2½: 5 or 1: 2: 4.

The mixing is done by 10 cu. ft. heavy duty mixers; the gauging being done in the ordinary concrete hand-carts.

The mix is kept .as dry as possible, but owing to the awkwardness of the placing it has to be fairly wet to be run into its position.

The formwork for double track and special construction tunnels, which go up to 35 ft. 0 in. span, is made up in somewhat similar manner to the single-track ribs. (See Fig. 15.) The ribs are built up of laminated pieces, 1½ in. thick, the outside joints being covered with cover plates as before. The tie beams are two 12 in. x 4 in. Oregons, spaced 4½ in. apart, and there are four 8 in. x 8 in. posts to each rib and a double waling piece of two 6 in. x 4 in.'s across the four posts at the level of the top of the shorter posts. The centre bay below the waling is diagonally braced with 8 in. x 5 in.'s. A short strut also of 8 in. x 5 in. is introduced from the bottom of the long legs to the top of the short legs. These larger ribs are made in two halves for convenience of handling and the tie beams are stiffened between the centre legs by a piece of 12 in. x 4½ in. hardwood between them bolted through.

The centre waling is also tied with a piece of 6 in. x 4½ in. hardwood.

The ends of these ribs rest on trestles as before, but a line of posts of 6 in. x 6 in. with cap pieces, is wedged up under each of the two centre uprights.

In only one or two instances has the placing of· the concrete in the tunnel arches been done by hand.

Between Goulburn Street and Liverpool Street, where the twin tunnels now being used, were constructed, the centre line ran skewwise across Elizabeth Street, which carries a double line of trams and heavy bus and motor traffic. In this case no holes could be bored from the surface.

Also the type of construction with the rail ribs, as previously described, made the placing of the concrete a fairly simple matter.

The ordinary type of arch form work was not used, the ribs being replaced by 4 in. x 3 in. T irons curved to the inside radius of the tunnel. These were suspended to their proper level at the same centres as the rail ribs, viz., 3 ft. 0 in., by ½ in .. bolts threaded at both ends, the top end being passed through a hole provided in a 3 in. x 3 in. x ⅜ in. angle cleat which was riveted on to the web of the rail rib. There were six of these bolts to each T rib, three being placed on each side of the flat of the T iron in order to keep it level. Nuts were put on, and the T irons levelled up to their proper position by slackening off or tightening on the nuts.

The laggings were laid on the T irons up to a height of 4 ft. 0 in. from the walls, the bottom lagging on each side being wedged against the brickwork. The laggings were made about 13 ft. 0 in. long and the length concreted each time was 12 ft. 0 in., the two arches, i.e., one in each of the twin tunnels being done simultaneously. The bulkheads were made in sections and curved to fit exactly between the rail rib and the laggings.

The concrete was mixed on the surface at the mouth of the tunnel and conveyed by a chute into skips which were wheeled along on a 2 ft. 0 in. gauge track on top of the centre dumpling of the tunnel, which had not been excavated.

The skips were tipped up under the arch and the concrete was shovelled in behind the formwork.

After the first four feet on either side had been filled, additional laggings were put in and filled until only a space of 28 in. on either side of the centre of the arch remained. For this section, pieces of lagging 3 ft. 0 in. in length had been prepared and the first set of these was put in at the back end of the arch, the ends resting on the T irons; This piece was then filled with concrete, the concrete being made fairly dry and packed and rammed hard up to the timber and rail ribs. After one 3 ft. 0 in. length had been filled, the next 3 ft. 0 in. set of laggings was put up and butted to the preceding set. This length was filled in, and, in a similar manner, the remaining 6 ft.0 in. up to the bulkhead.

To strip the timber, the nuts on the hanger bolts were taken off and the T irons lowered.

The fact that the centre wall had been built to 2 ft. 3 in. above springing level gave ample clearance for the T irons to be handled.

The laggings, which always adhere to the concrete, were knocked off and cleaned and stacked ready for further use.

The ends of the bolts, which were protruding about 4 in. from the concrete, were cut off with the oxy-acetylene cutter.

CONCRETE GUNS.

During the past year the concreting of tunnels in places where holes could not be bored from the surface has been done by means of concrete guns.

At the present time there are three of the guns in almost constant use on various parts of the works.

Two of the guns are of similar pattern and were built at the Departmental Workshops at Eveleigh from a design made in the City Railway drawing office. The third gun was purchased from a local manufacturer.

The two departmental guns consist of a circular hopper of 3 ft. 0 in. internal diameter and 12 in. high, tapering off to a flanged outlet of 10 in. diameter at the bottom in a height of 1 ft. 10 in. At the top, the circular hopper is reduced to a neck of 15 in. diameter, and then is opened out again to form an open circular top of 2 ft. 0 in. diameter. In the neck is formed a seating for a pear-shaped cast-iron valve which is lifted up into the closed position by a counter balanced lever and chain pivoted on the outside edge of the top hopper, the valve being made air-tight by means of a rubber neck ring.

At the bottom, a cast-iron bend tapering from 10 in. internal diameter to 6 in. is bolted to the flange on the bottom of the hopper. For the admission of air on the back of this bend there is a boss screwed to take a 2 in. diameter pipe. Also on the top of the hopper below the neck there is a hole tapped to take a 1 ½ in. diameter pipe. The hopper capacity is approximately 13 cubic feet, and the whole is mounted with 3 angle iron legs on a circular frame.

For the air supply to the gun, a 50 cu. ft. air receiver is mounted with the gun on a timber frame. This receiver is fed by a 3 in. pipe from the nearest supply pipe line and carries a safety valve, blow-off cock and pressure gauge. From the receiver, a 2 in. pipe is carried to the back of the bend at the bottom of the gun with an1½ in. reducing tee and pipe line to the inlet on the hopper. The 1½ in. and 2 in. lines have separate full way valves close together in a convenient position for operating.

From the 6 in. bend at the bottom, the pipes conveying the concrete are connected and are carried into the tunnel to the place it is desired to deliver the concrete. Flanged pipes with rubber insertion jointing are used and it has been found that steam pipes have better lasting qualities than the ordinary galvanised iron pipes.

A batch of concrete is poured into the gun, either from a hole or chute, an operator then washes the pear-shaped valve clean of any concrete by means of a compressed air and water jet and closes the valve.

Another operator then opens the 1½ in. valve admitting compressed air on to the top of the charge. This has the effect of forcing the concrete down into the 6 in. pipe line. He then opens the 2 in. valve and the rush of air forces the concrete along the pipe line. The opening of these two valves is really almost simultaneous, so quickly does the top air force the concrete down. The operator keeps the valves open until by the sound he knows that the batch of concrete is well on its way along the pipe line. He then closes the valves, and the top valve is opened to allow another batch of concrete into the hopper.

The pressure on the air receiver before firing is anything from 80 lb. to 100 lb. per square inch, the higher pressure, of course, being the most desirable. For shooting long distances, or over 6 chains, say, it has been found more satisfactory to have an additional air receiver at the gun giving a total stored capacity of from 100 to 150 cu. ft. of air. By this means the pressure is maintained at a higher point during the whole of the time that the valves are open, and there is not the likelihood of a batch being left in the pipe line, also the pressure builds up quicker and there is no waiting for air to shoot with.

When concreting single track tunnel arches by this means, the end of the pipe line is simply carried about 12 in. inside the bulkhead on the end of the arch and central to the tunnel. The pipe is kept as high up as possible and the concrete requires little, or no, packing by hand. The last piece of pipe is withdrawn when the whole of the arch has been filled.

For doing double track arches, a director was made to attach to the end of the pipe to divert the stream of concrete to the sides. This was made out of a piece of 1 in. mild steel, about 12 in. square, hinged on 3 in. x ¾ in. flat bars above and below the pipe. The plate was kept 12 in. clear of the end of the pipe and the flat bars were attached to the pipe by means of a circular clamp. The inch plate was held at any desired angle by means of a ¾ in. diameter rod attached to the end of it with an eye bolt. On changing from one side of the tunnel to the other, this bolt and rod are changed from one side of the plate to the other. When the haunches of the arch have been filled up, the director is taken off, and the concrete allowed to fill up the remainder of the arch from back to front.

The concreting of the tunnel sidewalls by means of the gun was at first considered impracticable as the concrete is projected from the end of the pipe with such tremendous force. However experiments were made and a baffle box was evolved which has proved quite satisfactory.

This box was made out of 1½in. hardwood in a 4 in. x 4 in. hardwood frame. The end opening in the box is 12 in. square. The sides and top are regular, whilst the bottom slopes away on a grade of 1 in 3. Inside the box are placed two baffle plates made out of 1 in. mild steel. These are placed one behind the other on opposite sides and make an angle of about 30 degrees with the sides of the box. The space between them on the centre line of the box is 9 in. The plates are 24 in. long, and protrude clear of the top of the box and can be easily removed for replacement.

Beyond the second of these plates, the top of the box is sloped sharply down to form a 9 in. square opening with the bottom and sides.

The force of the concrete from the gun is greatly reduced by the baffles and can be conveyed to any desired point by means of chutes from the bottom opening.

The boxes and baffle plates wear out from -time to time and have to be renewed, but their cost is small compared with cost of putting the concrete into the sidewalls by other means.

Concrete guns have proved an immense success on these works and have been the means of saving many thousands of pounds to the department.

ACKNOWLEDGMENT.

In conclusion, the author desires to acknowledge thanks' to Dr. J. J. C. Bradfield, M.I.E.Aust., Chief Engineer, Metropolitan Railway Construction, for having given permission for this paper to be presented, and also for the use of some of his slides.

Cookies help us deliver our services. By using our services, you agree to our use of cookies.