This appendix contains additional information beyond that which is contained in Chapter 9.
AT GRADE
The term "at grade" is normally taken to mean construction in cut, on fill or at ground level. Work at grade is usually the cheapest form of construction but the wide strip of right of way required, vertical alignment restrictions and the problems associated with land severance often limit its use in an urban area to short, isolated sections of route. The width of expensive right of way can be reduced by the use of retaining walls but, in that case, the cost may cease to be competitive with that of an overhead structure.
OVERHEAD
Overhead construction divides into three basic components: foundations, substructure and superstructure.
Where ground conditions are suitable, spread footings can be used but, in less favourable ground, foundations may have to be piled. The choice between the many different types of load-bearing piles is largely determined by the relative costs. In Hong Kong, the precast reinforced concrete pile is the most frequently used.
For two-track running line, the substructure normally consists of a single T-shaped column; where the superstructure is wider, as in overhead stations, two columns or a portal frame may be used. In either case, construction can be in steel or reinforced concrete.
The choice of superstructure depends largely upon the span; where foundations and substructures are costly it is usually economic to adopt longer spans. With spans up to about 50 feet, reinforced concrete is often used; for medium spans, the choice lies between prestressed concrete and composite steel and concrete designs while, with longer spans, steel is often the most economic material for construction.
A high degree of prefabrication is possible with both prestressed concrete and steel and this can be of considerable advantage when constructing in congested areas.
UNDERGROUND — CUT AND COVER
In this form of construction, the ground is excavated to the desired levels, the underground structure is built and the earth above is replaced.
The simplest method of excavation is an open cut with the sides inclined back at a stable slope. Where high water tables are met, the excavation is carried out so that it drains naturally, or else the ground is de-watered by wellpoints or other means. In soft ground, where the width and, hence, the volume of excavation, increase rapidly with depth, this method is uneconomical for all but comparatively shallow underground structures where there are few underground utilities and the resulting temporary severance during construction is acceptable.
The excavation for a rapid-transit line in a built-up area will usually be relatively deep, the available right of way is generally narrow and severance must be kept to a minimum. Thus the open-cut method of excavation is clearly not suitable and can be used only for short lengths of the proposed routes in undeveloped areas. In other methods of construction, the sides of the excavation are supported to limit width of construction and also reduce the volume of material to be excavated and later replaced.
File:MTS FigE-1.pngFigure E-1 — Cut and Cover Method of ConstructionIn Hong Kong much of the proposed rapid-transit system must be constructed under heavily trafficked streets which must remain in use during construction. Also many service utilities are located below ground. Figure E-1 illustrates the construction sequence of one method whereby these problems may be overcome.
Stage (1): Steel H-section soldier piles are driven (or inserted in bored holes) at intervals of about 6 feet along each side of the pro posed excavation. At stations, where the width of excavation is greater, it will often be necessary to drive an additional row of piles in the centre of the excavation.
Stage (2): A channel-section steel beam is bolted to the top of the soldier piles, the road surface is broken out and steel I-beams are placed at regular intervals spanning across the proposed excavation. The I-beams are bolted to the channel-section beams and cast iron (or steel) decking plates span between the I-beam to provide a temporary road surface.
Stage (3): The ground is excavated by hand to a depth below all the existing utilities. As the utility lines are exposed, they are suspended from the main road-deck beams and from further subsidiary beams.
All the work in Stages (1) to (3) can be done at night with total or partial closure of the road. Stage (4): Below the level of the services, mechanical methods of excavation can be used. Excavation can proceed below the deck during the day; the spoil being removed through shafts at the edge of, or beside, the main route. The sides of the excavation are supported by placing lagging boards between the piles as excavation proceeds. Also, the soldier piles are strutted apart to resist the lateral earth pressure. Stage (5): The underground structure is cast in the completed trench. Stage (6): Major services are supported off the top of the subway structure and then the earth is backfilled; lagging boards being removed concurrently. When backfilling is complete, the soldier piles are removed and the road surface is reinstated.
This method, using H-section soldier piles, is extremely adaptable in dealing with underground utilities crossing the sides of the excavation, but it is not normally suitable for excavations more than a few feet below the water table since it is difficult to make the lagging boards water tight. Also, in some soils, the floor of the excavation can become unstable due to the upward flow of water. The inflow of water can be reduced by pumping to lower the water table but this is often undesirable in built-up areas since it can lead to problems with the foundations of nearby buildings. Since the majority of the proposed underground structures are located beneath the water table this, and other similar methods, can be used only to a limited extent.
To build structures below the water table by cut-and-cover methods, therefore, it is normally necessary to provide watertight supports to the sides of the excavation. The supports are extended to some distance below the formation level to increase their stability and also to improve the stability of, and reduce the flow of water through, the bottom of the excavation.
The most commonly used support of this type is steel-sheet piling which can be driven to considerable depths except in very hard soils. The construction sequence is very similar to that of the method using H-section piles. In Stage (1) a trench is excavated along the edge of the proposed excavation in order to expose, and temporarily divert, services crossing the line of the wall. The piles are then driven to the required depth and, in Stage (2), the channel-section steel beam is bolted to the top of the sheet piles. Thereafter the sequence of events is the same; the piles can usually be withdrawn for subsequent reuse.
The noise commonly associated with pile driving would be objectionable in a built-up area, particularly if work had to be carried out at night, but this can be largely eliminated by the use of modern "noiseless" equipment. The main drawback to this method of construction is the inability of the sheet piles to penetrate underground obstructions, such as old foundations and boulders, which are frequently encountered in Hong Kong.
Another method is to place "bored in-situ" reinforced concrete piles along the sides of the excavation; the piles can be placed in a single row or staggered to form two rows, with each pile in contact with its neighbours over its full length. The construction sequence would be the same as for steel sheet piling. The holes for the piles are excavated by mechanical augers; in soft ground, the sides of the holes derive support either from continuous-flight augers or from "drilling mud". In the first case, concrete is pumped into the hole through the shaft of the auger as it is withdrawn and the reinforcement, formed into a rigid cage, is vibrated into position after the concrete has been placed. Where drilling mud is used, the cage of reinforcement is placed in position in the mud which is then displaced by concrete pumped into the bottom of the hole. Boulders and other underground obstructions can be overcome but the method becomes costly if these are frequent. Difficulty can be experienced, particularly at depths over 30 feet, in keeping the piles vertical and in their correct positions and it is difficult to make the excavation watertight if the piles are not in contact over their full length. Furthermore, the circular section of a reinforced concrete pile is not an efficient section to resist the bending forces imposed by the lateral earth pressure.
File:MTS FigE-2.pngFigure E-2 — Diaphragm Wall Method of ConstructionThe diaphragm wall method of construction is illustrated in Figure E-2. The construction procedure is as follows:
Stage (1): Trenches are excavated by hand along each side of the proposed excavations to a depth sufficient to expose any utilities; these are temporarily diverted. The top of the trench is then lined with concrete to provide a physical guide for the mechanical excavation.
Stage (2): Special machines excavate the trenches to the full depth required; the walls of the trenches being supported by the use of drilling mud. Steel reinforcement is formed into welded cages and is placed in position in the mud-filled trench; concrete is then placed from the bottom upwards through trémie pipes, thereby displacing the mud. Diaphragm walls are normally constructed in panels of 10 to 15 feet length. There is often no need to construct the diaphragm wall up to ground level; steel sheet piles or, if the water table is not too high, H-piles with timber lagging boards can be inserted in the trench to support the side of the excavation above the level of the permanent structure.
Stage (3): The temporary road deck is constructed, the underground utilities are suspended and material is excavated as previously illustrated. As the excavation proceeds the diaphragm walls are strutted apart to resist the lateral earth pressure.
Stage (4): The underground structure is built using the diaphragm walls to form the side walls, thereby partly offsetting the higher cost of walls compared with other methods of supporting the sides of an excavation. The standard of finish obtainable on the exposed face of the diaphragm wall is adequate for running tunnel construction but in stations it will be necessary to provide curtain walls.
Stage (5): The larger services are supported off the roof of the underground structure, the excavation is backfilled, and the road surface reinstated; sheet piles or H-piles used to support the sides of the excavation above the main structure can be recovered.
Diaphragm walls have been satisfactorily constructed to depths of over 100 feet, though, at the greater depths, difficulty can be experienced in forming efficient joints between panels and in keeping the walls vertical. This method of construction is suitable for most ground conditions and can be carried out well below the water table. Boulders and other obstructions reduce the rate of construction and increase costs but they are far easier to deal with than in the bored pile method of construction.
For all the methods listed above, the support to the side of the trench can be constructed at night when the road can be partially or wholly closed to traffic without causing any great inconvenience. However, this necessarily involves higher costs, because of the higher labour rates and also because of the length of time that plant must lie idle; this is particularly true for the diaphragm wall method which uses highly specialised and sophisticated machinery. Thus, wherever temporary traffic diversions can be arranged to enable the partial closure of a road during the day, the cost of rapid-transit construction will be reduced.
UNDERGROUND — TUNNELLING
Tunnelling can be adopted where underground structures are located below the economic depth for cut-and-cover construction or where it is desirable that the ground surface should not be disturbed. Except in the case of sound rock, tunnels are normally of circular cross section, this being the most efficient shape for the lining to resist the forces imposed by the surrounding ground. The cost of a tunnel is roughly proportional to the square of its diameter and, therefore, it is normally economic to provide separate tunnels for each of the railway tracks.
In sound rock, tunnels are often horseshoe-shaped in cross section and, if there are no ground-water problems, they can be excavated in free air. These tunnels are normally lined with in-situ reinforced concrete.
Where ground water is not a problem or where the water table can be sufficiently lowered, soft-ground tunnels can be excavated in free air. In good ground, such as undisturbed decomposed granite, tunnels can often be excavated without a shield and lined with either in-situ reinforced concrete or with prefabricated segments. In softer ground, a shield is needed to provide support at the working face and a prefabricated segment lining is required.
Where the tunnel is below the water table, it is shield-driven in compressed air. The air pressure is used to counter the hydrostatic head, thereby reducing the amount of water entering the tunnel to manageable quantities. There is a limit to the size of tunnel that can be driven in compressed air. Where large diameter tunnels are required, it is often easier and economic to drive one or more smaller tunnels and to enlarge them to form a bigger tunnel. In particularly difficult conditions, grouting of the soils can be carried out from a "pilot" tunnel prior to construction of the larger tunnel.
Where tunnels are driven in compressed air, costs increase considerably; apart from the expense of the additional equipment required, labour costs become higher. Men can work in compressed air for only limited periods and the change from compressed air to free air must be gradual to avoid compressed air sickness (the "bends"). In pressures up to 15 lbs/in2, an eight-hour shift is possible and the decompression period is fairly short. Compressed air work is seldom carried out at pressures above about 35 lbs/in2.
Reference has been made to the use of shields in driving tunnels. The main functions of the tunnel shield are to support the sides and roof of the tunnel until the lining is placed and to provide access to and support for the working face. There are two basic types of shield; mechanical excavating equipment is incorporated in one type and, with the other, excavation is carried out by hand. In both types, the shield is normally moved forward by hydraulic rams thrusting against the completed tunnel lining. The choice of shield type is largely dependent on the nature of the ground encountered; if the ground is hard or boulders are likely to be encountered a hand-excavation shield will normally be used but, if the ground is softer and fairly consistent in composition, mechanical excavation may be suitable. Although the basic design of all shields is similar, most tunnelling contractors prefer to use their own designs which incorporate features to suit their own techniques. It takes about a year to design and fabricate a tunnel shield and this period is used to complete the many works which are required prior to starting the tunnel drive.
The cost of tunnelling is largely influenced by the type of lining, the choice of which depends on the forces which will be exerted on the completed structure. The types of segmental linings in order of increasing strength and cost are precast reinforced concrete, cast iron and steel; the latter type is required wherever movement of the completed tunnel is likely. The segments are bolted together, the spaces between the lining and the excavation are grouted and the joints between the segments are caulked.
Tunnelling can be carried out continuously with little interference to surface traffic or underground utilities and noise nuisance is largely eliminated. However, at shallow depths, cut-and-cover construction is invariably cheaper. Twin tunnels require a greater width of construction and this may preclude their use where routes follow narrow roads.
UNDERGROUND — CAISSON
The caisson method of construction is expensive and is normally used only in poor ground where other methods of construction are impracticable. The caisson is built at ground level or in a shallow excavation which can be roofed over to provide a temporary running surface for traffic. The ground beneath the caisson is excavated from within and the caisson is sunk under its own weight, building and sinking being carried out alternately in stages. Where excavation is below the water table, it may have to be carried out in compressed air. The sides of the caisson support the surrounding ground and the caisson must, therefore, extend to the level at which sinking commences. This method is not normally economic for the construction of running line structures or stations. It is more commonly used to provide access to a deep tunnel; when the caisson can be incorporated in the works either as a permanent station access or as a ventilation shaft.
UNDERGROUND — IMMERSED TUBE
This method of construction is similar to that proposed for the Hong Kong cross-harbour road tunnel. It is a form of cut-and-cover construction which is often used to cross a water barrier. A trench is excavated below water level and prefabricated sections of the structure are floated into position and sunk onto the prepared bed of the trench. The sections are connected together to form a continuous structure which is then covered by backfilling.
Although this method has been used in other parts of the world for the construction of underground railways in waterlogged ground, its use in Hong Kong would probably be limited to the crossing of the harbour.
UNDERGROUND — FREEZING AND GROUTING
The techniques of freezing or grouting soil to increase its strength and reduce its permeability can be used with the open cut, cut-and-cover and tunnelling methods of construction. In the freezing method a coolant is circulated continuously through small-bore tubes inserted in the ground around the area to be excavated. The ground water in the vicinity of the tubes is thereby frozen, permitting excavation to be carried out without supporting or waterproofing the sides. Freezing can present problems due to frost heave in certain types of soil and by the freezing of underground utilities in built-up areas. Many different forms of grouting are available in which materials such as cement, clays or chemical gels, are injected into the ground to fill the spaces between the soil particles.
Large-scale use of either of these techniques is costly but they can be very useful in dealing with local obstructions in other methods of construction.
TYPICAL COSTS FOR RUNNING TRACK
At Grade
Costs are based on the following conditions:
Two-line construction.
Track level at average depth of 12 feet below ground level.
Good ground reasonably free from boulders with water table below level of excavation.
Reinforced concrete retaining walls required on both sides of excavation.
Cost per yard
Preliminaries
$0,200
H-piling and lagging boards
760
Earthworks
620
Retaining walls
2,170
Drainage
100
Services
200
Miscellaneous
400
Total
$4,450
Overhead
Costs are based on the following conditions:
Two-line construction.
Clearance from ground level 16 feet 6 inches.
Ground conditions typical of reclaimed areas with few boulders or underground obstructions.
Cost per yard
Preliminaries
$0,180
Foundations
1,140
Substructure
370
Superstructure
2,040
Miscellaneous
550
Total
$4,280
Cut and Cover with H-piles
Costs are based on the following conditions:
Two-line construction.
Track level at average depth of 31 feet below ground level.
Good ground reasonably free from boulders with water table below formation level.
Construction below bituminous-surfaced road with few underground utilities.
Full access to be maintained to adjoining properties except for four hours at night.
Cost per yard
Preliminaries
$00,580
H-piling and lagging boards
1,760
Earthworks
2,760
Structural works
4,850
Allowance for utilities, traffic, reinstatement and ground conditions
1,900
Miscellaneous
1,250
Total
$13,100
Under similar conditions but at depths to track level of 21 feet and 41 feet, the corresponding costs per yard would be $11,000 and $15,000, respectively.
Cut and Cover with Sheet Piles
Costs are based on the following conditions:
Two-line construction.
Track level at average depth of 31 feet below ground level.
Good ground with occasional boulders, with water table 10 feet below ground level.
Construction below concrete road with moderately congested underground utilities.
Full access to be maintained to adjoining properties except for four hours at night.
Cost per yard
Preliminaries
$00,750
Sheet piling
4,450
Earthworks
2,760
Structural works
5,010
Allowance for utilities, traffic, reinstatement and ground conditions
2,800
Miscellaneous
1,630
Total
$17,400
Under similar conditions but at depths to track level of 21 feet and 41 feet, the corresponding costs per yard would be $14,150 and $21,000, respectively.
Cut and Cover with Diaphragm Walls
Costs are based on the following conditions:
Two-line construction.
Track level at average depth of 31 feet below ground level.
High percentage of boulders (but no other rock) and underground obstructions precluding sheet piling with water table 10 feet below ground level.
Construction below concrete road with highly congested underground utilities.
Full access to be maintained to adjoining properties at all times.
Cost per yard
Preliminaries
$00,950
Diaphragm walls, including sheet piling above level of structure
8,820
Earthworks
2,530
Structural works (excluding diaphragm walls)
5,010
Allowance for utilities, traffic, reinstatement and ground conditions
4,700
Miscellaneous
2,000
Total
$22,300'
Under similar conditions but at depths to track level of 21 feet and 41 feet, the corresponding costs per yard would be $16,950 and $27,600, respectively. With double-storey, two-line construction at a depth of 51 feet to the lower track level, the corresponding cost per yard would be $33,400.
Tunnelling in Free Air without Shield
Costs are based on the following conditions:
Two-line construction in separate horseshoe-shaped tunnels.
Track level at depths below that economic for cut-and-cover construction.
Tunnelling in solid rock with no ground-water problems.
Access to tunnels from previously constructed stations.
Cost per yard
Preliminaries
$0,400
Excavation
1,800
Steel ribs and lagging boards
900
In-situ reinforced concrete lining
3,600
Miscellaneous
800
Total
$7,500
Under similar conditions, the cost of a single tunnel to accommodate two tracks would be about $9,000 per yard.
Tunnelling in Compressed Air without Shield
Costs are based on the following conditions:
Two-line construction in twin bored tunnels working two faces with two shields in compressed air at 15 lbs/in².
Total length of twin tunnel 1,600 yards.
Track level at depths below that economic for cut-and-cover construction.
Tunnelling in decomposed granite, with occasional boulders.
Access to tunnels from previously constructed stations.
Cost per yard per twin tunnels
Preliminaries
$01,000
Provision and installation of compressed air plant, air locks and shields
1,380
Provision of cast-iron lining segments
7,800
Excavation, erection of lining and caulking
10,670
Miscellaneous
1,150
Total
$22,000
With precast reinforced concrete lining segments, the corresponding cost would be reduced by about $5,200 per yard; cast-iron lining is required for tunnels underneath a road.
Ground conditions typical of reclaimed areas with few boulders or underground obstructions.
Cost
Preliminaries
$0,110,000
Foundations
520,000
Substructure
160,000
Superstructure
1,090,000
Finishings
470,000
Miscellaneous
260,000
Total
$2,610,000
Cut and Cover with Sheet Piles
Costs are based on the following conditions:
Two-line construction with side platforms and mezzanine.
Track level at average depth of 31 feet below ground level.
Good ground with occasional boulders, with water table 10 feet below ground level.
Construction below concrete road with moderately congested utilities.
Full access to be maintained to adjoining properties except for four hours at night.
Cost per yard
Preliminaries
$01,300
-
Sheet piling
4,450
Earthworks
3,550
Structural works
11,800
Allowance for utilities, traffic, reinstatement and ground conditions
4,000
Miscellaneous
2,500
Total
$27,600
The corresponding cost per yard of the station structure without mezzanine would be $24,800. For a typical station 600 feet long with a mezzanine 360 feet long, the cost would be:
Length with mezzanine (360 feet)
$3,320,000
-
Length without mezzanine (240 feet)
1,990,000
Finishings
340,000
Total
$5,650,000
Cut and Cover with Diaphragm Walls
Costs are based on the following conditions:
Two-line construction with central platform and mezzanine.
Track level at average depth of 31 feet below ground level.
High percentage of boulders (but no other rock) and underground obstructions precluding sheet piling with water table 10 feet below ground level.
Construction below concrete road with highly congested utilities.
Full access to be maintained to adjoining properties at all times.
Cost per yard
Preliminaries
$01,750
-
Diaphragm walls and sheet piling
8,850
Earthworks
4,360
Structural works (excluding diaphragm walls)
11,140
Allowance for utilities, traffic, reinstatement and ground conditions
6,050
Miscellaneous
2,850
Total
$35,000
The corresponding cost per yard of the station structure without a mezzanine would be $33,500. For a typical station 600 feet long with a mezzanine 400 feet long, the cost would be:
Length with mezzanine (400 feet)
$4,670,000
-
Length without mezzanine (200 feet)
2,230,000
Finishings
410,000
Total
$7,310,000
For a double-storey, twin-line station at a depth of 53 feet to the lower track level, the corresponding cost would be about $11,300,000.
