8. Assessment of Foundation Options 8.1 Site Conditions 8.2 Foundation Options 8.3 Central Tower Foundations 8.4 Flanking Tower Foundations 8.5 Side Span Foundations 8.6 Approach Span Foundations
8. Assessment of Foundation Options
8.11 Ground Conditions
A preliminary assessment of the ground conditions along the line of the crossing has been made from published information, the investigations carried out as part of the Setting Forth Study in 1993 and preliminary investigations carried out in 2007 for the Forth Replacement Crossing Study. These have included bathymetric and marine geophysical surveys, and marine boreholes south of the Forth Deep Water Navigation Channel, around the Beamer Rock and north of the Rosyth Channel. However the existing marine boreholes on the south side of the Firth of Forth do not extend as far north as the proposed location of the south tower of the bridge and those on the north side lie to the west of the proposed crossing alignment.
At the crossing location the Firth of Forth has been cut into predominantly sedimentary rocks of Carboniferous age. However igneous rocks intruded into the sedimentary rocks now form the headland at North Queensferry and the Beamer Rock within the estuary. The 1:50,000 scale geological map shows some west east trending faults, and folding giving an anticlinal structure on the south side of the estuary at the crossing location. The dips shown on the map vary from 12 to 20°.
The south abutment of the bridge will be located approximately 265 m south of the shore of the Firth of Forth. The ground level falls from the abutment location towards the shore beyond which the alignment crosses gently sloping tidal flats to the west of Port Edgar marina. These extend for about 500 m from the shoreline before the river bed falls more steeply at a gradient of about 7° at the southern margin of the Forth Deep Water Navigation Channel reaching a lowest level of about -45 mOD in the channel.
The bedrock underlying this section of the crossing is the West Lothian Oil Shale Formation which typically comprises sandstone, siltstone, mudstone and oil shale with thin coal seams and limestone beds. Dolerite sills, up to about 5m thick but occasionally thicker, have been intruded into the West Lothian Oil Shale Formation Near to the shore rockhead lies at or close to the river bed level but falls towards the north and is overlain by variable glacial deposits which are themselves overlain by more recent beach deposits or alluvium. The glacial deposits range from till with a high fines content to coarser grained materials all of which may contain cobbles or boulders. The recent deposits are weaker and vary from sands to soft clay.
Schematic geological section - southern approach
(Dolerite sills are indicative only)
The Beamer Rock is formed by a dolerite outcrop which reaches an elevation of about +3 mOD close to the existing lighthouse. It forms a ridge trending north west - south east and the area of rock exposed varies with the tide reaching about 45 m by 95 m at low water springs. The bathymetric surveys have shown that the south and east sides of Beamer Rock are very steep with slopes of 60 to 65°. The northeast and southwest edges are less steep with slopes of around 25 to 30°.
To the north of the Beamer Rock the river bed falls to about -33 mOD in the North Channel. The bed level then rises northwards to the Rosyth Channel which is dredged to a level of -12 to -16 mOD. The north margin of this channel rises at a gradient of about 6° towards the more gently sloping north foreshore which extends for about 300 m towards the northern landfall near Cult Ness.
On the north side of the Rosyth channel bedrock consists of the Sandy Craig Formation. This typically comprises sandstone siltstone and mudstone but the previous investigations have revealed layers of volcanic tuff within the Sandy Craig Formation in this area. The bedrock is overlain by glacial and recent deposits which decrease in thickness towards the north shore. These are generally similar to the deposits previously described on the south side of the estuary.
Schematic geological section - northern approach
(extent of Tuff indicative only)
Ground level rises steeply at the north shore of the Firth and is underlain by dolerite. The bridge alignment is partly sidelong to the topography in this area reaching the northern abutment some 140 m from the shoreline.
Further marine and land ground investigations are being undertaken to extend the coverage of the previous investigations and further consideration of the foundation options described below will depend on the findings of these investigations.
8.1.2 Water conditions
The tidal range at Rosyth from MHWS to MLWS is 5 m and the Admiralty Chart indicates that peak tidal currents vary up to about 2.3 kts.
8.1.3 Constraints to foundation arrangements
Within the Firth it is proposed that the top of the foundations should be below the river bed level or -5 mOD, whichever is the higher. This is to ensure that the foundations will not be visible at any state of the tide whilst at the same time limiting the hazard that submerged foundations could pose to small craft. Suitable markers and warning lights will be provided to indicate the extent of the foundations.
8.2.1 Key Considerations
The selection of foundation scheme will depend on
- Ground conditions, and in particular the depth to a competent bearing stratum or bedrock
- Water depth
These factors, which vary along the crossing alignment, will determine whether in situ construction within temporary cofferdams or precast caissons or pile caps installed under water are preferred.
A further key consideration in the design of the foundations will be the capacity to resist ship impact loads. These will be most onerous at the south tower location.
In the FRCS Reference Design caisson foundations taken down to rockhead are shown for the main tower foundations. Precast caisson foundations have been used for recent major bridges including Oresund Bridge and the Second Severn Crossing where units have been positioned on a prepared rock formation. In both cases rock was at or close to the sea bed. Where a suitable bearing stratum is only encountered at significant depth below bed level caissons constructed and sunk in situ have historically been adopted for several major bridges, including the south tower of the existing Forth road bridge.
At the south tower of the existing Forth road bridge a pair of caissons were sunk from within a temporary cofferdam to bedrock at -29 mOD. Provision was made for excavation under compressed air, but the glacial deposits were found to consist of boulder clay of low permeability and the use of compressed air was not required. The water depth and anticipated thickness of soft alluvial deposits at the new crossing north and south tower locations are expected to be greater than at the existing bridge south tower, and at the new crossing south tower substantially greater. Furthermore, the glacial deposits appear likely to be more variable comprising interbedded fine grained and coarse grained materials. These conditions are much more onerous and sinking a caisson to bedrock would be challenging. The use of compressed air may not be practical at the depths required for a caisson at the south tower and alternative means of excluding water and maintaining a stable base to the excavation such as dewatering, grouting or ground freezing, would be difficult to implement and involve significant risk.
Precast caissons could be an attractive solution for some of the approach span piers where rockhead is at shallow depth, and could also be considered for the central tower.
An alternative foundation solution is large diameter piles. Current piling technology enables piles up to 4 m diameter or more to be drilled into rock using reverse circulation drills (RCD), and piles of 3.85 m diameter have recently been constructed for the new Kincardine bridge. For initial studies of foundation schemes 3 m diameter piles have been considered.
Large diameter piles with relatively high axial and bending resistance could be constructed by initially driving a steel tube to bedrock (or as deep as practical if boulder obstructions are encountered). An RCD would then be mounted on the steel tube and the plug of soil inside the tube drilled out. If the tube has stopped short of bedrock it would have to be taken down to rockhead with the drilling operation. A socket into the rock could then be drilled below the base of the tube and the socket and steel tube filled with concrete reinforced as necessary.
Layouts with both vertical and raking piles have been investigated but the preliminary studies have indicated that arrangements with vertical piles only are most practical and likely to be most efficient.
A key consideration with piled foundations is the method of pile cap construction as the pile caps will be fully submerged. Although one option would be in situ construction within temporary cofferdams, a cofferdam would be very difficult to construct in the deep water and ground conditions anticipated at the south tower location. An alternative would be to use precast pile caps and preliminary studies have indicated that this should be feasible using technology developed in the offshore industry.
These studies have considered a cellular reinforced concrete structure incorporating sleeves to take the piles. The size of the cap required will depend on the form of the tower and design ship impact force. Preliminary sizing for the Needle or Inverted Y towers gives overall plan dimensions about 45 m by 30 m and a depth of 6 to 8 m resulting in a total structure weight of 7,000 to 9,000 t. This could be constructed in a suitable dry dock or on a submersible barge and floated into position using temporary buoyancy towers to ensure that the unit floats in a stable position. Alternatively construction in sections that are then stressed together afloat could be investigated.
Further important considerations with piled foundation schemes will be the method of forming the connection between the piles and cap and construction tolerances. For the precast pile cap solution a possible construction method could be to initially install some locating piles using a sea bed template to accurately position the piles. The pile cap unit would then be floated over the piles during slack water, ballasted onto the piles and the sleeves grouted. The remaining piles could then be installed using the pile cap as a guide with connections also formed by grouting the sleeves.
8.2.4 Pad foundations
In areas of shallow water or on land where rock or competent strata are encountered at shallow depth pad foundations constructed in situ are a possible solution.
Foundations for the central tower will bear directly on Beamer Rock with the Needle, Inverted Y and H Shape tower forms imposing different constraints on foundation geometry and founding level. Some rock excavation will be required to reduce the existing surface profile to the required founding level and construction in water depths of up to about 10 m may be necessary depending on the tower scheme.
One solution would be to reduce the level of the rock by underwater excavation and float a precast caisson into place. Once ballasted on to the prepared platform a grouted contact with the rock would be formed at the base of the caisson.
A temporary cofferdam would be required to permit the alternative of in situ construction of a pad foundation in dry conditions. The temporary works required for this will potentially be difficult as toe penetration of driven piles into the rock will not be practical. A scheme using concrete walls with vertical anchors into the rock was however successfully used at the Mackintosh Rock for the north tower cofferdam for the existing Forth Road Bridge where conditions were similar.
8.4.1 South Tower
The south tower will be located in deep water - with 650 m main spans it will be located where the river bed level is about -22 mOD (shown as ST on the geological section). The tower location lies to the north of the corridor previously investigated during the Setting Forth study in an area where the geophysical survey results from the Setting Forth and Forth Replacement Crossing Study investigations were inconclusive. Whilst there is at this stage considerable uncertainty as to the ground conditions it is likely that there will be a substantial thickness of soft alluvium below the river bed and extrapolating from existing information the top of the glacial deposits has tentatively been assumed at - 35 mOD and it has been assumed that rockhead level could be as low as -55 mOD. Additional investigations are being undertaken to establish the ground conditions at this location..
It is proposed that a piled foundation scheme with precast cap should be developed further in the next stage for this location.
8.4.2 North Tower
The north tower location lies to the east of the corridor previously investigated during the Setting Forth study. With 650 m main spans it will be located where the river bed level is about -9 mOD (shown as NT on the geological section). Based on the existing information it has been assumed that the river bed is underlain by soft alluvium with the top of the glacial deposits tentatively assumed to be at -15 mOD and rockhead at -35 mOD. Further investigations are being undertaken to confirm the ground conditions at this location.
A piled foundation scheme with a precast cap could also be constructed at this location following some dredging to provide sufficient draft to float the cap into position. Alternatively an in situ pile cap constructed within a temporary cofferdam may also be feasible. It is proposed that both of these options should be investigated further in the next stage.
The locations of the side span piers (S2, S1, N2 and N1) will depend on the structural form adopted. Some of the side span foundations are required to carry both compression and tension loads in which case rock socket piles would provide a suitable foundation solution. Where the foundations are required to carry compression loads only and rock is at shallow depth a pad foundation constructed insitu or precast caisson foundations may be an alternative. Pile caps are likely to be permanently submerged on the south side of the bridge but may be within the intertidal zone or on land on the north side of the bridge. It is proposed that solutions for the marine piers in which the pile caps are either precast and lifted into place or constructed insitu within a temporary cofferdam are investigated further in the next stage.
The spans of the approach viaducts and locations of the piers will depend on the structural form adopted. Several alternative solutions may be considered for the approach span foundations depending on the water depth at the marine piers and depth to a competent bearing layer. On land and near to the south shore where rockhead is shallow, pad foundations are likely to be feasible whilst with increasing distance from the shore and increasing thickness of alluvium and/or variable glacial deposits piled foundations will be required. Close to the shore pad foundations would probably be constructed in situ within temporary cofferdams but the alternative of precast caissons lifted onto a prepared base within a dredged pocket could be considered. Similarly where piled foundations are adopted either pile caps constructed in situ within temporary cofferdams or precast units lifted into place could be considered. These options should be further investigated in the next stage.