The Forth Bridge/South Approach Viaduct Piers

by Wilhelm Westhofen

South Approach Viaduct Piers.

On the South Queensferry shore the foundations of piers 3, 4, and 5 were first started, as they were all dry at low water. These three piers are founded on the freestone rock prevalent in ridges all along this shore. Tidal work had to be resorted to without any protection by cofferdam or caisson, and all that was required was to remove the soft or rotten portions of the rock and cut flat steps into any sloping faces met with. The hollows were then levelled up with concrete, and upon this a concrete base was laid enclosed by planking. When this had properly set the granite masonry was at once begun. The concrete base was 61 ft. long by 31 ft. wide, and was the same for all viaduct piers from 3 to 9. The thickness of this concrete foundation varied according to position, from 4 ft. in the shore piers to 11 ft. in No. 9 pier. Piers 2 and 1, as also the abutment, were situated on the hillside. The first was founded on the freestone rock; but the other two were placed on a stiff blue clay of sufficient solidity. Pier 6 required the construction of a cofferdam, the bottom being rock with a thin layer of gravel and clay. Owing to this the cofferdam could not be carried to more than half-tide, the difficulty being to get the piles to hold. A great deal of trouble was experienced with this foundation through the want of a tight joint at bottom; but it was, of course, only a matter of patience and perseverance. The foundation of pier 7 also caused some trouble owing to the same difficulty at bottom, and a half-tide cofferdam had to be used; but in the foundations of piers 8 and 9 the layer of clay at bottom was of sufficient thickness to allow good piling to be done, and whole-tide cofferdams could be constructed. Pier 7 is founded on the rock, but piers 8 and 9 are founded on the hard clay.

The south cantilever pier required a cofferdam of abnormally large proportions and of great strength, for the base upon which this great masonry pier had to be raised was not less than 115 ft. long by 60 ft. wide. It was at first divided into halves by a double row of piles with puddle clay filling; but, after a good portion of the concrete base had been laid all round the side of the dam, this partition was cut down, the piles being sawn off flush with the bottom and the concrete base carried right across. In this pier the boulder clay at bottom was found nearly dry and extremely tough to handle, and it gave an indication of the ground into which the large caissons would have to be sunk.

Table IV., given below, shows the levels at which the foundations were started of all the viaduct piers from 1 to 9, of the south cantilever pier, the north cantilever pier, and the abutment, as also the nature of the ground under foundations.

Figs. 23 to 28 show views of the cantilever end pier and the viaduct piers, with the principal dimensions. The cutwaters with a coping of dressed granite reach to the same height above high water as the circular piers, namely 18 ft., and up to this level all the piers from 3 to 9 were built with a hearting of concrete; above this level the granite was backed by masonry of Arbroath rubble in cement carried up to the top. The hearting in piers 1 and 2 was of concrete up to the level at which the viaduct girders were erected—namely, at about 47 ft. above high water, from this level to top they were, like the other viaduct piers, built with a hearting of Arbroath rubble.

To facilitate the building of these piers, staging was erected alongside and connected with the jetty, and cranes were set up to lift the granite blocks from the jetty adjoining and deposit them in their places. All materials were brought along the jetty, where also the mortar was prepared and the concrete mixed. Two courses of granite were generally laid, and the hearting of concrete filled in. The mortar used consisted of one part cement and two parts sand.

The concrete consisted of 27 cubic feet, or 1 cubic yard of broken whinstone, 5+¹⁄₂ cubic feet of cement, and 5+¹⁄₂ cubic feet of sand, mixed dry by hand and then charged into a concrete mixer where the water was added. From the mixer it was delivered into barrows and tipped where required.

All the south viaduct piers from 3 to 9, and the south cantilever end pier, were built up to a level of 18 ft. above high water, and further work upon them had to be delayed until the girders had been erected and rivetted up, and were ready for lifting. The dates relating to the building of the piers, the raising of the girders and the quantities of materials used are given in a summary on another page.

Piers 1 and 2 were carried up to level 47 ft. above high water, and the girders built at that level. The south abutment was raised to 119 ft. above high water, and the girder erected at that level on staging reaching close up to No. 1 pier.

TABLE No. IV.—Particulars of Foundations for Viaduct and Cantilever Piers, and Nature of Ground

For General View of Piers see Plate III., Figs. 1 and 2.							Level of Foundations referred to High Water.		Nature of Ground.
South abutment		...	...	...	...	...	109 ft.	above	Blue clay
Approach viaduct pier		1	...	...	...	...	32 ft.	"	"
"	"	2	...	...	...	...	7 ft.	"	Freestone rock
"	"	3	...	...	...	...	13 ft. 6 in.	below	"
"	"	4	...	...	...	...	14 ft.	"	"
"	"	5	...	...	...	...	17 ft.	"	"
"	"	6	...	...	...	...	22 ft.	"	"
"	"	7	...	...	...	...	30 ft.	"	"
"	"	8	...	...	...	...	32 ft. 6 in.	"	Hard clay
"	"	9	...	...	...	...	38 ft.	"	Boulder clay
Cantilever end pier,		south	...	...	...	...	33 ft. 6 in.	"	"
"	"	north	...	...	...	...	21 ft.	above	Whinstone rock
Approach viaduct pier		10	...	...	...	...	25 ft.	"	"
"	"	11	...	...	...	...	22 ft.	"	"
"	"	12	...	...	...	...	7 ft.	below	Part whinstone
"	"	13	...	...	...	...	7 ft.	"	Part freestone
North abutment		...	...	...	...	...	92 ft.	above	Whinstone rock