INTRODUCTION
Design of bridge piers and abutments is an important civil
engineering task that will fall at some time to the lot of most
practicing civil engineers. History is replete with many examples
of substantial bridge works such as the piled foundation of the
Roman bridge across the Rhine, and London Bridge, over the Thames.
According to a third-century Roman writer, there was a bridge
across the Thames just above its mouth as early as A.D. 43. On its
arrival in A.D. 1014 to aid King Ethelred of England against the
occupying Danes, the fleet of King Olaf (St. Olaf) "rowed quite up
under the bridge and then rowed off with all the ships as hard as
they could downstream (having secured ropes to the piles supporting
the bridge). The piles were then shaken at the bottom and were
loosened under the bridge," which gave way, throwing all the
defenders ranged upon it into the river.The London Bridge so
well-known through illustrations in history books appears to have
been completed in the early part of the thirteenth century. The
waterway was so reduced by this multiarched structure that swift
rapids developed and many persons lost their lives in passing
through. The old saying was that "London Bridge was made for wise
men to go over and fools to go under. An act of Parliament h 1756
ordered all the buildings on the bridge to be removed and the two
central arches rebuilt into one arch. This work inevitably diverted
the main flow through the opening and set up serious scouring,
which eventually led to demolition of the bridge and replacement
with a modem structure. This is but one of the ancient bridges in
which piers have caused trouble. Records are scarce, unfortunately,
but it can safely be said that scouring out of the foundation beds
adjacent to bridge piers has been a major cause of trouble in the
past. The piers of ancient bridges rarely failed because of
excessive loading on the foundation beds, if only because of the
limitation of span length imposed by the structural materials
available. The two defects mentioned can be regarded as the two
main possibilities of failure to be investigated in the design of
bridge piers. Both are essentially geological in character.
Figure 8.1 The ancient London Bridge supported by piled
foundation.
Figure 8.2 Model of Caesars bridge across the Rhine (Museo
nazionale della civilta romana, Roma)
IMPORTANCE OF BRIDGE FOUNDATIONS
However scientifically a bridge pier may be designed, the whole
weight of the bridge itself and of the loads that it supports must
ultimately be carried by the underlying foundation bed. Although
piers and abutments may be relatively uninteresting to structural
engineers, the careful consideration of foundation materials is as
challenging as the determinate mathematical calculations relating
to the arrangement of steel, reinforced concrete, or timber to be
used for the superstructure. Sometimes it is assumed that the cost
of foundations, compared with the total cost of a bridge, is
relatively small. Actual cost records, however, show that the cost
of foundations (piers and abutments) often almost equals the cost
of superstructure, even on large bridges.It has not always been
fully recognized that concern should always be given to the pier-
and abutment-bearing surfaces and whether they can support the
structure without fear of any serious movement in the future. Dr.
Terzaghi once said that:On account of the fact that there is no
glory attached to the foundations, and that the sources of success
or failure are hidden deep in the ground, building foundations have
always been treated as stepchildren and their acts of revenge for
lack of attention can be very embarrassing.Of no group of
foundations is this more true than of those for bridges.
SPECIAL PRELIMINARY WORKThe first considerations in bridge
location are ge4erally those of convenience and economy. Foundation
conditions usually take a subsidiary place, for the prime
requirement of a transportation route is that it connects its
terminal points by the shortest convenient route consistent with
topography. For crossings of deep canyons, considerations of cost
usually limit the choice to the site requiring the shortest
possible structure. The bridge engineer must therefore often fit
design to available foundation conditions. The limitation of site
selectivity necessitates acquisition of the most complete geologic
information possible.A still more compelling reason for obtaining
full geologic information is that once the construction of bridge
piers is started, their respective locations cannot be changed
except in most unusual circumstances. More than the usual degree or
certainty must therefore be attached to the design and anticipated
performance of bridge piers and abutments. There is yet a further
reason for this special care in preliminary investigations.
Bridges, as a rule, are constructed to cross river or
othervalleys-topographic depressions that generally exit because of
departure from normal geologic structure. Terrain covered by
glacial debris may now conceal an older riverbed or other
depression well below the existing riverbed. Such conditions are
common, and even if known in advance can have serious effects on
design.Riverbeds contain many types of deposits, including boulders
and if preliminary geologic work is not done carefully, an
extensive boulder deposit can easily be mistaken for solid rock. A
telling example is that of the Georges River Bridge, Sydney in New
South Wales, Australia. Construction of a toll highway bridge to
replace existing vehicular ferries was begun in 1923. Three
possible sites were explored by an experienced drilling foreman.
The borings at the site finally selected showed solid rock at
depths below bed level varying between 10.5 and 14.1m (35 and 47
ft.) at regular intervals across a river section about 450 m
(1,500ft) wide, the rock at the sides of which was known to dip
steeply. On the basis of this information, a through-truss bridge
of six main spans supported on cylinder piers was designed, and a
lump-sum contract was awarded. During construction, rock was found
at only two of the seven main piers. Additional borings taken to
depths up to 39 m (130 ft.) failed to disclose any solid rock at
all the other pier sites, and what is even more strange, they
disclosed no stratum harder than "indurated sand". Construction had
to be stopped and designs changed; in consequence, the bridge took
five years to build instead of two and cost 27 .6 percent more than
the contract price.Discussion of the paper in which this work was
reported to the Institution of Civil Engineers naturally emphasized
the rigid necessity of having borings most carefully watched by a
trained observer. The absence of geological references in both
paper and discussion suggests that neglect of geologic features may
have been contributory cause of the trouble experienced.
Although this is an unusual and possibly exceptional example,
the construction of the Georges River Bridge is a telling reminder
of the supreme importance of preliminary geological information in
bridge design and of the vital necessity for professional
supervision of test boring work.Another reason for devoting unusual
care to geologic investigations at bridge sites in all cases of
river crossings is the fact that so much of the ground surface
involved is hidden below the water. The results of the underwater
borings must be correlated with geologic observations secured at
the adjacent shores. Where sound rock is encountered, this calls
for no unusual attention, provided the exposed surfaces of the rock
show no signs of weathering or frequent fracturing, but if any part
of the foundation bed consists either wholly or partially of clay,
then it is desirable-in most cases imperative-to obtain samples of
the clay in as undisturbed samples of clay and other unconsolidated
materials, even through great depths of water.The cities of San
Francisco and Oakland are separated by the entrance to San
Francisco Harbor. Yerba Buena Island stands in the center of the
harbor and divides it into the East Bay and the West Bay. For many
years, transportation across the harbor was restricted to ferries,
but a bridge reached the construction stage in 1993, being
officially opened on November 12, 1936. Early in their planning,
the engineers decided upon a program of borings and soil testing to
enable them (1) to determine the nature of the subsurface
materials, (2) to ascertain the most desirable location for the
center line of the bridge, (3) to determine the best location for
individual piers, (4) to select basis for the design of the piers.
Preliminary jet borings provided the basis for contouring the top
of the rock surface of the harbor. With the aid of additional
wash-pipe borings and diamond core borings into the rock, they
prepared a final design for the West Bay crossing. Piers were
located and designed; all were founded on solid rock and
constructed by means of caissons, the behavior of which could be
accurately foretold.The East Bay crossing presented quite distinct
problems. Since rock was not found by borings at practicable
depths, it became necessary to rely on the overlying unconsolidated
material. Cores were obtained and hermetically sealed in the
sampling tube, right on the deck of the drill barge; they were soon
tested at the University of California. When the containers were
opened, perfect cores were generally found, although in some cases
a slight swelling was noticed, possibly due to the change in
internal pressure in the sample as it came up to the surface.
Material was obtained in this way from depths of 82m (273 ft) below
water level.This unique example still has many ordinary features of
preliminary investigations. Adequate borings, not only along the
line of the selected bridge site, but also on either side of it;
careful study of core samples; and correlation of this information
with the geologic structure of the adjoining dry ground should
present a reasonably accurate structural picture of the foundation
beds. This information will enable the designing engineer to locate
and design accurately the bridge abutments and piers.Finally, the
necessity of taking all borings deep enough below the surface of
solid material (and especially of unconsolidated material) must be
stressed. Loadings from bridge supports are always relatively
concentrated and often inclined to the vertical. It is therefore
doubly necessary to be sure that no underlying stratum may fail to
support the loads transmitted to it, even indirectly, by the strata
above.An interesting example of trouble due to this cause is the
failure of a highway bridge over the La Salle River at St. Norbert,
Manitoba. Thebridge was a single reinforced-concrete arch, with a
clear span of 30m ( l00ft.); the spandrels were earth filled. The
roadway was about 9m (30ft) above the bottom of the river, and the
height of the fill placed in each approach was about 6m (20 ft.).
The bridge abutments were founded on piles driven into the stiff
blue clay exposed at the site and thought to overlie limestone
bedrock, as shown by preliminary auger borings and the record of an
adjacent well. Failure occurred by excessive settlement. The north
abutment dropped l.2m (4ft), and bearing piles were bent and
broken. Subsequent investigations disclosed the existence of a
stratum of "slippery white mud" (actually bentonitic clay) about
7.5 m (25 ft.) below the original surface; this material failed to
carry the superimposed load.Local soils were formed in an ancient
glacial lake and usually overlie lodgment till, under which is
limestone carrying subartesian water. The existence of this water
complicated the underpinning of the bridge foundation, but the work
was successfully completed, and the bridge was restored to use. The
bentonitic clay was previously unknown in the vicinity, and
illustrates the uncertainty of glacial deposition. The occurrence
has a special interest for engineers; although the bearing piles
were driven into the lodgment till ("hardpan"), settlement of the
abutment occurred as the result of failure of soft material
underlying this.
DESIGN OF BRIDGE PIERSGenerally speaking, there are four types
of bridge-pier loading, one or more of which may have to be
provided for in design: (1) vertical loads possibly of varying
intensity, from truss or girder spans or suspension bridge towers;
(2) inclined loads, again of varying intensity and possibly varying
direction, for arched spans; (3) inclined tensions, from the cables
of suspension bridges; and (a) horizontal thrusts due to the
pressure of ice or possible debris, the flow of water impinging on
the pier, and the wind acting on the bridge superstructure and
piers. In earthquake regions allowance must also be made for
seismic forces that may act upon the piers. Combinations of these
several loads will give rise to certain maximum and minimum unit
pressures to be taken on foundation beds. From considerations of
these results and of the nature of the strata to be encountered,
the type of foundation can be determined.Estimation of foundation
load at the site of a bridge pier is generally similar to the same
operation for other foundation work. Aside from concern for weak
strata below the surface, there are two unusual features that may
require reductions of the calculated net load on the base area. The
first is the allowance for the natural material excavated and for
the displacement by the pier of water; and the second is the
reduction for skin friction on the sides of the pier because of the
usually large surface area exposed as compared with the base are.
These two factors are obviously dependent on the nature of the
foundation strata. Estimation of the first is straightforward, but
that of the second is generally a matter of experience or of
experiment during pier sinking, tempered by the results of careful
laboratory soil tests.Weaker strata may even dictate the use of
hollow piers to reduce unit loads or of such unusual structures as
the open reinforced-concrete framework abutment supports adopted
for the Mortimer E. Cooley Bridge across the Manistee River in
Michigan. This singularly beautiful bridge, consisting of two
37.5-m (125-ft) deck truss steel cantilever arms supporting a l5-m
(50 ft) suspended span and balanced by two 37.5-m (125-ft) anchor
arms, has its deck level about 18 m (60 ft.) above the level of the
ground on either side of the river. The foundation of varying
strata of consolidated materials was accurately explored and
foundation loads were kept to a minimum through use of the open
framework design.When preliminary investigations indicate
foundation material of poor bearing capacity, consideration may be
given to the use of artificial methods of consolidating such
material to improve its bearing capacity. This is no new expedient.
The account given by Leland (antiquary to King Henry VII) in 1538
concerning the Wade Bridge in England revealed that the foundation
of certain of tharches was first sette on so quick sandy ground
that Lovebone (Vicar of Wadebridge) almost despaired to perform the
bridge ontylsuchtyme as he layed pakkes of wollefor foundation".
Although this use of wool has been disputed, the record
demonstrates that some artificial means was used to improve bearing
capacity. Modern methods include grouting chemical consolidation,
or leaving t[e steel piling of the pier cofferdam in place to
confine the foundation-bed material and thus prevent lateral
displacement. In this way bearing capacity will be increased to
some extent.The foundations for the Tappan Zee Bridge that carries
the New York Thruway across the Hudson River for a distance of 4.5
km (2.8 mi) between Nyack and Tarrytown, New York, provide an even
more unusual approach to the problem of minimizing loads on weak
strata. The bridge site selected through careful studies had to be
accepted even though the bedrock drops off under the bridge to
depths as great as 420 m (1,400 ft.) below water level-a depth too
great to be reached by end-bearing piles. The approach spans are
therefore carried on friction-bearing piles, driven into the silt,
sand, and gravel that form the riverbed.The main piers carrying the
363.3 m (1,212-ft) cantilever main-channel span, however, are
founded on buoyant, reinforced-concrete boxes, carrying about
two-thirds of the dead load of the superstructure. The remaining
part of the dead load, and the live load, are taken by 75-cm
(30-in) concrete-filled pipe piles for the four main piers and by
35-cm (14-in) steel H piles for the four buoyant boxes; in each
case piles and boxes are ingeniously connected together. The steel
piles had to be driven to depths up to 52.5 m (175 ft.), but the
concrete pipe piles went as deep as102.0 m (340 ft.) below water
level, being driven through clay and then gravelly clay after the
sand and gravel had been penetrated. The hollow piles were mucked
out to full depth by water jet and airlift techniques and then
grouted into preplaced aggregate. The grouting consolidated the
sheared gneiss and decomposed sandstone bedrock.
Figure 8.3 The Tappan Zee Bridge over the Hudson River, New
York, looking east, showing the spans which are supported by the
special piers, described in the text.Details of the piers are
admittedly an engineering matter, but it was geology of the site
that dictated such bold design.Geologic information can be applied
to predict settlement of loaded piers. What happens when uneven
settlement does take place is well illustrated by the failure of
piers 4 and 8 of Waterloo Bridge, London; the whole bridge had to
be taken down and a new structure erected. Described by Canova as
"the noblest bridge in the world worth a visit from the remotest
corner of the earth", Waterloo Bridge was constructed from l8ll to
1817. Timber rafts on timber piles bearing on gravel were designed
to protect the pier foundations against scour. Progressive
settlement became serious in 1923; the total settlement of pier 4
exceeded 75 cm (2ft) and naturally caused an arching action between
piers 3 and 5.
Figure 8.4 The old Waterloo BridgeSettlement may occur from one
or more of the following causes: (1) displacement by scour, (2)
lateral displacement due lack of restraint, (3) consolidation of
the underlying material, or (4) failure of an underlying stratum.
Only condition 3 can be controlled; the other three types are of a
nature that may cause serious trouble on the structure. All types
can be predicted on the basis of adequate preliminary geologic
information.Provision against unequal settlement of piers has
assumed considerable importance in recent years owing to the
development of the rigid-frame type of structure, requiring
"unyielding" abutments and uniform settlement of piers. Rigid-frame
structures founded on clay require isolation of the bridge
foundation from the bearing piles and load transmittal through a
tamped layer of crushed rock (employed at a Canadian National
Railways bridge at Vaudreuil in Quebec). Uneven foundation-bed
loading, especially that caused by irregular construction
scheduling, must also be carefully considered in design. During the
1932 construction of the Broadway Bridge, Saskatoon, Saskatchewan,
concreting of the six arches proceeded in varying stages. As a
result, the piers tilted when carrying the dead load of only one
adjacent arch rib. Amaximum deflection of l5mm (0.6 in) was
recorded as anticipated.Inclined tensions of the third type of
loading are generally transmitted to anchorages in solid rock. This
design approach provides for shearing resistance in the rock,
which, together with allowance for the dead weight of the
anchorage, will be sufficient to balance the tensile forces in the
bridge cables. Some inclined tension in bridge cables is taken up
wholly by concrete piers, as at the Ile d'Orleans (suspension)
Bridge, in Quebec. The suspension span is carried into the long
approach structures and secured in anchor piers, one of which is
founded on rock, but the other on sand. The stability calculations
for these piers had to keep the unit toe and heel pressures within
the limit for the foundation-bed material. Frictional values of
concrete-to-rock and concrete-to-sand provided the basis for this
anchorage. Inclined H-beam piles wore driven into the sand
underlying one of the anchor piers and were left to project
outwardly in counterforte, into the concrete of the finished pier
to give the necessary increase in stability.As a final example,
Burford Bridge across the river Mole in Surrey, England, was
designed to accommodate an unusual geological condition. The bridge
is a single reinforced-concrete arch span of 24 m (80ft.), 30m (100
ft.) wide between parapets, with specially selected brick facing.
The Mole Valley, located some 40 km (25 mi) to the south of London,
flows through chalk formation, which is highly susceptible to
groundwater dissolution. Underground cavities here are so large as
to receive the whole normal flow of the stream. Borings were put
down to see if any such "swallow holes" in the chalk were revealed.
Two soft spots were located which proved to be dissolution channels
having most vertical sides and filled with alluvial matter.
Concrete domes were constructedover each of the holes, domes
founded on circular ledges cut in the chalk around the tops of the
excavated channels; the largest dome was 17.4 m (58 ft.) in
diameter with a rise of 2.4 m (8 ft.). The holes were filled up to
the undersides of the domes, and the filling was then covered with
waterproof paper and used as the lower form for concreting the
domes. Each dome was furnished with an access shaft con1ecting to a
manhole at road level by means of which engineers may inspect the
swallow holes from time to time to see that no dangerous
undercutting or further erosion of the chalk is taking place.