8/17/2019 Caissons Cofferdams Report
1/24
1
B.TECH SEMINAR ON
CAISSONS AND COFFERDAMS
FOR THE SUBJECT OF
ADVANCED FOUNDATION ENGINEERING
BY
RAJAT JAIN (11BCL020)
KARTIK GUPTA (11BCL047)
SUBMITTED TO
Dr. TRUDEEP N. DAVE
DEPARTMENT OF CIVIL ENGINEERING
SCHOOL OF TECHNOLOGYPANDIT DEENDAYAL PETROLEUM UNIVERSITY
GANDHINAGAR, GUJARAT
8/17/2019 Caissons Cofferdams Report
2/24
8/17/2019 Caissons Cofferdams Report
3/24
3
1. I ntroduction
1.0 GENERAL
During the construction of bridges, dams or any other structure where the foundation part
of the structure is most likely to lie underwater, we have to opt for underwater
construction. Construction in water poses many difficulties especially in the places where
there the depth is considerable. During underwater construction our main objective is to
create dry and water free environment for working in such manner that the structural
stability of the structure is not compromised.
Fig- 1 Flowchart showing techniques of underwater construction
Underwater Construction
Caissons
Cofferdams
8/17/2019 Caissons Cofferdams Report
4/24
4
2. C aissons
2.0 GENERAL
The name „caisson‟ is French and is to be translated as a „large chest‟, which refers to thegeneral shape of the caissons. In geotechnical engineering, a caisson is a watertight
retaining structure used, for example, to work on the foundations of a bridge pier, for the
construction of a concrete dam, or for the repair of ships. These are constructed such that
the water can be pumped out, keeping the environment dry.
2.1 TYPES
Caissons are divided into three major types:
i. Open Caissons
ii. Box Caisson (or closed caissons)
iii. Pneumatic Caissons
Open caissons (Fig-2) are concrete shafts that remain open at the top and bottom during
construction. The bottom of the caisson has the cutting edge. The caisson is sunk into
place, and soil from the inside of the shaft is removed by grab buckets until the bearing
stratum is reached. The shafts may be circular, square, rectangular or oval. Once the bearing stratum is reached, concrete is poured into the shaft (underwater) to form a seal at
its bottom. When the concrete seal hardens, the water inside the caisson shaft is pumped
out. Concrete is then poured into the shaft to fill it. Open caissons can be extended to
great depths, and the cost of construction is relatively low. However, one of their major
disadvantages is the lack of quality control over the concrete poured into the shaft for the
seal. Also, the bottom of the caisson cannot be thoroughly cleaned out. An alternative
method of open-caisson construction is to drive some sheet piles to form an enclosed
area, which is filled with sand and is generally referred to as a sand island. The caisson is
then sunk through the sand to the desired bearing stratum.
8/17/2019 Caissons Cofferdams Report
5/24
5
Fig-2 Open Caisson
Box caissons (Fig-3) are caissons with closed bottoms. They are constructed on land and
then transported to the construction site. They are gradually sunk at the site by filling the
inside with sand, ballast, water or concrete. The cost for this type of construction is low.
The bearing surface must be level, and if it is not, it must be leveled by excavation.
8/17/2019 Caissons Cofferdams Report
6/24
6
Fig-3 Box Caisson
Pneumatic caissons (Fig-4) are generally used for depths of about 50-130 ft (15-40 m).
This type of caisson is required when an excavation cannot be kept open because the soil
flows into the excavated area faster than it can be removed. A pneumatic caisson has a
work chamber at the bottom that is at least 10 ft (≈ 3 m) high. In this chamber, the
workers excavate the soil and place the concrete. The air pressure in the chamber is kept
high enough to prevent water and soil from entering. Workers usually do not counter
8/17/2019 Caissons Cofferdams Report
7/24
7
severe discomfort when the chamber pressure is raised to about 15 lb/in 2 (≈ 100 KN/m 2)
above atmospheric pressure. Beyond this pressure, decompression periods are required
when the workers leave the chamber. Workers enter and leave the chamber through a
steel shaft by means of a ladder. This shaft is also used for the removal of excavated soil
and the placement of concrete. For large caisson construction, more than one shaft may
be necessary, an airlock is provided of each one. Pneumatic caissons gradually sink as
excavation proceeds. When the bearing stratum is reached, the work chamber is filled
with concrete.
Fig-4 Pneumatic Caisson
8/17/2019 Caissons Cofferdams Report
8/24
8
2.2 FUNCTIONS
The main functions of caissons generally are soil or water retention and transfer of
vertical and horizontal loads into the subsoil. Less frequent functions are provision of
space for equipment or machinery, and locking through of ships, if the caisson is part of alock or barrier.
With respect to application:
i. Closure of breaches in dikes and dams (closed and flow-through caissons)
ii. Breakwater
iii. Quay wall
iv. Storage
v. Tunnel element
vi. Foundation for bridge pier, lighthouse, wind mill etc.
vii. Specials – casing for hydro-electric plant; Gate for a dry dock
2.3 THICKNESS OF CONCRETE SEAL IN OPEN CAISSONS
The concrete seal should be thick enough to withstand an upward hydrostatic force from
its bottom after dewatering is complete and before concrete fills the shaft. Based on the
theory of elasticity the thickness „t ‟, according to Teng (1962) is
Fig-5 Calculation of the thickness of the seal for an open caisson
8/17/2019 Caissons Cofferdams Report
9/24
9
t = 1.18 R i √(q/f c )………………………………………(Circular Caisson)
and
t = 0.866 B i √q/f c [1+1.61(L i /Bi )…………………… (Rectangular Caisson)
where;
Ri = inside radius of a circular caisson
q = unit bearing pressure at the base of the caisson
f c = allowable concrete flexural stress
Bi , L i = inside width and length, respectively, of rectangular caisson
The value of q can be approximated as
q ≈ Hγw - tγc
where,
γc = unit weight of concrete
The thickness of seal calculated by equations will be sufficient to protect it from cracking
immediately after dewatering. However, two other conditions should also be checked for
safety.
1) Check for Perimeter Shear on Contact Face of Seal and Shaft
According to Fig-5, the net upward hydrostatic force from the bottom of the seal
is − (where = 2 for circular caissons and A = for
rectangular caissons). So the perimeter shear developed is
v ≈ ( – )/
Where,
p i = inside perimeter of the caisson
Note that
p i = 2 ……………………………………..(For circular caissons)
8/17/2019 Caissons Cofferdams Report
10/24
10
And
p i = 2 (L i + B i)…………………………….. (For circular caissons)
The perimeter shear given by equation should be less than the permissible
shear stress, v u , or
v (MN/m 2) ≤ v u (MN/m 2) = 0.17 √f’ c (MN/m 2)Where
= 0.85
2 Check for Buoyancy
If the shaft is completely dewatered, the buoyant upward force, F u , is
F u = ( R 0 2 ) H ……………………………(For circular caissons)
And
F u = (B 0 L 0 ) H ……………………………(For rectangular caissons)
The downward force, F d , is caused by the weight of the caisson and the seal
and by the skin friction at the caisson-soil interface, or
F d = W c + W s + Q s
Where
W c = weight of caisson
W s = weight of seal
Q s = skin frictionIf F d > F u , the caisson is safe from buoyancy. However, if F d < F u , dewatering
the shaft completely will be unsafe. For that reason, the thickness of the seal
should be increased by Δt or
Δt = ( F u – F d )/A i
8/17/2019 Caissons Cofferdams Report
11/24
11
Example:-
An open caisson (circular) is shown in Fig. Determine the thickness of the seal that
will enable complete dewatering.
Solution:-
t = 1.18 R i √(q/f c )For Ri = 7.5 ft,
q ≈ (45)(62.4) -
with = 150lb/ft 3, q = 2808 – 150t and
f c = 0.1 f’ c = 0.1 x 3 x 10 3 lb/in 2 = 0.3 x 10 3 lb/in 2
So
t = (1.18)(7.5) √(2808 – 150)/(300 x 144)or
t 2 + 0.07 t – 5.09 = 0
t = 2.2 ft
Use t = 2.5 ft
Check for Perimeter Shear
v = ( 2 – 2 )/ 2 2
= ( )(7.5) 2[(45)(62.4)-(2.5)(150)]/(2)( )(7.5)(2.5) ≈ 3650 lb/ft 2
= 25.35 lb/in 2
8/17/2019 Caissons Cofferdams Report
12/24
12
The allowable shear stress is
vu = 2 √f c = (2)(0.85) √300= 29.4 lb/in 2 v = 25.35 lb/in 2 < v u = 29.4 lb/in 2 --- OK
Check against Buoyancy
The buoyant upward force is
F u = ( R 0 2 ) H
For R 0 = 10ft
F u = ( ) (10) 2(45) (62.4)/1000 =882.2 kip
The downward force, F d = W c + W s + Q s and
Wc = (R 02 – Ri2) ( )(55) = ( 10 2 – 7.5 2) (150) (55) = 1,133,919 lb ≈ 1134 kip
Ws = ( Ri2)t = ( ) (7.5) 2 (1) (150) = 26,507 lb =26.5 kip
Assume that Q s ≈ 0, So
F d = 1134 + 26.5 = 1160.5 kip
Because F u < F d , it is safe. For design, assume that = 2.5 ft.
2.4 CAISSONS THROUGH THE AGES
2.4.1 ANCI ENT TI M ES
Caissons in civil and military engineering have been used since the era of the Roman
Empire for various purposes. The first application of caissons found in the research, is in
about 250 BC, in Alexandria, Egypt, where watertight caissons have been used to
construct quay walls (Fig-6). (De Gijt, 2010)
Fig-6 Floating caisson used to transport a mortar block, Alexandria 250BC
8/17/2019 Caissons Cofferdams Report
13/24
13
Later on in history, 13 years BC, king “Herod the Great” ordered the construction of the
port of Caesarea, Judea, which became the largest on the eastern Mediterranean coast.
The mole (havenhoofd) was built on floating units; timber casing that were prefabricated,
transported over water (floating) and on the right location immersed by ballasting with
stone (Fig-7). The dimensions of these caissons were 15 x 5.5 x 2.7 m, the water
displacement was 220 tons. (Bernshtein 1994)
Fig-7 Timber caisson for the mole of the port of Caesarea, Judea, about 13 BC
Some centuries later, Robert Weldon, a British Engineer, invented a ship elevator, which
he called a “Hydrostatic Caisson Lock”. This caisson lock is a type of canal lock, and was
intended to raise and lower ships in the Shropshire Canal. The vertical transport of ship
took place in an immersed, sealed caisson box that moved up and down in a big water
container, a cistern (Fig-8)
8/17/2019 Caissons Cofferdams Report
14/24
14
Fig-8 Robert Weldon`s Caisson lock at Oakengates
Engineers found out that in the same way they could construct foundations for bridge
piers, which they first did in Vichy (France), later also in England (e.g. for the piers of
the Royal Albert Bridge in Cornwall, 1859 and the Firth of Forth railroad bridge in
Scotland, 1890) and the United Sates of America (e.g. the Brooklyn Bridge in New Yorkand the Mississippi Bridge in St. Louis), followed by other countries. (Nebel, 2007)
2.4.2 TWENTI ETH CENTURY
Another important field of application of caissons nowadays are ports and harbors. The
variations in dimensions through the years is shown in Fig-9
Fig-9 Various Caisson Dimensions in the Netherlands, 20 th Century
8/17/2019 Caissons Cofferdams Report
15/24
15
2.4.3 NOWA DA YS USE OF CA I SSONS
Caissons nowadays are used for a wide variety of applications. Pneumatic caissons, for
example, are still used for the construction of metro tunnels, like for the Amsterdam
Noord – Zuidlijn underpass of the Damrak.
Fig-10 Caisson for the metro of Amsterdam
Fig-11 Caisson pier foundation with watertight partition during construction of the HSL bridge over the
Hollandsch Diep
8/17/2019 Caissons Cofferdams Report
16/24
16
2.5 CONSTRUCTION
Standard caissons are generally prefabricated „in the dry‟ in a construction dock. When
ready, the dock is inundated and the caisson can be transported over water to the actual
site using its own buoyancy. There it is immersed to the river, sea or estuary bed and ballasted heavily enough to remain at its place and fulfill its function. The life cycle of
caissons consists of the following stages:
i. Idea/ Initiative
ii. Planning and design, laboratory tests
iii. Prefabrication
iv. Transport
v. In – situ constructionvi. Operation, Maintenance
vii. Upgrading, removal & reuse or demolition
8/17/2019 Caissons Cofferdams Report
17/24
17
3. C offerdams
3.1 GENERAL
A cofferdam is a temporary structure designed to keep water and/or soil out of the excavationin which a bridge pier or other structure is built. When construction must take below the
water level, a cofferdam is built to give workers a dry work environment. Sheet pile is driven
around the work site, seal concrete is placed into the bottom to prevent water from seeping in
from underneath the sheet piling, and the water is pumped out. The word “cofferdam” comes
from “coffer” meaning box, in other words a dam in the shape of a box. There are different
types of cofferdam, some are used to support excavation operation and some are enclosed
type box placed in water.
3.2 TYPES OF COFFERDAMS
1. Braced: - It is formed from a single wall of sheet piling which is driven into the
ground t o form a „„box‟‟ around the excavation site. The box is then braced on the
inside and the interior is dewatered. It is primarily used for bridge piers in shallow
water (30-35 ft depth)
2. Earth – Type: - It is the simplest type of cofferdam. It consists of an earth bank with
a clay core or vertical sheet piling enclosing the excavation. It is used for low levelwaters with low velocity and easily scoured by water rising over the top.
3. Timber – Crib: - Constructed on land and floated into place. Lower portion of each
cell is matched with contour of river bed. It uses rock ballast and soil to decrease
seepage and sink into place, also known as “Gravity Dam”. It usually consists of 12`
x 12` cells and is used in rapid currents or on rocky river beds. It must be properly
designed to resist lateral forces such as tipping / overturning and sliding.
4. Double – Walled Sheet Pile: - They are double walled cofferdams comprising two
parallel rows of sheet piles driven into the ground and connected together by a system
of tie rods at one or more levels. The space between the walls is generally filled with
granular material such as sand, gravel or broken rock.
8/17/2019 Caissons Cofferdams Report
18/24
18
5. Cellular: - Cellular cofferdams are used only in those circumstances where the
excavation size precludes the use of cross excavation bracing. In this case, the
cofferdam must be stable by virtue of its own resistance to lateral forces.
3.3 ADVANTAGES OF COFFERDAM
Performing work over water has always been more difficult and costly than performing the
same work on land. And when the work is performed below water, the difficulties and cost
difference can increase geometrically with the depth at which the work is performed. Below
some of the advantages of cofferdam are listed:
a. Allow excavation and construction of structures in otherwise poor environment
b. Provides safe environment to work
c. Contractors typically have design responsibilityd. Steel sheet piles are easily installed and removed
e. Materials can typically be reused on other projects
3.4 TYPES OF IMPOSED LOADS
A typical cofferdam will experience several loading conditions as it is being build and during
the various construction stages. The significant forces are hydrostatic pressure, forces due to
soil loads, water current forces, wave forces, ice forces, seismic loads and accidental loads.
In order to overcome the displaced water buoyancy, the tremie seal thickness is about equal
to the dewatered depth.
Fig – 12 Cofferdam schematic
8/17/2019 Caissons Cofferdams Report
19/24
19
3.4.1 H YDROSTAT I C PRESSURE
The maximum probable height outside the cofferdam during construction and the water
height inside the cofferdam during various stages of construction need to be considered.
These result in the net design pressure shown in Fig-13 below:
Fig-13 Hydrostatic forces on partially dewatered cofferdam
3.4.2 F ORCES DUE TO SOI L L OADS
The soil imposes forces, both locally on the wall of the cofferdam and globally upon the
structure as a whole. These forces are additive to the hydrostatic forces. Local forces are a
major component of the lateral force on sheet-pile walls, causing bending in the sheets,
bending in the wales, and axial compression in the struts.
Fig – 14 Soil force in typical weak mud or clays
8/17/2019 Caissons Cofferdams Report
20/24
20
3.4.3 CURRENT F ORCES ON STRUCTURE
With a typical cofferdam, the current force consists not only the force acting on the normal
projection of the cofferdam but also on the drag force acting along the sides. With flat sheet
piles, the latter may be relatively small, whereas with z-piles it may be substantial, since thecurrent will be forming eddies behind each indentation of profile, as shown in Fig – 15
Fig-15 Current flow along sheet piles
3.5 SCOUR
Scour of the river bottom or seafloor along the cofferdam may take place owing to river
currents, tidal currents, or wave-induced currents. Some of the most serious and disastrous
cases have occurred when these currents have acted concurrently.
A very practical method of preventing scour is to deposit a blanket or crushed rock or heavy
gravel around the cofferdam, either before or immediately after the cofferdam sheet piles are
set. A more sophisticated method is to lay a mattress of filter fabric, covering it with rock to
hold it in place.
3.6 SHEET PILE SHAPES
Larsen/”U” Type Flat/Straight Type Fig-16 Sheet Pile Shapes
Arch shaped and Lightweight
8/17/2019 Caissons Cofferdams Report
21/24
21
3.7 TYPES OF INTERLOCKS
Ball & Socket (BS)
Single Jaw (SJ)
Double Jaw (DJ)
Hook and Grip (HG)
Thumb and Finger one point contact (TFX)
Double Hook (DH)
Thumb and Finger three point contact (TF)
Fig-17 Types of Interlocks
8/17/2019 Caissons Cofferdams Report
22/24
22
3.8 EXAMPLES OF COFFERDAM
Cofferdam for the Sidney Lanier Bridge, Oregon
Installation of wale & strut system – Braced Cofferdam
Installation of strut system and driving the sheet piles
Fig-18 Cofferdam examples
8/17/2019 Caissons Cofferdams Report
23/24
23
REFERENCES
Bernshtein, L.B. (chief edt.) Tidal Power Plants Korea Ocean Research and Development
Institute. 1996.
Gijt, J.G. de A History of Quay Walls Doctoral dissertation Delft, 2010
Nebel, B. Die Caissongründung
http://www.berndnebel.de/bruecken/6_technik/caisson/caisson.html. 2007
FIGURES
Fig – 1: Flowchart showing techniques of underwater construction
Fig – 2: Drilled Shaft and Caisson Foundation, NPTEL
Fig – 3: Drilled Shaft and Caisson Foundation, NPTEL
Fig – 4: Drilled Shaft and Caisson Foundation, NPTEL
Fig – 5: Drilled Shaft and Caisson Foundation, NPTEL
Fig – 6: M.Z. Voorendt, W.F Molenaar, K.G. Bezuyen, Hydraulic Structures
Fig – 7: M.Z. Voorendt, W.F Molenaar, K.G. Bezuyen, Hydraulic Structures
Fig – 8: M.Z. Voorendt, W.F Molenaar, K.G. Bezuyen, Hydraulic Structures
Fig – 9: M.Z. Voorendt, W.F Molenaar, K.G. Bezuyen, Hydraulic Structures
Fig – 10: M.Z. Voorendt, W.F Molenaar, K.G. Bezuyen, Hydraulic Structures
Fig – 11: M.Z. Voorendt, W.F Molenaar, K.G. Bezuyen, Hydraulic Structures
Fig – 12: Professor Kamran M. Nemati, Tokyo Institute of Technology
Fig – 13: Professor Kamran M. Nemati, Tokyo Institute of Technology
Fig – 14: Professor Kamran M. Nemati, Tokyo Institute of Technology
Fig – 15: Professor Kamran M. Nemati, Tokyo Institute of Technology
http://www.berndnebel.de/bruecken/6_technik/caisson/caisson.html.%202007http://www.berndnebel.de/bruecken/6_technik/caisson/caisson.html.%202007http://www.berndnebel.de/bruecken/6_technik/caisson/caisson.html.%202007
8/17/2019 Caissons Cofferdams Report
24/24
Fig – 16: Professor Kamran M. Nemati, Tokyo Institute of Technology
Fig – 17: Professor Kamran M. Nemati, Tokyo Institute of Technology
Fig – 18: Professor Kamran M. Nemati, Tokyo Institute of Technology
Top Related