Caissons Cofferdams Report

8/17/2019 Caissons Cofferdams Report

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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


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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


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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.


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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.


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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


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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


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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


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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)


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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


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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


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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


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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)


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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


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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


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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


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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.


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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


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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


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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


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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


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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


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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 – 6: M.Z. Voorendt, W.F Molenaar, K.G. Bezuyen, Hydraulic Structures






Fig – 12: 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


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