4_mudmat[1]
TRANSCRIPT
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
OUTLINE FOR SESSION 10Mudmat
ConceptsStability RequirementsDesign
Special FoundationsBucket FoundationsGravity Foundations
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
MudmatMudmats are temporary floor support for the
jacket immediately after the jacket has been upended from floating horizontal position prior to supported by piles.
Need to designed with adequate surface area and sufficient strength strength to avoid excessive settlement of the jacket.
Usually made of steel plate and reinforced by steel beams. However, alternate materials like Timber and FRP has been used to reduce weight and cost
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Advantages of
FRP and Timber Mudmat
FRP and Timber mudmats are used when lift weight is a concern. They will reduce the weight considerably.
The design requirement for Cathodic Protection will also be reduced
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Large Timber Mudmat
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Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
FRP Mudmat
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
MUDMAT CONCEPTS
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Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Jacket with Rectangular Mudmat
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Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Triangular Mudmat
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Rectangular Mudmat
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Circular Mudmat
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Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Triangular Mudmat
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Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Mudmat Panels
Mudmat panels can be any one of the following.
Flate Plate (Steel)
Corrugated Plate (Steel)
Timber Plank
Profilled Panel (FRP)
These panels will be appropriately supported by steel structural members attached to the jacket structure
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Flat Steel plate
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Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Timber Plank
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Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Corrugated Steel plate
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
FRP PANEL
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Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Design Requirements
When the jacket is resting on seabed, it shall satisfy following requirements
Stability against bearing
Stability against sliding
Stability against overturning
Structural members shall have adequate strength
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Design Loads
Dead loads
Bouyancy Loads
Wave and Current Loads
Wind Loads
Loads from Pile stabbing sequence
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Design Requirements
When the jacket is resting on seabed, it shall satisfy following requirements (API RP 2A)
Stability against bearing
Stability against sliding
Stability against overturning
Sometimes it is also called “Unpiled Stability” since this is prior to the piling of the jacket after which the jacket is firmly fixed to the seabed by piles
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Stability Against BearingAs explained earlier, stability against bearing is to
have adequate bearing area to avoid excessive settlement of jacket / failure of mudmat. This has two parts.
Geotechnical Requirement
Structural Requirement
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Factor of Safety against BearingThe Factor of Safety against bearing shall be
calculated as below.
. . u
a
QF O SP
=
The minimum Factor of Safety shall be 2.0 for loads arising from dead weight of the jacket only and 1.5 for dead weight + environmental loads.
Where Qu is the ultimate bearing capacity of soil and Pa is the applied pressure
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Applied Mudmat Pressure (Dead Load)
The applied mudmat pressure can be calculated for dead loads alone very easily.
2S x S
a
M yy
W e W HPA I
= +
Where WS is the total submerged weight of the jacket including ballast water on any compartments of legs, bouyancy tanks and AM is the total mudmat area
If the Jacket is not symmetrical and has self weight acting at an eccentricity of ex, and not at the geometric centre of mudmat, then the effect shall be included as moment component.
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Applied Mudmat Pressure
(Dead Load + Environment Load)
The applied mudmat pressure can be calculated for dead loads alone very easily.
2 2S x S e
a
M yy yy
W e W F hH HPA I I
= + +
Where Fe is the total environmental loads from wave, current and wind and h is the height from seabed at which the environmental loads are applied and Iyy is the moment of inertia of the mudmat system about YY axis.
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Factor of Safety against SlidingThe Factor of Safety against sliding shall be
calculated as below.
. . e
s
FF O SWµ
=
Where Fe is the total environmental loads applied and µ is the friction coefficient between the soil and mudmat system.
The minimum FOS of 1.5 shall be required.
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Factor of Safety against OverturningThe Factor of Safety against Overturning shall be
calculated as below (for each edge).
. . e
s
F hF O SW x
=
Where x is the distance between the vertical load (jacket submerged weight) and the geometric centre of mudmat system at mudline.
The minimum FOS of 1.5 shall be required.
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Jacket SettlementMost of Settlement will take place immediately after the
jacket has been placed on seabed.
Hence the only immediate settlement using elastic theory will suffice.
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
W Fe
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Jacket SettlementSettlement of jacket is an important criteria in designing
the mudmat system as excessive settlement woill lead submergence of bottom framing in to the soil. This will lead following issues.
The mudline framing will be subjected to constant upward force on the members
The conductor guide if any will be submerged in to mud thus driving conductors will become difficult
Boulder if present at shallow depth may damage structural braces
The jacket cut-off level will get affected
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Jacket SettlementElastic settlement of jacket on to the seabed can
be calculated as below.
2(1 )s
qB IE
δ ν= −
Where q is the uniform applied pressure, B is the width of the mudmat, E is the Modulus of the soil, ν is the poissons ratio and Is is the influence coefficient and shall be calculated depending on the shape of the mudmat.
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Settlement of Circular Footing
Vertical settlement of circular footing is given by
QGR
⎟⎠⎞
⎜⎝⎛ −
=41 γδ
QGR
uv ⎟⎠⎞
⎜⎝⎛ −
=41 υWhere
uv,un = vertical and horizontal displacement
Q, H = Vertical and horizontal loads
G = elastic shear modulus of the soil
υ = poisson’s ratio of the soil
R = radius of the base
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
bhAm 4=
23
)2/2/(412
4 bBbhhbI yy −+=
yyxxm
sa I
xMI
yMAWP )()(
+−=
23
)2/2/(412
4 hHbhbhIxx −+=
Where x and y are co-ordinates of points at which the mudmat pressure is required
Rectangular Mudmat system
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
( )224
244
644 BDDI yy
ππ++=
( )224
244
644 HDDIxx
ππ+=
Circular Mudmat system
2
44 DAmπ
=
yyxxm
sa I
xMI
yMAWP )()(
+−=
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
( )23
32
2236
4 bBbhbhI yy −++=
( )23
32
2236
4−+= HbhbhIxx
Triangular Mudmat system
24 bhAm =
yyxxm
sa I
xMI
yMAWP )()(
+−=
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
( )23
22483 bBbhbhI yy −+=
( ) ( )223
31
32
232
2363 hHbhhHbhbhIxx −+−+=
Triangular Mudmat system
24 bhAm =
yyxxm
sa I
xMI
yMAWP )()(
+−=
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
35
Mudmat Concepts and Design
BEARING CAPACITY OF MUDMATS
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
36
Mudmat Concepts and Design
BEARING CAPACITY
The ultimate bearing capacity (qu) is defined as the least pressure which would cause shear failure of the supporting soil immediately below and adjacent to a formation.
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
37
Mudmat Concepts and Design
MODES OF FAILURE
a) General failureb) Local shearc) Punching failure
The mode of failure depends on the following - Foundation type and geometry- Soil compressibility
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
38
Mudmat Concepts and Design
MODES OF FAILURE
a) general shear b) local shear c) punching shear
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
39
Mudmat Concepts and Design
THEORY OF PLASTICITY
A suitable failure mechanism shall be found by either inspection, trial or limit theorems. Two bounds can be defined.
Lower BoundTrue failure load is large than the load corresponding to an equilibrium system
Upper BoundThe true failure load is smaller than the load corresponding to a mechanism if that load is determined using the virtual work principle
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
40
Mudmat Concepts and Design
EQUILIBRIUM SYSTEM
An equilibrium system, or a statically admissible field of stresses is a distribution of stresses that satisfies the following conditionsa) it satisfies the conditions of equilibrium in each point
of the bodyb) it satisfies the boundary conditions for the stresses
c) the yield condition is not exceeded in any point of the body.
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
41
Mudmat Concepts and Design
MechanismA mechanism, or a kinematically admissible field of displacement is a distribution of displacements and deformations that satisfies the following conditions.
a) the displacement field is compatible, i.e. no gaps or overlaps are produced in the body (sliding of one part along another part is allowed)b) it satisfies the boundary conditions for the displacementsc) wherever deformations occur the stresses satisfy the yield conditions
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
42
Mudmat Concepts and Design
IDEALIZED STRESS-STRAIN RELATIONSHIP
Shea
r str
ess
Shear strain
Y’
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
STATE OF PLASTIC EQUILIBRIUM
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
φφσφσ
φσσ
σσφ
cos2)sin1()sin1(
)cot2(21
)(21
sin
13
31
31
c
c
−−=+∴
++
−=
⎟⎟⎠
⎞⎜⎜⎝
⎛+−
−⎟⎟⎠
⎞⎜⎜⎝
⎛+−
=∴
+−
−⎟⎟⎠
⎞⎜⎜⎝
⎛+−
=∴
φφ
φφσσ
φφ
φφσσ
sin1sin12
sin1sin1
sin1)sin1(2
sin1sin1
13
2
13
c
c
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
45
Mudmat Concepts and Design
LOWER BOUND SOLUTION
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
46
Mudmat Concepts and Design
( )
( ) ( )
cqqcccqq
cqcq
c
ult
ult
ult
4422
)1(2)1()1(2)1(
0for 12/45tan2/452
45tan22/45tan
1.31.1
1.31.2
2
231
=+=++=
+==+==
==+=+
⎟⎠⎞
⎜⎝⎛ +++=
−−
−
σσσσ
φφφ
φφσσ
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
−
−
+=
=Χ
−Χ−Χ
qcq
BqBBBcBBq
ult
ult
π
π
2
022
UPPER BOUND SOLUTION
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Simplified bearing capacity for a ø – c soil
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
yqcult
pppp
pp
ult
pult
pppp
H
O
H
Op
yBNNqcNq
KKyBKK
qKK
cq
PcAHByBq
KcHKHqKyHP
dzcqyzdzP
++=
⎥⎥⎦
⎤
⎢⎢⎣
⎡−++⎥
⎦
⎤⎢⎣
⎡+=
=−−+Χ
++=
⎭⎬⎫⎟⎠⎞
⎜⎝⎛ ++⎟
⎠⎞
⎜⎝⎛ +
⎩⎨⎧ +==
−
−
−
−
∫∫
φφφ
φρρ
φφσ
cos4coscos2
0cossin
cos2
.22
.2..2
245tan2
245tan)()(
2
2
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
FAILURE UNDER A STRIP FOOTING
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
FOOTING AT DEPTH D BELOW THE SURFACE
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
52
Mudmat Concepts and Design
Susceptible to long-term consolidation settlement
300 – 600150 – 30075 – 150<75–
Very stiff boulder clays and hard claysStiff claysFirm claysSoft clays and siltsVery soft clays and silts
Width of foundation (B) not less than 1 m. Water table at least B below base of foundation
>600200 – 600
<200>300100 – 300<100
Dense gravel or dense sand and gravelMedium dense gravel or medium dense
sand and gravelLoose gravel or loose sand and gravelCompact sandMedium dense sandLoose sand
Remarks Bearing value(kN/m²)
Soil type
PERSUMED BEARING VALUS (BS 8004: 1986)
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
53
Mudmat Concepts and Design
factorscapacity bearing and ,Depth
Breadth capacity bearing ultimate the
21
====
++=
qc
u
qcu
NNND Bq
DNcNBNq
γ
γ γγ
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
54
Mudmat Concepts and Design
factorscapacity Bearing ,
)4.1tan()1(
tan)1(80.1N
factorscapacity Bearing ,
cot1
/2) 45( tan) tan ( exp 2
=
−=
−=
=
−=
+=
γ
γ
γ
φ
φ
φφπ
NN
NN
N
NN
φ) (NN
N
q
q
q
cq
qc
oq
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
qcf DNcNBNq γγ γ ++= 2.14.0
Length Breadth
capacity bearing ultimate The
2.13.0
==
=
++=
LB
q
DNcNBNq
f
qcf γγ γ
Circular footing
Square footing
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Skempton’s values of Ncfor øu = 0 (Reproduced from A.W.Skempton (1951) Proceedings of the Building Research Congress, Division 1, p.181, by permission of the Building Research Establishment, ©Crown copyright)
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and DesignRECOMMENDED BEARING CAPACITY FACTORS
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
ECCENTRICALLY-LOADED FACTORS
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Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
AREA REDUCTION FACTORS
ECCENTRICALLY-LOADED FACTORS
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
61
Mudmat Concepts and Design
42 – 58 85 – 100 Very dense> 50
25 – 42 65 – 85 Dense30 – 50
8 – 25 35 – 65Medium dense10 – 30
3 – 8 15 – 35 Loose4 – 10
0 – 3 0 – 15 Very loose 0 – 4
(NI)60Id (%)ClassificationN Value
DENSITY INDEX OF SANDS
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
62
Mudmat Concepts and Design
Bearing capacity calculations by Davis and Booker
The bearing capacity can be calculated when the soil profile is varying linearly with depth
)1(4 ccuoru SBNCFq +⎟⎠⎞
⎜⎝⎛ +=
ρfactor Shape==
LB
NN
Sc
cγ
NC= 5.14 for strip footing
Nγ = 1 for footing at top of soil
Fr = shear strength factor depends on the variation of the soil profile
ρ= rate of increase of shear strength
B = width of footing
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
63
Mudmat Concepts and Design
F r - Shear Strength F acto r
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000 8.000
Rho
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
SPECIAL FOUNDATIONS
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Special Foundations
Suction Anchor (Bucket Foundation)Gravity Foundation
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
66
Mudmat Concepts and Design
Suction Anchors (Piles)
A suction anchor is an inverted top capped hollow cylinder of fairly large diameter with a length to diameter ratio (L/D) of 1.0 to 2.0 that is embedded into the sea bed. Self-weight and differential water pressure can facilitate easy installation of this type of anchor into the sea bed. This differential water pressure (active suction) can be created by pumping out the water from the interior of the anchor.
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
67
Mudmat Concepts and DesignThe main pile advantages of this anchor over tension piles are due to the weight of the soil plug inside and the freely available high ambient water pressure which offers two advantages; easy installation of the anchor with its active suction arrangement and mobilization of passive suction force at the anchor bottom during uplift. Further, the large-diameter sealed top provides a substantial space for additional ballast, which can increase thebreakout resistance
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
68
Mudmat Concepts and Design
Suction Breakout FactorsFrom the equilibrium considerations (referring to figure 1) the uplift pullout capacity of the suction anchor is given by
Pu = Wa + Fext + Ws + Wb + Rb
WhereWa = is the weight of the anchorFext = is the shear resistance along the external wallWs = is the weight of the soil plugWb = is the weight of the ballast (if any) at the topRb = is the suction-induced reversed end bearing
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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
69
Mudmat Concepts and Design
30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering
Indian Institute of Technology Madras-36
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Mudmat Concepts and Design
Pu = Wa + Ws + Fext + Rb
Rb1 = Pu – (Wa + Ws + Fext)
From consideration of rupture in clay under tensile loading (Vesic, 1971) the bottom breakwater resistance is expressed in a non-dimensional form as
Fext = αCu Ase
From the plug equilibrium (refer to figure 13) equations can be written as:
Rb2 + Ws - Ps + Fint
Rb2 = Ps + Fint - Ws
Rb2 = Nb2 Cu AbWhere
Nb1 and Nb2 are bottom breakout factors from overall and plug equilibrium, respectively, ps is suction pressure measured at the top of soil plug, Ab is the base area of the anchor, α is an adhensionfactor, Fint is internal skin friction, Asi is the area of internal skin friction and Ase is the area of external skin friction