Pile Design Calculation

(1)

DESIGN CHECK FOR VERTICAL, LATERAL AND MOMENT CAPACITY. (for pile at locations B & C)

DESIGN DATA

Maximum axial load from the analysis = P = 1198.0 kN

= M = 391.0 kN.m

Maximum lateral force at fixity from the analysis = 24.0 kN

SOIL DATA

Reference - Geotechnical Investigation prepared by Gulf Consult

From the bore hole logs at the two locations and as well as from the report it is very clear that the piles are founded on cohesionless Very dense silty sand. Hence calculations are performed based on cohesionless soil. The following calculation represents the pile design at grid B & C.

Type of soil =

Average peak value of cone resistance = ckd = 21576.0 kN/m2

DIMENSION

Diameter of the pile = 914.4 mm

Thickness of pile = 17.7 mm

Theoretical thickness of pile = 11.7 mm

Seabed level = -3.5 m

Depth of fixity below seabed level = 1.0 m

Assumed depth of pile below fixity point = 6.5 m (SP-SM)

Assumed depth of pile below fixity point = 3.0 m (SM)

Angle of internal friction below the fixity point = f = 36.0(Lower value)

= g1 = 10.2 kN/m3 (SP-SM)

= g2 = 11.2 kN/m3 (SM)

= E = 2.05 x 105 N/mm2

Minimum yield strength of pile Fy = 400.0 N/mm2

MEMBER CHECK

Internal diameter of the pile = d2 = 878.96 mm

Pile outer dia. after corrosion = D = 902.44 mm

Moment of Inertia of pile = I = p (D4 - d4))/64

= 0.00326 m4

Stress due to axial load = P/a

= 36.5 N/mm2

Stress due to bending = (M*y / I)

= 54.1 N/mm2

(Referring to BS 6349, all stress is 0.6 Fy) < 240.0 N/mm2 OKAY

Maximum compression = (P/a) + (M y / I)

= 90.6 N/mm2

Maximum resultant bending moment

Very dense Silty Sand

Submerged unit weight of soil below the fixity point

Youngs modulus of elasticity

Submerged unit weight of soil below the fixity point

(2)

Maximum tensile stress = (M y / I)

(By Ignoring axial compression) = 54.1 N/mm2

DESIGN CHECK FOR VERTICAL CAPACITY

From the available soil data it is evident that the piles at these locations are founded at a very dense silty sand layer, at a depth of 10.5 m from the sea bed. Hence the capacity is calculated based on the cohesionless soil property at these locations.

Referring to API RP2A and 'Pile design Construction practice by Tomlinson'

Frictional resistance in cohesionless soil

Skin friction at outside the shaft = Qs = Ks pd tand As

Where,

Coefficient of earth pressure based on relative density of soil.

Efective overburden pressure(average for 7.5 deep) = pd1 = 38.3 kN/m2 (SP-SM) Efective overburden pressure(average for 7.5 to 10.5 deep) = pd2 = 110.1 kN/m2 (SM)

Friction angle between pile and soil = d1 = 300 (SP-SM)

Friction angle between pile and soil = d2 = 350 (SM)

Outer Area of pile in contact with soil = Aso1 = 21.6 m2 (SP-SM)

Outer Area of pile in contact with soil = Aso2 8.6 m2 (SM)

Skin friction at outside the shaft = Qso1 = 429.4 kN (SP-SM)

Skin friction at outside the shaft = Qso2 = 599.6 kN (SM)

Total skin friction at outside the shat = Qso = 1029.0 kN

As the end bearing resistance for the entire base area is much more than the internal skin friction, the pile behaves unplugged. Hence the internal skin friction is considered for resistance calculation. For end bearing calculation, only the pile annulus area is considered.

Inner Area of pile in contact with soil = Asi1 = 20.8 m2 (SP-SM)

Inner Area of pile in contact with soil = Asi2 = 8.3 m2 (SM)

Skin friction at inside the shaft = Qsi1 = 412.7 kN (SP-SM)

Skin friction at inside the shaft = Qsi2 = 576.3 kN (SM)

Toatl skin friction at inside the shaft = Qsi = 989.0 kN

End bearing resistance

Base area of pile = Ab = 0.050 m2

End bearing resistance = Qb = Ckd Ab

= 1078.3 kN

However referring to API, Table 6.4.3.1

Limiting unit end bearing stress = 12.0 N/mm2

Limiting end bearing resistance Qbl = 599.7 kN

Total pile resistance = Q = Qso+Qsi+min(Qb&Qbl)

= 2617.7 kN

Safety Index = 2.19

> 2.0 OKAY

= Ks = 0.9

(3)

CHECK FOR LATERAL CAPACITY

The lateral resistance of the short piles, is checked based on the assumption that the the passive soil resistance for a depth equal to the depth of embedment should be sufficient to carry the lateral load

Passive earth pressure = qp = kp g h

1+sinf 1-sinf i.e. kp = 3.85

Passive pressure at -14.0 m level = qp = 373.2 kN/m2

9.5 m

373.2 kN/m2

Effective perimeter = P = 1.42 m

Lateral capacity due to passive earth pressure = FP = P* h * qp

Lateral capacity due to qp = 1507.9 kN

Safety Index = 62.83 kN

> 2.00 OKAY

CHECK FOR ANCHORAGE LENGTH

Effective perimeter = 1.42 m

Moment capacity due qp = 9550.1 kN.m

Safety Index = 24.42

> 2.00 OKAY

Where = kp =

(4)

DESIGN CHECK FOR VERTICAL, LATERAL AND MOMENT CAPACITY.

(for pile at locations D)

DESIGN DATA

Maximum axial load from the analysis

=

P

=

582.0 kN

=

M

=

364.0 kN.m

Maximum lateral force at fixity from the analysis

=

26.0 kN

SOIL DATA

Reference - Geotechnical Investigation prepared by Gulf Consult

From the bore hole logs at the two locations and as well as from the report it is very clear that the piles are founded on cohesionless Very dense silty sand. Hence calculations are performed based on cohesionless soil. The following calculation represents the pile design at grid B & C.

Type of soil

=

Average peak value of cone resistance

=

c

kd

=

21576.0

kN/m

2

DIMENSION

Diameter of the pile

=

914.4 mm

Thickness of pile

=

17.7 mm

Theoretical thickness of pile

=

11.7 mm

Seabed level

=

-8.25

m

Depth of fixity below seabed level

=

1.0 m

Assumed depth of pile below fixity point

=

1.75 m

(SP-SM)

Assumed depth of pile below fixity point

=

6.00 m

(SM)

Assumed depth of pile below fixity point

=

2.00 m

(SC)

Angle of internal friction below the fixity point

=

f

=

36.0

(Lower value)

=

g

1

=

10.2 kN/m

3 (SP-SM)

=

g

2

=

11.2 kN/m

3 (SM)

=

g

3

11.0 kN/m

3 (SC)

=

E

=

2.05 x 10

5

N/mm

2

Minimum yield strength of pile

F

y

=

400.0 N/mm

2

MEMBER CHECK

Internal diameter of the pile

=

d

2

=

879.0 mm

Pile outer dia. after corrosion

=

D

=

902.4 mm

Moment of Inertia of pile

=

I

=

p

(D

4

- d

4)

)/64

=

0.0033

m

4

Stress due to axial load

=

P/a

=

17.7 N/mm

2

Stress due to bending

=

(M*y / I)

=

50.4 N/mm

2

(Referring to BS 6349, all stress is 0.6 Fy)

<

240.0 N/mm

2 OKAY

Maximum resultant bending moment

Very dense Silty Sand

Submerged unit weight of soil below the fixity point

Youngs modulus of elasticity

(5)

Maximum compression

= (P/a) + (M y / I)

=

68.1 N/mm

2

<

120.0 N/mm

2 OKAY

Maximum tensile stress

=

(M y / I)

(By Ignoring axial compression)

=

50.4 N/mm

2

<

120.0 N/mm

2 OKAY

DESIGN CHECK FOR VERTICAL CAPACITY

From the available soil data it is evident that the piles at these locations are founded at

a very dense silty sand layer, at a depth of 10.5 m from the sea bed. Hence the

capacity is calculated based on the cohesionless soil property at these locations.

Referring to API RP2A and 'Pile design Construction practice by Tomlinson'

Frictional resistance in cohesionless soil

Skin friction at outside the shaft

=

Q

s

= K

s

p

d

tan

d

A

s

Where,

Coefficient of earth pressure based on relative

density of soil.

Efective overburden pressure(average for 7.5 deep)

=

p

d1

=

38.3 kN/m

2 (SP-SM)

Efective overburden pressure(average for 7.5 to 10.5 deep)

=

p

d2

=

110.1 kN/m

2 (SM)

Efective overburden pressure(average for 7.5 to 10.5 deep)

=

p

d2

=

152.0 kN/m

2 (SC)

Friction angle between pile and soil

=

d

1

=

30

0 (SP-SM)

Friction angle between pile and soil

=

d

2

=

35

0 (SM)

Friction angle between pile and soil

=

d

3

25

0 (SC)

Outer Area of pile in contact with soil

=

A

so1

=

7.9 m

2 (SP-SM)

Outer Area of pile in contact with soil

=

A

so2

17.3 m

2 (SM)

Outer Area of pile in contact with soil

=

A

so3

5.8 m

2 (SC)

Skin friction at outside the shaft

=

Q

so1

=

157.4 kN

(SP-SM)

Skin friction at outside the shaft

=

Q

so2

=

1199.2

kN

(SM)

Skin friction at outside the shaft

=

Q

so3

=

367.5 kN

(SC)

Total skin friction

=

Q

so

=

1724.1

kN

Inner Area of pile in contact with soil

=

A

si1

=

7.6 m

2 (SP-SM)

Inner Area of pile in contact with soil

=

A

_si2

₌

_16.6

_m

2 _(SM)

Inner Area of pile in contact with soil

=

A

si3

=

5.5 m

2 (SC)

Skin friction at outside the shaft

=

Q

si1

=

151.3 kN

(SP-SM)

Skin friction at outside the shaft

=

Q

si2

=

1152.6

kN

(SM)

Skin friction at outside the shaft

=

Q

_si2

₌

_353.2

kN

(SC)

Toatl skin friction at outside the shaft

=

Q

si

=

1657.2

kN

Total pile resistance

=

Q

=

Q

so

+Q

si

=

3381.3

kN

Safety Index

=

5.81 >

2.0

OKAY

0.9 =

K

s

=

(6)

CHECK FOR LATERAL CAPACITY

The lateral resistance of the short piles, is checked based on the assumption that the

the passive soil resistance for a depth equal to the depth of embedment should be