Soil data in Orcaflex

Prepared by Y.T Kim

Offshore Engineering Lab. Seminar No. 6

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1. API RP 2A-WSD

1. Winkler’s hypothesis

• Most of the theoretical solutions for laterally loaded piles involve the concept of modulus of subgrade

reaction

• Modulus of subgrade reaction (the pressure distribution which is the result or ‘reaction’ of the

‘subgrade’ to a load imposed upon the top of a foundation structure)

• unit – kN/m2/m

• Because of the limitation of available data and the uncertainty of soil condition, it was also

proposed to use the empirical equations. The following empirical equations (Widjaja, 2008) were

suggested to estimate k value both in clay and sand:

• Clay : k = 40 – 50 su (t/m3)

• Sand : k = 70 – 100 NSPT (t/m3)

• Winkler’s hypothesis-The reaction at any point on the base of the beam in Fig.1 depends only on the

deflection at that point.

• The same principle applies to batter piles.

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1. API RP 2A-WSD

1.1 Winkler’s hypothesis

• A series of nonlinear - the force deformation characteristics of the soil.

• The springs attached to the blocks of different sizes - reaction increasing with deflection and then

reaching a yield point, or a limiting value that depends on depth.

• The taper on the springs – a nonlinear variation of load with deflection

• The gap btw the pile and the springs – the molding away of the soil by repeated loadings

Fig.1

Fig.2

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1. API RP 2A-WSD

1.2 Lateral bearing capacity for soft clay (API RP 2A 6.8.2)

• For static lateral loads, the ultimate unit lateral bearing capacity of soft clay pu 8c~12c

• Cyclic loads cause deterioration of lateral bearing capacity

• The following is recommended,

' tanf c

=critical shear stress

cohesion

friction angle

f

c

3 , 0u R

cXp c X J X X

D

9 u Rp c X X

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1. API RP 2A-WSD

1.3 Load-deflection (p-y) curves for soft clay (API RP 2A 6.8.3)

• Lateral soil resistance-deflection relationships in soft clay – non-linear

• The p-y curves for short-term static load case as following table:p/pu y/yc

0.000.230.330.500.721.00

0.00.10.33.08.0∞

P=actual lateral resistance, psi(kPa)

y=actual lateral deflection, in.(m)

yc=2.5εcD, in.(m)

εc=strain which occurs at one-half the maximum stress on laboratory

unconsolidated undrained compression tests of undisturbed soil

samples.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

5

10

15

p-y curves

p/pu

y/y

c

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2. Orcaflex p-y Models

Model type

One of the following options:

• The API RP 2A Soft Clay option uses the load-deflection curves for piles in soft clay under short-term static loading as

specified in API RP 2A (2000).

• Similarly, the API RP 2A Sand option uses the API RP 2A (2000) load-deflection curves for sand.

• The P-y Table option allows you to specify the load-deflection curves directly in tabular form.

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3. Orcaflex Seabed data

2.1 Linear Model

• Treat the seabed as a simple linear spring

• Seabed resistance is proportional to the penetration

• Normal seabed stiffness-the spring reaction force, per unit are of contact, per unit depth of penetration, (e.g., rock-high but

clay-very low)

• Shear seabed stiffness-used by the friction calculation. A value of '~' results in the Normal Seabed Stiffness being used.

• The Seabed Damping-a percentage of critical damping. Equals “0” when using the implicit integration scheme.

The normal stiffness reaction force has magnitude = KnAd Kn = seabed normal stiffnessA = penetrator contact aread = depth of penetration into the seabed.

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3. Orcaflex Seabed data

2.2 Non-linear Model

• Prof. Mark Randolph, University of Western Austrailia

• It is a development from earlier models that proposed and used a hyperbolic secant stiffness formulation

Fig.4 Soil model characteristics for different modes

• More sophisticated than the linear model

• Models the non-linear and hysteretic behavior of seabed soil

including modelling of suction effects.

• The non-linear modelling only applies to the seabed normal

direction. Linear modelling of lateral directions

Fig.3 Soil model penetration modes

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• Remains of the non-linear seabed modelling of Orcaflex

• Seabed stiffness & linear vs. nonlinear modelling parametric study using Orcaflex

and SHEAR7

6. Future Plan