IV - Part 6 Section 7 C Foundations Chapter 4

GL 2007 Page 7–1

Section 7

Foundations

A. General – in situ cone penetration tests (or other equivalent

in situ test) performed, if possible, to a depth
1. Scope that will cover the critical shear zone
This Section applies to the foundations of fixed off- The actual extent of a geotechnical field investigation
shore structures and covers piled as well as gravity has to be set up individually for each site.
type designs. Requirements regarding self-elevating
platforms (mobile units) see Chapter 2 – Mobile Off- 2. Boring vessel
shore Units, Section 2.
2.1 Positioning system
2. Foundation design
The boring vessels shall be equipped with a position-
The foundation design of fixed offshore structures
ing system which ensures an adequate accuracy of
shall consider all types of loads which may occur
borehole positioning.
during installation and operation of the structure.

3. Soil 2.2 Laboratory and personnel

The soil behaviour and its interaction with the founda- The boring vessel shall contain a high quality soil
tion shall be investigated with special regard to static, laboratory for classification and testing of soil speci-
dynamic and transient loading. mens. The opening of samples on board, the visual
inspection and the classification of the soil shall be
performed by qualified geotechnical personnel.
4. Evolution of foundations
The design practise for foundations has proved to be 3. Samples
very innovative and this evolution is expected to con-
tinue. Therefore circumstances may arise when the The handling, storage and transport of soil samples
procedures described herein are insufficient on their shall be such as to retain a satisfactory sample quality.
own to ensure that a safe and economical foundation
design is achieved. In such a case new requirements
have to be discussed and agreed by GL.
C. General Design Considerations

B. Geotechnical Investigations 1. Influences to be considered

The design of foundations shall take account of all
1. Field investigation effects which may be of influence on deformations,
The field investigation shall be carried out at the ac- bearing capacity and installation of the foundation
tual site, defined as the area within which the structure system.
may be installed, including accepted tolerance limits
from theoretical centre. The field investigation pro- 2. Characteristics of soil layers
gramme shall comprise but not necessarily be limited
to the following: For each soil layer the physical properties shall be
thoroughly evaluated with the help of in-situ and labo-
– site geology survey in order to establish geo- ratory testing.
logical conditions in general
The main parameters for the foundation design are:
– geophysical explorations for the purpose of
extending the localized information from the For sand:
borings and in situ testing
– effective friction angle Φ' [degrees]
– bottom topography including registration of
rocks or hard objects on the bottom – submerged unit weight γ' [kN/m3]
– soil sampling to ensure a sufficient number of – Youngs modulus E [Mpa]
good quality samples, that will allow the proper-
ties of all important layers to be determined – initial soil modulus K [MN/m3]
Chapter 4 Section 7 D Foundations IV - Part 6
Page 7–2 GL 2007

For clay: 6.3 Especially for soft, normally consolidated

clays or loose sand deposits consideration of seafloor
– undrained shear strength cu [kN/m2] instability is required.
– submerged unit weight γ' [kN/m3]
6.4 Seabed movements due to action of waves,
– Youngs modulus E [Mpa] earthquake or operational effects, e.g. drilling, dredg-
ing, may cause reduced resistance or increased loading
– Strain at 50 % of failure [e50] and shall be investigated.

3. Cyclic loads 7. Settlements and displacements

Long term settlements and displacements of the struc-
3.1 The effect of cyclic loading, which may cause ture as well as the surrounding soil shall be analyzed.
a reduction of shear strength and bearing capacity of
the soil, shall be investigated in the case of gravity 8. Load deformation behaviour
type foundations.
The load deformation behaviour of a piled foundation
3.2 Especially for structures installed in seismi- is to be analyzed with special regard to possible struc-
cally active areas the proneness of the soil to partial tural interaction.
liquefaction and possibly resulting reduction in soil
strength and stiffness have to be evaluated with great
care.
D. Pile Foundations
4. Scour
1. General
4.1 The possibility of scour or undermining
around the foundation shall always be investigated. 1.1 Among several existing types of pile founda-
tions the following are most frequently used offshore:
In case that scouring may occur, – open ended and driven piles
– the foundation has to be protected by suitable – driven and underreamed piles
means
– drilled and grouted piles
– alternatively, the foundation has to be consid-
ered to be partly unsupported Many other designs are used to suit the needs of the
individual soil conditions.
4.2 If no other data are available for the specific 1.2 For each type of pile foundation the soil-pile
site conditions, the scour depth at pile foundations interaction and pile capacity have to be evaluated with
may be estimated as 2,5 × d (d = pile diameter) for due regard to site specific soil conditions.
design purposes.
2. Pile design, design methods
4.3 The design criteria shall be verified by regu-
lar surveys, see Chapter 1 – Classification, Certifica-
2.1 The design of piled foundations has to fulfil
tion and Surveys, Section 5. Countermeasures shall be
requirements for axial and lateral load capacity and
taken in case of exceeding the limits established in the
stiffness.
design.
2.2 Among others the following factors will
5. Hydraulic stability affect the design: diameter, penetration, spacing,
mudline restraint, material, pile footing and installa-
The hydraulic stability shall be investigated for those tion method.
types of foundations which may exhibit significant
hydraulic gradients within the supporting soils. 2.3 The design method has to incorporate the pile
geometry, properties and arrangement. It should be
6. Soil stability capable of simulating the non-linear properties of the
soil and be compatible with the load deflection behav-
6.1 If necessary, due to existing slope or due to iour of the structure and pile foundation system.
installation effects, the risk of slope failure or the
possibility of a deep slip shall be investigated. 2.4 Deflections and rotations shall be checked for
individual piles and pile groups.
6.2 Total or effective stress analysis shall be used The design shall consider bending moments as well as
for soil stability investigations. axial and lateral loads.
IV - Part 6 Section 7 D Foundations Chapter 4
GL 2007 Page 7–3

3. Axially loaded piles For plugged piles (i.e. closed end piles) Rd may be
calculated as follows:
3.1 Design loads
(2) Rd = ΣfC0 ⋅ A0 + q ⋅ Ap
The design of the pile penetration shall provide suffi-
cient capacity for the maximum axial compression and fC0 = outer unit skin friction capacity for compres-
tension loads. The loading conditions of Table 7.1 sion
have to be investigated.
A0 = outer pile shaft area

Table 7.1 Safety factors γg for different loading q = unit end bearing capacity
conditions (compare Table 3.2) Ap = gross end bearing area
Loading conditions = 0,25 ⋅ π ⋅ d02
Safety factor γg
No. Designation d0 = outer pile diameter
2 Operating loads 2,0 For unplugged piles the following formula is to be
3 Extreme environmental loads 1,5 used
with specified drilling or (3) Rd = ΣfC0 ⋅ A0 + ΣfCi ⋅ Ai + q ⋅ Aw
producing loads
3 Extreme environmental loads 1,5 fci = inner unit skin friction for compression
with minimum weights Ai = inner pile shaft capacity area
AW = net bearing area
The design requirement may be expressed as follows:
= π ⋅ (d0 – t) ⋅ t
RK
γ ⋅ FA < t = pile wall thickness
γm
Plugged condition is given if
RK = ultimate resistance (4) ΣfCi ⋅ Ai > q ⋅ APi
FA = axial pile load
APi = 0,25 ⋅ π ⋅ di2
γ = γg global safety factor as given in Table 7.1 di = inner pile diameter
when deterministic method is used
In this case equation (2) is to be used.
γ = γf partial safety factor when limit state design is
Unplugged condition prevails if
used
(5) q ⋅ APi > ΣfCi ⋅ Ai
γm = 1,0 when deterministic method is used, see
Section 3, B.3. Equation (3) applies in this case.

3.2 Calculation of ultimate resistance When calculating the design resistance of pile founda-
tions the weight of the pile soil plug system shall be
For the calculation of the ultimate pile resistance vari- considered.
ous methods may be used. One acknowledged method
Due regard shall be given to the possibility that full
is outlined in the following parts of this Section.
resistance of skin friction and tip capacity may be
Other design methods may be used. Their applicability mobilized at different deflections and are not necessar-
for the specific site conditions and their limitations ily additive.
shall be carefully checked and agreed upon with GL.
3.4 Ultimate resistance of piles in tension
3.3 Ultimate resistance of piles in compression When calculating the ultimate resistance for pullout
The ultimate resistance of piles in compression Rd loads, the effective weight of the pile and the soil plug
may be defined as follows: may be considered.
The pullout resistance is calculated from the expres-
(1) Rd = Rf + Rp sion

Rf = skin friction resistance (6) Rt = ft ⋅ A0 + G's + G'p

Rp = total end bearing (tip) resistance ft = unit skin friction capacity for tension, see 3.5
Chapter 4 Section 7 D Foundations IV - Part 6
Page 7–4 GL 2007

G's = effective steel weight of pile Table 7.2 Skin friction and end bearing capaci-
ties for silica sand
G'p = effective weight of plug
It has to be observed that for tension loads the skin Unit skin friction Unit end bearing
friction capacity normally is considerably lower than capacity f capacity q
Type of soil
for compression. Unless it can be proven otherwise, [kPa] [Mpa]
the skin friction for tension should be taken not more limit limit
than 2/3 of the skin friction for compression loading. Very dense sands
115 12,0
and gravel
3.5 Skin friction and end bearing capacity Dense sand and
very dense silty 96 9,6
3.5.1 Driven piles sand
The design values for skin friction (f) and end bearing Medium dense
capacity (q) may be established either on the basis of sand and dense 81 4,8
test results or according to empirical methods. silty sand
For driven piles in cohesive and cohesionless soils a Loose sand,
medium dense
proven analytical method is described in API RP 2A 1. 67 2,9
silty sand and
3.5.2 Drilled and grouted piles dense silt
Very loose sand,
The skin friction of drilled and grouted piles in rock is loose silt sand and 48 1,9
limited by the shear strength of the rock. (Normally medium dense silt
the unit skin friction will be much less than the shear
strength of the rock.)
When establishing the design values, the installation 3.6 Piles driven in non-homogeneous soil
methods, e.g. drilling or jetting, are to be taken into For piles penetrating non-homogeneous soils an aver-
account, which may greatly affect the rock strength age value of the tip resistance should be chosen as
and rock/grout bond. follows:
The capacity of steel/grout bond and shear key design
has to be examined with great care, see 7. 3.6.1 Pile tip penetrating a dense layer from a weak
layer, see Fig. 7.1
3.5.3 End bearing capacity
For the unit end bearing capacity the following is valid
The end bearing capacity should be specified on the
basis of the triaxial shear strength. The bearing capac-
ity factor shall be selected with good engineering q1 − q 0
q = q0 + Db ⋅ ≤ q1
judgement. Due regard shall be given to a reduction of 10 ⋅ D
bearing capacity in fractured rock.

3.5.4 Limits for skin friction and end bearing

capacity
3.5.4.1 For silica sand the limit values shown in qo Weak Soil
z

Table 7.2 for unit skin friction and unit end bearing
capacity shall be observed unless it can be proven by
in-situ tests that other values are applicable.
Db

10 D

3.5.4.2 The selection of design parameters in cal-

q
careous sands or other soils which are easily crushable
and compressible has to be done with great care be-
cause the skin friction may be reduced significantly. q Dense Sand

The compressibility and carbonate content of calcare-

ous soils may serve as an indicator for the risk of re-
qo limiting unit end bearing capacity in weak soil
duced side adhesion. Further properties, e.g. grain
crushing and degree of cementation, are to be studied q limiting unit end bearing capacity in dense sand
as a basis for the selection of the design parameters. D pile diameter
–––––––––––––– Db depth of penetration into dense layer
1 For flush piles (no inner driving shoe) the inner and outer unit z penetration depth
skin friction can be assumed to be of the same magnitude.
For piles with an inner driving shoe typical values for inner
unit skin friction above the shoe are 50 % to 70 % of outer unit Fig. 7.1 Pile tip penetrating a dense layer from
skin friction. a weak layer
IV - Part 6 Section 7 D Foundations Chapter 4
GL 2007 Page 7–5

3.6.2 Pile tip penetrating a thin dense layer laying The p-y data may be derived using stress-strain data
over a weak layer, see Fig. 7.2. from laboratory tests on soil samples.
Possible punch-through effects are to be considered. A proven numerical method to calculate the p-y
curves is given in API RP 2A.
The unit end bearing capacity may be calculated as
follows:
5. Pile groups
q1 − q 0
q = q0 + H ⋅ ≤ q1 5.1 For pile groups with pile spacing of less than
10 ⋅ D eight pile diameters, pile group effects have to be
evaluated.
For axial pile loads the group capacity in clay may be
less than the sum of the single pile capacities, whereas
q Weak Soil the group capacity in sand may be greater than the
z

sum of the individual pile capacities.

Dense Sand 5.2 For lateral loads the group will normally
10 D

undergo larger deflection than a single pile under

corresponding average loading.
H

q
Weak Soil
5.3 The analysis of a pile group may be carried
qo out according to acknowledged methods 2 as agreed
with GL.

5.4 All piles of a pile group should end in the

H distance of pile tip from weak soil layer
same soil layer.

Fig. 7.2 Pile tip penetrating a thin dense layer 6. Pile structure design
laying over a weak layer
6.1 General
4. Laterally loaded piles The pile structure design shall consider all types of
loads which may occur during installation and opera-
4.1 Task tion phases.
It has to be ensured that the pile foundation is capable
to sustain all static and cyclic loads acting in lateral 6.2 Stresses in piles during installation
direction. 6.2.1 During pile installation the stresses including
The soil resistance in the vicinity of the mudline con- buckling shall not exceed the allowable values ac-
tributes significantly to the lateral capacity of a pile cording to loading condition 2, see Section 3, D.
foundation.
6.2.2 Consideration shall be given to axial loads
Any disturbance of the soil e.g. due to scouring, or to and bending moments due to pile weight and full
the installation of piles or conductors has therefore to hammer weight. Bending moments due to pile batter
be considered with great care. eccentricities are to be accounted for. For vertical piles
2 % of pile and hammer weight shall be assumed as if
4.2 Design criteria acting laterally to account for possible impacts and
eccentricities during hammer placement.
The design has to fulfil two main criteria:
6.2.3 Consideration shall be given to the stresses
4.2.1 The load-deflection behaviour has to meet the that occur during driving. Combined stresses during
requirements of platform operability. driving, i.e. static plus dynamic portion, shall not
exceed yield stress.
4.2.2 The allowable pile stresses shall not be ex-
ceeded. 6.2.4 Pile wall thickness
6.2.4.1 The ratio of pile diameter and wall thickness
4.3 p – y curve
shall be small enough so that local buckling during
A commonly accepted method makes use of the p-y
curve. In this method the load (p)-deflection (y) rela-
––––––––––––––
tionship of the soil is established for each soil layer. 2
Based on numerical methods the soil stiffness is used See e.g. Focht and Koch, OTC 1896, 1973. This procedure
combines the subgrade reaction (p-y) analyses and elastic half
to calculate deflection and bending moments of the space procedures. It results in modified p-y curves for an iso-
pile foundation system. lated pile to represent the single pile behaviour in a pile group.
Chapter 4 Section 7 E Foundations IV - Part 6
Page 7–6 GL 2007

installation is avoided. For piles driven in hard soils 7.3 For geometries not covered by the existing
the following relation may serve as a guideline: formulae the strength of the grouted connection has to
be proven by suitable calculation methods or by test.
D
t = 6,35 +
100 7.4 Grout quality

t = wall thickness [mm] 7.4.1 Prior to the installation, the suitability of the
grout mix has to be proven. The compressive strength
D = pile diameter [mm] has to be confirmed by laboratory tests on grout sam-
ples which were mixed and cured under field condi-
6.2.4.2 The pile wall thickness may vary along its
tions.
length according to the stress level in the different
sections. Thickened portions should be reasonably 7.4.2 The design grout strength has to be verified
extended in order to allow for underdrive or overdrive. by a representative number of specimens taken during
6.2.4.3 For piles to be driven in hard soils a driving grouting operations. It has to be ensured that the
shoe should be provided with a length of about one specimens will be cured simulating in-situ conditions.
pile diameter and a wall thickness of 1,5 times the
value established according to 6.2.4.1.
E. Gravity Type Foundations
6.3 Stresses in piles during platform operation

6.3.1 All loads resulting from the design load con- 1. General
ditions of the platform are to be considered for the pile
structure design. 1.1 Characterization
Normally these loads will control the pile design in the Gravity type foundations are characterized as founda-
mudline area. tions with relatively small penetration into the soil,
compared to the width of the foundation bases, and
6.3.2 The soil stiffness, see 4., has to be accounted relying predominantly on compressive contact with
for when calculating the bending moment distribution. the supporting soil.
Due regard shall be given to the effect of scour and
lack of soil adhesion due to large pile deflections. 1.2 Skirts
6.3.3 The axial load transfer may be calculated If soil layers of sufficient strength are found at some
according to the "axial soil resistance - pile deflection" depth below the sea bed, skirts may be required in
characteristics or alternatively according to the skin order to ensure the suitability of the foundation (see
friction capacity of the pile, see 3. e.g. 2.3.4.3). In some cases skirts will be used to pro-
The relevant safety factors specified in 3.1 have to be tect the foundation against scouring.
accounted for when calculating the axial load transfer.
1.3 Scope of design
6.3.4 The pile stresses during platform operation The design of the foundation shall include the follow-
shall not exceed the allowable values according to
ing considerations:
Section 3, D.
– stability
For piles having large horizontal deflections a stress
increase due to second order effects shall be consid- – static deformation
ered.
– dynamic behaviour
6.3.5 For pile sections embedded in the soil nor- – hydraulic instability
mally column buckling need not to be investigated.
– installation and removal
7. Grouted pile to structure connection
1.4 Design loads
7.1 Platform loads may be transferred to leg and The design foundation load shall not be greater than
sleeve piles by grouting the annulus. the bearing capacity for the relevant type of loading:
Shear keys may be added in order to improve the
capacity of the steel grout bond. QK
γ ⋅ FG ≤
γm
7.2 Grouted pile to structure connections may be
designed using parametric formulae, e.g. as provided FG = foundation load
by API RP2A. Due regard shall be given to the suit- QK = characteristic bearing capacity
ability of the formula for the geometric configuration
investigated. γ, γm = see remarks in D.3.1 and Table 7.3
IV - Part 6 Section 7 E Foundations Chapter 4
GL 2007 Page 7–7

Table 7.3 Global Safety Factor γ

Loading conditions acc. to Section 3, Table 3.1

2 3 4 On bottom stability
Permanent loads Operating loads Accidental loads (unpiled jacket)
Sliding 1,5 1,3 1,1 1,3
Bearing 2,0 1,5 1,25 1,5
Overturning See 2.2 1,2
Buoyancy 1,25 1,1 1,05 1,1

The values shown in the Table may be used as well if radius of 1st core area re = 0,25 r
cyclic loading effects have been taken into account.
They may have to be increased when geotechnical radius of 2nd core area re = 0,59 r
data are uncertain.
2.3 Bearing capacity and sliding resistance
2. Stability
2.3.1 Different methods will be applicable to check
2.1 Stability requirements the bearing capacity and sliding resistance of shallow
foundations.
The foundation system shall ensure the stability in
respect of overturning, bearing capacity and lateral Provided the limitations given in 2.3.4 are fulfilled,
sliding resistance. the equations given in 2.4 and 2.5 may be used. When
calculating the bearing capacity the effective founda-
For structures without skirts the contact stresses be- tion area according to 2.4.1 is to be used.
tween foundation and soil shall always be compres-
sive. 2.3.2 The bearing capacity and the sliding resis-
tance of the foundation system shall be evaluated
2.2 Overturning considering the following:
For the calculation of overturning moments the most a) the shape of the foundation base
unfavourable load combination and extreme position
of loads has to be accounted for. b) loads acting on the foundations and their varia-
tion in time
The dimensions of foundation surfaces have to be
chosen as follows: c) surface characteristics of sea bottom
The eccentricity e of the resultant of permanent loads d) geophysical characteristics of the soil layers
shall be within the 1st core area of the bottom as de- concerned
fined in Fig. 7.3 to avoid gaping.
e) possible rupture surfaces in the soil
For the total loads and most unfavourable load condi-
tions, the eccentricity e of the resultant has to be f) effective stress reduction due to cyclic loading
within the 2nd core area defined in Fig. 7.3. That
means a maximum gaping is allowed only up to the g) pore pressure variation corresponding to the
centre of gravity of the bottom area. actual stress level of the soil.

For rectangular foundation areas: 2.3.3 For the bearing capacity analysis either un-
drained or drained condition applies.
Proof that the resultant is within the 1st core area:
For rapid loading, where no drainage and consequently
ex ey 1 no dissipation of excess pore pressure occur, an
+ ≤
bx by 6 undrained analysis is to be performed. In this case the
internal friction of soil is considered to be zero, Φ' = 0°
Proof that the resultant is within the 2nd core area: The bearing capacity will depend on the undrained
2 shear strength c of the soil.
⎛ ex ⎞
2
⎛ ey ⎞ 1
⎜ ⎟ + ⎜⎜ ⎟⎟ ≤ Where, on the contrary, the rate of loading is slow
⎝ bx ⎠ ⎝ by ⎠ 9 enough, a complete drainage occurs and excess pore
pressures will not develop. In this condition the bear-
For circular foundation areas with radius r: ing capacity of the foundation is determined by the
Chapter 4 Section 7 E Foundations IV - Part 6
Page 7–8 GL 2007

Boundary of 1st core area ex Boundary of 2nd core area

ey

by
by
6
by
6
bx bx
6 6
bx

Fig. 7.3 Permissible eccentricity of permanent and total loads

drained shear strength of the soil, which may be de- 2.4 Bearing capacity
termined from Mohr's failure envelope, i.e.
2.4.1 Bearing capacity, undrained condition
s = c + σ' ⋅ tan Φ'
The maximum bearing capacity QV is given by the
c' = effective soil cohesion following formula:
σ' = effective normal stress QV = (cu ⋅ NC ⋅ KC + γ ⋅ d) ⋅ A'
Φ = effective angle of internal friction
cu = undrained shear strength of the soil [kN/m2]
2.3.4 Limitations
NC = dimensionless factor
2.3.4.1 The formulae given under 2.4 and 2.5 may (NC = 5,14 for Φ' = 0, see Fig. 7.5)
only be used provided the following limitations are
observed: KC = correction factor which accounts for load
inclination, shape of footing, penetration
– the soil is homogeneous, isotropic and fully plastic depth, inclination of the foundation base and
– loading rates result in clearly drained or un- of the ground surface, see 2.4.3
drained conditions
γ = total unit weight of soil [kN/m3]
– the actual loading is close to the simple condi-
tions assumed in the stability formulae d = penetration depth of foundation [m]

– low torsional stress levels A' = effective area of the foundation, depending
on the load eccentricity, see Fig. 7.4 [m2]
– the foundation geometry is regular
B' = effective breadth of the foundation
2.3.4.2 It has to be observed that the effective soil
stresses may be reduced due to cyclic loading or hy- L' = effective length of the foundation
draulic gradients, induced for example by foundation In the rather frequently encountered cases of
rocking.
– vertical centric load
2.3.4.3 The sliding analysis according to 2.5 is only
applicable where suitably spaced skirts are provided in – horizontal foundation base
order to ensure a horizontal failure plane in the soil – horizontal sea bed
rather than a failure at the interface of the foundation
base and the soil. the above formula may be reduced as follows for cir-
cular or square footings:
2.3.4.4 Where the above conditions are not satisfied,
more conservative methods of analysis and / or in- QV0 = 6,17 ⋅ cu ⋅ A
creased safety factors shall be used, or more refined
techniques shall be adopted. A = actual foundation area [m2]
IV - Part 6 Section 7 E Foundations Chapter 4
GL 2007 Page 7–9

e 2.4.2 Bearing capacity, drained condition

M
Q Q The maximum bearing capacity is given by the fol-
lowing formula:

Q'V = (c' ⋅ NC ⋅ KC + q ⋅ Nq ⋅ Kq +
B Reduced area A'
0,5 ⋅ γ' ⋅ B ⋅ Nγ ⋅ Kγ) ⋅ A' [kN]
Y
c' = effective cohesion intercept of Mohr-Coulomb
failure envelope [kN/m2]
0 X NC = (Nq – 1) ⋅ cotan Φ ' see Fig. 7.5
L

e1

L'

e2
⎛ Φ'⎞
Nq = eπ.tanΦ' ⋅ tan2 ⎜ 45° + ⎟ see Fig. 7.5
⎝ 2 ⎠
B'
Nγ = 2 ⋅ (Nq + 1) ⋅ tan Φ' see Fig. 7.5
Rectangular footings: Φ' = effective friction angle of Mohr-Coulomb
A' = L' ⋅ B', L' = L – 2e1, B' = B – 2e2 failure envelope [°]
γ' = effective unit weight of soil (in water)
Y [kN/m3]
C q = effective overburden pressure [kN/m2]
= γ' ⋅ d
R

d = penetration depth of foundation [m]

B C 0' D X B = minimum lateral foundation dimension [m]
ey
A' = effective area of foundation, see 2.4.1 [m2]
KC , Kq , Kγ = correction factors, see 2.4.3

A 1000
800
Circular footings: 600
For a circular footing as shown above the effective 400
area A' is considered to be two times the circular seg-
ment ADC:
200
⎡ ⎤ Ng
⎛ ey ⎞
A' = 2 ⋅ ⎢ R 2 ⋅ cos −1 ⎜⎜ ⎟⎟ − e y ⋅ R − e y ⎥
2 2
100
⎢⎣ ⎝R ⎠ ⎥⎦
80
60
In addition, for further calculations, the effective area
may be considered to be rectangular with a length-to- 40
Nc, Nq, Ng

width ratio equal to the ratio of the line length AC to

BD . The effective dimensions are therefore: 20
Nc
Nq
A' = B' ⋅ L' 10
8
where:
6
0,5
⎛ R + ey ⎞ 4
L' = ⎜ A' ⋅ ⎟
⎜ R-e y ⎟
⎝ ⎠ 2
R-e y
B' = L' ⋅ 1
R + ey 0 10 20 30 40 50
f [°]

Fig. 7.4 Reduced footing area Fig. 7.5 Bearing capacity factors
Chapter 4 Section 7 E Foundations IV - Part 6
Page 7–10 GL 2007

For cohesionless soils and in case of centric load, B'

simplified formulae may be used for circular or square sq = 1 + ⋅ tan Φ '
L'
footing, e.g.:
B'
QV = 0,3 ⋅ γ' ⋅ B ⋅ Nγ ⋅ A [kN] sγ = 1 − 0, 4 ⋅
L'
B = breadth or diameter of the foundation [m] Nq see Fig. 7.5
A = actual foundation area [m2]
– Circular base with centric load:
2.4.3 Calculation of correction factors Nq
Correction factors KC , Kq and Kγ are usually written sc = 1 +
NC
as follows:
sq = 1 + tan Φ'
KC = ic ⋅ sc ⋅ dc ⋅ bc ⋅ gc
sγ = 0,6
Kq = iq ⋅ sq ⋅ dq ⋅ bq ⋅ gq
For a circular base with eccentric load, the formulae
Kγ = iγ ⋅ sγ ⋅ dγ ⋅ bγ ⋅ gγ for an equivalent rectangular base shall be used.
where i, s, d, b and g are individual correction factors
related to load inclination, foundation shape, penetra- 2.4.3.3 Depth factors d
tion, depth, base inclination and ground surface incli- 1 − dq
nation, respectively. dc = d q −
N C ⋅ tan Φ '
Their recommended values are:
d
2.4.3.1 Inclination factors i dq = 1 + 2 ⋅ tan Φ ' ⋅ (1 − sin Φ ')2 ⋅
B'
m
⎡ H ⎤ dγ = 1,0
iq = ⎢1 − ⎥ Φ' > 0
⎣ Q + B ' ⋅ L ' ⋅ c ⋅ cotan Φ ' ⎦
2.4.3.4 Base and ground surface inclination fac-
m +1
⎡ H ⎤ tors b and g
iγ = ⎢1 − ⎥ Φ' > 0
⎣ Q + B ' ⋅ L ' ⋅ c ⋅ cotan Φ ' ⎦ bq = b γ = (1 − α ⋅ tg Φ ') 2 Φ' > 0
1 − iq 1 − bq
ic = iq − Φ' > 0 bc = b q − Φ' > 0
N C ⋅ tan Φ ' N C ⋅ tg Φ '
m⋅H α
ic = 1 − Φ' = 0 bc = 1 − 2 ⋅ Φ' = 0
B ' ⋅ L ' ⋅ c ⋅ NC NC
H = horizontal load [kN]
gq = g γ = (1 − tan β)2 Φ' > 0
Q = vertical load
1 − gq
m = mL ⋅ cos2 δ + mB ⋅ sin2 δ gc = g q − Φ' > 0
N C ⋅ tg Φ '
δ = angle between longitudinal axis of the base
and force H β
gc = 1 − 2 ⋅ Φ' = 0
NC
2 + L '/ B'
mL =
1 + L '/ B ' where α and β are the base and ground inclination
angles, in radians, in respect to the horizontal plane.
2 + B '/ L '
mB =
1 + B '/ L ' 2.5 Sliding resistance
NC = see Fig. 7.5 The maximum sliding resistance is given by the fol-
lowing formulae.
2.4.3.2 Shape factors s – Undrained condition:
– Rectangular base: QH = cu ⋅ A [kN]

B ' Nq – Drained condition:

sc = 1 + ⋅
L ' NC Q'H = c' ⋅ A + Q ⋅ tan Φ' [kN]
IV - Part 6 Section 7 E Foundations Chapter 4
GL 2007 Page 7–11

A = actual foundation area 4. Dynamic behaviour

Q = vertical load 4.1 Dynamic loads

2.6 Soil reaction on foundation structure Dynamic loads due to waves, earthquake, etc. may
significantly influence the integrity of the foundation.
Grouting of voids beneath the foundation base may be Their effect on the foundation behaviour has to be
required in order to ensure the predetermined load thoroughly evaluated.
distribution and sufficient bearing capacity. Materials
and methods used for filling of voids have to be 4.2 Low stress level
agreed upon with GL, see also Section 5.
In many cases, especially when the stress level is
rather low, the dynamic foundation behaviour may be
3. Static deformations investigated using the continuous "half space" ap-
proach which assumes the soil to be a homogeneous,
3.1 Maximum foundation deformation linearly elastic material.
The maximum foundation deformation has to be in- 4.3 Non-uniform soil profiles
vestigated with special regard to the structural integ-
rity and possible effects on components attached to the In case of non-uniform soil profiles with the risk of
structure or embedded in the soil, e.g. risers, conduc- energy reflection at the interfaces of soil layers, or of
tors. dynamic loads with large amplitudes which cause non-
linear soil behaviour, more appropriate analyses are
3.2 Short time deformations for circular foun- required which are to be agreed by GL.
dation
5. Hydraulic instability
For the condition where the foundation of the structure
base is circular, rigid, subject to static loads or to loads 5.1 Scour
which may be considered as static, and rests on iso-
tropic and homogeneous soil, the short time Depending on current conditions (influence of the
(undrained) deformations may be evaluated by the geometry of the structure) and soil conditions, meas-
following formulae: ures shall be taken to prevent scouring and wash-out
phenomena, e.g. with the help of scour skirts, concrete
1− ν mats, rock riprap or other suitable means.
Vertical: uv = Q ⋅
4⋅G⋅R
5.2 Piping
7 −8⋅ν The foundation design shall take account of possible
Horizontal: uh = H ⋅
32 ⋅ (1 − ν ) ⋅ G ⋅ R excessive hydraulic gradients in the soil with conse-
quent formation of piping channels and a reduction of
1− ν bearing capacity, see also 2.3.4.2.
Rocking: δr = 3 ⋅ M ⋅
8 ⋅ G ⋅ R3
6. Installation and removal of gravity foun-
Mt dations
Torsion: δt = 3 ⋅
16 ⋅ G ⋅ R 3 6.1 Installation
uv, uh = vertical and horizontal deformations [m] 6.1.1 Careful planning is necessary in order to
ensure a proper installation of the foundation base.
Q, H = vertical and horizontal loads [kN]
When scour skirts and other installation aids (dowels,
δr , δt = overturning and torsional rotations [rad] etc.) are required to penetrate into the sea bottom, a
penetration analysis has to be performed using the soil
M, Mt = bending and torsional moments [kN ⋅ m] characteristics of the relevant soil layers.
The requirements on ballasting facilities have to be
G = elastic shear modulus of the soil [kN/m2] investigated in order to ensure a well balanced seating
ν = Poisson's ratio of the soil of the foundation base without excessive disturbance
of the supporting soil.
R = radius of the base [m]
6.1.2 The resistance to penetration R [kN] of scour
skirts and non-plugged dowels is given by their end
3.3 Square foundations resistance and skin friction resistance and may be
These equations may also be used to estimate the calculated by the following formula:
response of square foundation shapes with equivalent
area. R = Kp(d) ⋅ Ap ⋅ qc(d) + AS ⋅ 0∫d Kf(z) ⋅ qC(z) ⋅ dz [kN]
Chapter 4 Section 7 E Foundations IV - Part 6
Page 7–12 GL 2007

z = depth of the soil layer under consideration Table 7.4 Coefficients for penetration resistance
[m]
Type of End resistance Skin friction
d = penetration depth [m]
soil coefficient Kp coefficient Kf
Kp = empirical coefficient relating to end resis- Clay 0,6 0,05
tance
Sand 0,6 0,003
Kf = empirical coefficient relating to skin friction
qC = cone penetration resistance [kN/m2] 6.2 Removal

Ap = end area of scour skirt [m2] 6.2.1 It should be clarified between Owner/Opera-
tor, Designer and the relevant Administration whether,
AS = skin area of scour skirt per unit penetration and to what extent, a removal of the structure will be
depth [m2/m] required.
For KP and Kf the coefficients shown in Table 7.4 6.2.2 If a removal is planned, investigations are to
may be taken to calculate an upper limit of penetration be carried out regarding, e.g.,
resistance.
– method and phases of removal
For penetration depths lower than 1 to 1,5 m, the val-
ues shown in Table 7.4 may be reduced by 25 to 50 % – environmental conditions
due to local piping or lateral movements of the plat- – risks involved
form.
– necessary equipment to be provided during
When friction reducers both at the inside and outside operations
skin of dowels are used, the coefficient Kf for sand
may be reduced. – structural arrangements and mechanical devices
(pipes, fittings, etc.) to be provided already dur-
6.1.3 The end resistance of plugged dowels may be ing the construction phase, and measures re-
calculated in the same way as for plugged open ended quired to ensure that removal operations will be
piles, see D.3.3. possible at the expected time