Platform loads may be transferred to steel piles by grouting the annulus between the jacket leg (or sleeve) and the pile. The load is transferred to the pile from the structure across the grout. Experimental work indicates that the mechanism of load transfer is a combination of bond and confinement friction between the grout and the steel surfaces and the bearing of the grout against mechanical aids such as shear keys.
Centralizers should be used to maintain a uniform annulus or space between the pile and the surrounding structure. A minimum annulus width of 38 mm (1 1/2 in.) should be provided where grout is the only means of load transfer. Adequate clearance between pile and sleeve should be provided, taking into account the shear keys’ outstand dimension, h. Packers should be used as necessary to confine the grout. Proper means for the introduction of grout into the annulus should be provided so that the possibility of dilution of the grout or formation of voids in the grout will be minimized. The use of wipers or other means of minimizing mud intrusion into the spaces to be occupied by piles should be considered at sites having soft mud bottoms.
10.4.2 Factors Affecting the Connection Strength
Many factors affect the strength of a grouted connection. These include, but are not limited to, the unconfined compressive strength of the grout; size and spacing of the shear keys; type of admixture;
method of placing grout; condition of the steel surfaces, presence of surface materials that would prevent bonding of grout to steel; and the amount of disturbance from platform movement while the grout is setting. For high D/t ratios the hoop flexibility of the sleeve and the pile is also known to be a factor.
10.4.3 Computation of Applied Axial Force
In computing the axial force applied to a grouted pile-to-structure connection, due account should be taken of the distribution of overall structural loads among various piles in a group or cluster. The design load for the connection should be the highest computed load with due consideration given to the range of axial pile and in situ soil stiffness.
10.4.4 Computation of Allowable Axial Force 10.4.4.1 General
In the absence of reliable comprehensive data that would support the use of other values of connection strength, the allowable axial load transfer should be taken as the smaller value (pile or sleeve) of the force calculated by a multiplication of the contact area between the grout and steel surfaces and the allowable axial load transfer stress fba, where fba is computed by the appropriate value in Equations (10.1) or (10.2) for the grout/steel interface. This allowable axial force should be greater than or equal to the applied axial force computed according to 10.4.3.
10.4.4.2 Plain Pipe Connections
The value of the allowable axial load transfer stress, fba, should be taken as 138 KPa (20 psi) for Loading Conditions 1 and 2 and 184 KPa (26.7 psi) for Loading Conditions 3 and 4 (see 5.2.2).
10.4.4.3 Shear Key Connections
Where shear keys are used at the interface between steel and grout, the value of the nominal allowable axial load transfer stress, fba, for Loading Conditions 1 and 2 may be calculated as:
In SI units:
f . f h
= + ×s
ba 138 KPa 0 5 cu (10.1)
In USC units:
f . f h
= + ×s
ba 20 psi 0 5 cu
For Loading Conditions 3 and 4, fba may be calculated as:
In SI units:
f . f h
= + ×s
ba 184 KPa 0 67 cu (10.2)
In USC units:
ba 26 7 psi 0 67 cu h
f . . f
= + ×s
where
fcu is the unconfined grout compressive strength, in MPa (psi) in accordance with 11.4.1;
h is the shear key outstand dimension, in mm (in.) (see Figure 10.1 and Figure 10.2);
s is the shear key spacing, in mm (in.) (see Figure 10.1 and Figure 10.2).
Shear keys designed according to Equations (10.1) and (10.2) shall be detailed in accordance with the following requirements.
a) Shear keys may be circular hoops at spacing “s” or a continuous helix with a pitch of “s.” See 10.4.4.4 for limitations.
b) Shear keys should be one of the types indicated in Figure 10.2.
c) For driven piles, shear keys on the pile should be applied to sufficient length to ensure that, after driving, the length of the pile in contact with the grout has the required number of shear keys.
d) Each shear key cross section and weld should be designed to transmit that part of the connection capacity that is attributable to the shear key for Loading Conditions 1 and 2 in 5.2.2. The shear key and weld should be designed at basic allowable steel and weld stresses to transmit an average force equal to the shear key bearing area multiplied by 1.7fcu, except for a distance of 2 pile diameters from the top and the bottom end of the connections where 2.5fcu should be used.
10.4.4.4 Limitations
The limitation 17 MPa (2,500 psi) ≤ fcu ≤ 110 MPa (16,000 psi) should be observed when designing a connection in accordance with 10.4.4.2 or 10.4.4.3.
The limitations in Table 10.1 should be observed when designing a connection according to 10.4.4.3 (see Figure 10.1 and Figure 10.2).
Figure 10.1—Grouted Pile-to-structure Connection with Shear Keys
Figure 10.2—Recommended Shear Key Details Table 10.1—Connection Design Limitations
Attribute Limitation
Sleeve geometry D
ts≤
s
80
Pile geometry D
tp ≤
p
40
Grout annulus geometry D
≤ tg≤
g
7 45
Shear key spacing ratio a . D
≤ sp≤
2 5 8
Shear key ratio h s≤0 10. Shear key shape factor . w .
≤ ≤h 1 5 3 0 Product of fcu and h/s ≤5.5 MPa (800 psi)
a For helical shear keys only.
10.4.4.5 Other Design Methods
Other methods, which are based on testing and verification, may be used for calculating the allowable load transfer stress fba. One such method is described in B.10.4.4.5.
10.4.5 Loadings Other Than Axial Load
Grouted pile to sleeve connections will be subjected to loading conditions other than axial load, such as transverse shear and bending moment or torque. The effect of such loadings, if significant, should be considered in the design of connections by appropriate analytical or testing procedures.