5. Monopile Penetration Depth

Rules of Thumb & Thinks to Remember

5.1. Penetration depth

The penetration depth must be sufficient for the transfer of the ULS Moment, Shear, and Axial to the soil without:
(1) without exceeding the soil
(2) exceeding the allowable deflection

Allowable deflection limits:
Maximum deflection at pile toe: ≤ 0.02m
Maximum deflection at seabed: ~ 0.03·Dpile=0.19 m
– Rotation at seabed: ≤ 0.1°

The check is done of the M, V, N as calculated at seabed level (section D). The required penetration depth for both factor sets (a) and (b) is calculated. The design load are:
– Factor Set (A): M=222 MNm, V=6.4 MN, N=15.7 MN
– Factor Set (B): M=300 MNm, V=8.6 MN, N=12.8 MN

First, we calculate the penetration depth to resist the Moment and Shear. Then we check if the depth is enough to resist the axial load and if need we further increase.

We used the tools below to calculate the nonlinear response of the pile for the design loads at seabed level. The input is the soil characteristics (su, γ, φ, ε). The results are the displacements of the pile. We evaluate them against the criteria shown in Table 23 and if the displacements are higher, we increase the penetration depth or the pile diameter. We assume that scour protection is used so the total pile length contributes to the bearing capacity. The required penetration depth was found to be 40 m.

Figure: Pile displacement. Horizontal Load plus Moment

Regarding the axial capacity, it is easy to show that the penetration depth of 40m is sufficient. Following the API (2000) Main Text Method for predicting the ultimately unit shaft resistance in sand we derive the result shown in Table 24. We assumed that the soil is uniform (silty-sand) with an ultimate unit shaft resistance equal to the average value obtained from the soil investigation. This assumption is logical and since the soil properties over the range of penetration depth considered here do not vary much.

From the soil investigation data, we can see that the site can be divided in three areas with regard to the ultimate unit shaft resistance of the soil.

Areaτsf,ult  Range [MPa]τsf,ult  , average of top 50 m [MPa]
B6 – 1510

Unplugged failure is assumed and considered only the shaft resistance of the pile

For the three soil types mentioned above the axial capacity of the pile is calculated.

Penetration depth = 40 m

τsfτsf,limit [MPa]Vult [MN]

Result: The external shaft resistance is in all cases higher than the vertical design load at the foundation. In area Β, although the capacity is sufficient, an increase in penetration depth might be desirable in order to increase the factor of safety.

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