CLIENT: Norwegian Cruise Line
SERVICES RENDERED: geotechnical modeling with interpretation of dilatometric tests and calculation of the bearing capacity of square section foundation piles. Foundation study of the lighthouse structure with Boundary Element Method (BEM) approach and Repute software.


Twenty-six DMT dilatometric tests were performed in the area under investigation. All investigations were interpreted and 7 different geotechnical models were defined to describe the discriminated areas in terms of homogeneity of geotechnical conditions.

The following plan of the area under investigation shows the DMT tests performed and identifies the different geotechnical models.

1. Type of pile
Prefabricated precast reinforced concrete piles with a square section and 14″ (approx 355 mm) side. These poles will be installed by beating.

The project under consideration was executed by referring entirely to American units of measurement.

2. Geotechnical Modeling
The interpretation of the 5 homogeneous DMT dilatometric tests that are part of the geotechnical model A is shown.

Lithology comparison
Undrained resistance
Oedometric modulus

3. Lighthouse foundation project
The layout of the foundation of the lighthouse, provided by its designer, consists of 32 beaten piles, divided into groups of 8, arranged symmetrically with respect to the axes.

A study has been carried out with the Repute program of the Geocentrix company, software that allows to take into account the real stratigraphy of the intervention site, the non-linear behavior of the ground, avoiding stress concentrations in the most centrifugal piles of the group. This program also allows to take into account phenomena, such as shadowing, typical of the interaction between the piles of the group. This not only produces greater accuracy, but also allows the effects of soil-structure interaction to be considered.
The Repute software is a BEM-type calculation code that provides for the discretization only of the interface (hence their name) between piles and soil.
The nonlinear solution allows to:

  • Obtain the most realistic constitutive model of the soil;
  • Consider the plasticization of the single piles of the group: when the most loaded one has reached its maximum axial load, it is no longer able to increase its load and therefore the stresses are redistributed to the other piles of the group, as can be expected in reality;
  • Getting the maximum load on the most loaded pile more accurate.
    The piles in Repute were modeled as equivalent circular beaten piles.

For the pile connection slab, an equivalent weight slab was considered because it is not possible to define a geometry as provided by the designers.

Modello in Repute

The foundation of the lighthouse initially subjected only to a centered load of vertical compression (the only input received in this sense by the designers of the lighthouse) generates an axial load on the piles not uniformly distributed but variable due to the effects of interaction between them. This fact cannot be represented unless a geotechnical calculation method is used, i.e. one that takes into account the mechanical properties of the soil, such as the one presented here.

The vertical distribution of the axial load acting on the piles is shown as.

Maximum actions acting in the piles of a quarter of the Foundation

The load-bearing capacity of the piles as they vary in length was evaluated for the lighthouse area (geotechnical model A).
This is because a stiff substrate, at depths from ground level varying between 10.0m and 17.0m, is found to prevent pile driving.

The 12.0m long pile is almost totally exploited only for the vertical loads transmitted by the lighthouse.
The maximum bending moment borne by the pile group was then researched, obtaining a value of 1491 kNm. This is provided that attention is paid to the layout of the foundation.
If the foundation is built where it is only possible to place piles of the minimum length (12.0m), the resistance of the foundation must be increased by providing a greater number of piles.
A possible solution involving additional piles is shown in the figure. The additional piles, shown in green, have a greater centrifugal force and are therefore able to absorb a greater proportion of the bending action.

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