CLIENT: Agricultural Cooperative C.A.P.A. Cologna
LOCATION OF INTERVENTION: Municipality of Berra (FE), via Fossa Lavezzola, 83
SERVICES RENDERED: project as mixed foundation (piled raft foundation) to optimize the number of foundation piles and to control the subsidence of the work

The foundations of five new silos for the storage of grain have been designed and realized inside the headquarters of the agricultural cooperative C.A.P.A. Cologna, located in via Fossa Lavezzola, 83 in the municipality of Berra (FE). The foreseen silos have a circular section with a diameter of 16.4 m, a height of 17.65 m and a height at the ridge of the conical cover equal to 22.65 m.
The bottom structure is also designed to house 6 trusses to support the upper walkway of the silos and an end truss for loading and unloading grain.

A view of the elevated structure of the silos is also shown.

The foundations were calculated according to non-traditional criteria, now also covered by the technical standards NTC2008; in particular reference was made to mixed foundations, or as it is more correct to say, piled raft foundations [PRF].
Considering a piled raft it is possible to optimize the design of the foundation in terms of resistance (the bearing capacity of the complex of piles plus slab is adequate to transfer to the ground the load on the structure in elevation), that of functionality (the arrangement, number and geometry of the piles are optimized to obtain the desired/prescribed performance in terms of both absolute and differential settlement) and cost (you get a significant reduction in the number and/or length of the piles considering in the design of the foundation the contribution offered by the slab, in reality always present but constantly neglected).
It is clear that, on the other hand, a greater geotechnical screening of the soil and a geotechnical analysis and modeling capacity of adequate level and great depth is necessary.

The foundation analysis criteria that have been adopted are:
1) PDR (Poulos, Davis & Randolph) analytical method.
2) Finite element calculation using Plaxis 2D software.
The Teleios study has developed an automatic calculation process that traces the PDR method, which can be seen as an evolution of it as it considers the non-linearity of the behavior of the piles (Fiorelli & Franceschini, 2014).
Note in the following figure how, compared to the tri-linear constitutive bond of the bibliographic PDR, its refinement contemplates, in the range of interaction between piles and slab, a non-linear response of the piled raft.

The slab that houses the 5 new grain storage silos has a rectangular plan and planimetric dimensions of 18.40 x 94.0 m approximately.

The section of the slab is instead studied in a peculiar way.
There is a portion of bedplate, with a thickness of 80.0 cm, having a purely structural function of connection of the foundation piles, which is reinforced to withstand the stresses generated by the latter and that does not accommodate plants or holes of any kind so that the resistant section is not compromised. Above is a portion of the slab, suitably reinforced, dedicated to systems (ventilation, discharge silage, etc.).

In the project have been used prefabricated piles with longitudinal section cylinder-tronchoconic (CTC). The cylindrical section has a diameter of 500 mm while the truncated cone section, 12.0 m long, will have a diameter varying between 500 mm at the top and 320 mm at the tip.
In particular, for the foundations of the 5 silos, CTC driven piles of three different lengths have been adopted:

  • Poles Length 20.0 m CTC2000/32-50-50 1 silo
  • Piles Length 22.0 m CTC2200/32-50-50 1 silo
  • Poles Length 25.0 m CTC2500/32-50-50 3 silos

The length was calibrated according to the stratigraphic variability found, as shown in the image that following.

1. Modeling
The subsidence was calculated, for all possible loading conditions, with the Plaxis code using the Hardening Soil constitutive model.

Calculation model

Excavation for the realization of rafts

Realizzazione pali

Poles realization

2. Load conditions
All possible load conditions were calculated. For example:

1 central silo

5 silos

3. Comparison of results
A comparison was made in terms of the failure of the mixed foundation of a silo using the PDR method and the Plaxis FEM program.
The following deformed configuration was obtained from the FEM calculation:

With PDR it was obtained:

  • Calculation in LINEAR regime of the pile-soil complex: δ = 46.4 mm
  • Calculation in NON-LINEAR regime of the pile-soil complex: δ = 34.0 mm

This last approach is the one developed within Teleios and consists in a non-linear development of the analytical method PDR (from the name of the authors, Poulos, Davis & Randolph).
We report the comparison of the above results in graphical form so that it is clearer.

The order of magnitude of failure is optimally found by the PDR analytical method.

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