GEOTECHNICAL AND FOUNDATION ANALYSIS FOR 80MT HIGH WIND TOWERS AT THE HUB IN SAN MARTINO IN PENSILIS
CLIENT: Sorgenia S.p.a, Via del Pozzo, 10 – S. Giuseppe di Comacchio (FE)
LOCATION OF INTERVENTION: San Martino in Pensilis (CB)
SERVICES RENDERED: survey
BEGINNING/END OF WORKS: 2009
CONTRACTOR: Georeflex S.r.l, Via Fioruzzi 15 – Piacenza
The work in question was carried out to evaluate, on the basis of geotechnical analysis in situ and in the laboratory, the types of foundations that can be used in the executive and constructive design for the foundations of n.6 wind towers, of an anemometer and for an SSE. The wind towers are of the type: WGT REPOWER MM92 – Blades LM/RE – Hub height 80 m – IEC class 2. The survey campaign was carried out by Georeflex of Piacenza; each stake included: geophysical analysis down hole type, survey with laboratory tests, CPT.
With the help of these tests, for each picket examined, a stratigraphy was constructed that was completely adequate for the calculation of the bearing capacity of the surface foundations or the axial and transverse bearing capacity for foundation piles. The dimensions of the foundations assumed for the verification are 17 m x 17 m and the laying plane of the foundation at 3 m from ground level; these dimensions respect the EQU verification provided in NT2008.
The calculation of the load-bearing capacity has been carried out in the static field both with the approach 1 and with the approach 2 foreseen in the NT2008; for the seismic field the calculation criterion of Maugeri exposed in the document: “M.Maugeri – D. Novelty: “Numerical model for the evalutation of the soil inertia effects on bearing capacity- Proc. Int. Conf. On Soil Dynamics and Earthquake Eng. 2004” and also explicated in the document: Franceschini – Carbonella “Comparison between the various methods of calculation of the bearing capacity of superficial foundations in sandy soils in seismic zone”. – INARCOS n.666 – January/February 2006.
For the evaluation of the settlements of the foundations, given the extreme sensitivity of these structures to differential settlements and for the determinant character on the choice of the typology of the foundations, it has been opted for an analysis to finite elements with the program Plaxis vers. 9.0. that allows a high reliability of the results.
The FEM analysis allows a higher degree of accuracy especially in problems where we must compare with the analysis by phases of excavation and as a function of time.
The problem posed by the general designer is that of the evaluation of settlements in the following situations:
1. Applied vertical loads due to the weight of the plinth and tower / 2. Applied vertical permanent loads due to the off-axis of the tower.
The stratigraphic soil model has dimensions 47 m x 30 m depth. In order to perform the analysis, three variants of the main model were analyzed:
A. Model with the soil and wind tower structure simulated as an infinitely rigid pillar in order to properly place the eccentric load required by the
general design and understand the heeling of the same.
B. Model with the soil and with the eccentric load evaluated by model A; this modeling allows to perform the consolidation analysis with the real plate and the evaluation of the displacements and the inclination of the foundation through an analysis that also allows to evaluate the time of exhaustion of the failures themselves.
C. Model with the soil and with an eccentric load evaluated by A, which allows to perform a time-independent plastic analysis for the evaluation of displacements at infinite time. This model is controlling with respect to the results obtained in B.
Excavation phase for consolidation analysis:
Excavation (plastic analisys) / Foundation laying and filling (consolidation 30 day) / Tower laying (consolidation 30 day) / Tower operation and off-axis (plastic
analisys) / Minumum pore pressure up to 1 kP.
The work also includes foundation hypothesis on drilled piles / Notes on soil-structure interaction analysis / Pile group analysis in non-linear field.