This paper presents the experience obtained with large closed-ended piles in Campos Basin, offshore Brazil.
Nonconventional soluctions were tested and closed-ended concept proved itself feasible.
Finite element analyses was used to verify the optimum cone geometry tip and the compacted soil mass zone due to pile group driving.
The installation of 72 piles with 66" & 80" o.d. showed this as one of the most recommended soluctions for pile foundations in similar carbonatic origin soils, without very high bearing capacity superficial layers.
Campos Basin Northeast Pole has an oil production system consisting of 7 fixed four-legged steel jackets, located on Pargo, Carapeba and Vermelho sites (fig. 1), where water depth varies from 80 to 101 m.
Geotechnical profile is composed of an upper layer of silty quartzous fine to medium sand, with a thickness in the range of 13 to 28 m, according to the location. A biodetritic calcareous sand stratum, with localized fragments of valves and coral, from non-cemented to highly cemented, underlay the upper sheet, down to 97 to 115 m depth (fig. 2).
The foundations basic design consisted of eight driven steel opened-ended pipe piles - four main piles 66" o.d. and four skirt piles 80" o.d. - with approximately 70 m penetration.
While the engineering phase went on, the stability of the foundation was questioned, specially in what concerned the high values of unit skin friction estimated for the carbonatic origin stratum. Such values were based in Cone Penetration Test (CPT) and in laboratory test results using molded or undisturbed samples. In order to obtain reliable parameters, static pull-out tests were made, using 20" o.d. piles in segments of 15 m deep layers located on Carapeba-1 site. The piles were driven, and after one month set-up pullout tests took place. A reduction factor of 30% was applied to the test results, in order to consider the ciclic degradation of the soil shear strength. The interpreted limit design parameters were:
18 KN/m2 for the layer between 19 and 34 m;
17 KN/m2 for the layer between 35 and 50 m, and
32 KN/m2 for the layer between 57 and 72 m (This highest value is due to the silicious content).
In the design review, four new skirt piles were added for each foundation making-up the final configuration shown in fig. 3. Penetrations were also modified, to 67 - 118 m, with each pile deadweight averaging up to 22% of the maximum axial load for extreme design condition.
To reduce foundation costs, another test campaign was initiated utilizing four non conventional offshore solutions. The primary scope of these tests was not only to ascertain the solution bearing capacity but also its feasibility.
From the four tested solutions, only one - a closed-ended pipe pile - proved feasible, during the test. The others required more technical development before practical use.
