1. ICGI Wollongong 2012 (30 October – 2 November, 2012)
GEOTECHNICAL CHARACTERISTICS AND DESIGN RESTRICTIONS OF TAVIRA (PORTUGAL) FISHING PORT: SOIL IMPROVEMENT FOR VERTICAL QUAYS
C. Rocha1, A.P.F. Silva2 and A. Santos-Ferreira3
1Graduate student Faculty of Sciences and Technology (FCT), Univ. NOVA Lisboa, Caparica, Portugal;
2CICEGe/Dept. Ciências da Terra, Faculty of Sciences and Technology (FCT), Univ. NOVA Lisboa, Caparica, Portugal;
3IPTM, IP – Instituto Portuário e dos Transportes Marítimos, Lisboa, Portugal;
INTRODUCTION
Designed to be implemented on the right bank of the Gilão River, city of Tavira, south of Portugal (Figure 1), the project of the new fishing port has undergone some changes. This included, among other maritime infrastructures, two rubble mound breakwater at the entrance of the harbor basin.
These structures are relatively flexible and transmit small loads to the foundation. The breakwaters would enter the river bed, reducing, although slightly, the flow rate and a fuel refilling post would be located outside the west breakwater of the river increasing environmental risks associated with a potential fuel spillage. So, it was proposed, to replace the breakwaters for a vertical quay and to place the fuel refilling post within the harbor basin. As these structures are more rigid than the previous ones, it was necessary to complement the existing geotechnical studies to account for the deformability and strength of existing foundation layers.
To improve the soft soil characteristics, several solutions were evaluated, namely concrete piles, pre-fabricated vertical drains (PVD) and stone columns; the chosen method was the use of stone columns..
GEOTECHNICAL MODELING
The overall geotechnical behavior of the foundation of the vertical concrete wall was assessed by both the design team and the authors (Table 3), although the values defined were slightly different (Table 4), since they considered 20 kN/m3 and 23 kN/m3, respectively, for the concrete bulk density. As the earthfill layer will be completely dredged, it was not considered by both analyses.
Both teams initially calculated the overload induced by the structure and the correspondent settlements (Table 4). Afterwards, they evaluated the behavior of the foundation soft soils reinforced with stone columns under the quay walls and surrounding area.
The design team considered two triangular grids for the stone columns; the authors considered the same grid. Both the design team and the authors applied the Priebe (1995) method; the authors also used the FEM for a stress – deformation analysis (Table 4, Figures 3 and 4). In Table 4, the amount of settlement after finishing the construction of the vertical quay on top of the stone columns was assessed, considering that an average of 50% of the total one should have been reached.
This paper introduces the projected solution, in stone columns and briefly discusses the assumptions made by the design team for the geotechnical parameters and also for the estimated final settlements related only with the eastern quay, and compares them with the reappraisal made by the authors.
Figure 1. Geotechnical survey of Tavira new fishing harbor. Initial geotechnical elements of the harbor area and the latest site investigation results
The estimated settlements and their evolution in time are discussed, as well as the observational system to be implemented for their control and validation of design assumptions.
GEOTECHNICAL CHARACTERIZATION
Geotechnical investigation encompassing borehole, field and laboratory tests was made in both study phases in order to identify and characterize the different layers of the foundation below a superficial heterogeneous earthfill, detecting a set of organic beds of clayey sands, sandy clays and soft soils. In the overall, 9 trial pits, 9 percussion borings and 9 light dynamic penetrometer tests (DPL) were performed in 1997 and additional 5 boreholes, 10 unaltered soil samples, 71 Standard Penetration Tests (SPT), 6 Vane Shear Tests (VST) were conducted in To obtain the index and mechanical soil properties, the undisturbed samples were submitted to lab tests, which include triaxial tests (CU).
Table 1 presents the geotechnical zoning for the eastern quay and the main characteristics of each zone as defined by the authors; Table 2 includes the same data in accordance with the design team. Figure 2 presents the geotechnical zoning for this case study according with the authors.
Figure 3. FEM total settlements for the structure before treatment.
Figure 4. FEM total settlements after soft soils reinforcement with stone columns.
DISCUSSION OF THE RESULTS
Although both analyses obtained similar results for the settlements evaluation (Table 4), one must emphasize that the design team made several assumptions which clearly underestimated the final settlements as they began to misjudge the overload induced by the quay construction on the soft soils. They also overestimated the soft soils thickness as well as the value for their saturated bulk density and Young modulus. Fortunately, all the mistakes balanced between each other.
The FEM analysis obtained a total amount of 12cm for the maximum settlement after soft soil reinforcement, and since part of it should occur during construction, it is an acceptable value for the rigid vertical quay. This stress deformation analysis emphasizes the role played by the stone columns – they transfer the surcharge to the deep, frictional, more resistant geotechnical unit (GU4), Figure 4. Santos-Ferreira and Santos (2011), have demonstrated that the structure is safe, both under static and seismic conditions, and the probability of failure is almost null.
Both set of data were used in the geotechnical modeling. Nevertheless, one must emphasis some differences on the base data used by the design team and the authors, namely (Tables 1 and 2):
bulk density is overestimated in 3 kN/m3;
Young modulus is overestimated 60 times.
Figure 2. Profile of the geotechnical zoning adopted by the authors for Tavira eastern quay.
FINAL REMARKS
The authors have undertaken a reaprecciation of the design solution presented by a firm to the new eastern quay. The new analyses was launched because anomalies were detected by the Owner in the assumptions made for the geotechnical modeling developed to assess the settlements induced by the constructions of the vertical quays on top of thick organic soft soils. In fact, the authors analyses shows that the design had both safe and unsafe considerations and assumptions but, fortunately, the final settlements estimated before and after soft soil reinforcement are of the same order. Nevertheless, during construction phase, quality control of the project assumptions and estimations shall be implemented, namely during constructions of:
the stone columns - the validation of the thickness of the relevant geotechnical units (GU2 and GU3), should be verified;
the vertical quay – daily topographical survey marks should be motorized to verify the settlements evolution, as well as during the week after finishing the quay.