Offshore Foundation with Large Diameter Piles Anchored in Rock - Case of EEAB Jaguari

Danielle MELOa , Thomaz LEITEa , Habib JARROUGE NETO a,1, Izabel BASTOSa and Roberto KOCHENb aGeoCompany Technology, Engineering and Enviroment bProf. Dr. Escola Politécnica da Universidade de São Paulo 


Abstract. The objective of this article is to present the foundation’s solution adopted in the raw water intake structure named by EEAB (RWPS) Jaguari, an integral part of the interconnection between the Jaguari and Atibainha Dams. The structure is located within the reservoir of the Jaguari dam, so that the whole foundation was executed with the aid of boats, floating and professional divers, due to the complexity for its execution. The foundation was executed through large diameter piles, 4.20m and pins of 1.80m, excavated in rock. The foundation execution took place in order to guarantee the quality in the execution, assuring the performance of the work.


Keywords. Offshore foundation, large diameter piles, foundation in rock.  


1. Introduction 


In 2014, a water crisis began in the Metropolitan Region of São Paulo (RMSP), Brazil's main financial center, which occupies an area of around 8,000 km2 with 19 million citizens, leading to a critical water supply situation. The crisis was addressed through several measures, such as the implementation of rotation in water supply, intensification of the program to combat losses, bonus program for the reduction of consumption, execution of works aimed at capturing the technical reserve of the Cantareira System and water transfers from other producer systems. One of the large works carried out to increase the water security of the RMSP was the Interconnection of the basins of the Paraíba do Sul (Jaguari reservoir) and Cantareira (Atibainha reservoir). This work can be divided into 3 main elements, namely the Jaguari Raw Water Pump Station (EEAB-RWPS), EEAB (RWPS) Atibainha and the walking between such structures, which consisted of 13.3 km of pipeline and 6.1 km of tunnels. The pipeline system is designed to operate with two flow directions. In the JaguariAtibainha direction the operation takes place through forced duct, whereas in the Atibainha-Jaguari direction the adduction is by gravity, taking advantage of the favorable ?geometric difference. The adduction takes place through steel pipe with a diameter of 2200mm. The transport speed is 2.2m/s in the Jaguari-Atibainha direction and 3.2m/s in the reverse direction. The objective of this article is to present the solution adopted for the foundation of EEAB (RWPS) Jaguari, whose structure has external dimensions of 30.2 x 29.7 meters. The foundation was made completely submerged, inside the Jaguari dam reservoir, through the support of floating, hoisting and crimping hammers, as well as the constant support of a professional diver’s team to monitor the services. At the time of construction, the water slide at the structure site had a height relative to the bottom of the reservoir of approximately 20.00 m. Figure 1 shows a photo of EEAB (RWPS) after the construction stage, in which one can observe the location of the work and part of the support structure. The connection between the EEAB (RWPS) platform and the reservoir margin was carried out by a Service Bridge, 5.80m wide and 70m long, executed through a metallic structure. 


2. EEAB (RWPS) Jaguari 


The Jaguari Raw Water Pump Station is composed of 6 vertical motor pump sets of 5000cv of power, arranged in two parallel lines with 3 pumps each, and the tubing in the shape of a fishbone. In the center of these lines there is a pickup manifold (easel) with DN 2200mm that will receive the discharge of the 6 pumps. The set has the capacity to transpose an average flow of 5.13m³/s and a maximum flow of 8.5m³/s to a geometric difference of 176m in the Jaguari-Atibainha direction. The catchment wells have double functionality, act as a catchment structure, in which vertical pumps for water intake will be installed, and also serve as a load transmission element, from the structure to the foundation. The EEAB (RWPS) Jaguari structure is supported on two blocks and six catchment wells. Each of the blocks is supported by 4 piles with a diameter of 1.20m in the soil layer and of 1.00m in the rock layer. Under each catchment well the foundation was given through a large diameter station, 4.20m, anchored in sound rock through a pin of 1.80m in diameter. Figure 2 shows a structure perspective, in which it is possible to visualize the supports described. ?



3. Constructive Aspects

The construction of the wells and their foundations was made completely submerged inside the Jaguari dam reservoir through the support of floating, hoisting and crimping hammers, as well as the constant support of professional divers to monitor the services. The main float used had dimensions of 40 x 70m, the largest in operation in Brazil, with a hoisting of 220tf capacity operating on board. The main constructive difficulty was the option of executing the structure inside the reservoir, without the aid of a cofferdam. This constructive measure was taken to make it possible to meet the short term of construction, considering that the work was a response of the Government of the State of São Paulo to the water crisis that hit the State in recent years. Figure 3 shows a construction situation in which it is possible to visualize the difficulty inherent in the construction of an offshore foundation. The water slide at the site of the structure had a height relative to the bottom of the reservoir approximately 20 meters deep. Initially it was estimated that the water level would remain at 615.00m, however, during construction the level reached 620.00m, which increased the difficulty for the maneuvers required to carry out the work.  




 Each of the collection wells has the same configuration, as shown in Figure 4, in the typical cut of the foundation structure.  


In this section it is possible to observe that the structure consists of the following elements: foundation pin with a diameter of 1.80 m embedded in sound rock, continuation of the foundation through a pile with a diameter of 4.20 m concreted through the support of a metallic casing and well with external diameter of 3.80 m and internal diameter of 3.00 m which was also concreted using metallic casings.  

For the construction of the foundations, the first step was the driving of a 4.20m diameter metallic casing until the rocky top was found. This procedure found difficulty in the performing, since the rocky top is variable, and the metallic casing had to remain undisturbed and in the plumb. To ensure performing as designed, strict quality control was required during the driving of these casings. The driving was done through a hydraulic impact hammer with a helmet on the casing to distribute the energy, specially designed for this work. Figure 5 shows this step of the construction.  

After the driving was performed the excavation in soil and rock with Wirth type drill with diameter of 4.20m up to the quota specified in the project. The drill length with diameter of 4.20m for wells 1 to 6 was 13.08m, 6.1m, 10.3m, 13.56m, 12.85m and 9.54m, respectively, the mean length was 10.91m. 


With the conclusion of the section found in soil and in a modified rocky massif, the excavation of the foundation pins began, with a diameter of 1.80 m, excavated through a Wirth type drill with cleaning through reverse circulation. The foundation pin length for wells 1 to 6 was 10m, 10m, 10m, 4m, 5.15m and 10m, respectively, with an average length of 8.2m.  

After excavation and cleaning of the pin, the lowering of the frame cage was started. The frame of the pins was assembled together with the frame of the 4.20m diameter stations, forming a chalice-shaped structure, as shown in Figure 6. After the positioning of the frame was made the concreting in steps, which was performed submerged through tremie tube. The concreting was carried out in several stages with strict control of concrete technology in order to maintain the specifications of concrete strength and durability. The average concrete consumption per support was 225m³, considering the foundation pin at the beginning of the catchment well wall.  


After the concreting of the foundation, the catchment wells were started up. The catchment well structure is composed of two metallic casings, the inner casing has a diameter of 3.00m and the outer one has a diameter of 3.80m, so that the wall thickness of the well is 40cm. 

The performing sequence of the catchment well was as follows: descent of the metal casing of 3.80m in diameter, descent of the well wall frame and concreting of the inner region, descent of a 3.0m metal casing and concreting of the catchment well wall. 

The wall of the wells was 40cm thick, containing openings, where gates were installed to allow the water to be captured. The positioning of the openings in the metal casings had to have millimetric precision.  

Figure 7 shows the executive sequence of the construction of each of the catchment wells. The performing of the foundation was accomplished in approximately 6 months, term to meet the project requirements. 


4. Heading 

To define the structure foundation, a drilling investigation campaign was carried out with a mixed drilling on the axis of each of the EEAB (RWPS) catchment wells, and two more mixed boreholes in the area around them. Figure 8 shows the distribution of the foundation elements and the location of the mixed surveys conducted to characterize the subsoil. 

D. Melo et al. / Offshore Foundation with Large Diameter Piles Anchored in Rock


Based on the analysis of the bulletins obtained in the surveys, it is observed that the geological profile is composed of an alluvium layer (thin to thick sand, with fine to medium gravel, with stretches of silty clay, yellowish brown to variegated gray) on a residual soil layer (silt little sandy, with mica, dark brown to yellowish brown) on rocky massif (granite-gnaisse milonitized belonging to the intrusive granitoid Santa Isabel (neoproterozoic), presenting a characteristic of sound rock to little alteration, with slat stretches, with quartz porfiroblasts and k-feldspar, foliation inclined, subvertical and incipient, granulepideblastic, phaneritic thin to medium, with very modified stretches, dark gray and white). Figure 9 shows the typical longitudinal profile obtained.  



The depth definition of the rocky massif and sound rock was performed according to the analysis of the bulletins and the testimonials of the mixed surveys conducted. These measurements were used to determine the inlay length of the piles, as shown below. As the foundation pins will be supported by rock, they have varying dimensions. Figure 10 is a three-dimensional representation of the geology found at the site of the catchment wells, indicating the extent of the water depth, soil layer, modified rock, and sound rock. 


5. EEAB (RWPS) Jaguari Foundation

Each of the six catchment structures was supported by a 4.20m diameter station with a foundation pin of 1.80m diameter and the 2 columns were supported by a block supported in four piles of 1.20m diameter in the section in soil and 1.00 m in the rock section.  

For the definition of the working loads, the STRAP 2009 software, a finite element structural analysis program was used, considering the closing and cover structure, adopted TB-45 mobile load and the 30tf capacity crane, whose weight is 15tf. The loading obtained in the foundation varied between 1081 and 1771tf for the pins and between 166 and 229tf for the columns. Considering the high load and tension acting on the piles, it was defined that the support of the piles would be in rocky massif of high competence, that is, sound rock.  

Although the pile crosses layers of residual soil, due to the great difference between the soil/rock rigidity coefficients and in favor of safety, only the load capacity of the rock section was considered, and the soil contribution (lateral friction) was neglected.

The method proposed by Cabral-Antunes, presented at the 4th Engineering Seminar of Special Foundations and Geotechnics (2000) [1], was used for sizing the pile load capacity. This method, as well as the classical methods of sizing piles (Aoki and Velloso, Decourt Quaresma) [2], establishes that the load capacity of piles embedded in rock can 1426 D. Melo et al. / Offshore Foundation with Large Diameter Piles Anchored in Rock be considered as the sum of the portion of lateral friction resistance with the portion of tip resistance.  

The above-mentioned method depends on the rocky massif quality factor, which can be characterized by tests of Simple Compressive Strength ( = ) and also the cleaning of the pile tip, where the concrete-rock contact occurs, using a safety coefficient rating of 3.0 or greater to obtain the allowable stress in the rock under simple compression. Based on the information from the drillings carried out at the EEAB (RWPS) Jaguari site, the rock of the site (Gnaisse) being of type 1, igneous and metamorphic rocks. 

From the rock type and considering the massif alteration degree, Cabral and Antunes present the tip rupture stress and the simple admissible compression stress in the rock through tables. For the value presented, a minimum safety factor of 3.0 is adopted.  

The methodology recommends that the maximum permissible contact stress between the foundation base concrete and the rock should be less than 0.40 . The required design for the piles is 30MPa, however, NBR-6122 (2010) establishes that the maximum to be used in the design calculations of molded piles in loco is of 20MPa, being used such a value to limit the allowable stress, ie maximum stress of 8MPa.  

The methodology proposed by Cabral and Antunes predicts that the lateral friction represents 2.5% to 3.5% of the tip resistance of the foundation. Due to the executive characteristics, the methodology also recommends that the lateral friction stress be lower than the /15; that is, for the 20MPa calculation, the maximum allowable contact stress between concrete and rock in lateral friction is 1.3MPa.

In relation to the minimum inlay length ( ) Cabral and Antunes recommend the adoption of a multiplication factor of foundation diameter ( ) that takes into account the quality of the support rock and the confidence level of the tip cleaning, which varies between 0.5D and 4.0D. 

Considering the most loaded well, with a workload of 1771tf, the total inlay length of the rock pile of 9.0m was obtained, being 4.0m of sound rock inlay. 

In the definition of lengths, according to the current practice of foundations and recommendation of the sizing method used, the minimum inlay length in rocky massif of high competence (sound rock) of 4.0m was adopted.


6. Conclusions

This work presented a description of the offshore foundation in rock with large diameter piles carried out in the catchment structure of EEAB (RWPS) Jaguari, in the work of Interconnection between the Jaguari and Atibainha Dams.

In order to serve as a basis for the catchment wells, the foundation was made through stations of 4.20 m in diameter and pins with 1.80 m diameter. Each of the two columns was supported by 1 one block with 4 four piles, these piles have a diameter of 1.20 m in the soil stretch and 1.00 m in the rock stretch.  

The design and execution of this foundation presented unique characteristics, such as the difficulty in the execution logistics inside the dam, with the positioning of the casings, frame cages and the concreting itself, which had to be carried out in stages.  

With the innovative solutions, commitment in the project elaboration and in the work performing, the enterprise can be delivered within the stipulated term, guaranteeing the water security of the metropolitan region of São Paulo. 

Acknowledgments  

The present work was developed from the data of executive project and technical monitoring of work. The authors would like to thank SABESP - Companhia de Saneamento Básico do Estado de São Paulo and the Bacias Paraíba do Sul and Cantareira Consortium for making the data available for publication.


References


[1] Cabral, D. A.; Antunes, W. R. Sugestão para Determinação da Capacidade de Carga de Estacas Escavadas Embutidas em Rocha. In: SEMINÁRIO DE ENGENHARIA DE FUNDAÇÕES ESPECIAIS E GEOTECNIA, n. 4, 2000, São Paulo. Anais... São Paulo: ABEF-ABMF, 2000, v. 2, p. 169-173. 

[2] Hachich, W. Et Al. FUNDAÇÕES Teoria e Prática. Editora PINI, 1ª Edição, 1996. 

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