Abstract:
This paper presents the engineering solution adopted for the construction of a substation on a suspended reinforced concrete slab fully supported by helical steel piles, built in the city of Afuá, located on Marajó Island, in the state of Pará, Brazil.
Afuá is an island; therefore, the only means of access are either regular boats operating between Afuá and Macapá or small aircraft. Although it belongs to the State of Pará, the nearest capital is Macapá, located approximately 84 km away. The city is known as the “Venice of Marajó†because its streets and houses are built on wooden stilts and walkways due to the waterlogged terrain of the region. The local population lives in harmony with the natural environment, adapting to the tidal cycle of the rivers, which rises and falls throughout the year. Bicycles are the main means of transportation, as cars and motorcycles are not allowed in the city. The municipality is surrounded by several rivers and streams that form a natural network of waterways, which, together with the swampy ground, shape the local way of life. The Afuá River is the main watercourse that gives the city its name and plays a major role in transportation, fishing, and water supply. In view of this context, the challenge of the solution presented in this study begins with the project logistics itself, since there is no land access for construction equipment and materials.
The site investigations revealed extremely low soil bearing capacity, with an upper fill layer of at most two meters, presenting very low resistance (maximum of 2 SPT blows), which made the use of shallow foundations unfeasible. Beneath this layer, there is an average of 50 meters of organic clay with zero SPT blows, where some low resistance begins to develop only at depths close to 60 meters, where bearing capacity is finally achieved. From a geotechnical standpoint, this represented a major challenge, as overcoming such extremely soft soil would require a solution with very deep piles and large cross-sections to ensure both penetration depth and lateral stability. The use of conventional heavy equipment (drilling rigs, pile drivers, etc.) was practically unfeasible due to the access logistics to the site and the limited conditions at the local pier. In addition, the seasonal rise in water level, which during certain times of the year reaches up to one meter above the substation platform elevation, required the adopted solution to be elevated.
Therefore, a suspended slab solution was adopted, where the electrical equipment is supported on a reinforced concrete slab resting on helical steel piles. The choice of helical piles proved to be efficient, as they could be installed using backhoes—lighter equipment with greater mobility—allowing easier transport to the site by barges. In addition, helical steel piles are composed of modular sections connected by bolts, which facilitates transportation, while their installation by torqueing (similar to a screw) allowed better execution and control in very low-strength geological conditions (more than 50 meters of organic clay). The same solution was also adopted for the construction of the access pier.
Thus, the objective of this paper is to detail this engineering solution for the construction of a substation suspended on piles in a soil of extremely low resistance, presenting, step by step, the technical challenges encountered and the proposed engineering approach. It is understood that this represents an important technical contribution to Brazilian power transmission engineering by demonstrating an atypical and unprecedented solution.