The Tennessee Valley Authority (TVA), a federal corporation, is the nation’s largest public power company. It operates fossil fuel, nuclear and hydropower plants, and produces energy from renewable sources, in an area covering seven states. The 1990 Clean Air Act Amendment requires U.S. utilities to reduce or remove sulphur dioxide emissions. To be in compliance, TVA built new scrubbers in its Cumberland power plant. The new building facility also required the installation of a series of Induced Draft Fans (I.D. Fans).
Foundation design is critical to stability due to the vibration levels that are produced during operation. I.D. Fans are supported on an inertia block foundation system, and the design of a dynamically loaded structure requires expert knowledge of the dynamic properties of the foundation and soil interaction. TVA contacted GeoSonics/Vibra-Tech for structural dynamic engineering services to ensure proper foundation support.
To deliver the information TVA needed, GeoSonics/Vibra-Tech first conducted an extensive evaluation and study:
- Calculations were made of the site soil shear propagation velocity by utilizing our Multi-Channel Analysis of Surface Waves (MASW) technique. This evaluates soil dynamic shear modulus of elasticity and damping.
- Based on the fan properties and the foundation dimensions, we then calculated the theoretical natural frequencies of the foundation system in vertical, horizontal, rocking and torsional.
The project required further demonstration and verification that the structure would withstand outside vibration. To calculate the dynamic properties of the foundation and experimentally evaluate the structure, an existing foundation similar to the proposed one was excited by detonation of a nearby small single-hole blast in the ground. The ground and foundation blast-induced vibrations were measured simultaneously in three orthogonal directions with multiple vibration recording instruments. We then calculated the frequency spectrum of the recorded blast wave. Any magnification per frequency of the foundation as compared to the ground indicates the natural frequency of the foundation and can be used in development of the inertia block.
Knowing the dynamic properties of a machinery foundation is essential for the lifetime and safety of the machine. If the frequency of excitation force coincides with one of the natural frequencies of the machine/foundation system, resonance is encountered, a phenomenon in which the vibration amplitude builds up to a dangerously high level.
In this project GeoSonics/Vibra-Tech successfully calculated the dynamic properties of the machinery foundation both theoretically and experimentally. Our calculations were used as a key part of the final foundation design and ultimately provided the support needed for the scrubbers.