Tennessee Valley Authority

The Tennessee Valley Authority (TVA), a federal corporation, is the nation’s largest public power company. It operates fossil fuel, nuclear and hydropower plants. When the Cumberland facility needed to build new scrubbers and install a series of Induced Draft Fans (I.D. Fans) in its power plant, they contacted GeoSonics/Vibra-Tech for structural dynamic engineering services.

Foundation design is critical to the stability of the I.D. Fans due to the vibration levels that are produced during their operation. The I.D. Fans are supported on an inertia block foundation system. The design of a dynamically loaded structure, such as this, requires expert knowledge of the dynamic properties of the foundation and its interaction with the soil. GeoSonics/Vibra-Tech conducted an extensive evaluation to deliver critical information for the final foundation design. Measurements were made of the shear wave propagation velocity of the soil by utilizing the Multi-Channel Analysis of Surface Waves (MASW) technique. Based on the soil shear wave velocity, the dynamic shear wave modulus of elasticity was calculated. The shear wave modulus of elasticity was a critical parameter used in the Finite Element Model (FEM) that was developed for the inertia block foundation system. The FEM model was used to theoretically calculate the natural frequency and damping of the inertia block foundation system.

In addition to the theoretical calculations of the natural frequency and damping, experimental measurements were also made. Experimental measurements were made on an existing foundation similar to the one proposed. The existing foundation was excited by the detonation of several nearby single-hole blasts. The blast induced vibrations were measured simultaneously on the ground and on the existing foundation. The recorded experimental data was used to calculate the natural frequency and damping of the existing foundation. These values were compared to the theoretical values from the FEM model. This comparison allows for verification of the parameters used in the theoretical FEM. Once the FEM is verified, changes to the foundation design can be evaluated through changes to the FEM.

Knowing the dynamic properties of a machinery foundation is essential for the lifetime and safety of the machine. If the operating frequency of the I.D. Fans coincide with the natural frequencies of the inertia block foundation system, resonance will be encountered. The resonance is a phenomenon in which the vibration amplitude can build up to dangerously high levels. In this project GeoSonics/Vibra-Tech successfully calculated the dynamic properties of the machinery foundation both theoretically and experimentally. These dynamic properties were a key part in determining the final foundation design needed to support the I.D. Fans.