Rock Blasting Induced Vibration Effects on the Integrity of the Dry Comal Creek Flood Retarding Structure

Capital Aggregates plans to develop a new rock quarry near the Dry Comal Creek Flood Retarding Structure (DCCFRS). The DCCFRS contains a RCC gravity dam and deep concrete cut off wall. Blasting will be used as the means of rock excavation. There is a concern that the ground vibration induced by rock blasting may induce excessive stress in these adjacent structures. Capital Aggregates hired Vibra‐Tech Engineers (VTE) to calculate the blast induced ground vibration effects on the structural integrity of DCCFRS and the deep concrete cut off wall, and to develop a vibration‐monitoring plan and criteria to prevent structural damage to the structures caused by future production blast induced ground vibration.
The characteristics of the blast induced ground vibration (amplitude, duration, and frequencies), are controlled by the geology between the blast area and the structure of concern. A total of three separate single hole blasts were detonated at various distances from the dam. The blast induced ground vibration and dam response to the single hole blasts were measured at several locations. A computer model of the cross section of the dam was developed based on its geometry and material properties. The natural frequencies of the dam cross section were calculated. The computer model was verified based on field vibration measurements of the dam structure.
The recorded ground vibration resulting from the detonation of the single holes were used to design a delay pattern for a production blast that results in a ground vibration that contains minimum energy at the natural frequencies of the dam (Vibra‐Map study). The production‐blast time history is applied to the foundation of the dam model. The dam response to the production blast is then calculated. The maximum allowable peak particle velocity and vibration monitoring locations were selected for the dam based on the dynamic stress analysis of the dam in response to the designed production blast.
The geology's compression and shear wave propagation velocities were utilized to define the blast induced ground vibration effects on the deep concrete cut off wall. Based on the material properties of the concrete cut off wall the stress waves were calculated.
Based on the result of this study and our professional opinion, the maximum allowable dam Peak Particle Velocity (PPV) should be limited to 3.0 in/sec. The conclusion section recommends vibration‐monitoring locations. The maximum ground vibration monitored near station 3+50 is limited to 1.35 in/sec. At these levels, the induced stress is well below their limits. These calculations are based on a safety factor of four.
Featured dams: Dry Comal County Flood Retarding Structure, New Braunfels, Texas. 35 pp.