| Summary: | 碩士 === 逢甲大學 === 土木工程所 === 96 === Weathered rockmass with its chemical and physical properties drastically changed possesses rock mechanical properties that deviate greatly from the original ones and frequently causes engineering mishaps such as foundation settlement and cave-in or rockfall in tunnels. Prior knowledge on the weathered rock formation is thus of utmost importance in any engineering project. The common practice in obtaining rock cores of weathered frock is through in-situ boring to retrieve cores for lab tests. However, such attempt tends to be limited in sampling coverage despite the high cost with a slow progress and is damaging to the site structure. Reflective and refractive shallow seismic surveys had the problem in that the signals from the longitudinal waves cannot be differentiated from shear waves. In the foothills area shallow reflective seismic survey is confronted by problems arising from reflected wave energy dispersion or from low propagation velocity of waves in weathered zone that cause poor resolution of the stacked profiles rendering it difficult to accurately interpret the data.
It is important for engineering construction and disaster prevention to obtain correct information of regolith depth, material deformation properties, shear wave velocities, strength properties, material characteristics. Use of spectral analysis of surface waves (SASW) and Ground Penetration Radar (GPR) methods in site investigation has long been an effective means in performing site inspection. Both these methods can effectively and economically investigate, without any destructive effects, the properties of rock formations and their thickness in the project site.
The thesis gives a summary of results from a recent experimental survey through use of GPR and SASW methods. The experiment was conducted at a site on County Highway Miaoli 119 south of the Tong Guang Elementary School in Houlong township. Rock formation underlying the site belongs to the Pliocene Hsianshan Sandstone of the Toukoshan Formation. The objective of the experimental investigation was to obtain relevant rock mechanical parameters of weathered rockmass through SASW method, and to reconstruct a structural profile of the weathered rock formations underground through GPR method.
Survey lines for SASW method ran superimposed along those for GPR method. The total length of these lines is 160 m. A 6kg hammer was used as the impulsive seismic source along with 4 lineally arranged receivers; a GPR system of SIR-3000 was used along with a 16-80MHz antenna.
The study results indicated: (1) In SASW tests, the coherence functions are close to 1 for the low and intermediate frequency ranges, which indicates high reliability in signals; (2) A value of 20 can be used as the signal/noise ratio threshold value for both signal quality indication and filtration requirement of SASW; (3) Composite dispersion curves can not readily reflect variations in stratigraphic boundaries, while some individual dispersion curve showed abnormal jumps at wave lengths of 8-18 m and these jumps may be related to occurrences of shallow seated weathered zones; (4) In GPR method, it reveals a reflection surface situated 10-13 m below the ground surface and it is taken to be the boundary for the weathered zone; (5) Profiles obtained through the GPR method bears better resemblance to the model for the weathering profile at the site.
Key Words : Regolith, Spectral Analysis of Surface Waves Method (SASW), Ground Penetration Radar Method (GPR)
|
|---|
