Application of geophysical results to designing bridge. over a large fault

Transcription

1 Application of geophysical results to designing bridge over a large fault Ho-Joon Chung 1, Jung-Ho Kim 2, Keun-Pil Park 2, Hyoung-Seok Kwon 1, Ho-Sik Choi 3, Ki-Seog Kim 4, Jong-Soo Kim 5 1 Manager, HeeSong Geotek Co., Korea 2 Principal researcher, Korea Institute of Geoscience and Mineral Resources, Korea 3 General Manager, HeeSong Geotek Co., Korea 4 President, HeeSong Geotek Co., Korea 5 General Manager, Hyundai Development Co., Korea hojoon@mantle.snu.ac.kr, jungho@kigam.re.kr, kpp@kigam.re.kr, hsgeotek@chollian.net, hsgeotek@chollian.net, hsgeotekk@chollian.net Abstract During the core drilling for the design of a railway bridge crossing over the inferred fault system along the river, fracture zone, extends vertically more than the bottom of borehole, filled with fault gouge was found. The safety of bridge could be threatened by the excessive subsidence or the reduced bearing capacity of bedrock, if a fault would be developed under or around the pier foundation. Thus, a close examination of the fault was required to rearrange pier locations away from the fault or to select a reinforcement method if necessary. Geophysical methods, seismic reflection method and electrical resistivity survey over the water covered area, were applied to delineate the weak zone associated with the fault system. The results of geophysical survey clearly showed a number of faults extending vertically more than 50m. Reinforcement was not desirable because of the high cost and the water contamination, etc. The pier locations were thus rearranged based on the results of geophysical surveys to avoid the undesirable situations, and additional core drillings on the rearranged pier locations were carried out. The bedrock conditions at the additional drilling sites turned out to be acceptable for the construction of piers. Introduction This paper shows a case history of an application of geophysical methods over the water-covered area

2 to evaluate the ground condition of bridge construction site. During the core drillings to investigate the bedrock for designing the railway bridge crossing the North Han River, large fracture zone filled with fault gauge was found down to the borehole bottom (figure 1). Since the foundation on such a fault might cause instability of bridge due to the possible excessive subsidence, the weak ground should be reinforced or other suitable location should be selected to achieve the stability of bridge. Reinforcing the weak ground could allow Figure 1. Core of the well BB-59 flexible selection of foundation location, but it would cause increasing cost, water pollution, and difficulty of construction work. Therefore, it would be more reasonable to find suitable foundation locations to minimize the effect of faults, through examining the ground condition thoroughly. It is known that Kyeong-Gang fault formed by strike slip movement lies along the North Han River. Since the fault is hardly observable at this site because of river water, the geophysical surveys, seismic reflection and underwater electrical resistivity surveys, were carried out to image the fault zone precisely. Then, optimal pier locations were chosen based on the geophysical and core drilling results. Geophysical surveys Underwater electrical resistivity survey Electrical resistivity method investigates subsurface by analyzing the electric potential distribution that reflects geo-electrical structure. The method is being successfully applied to various areas: the site evaluation for civil engineering, the geothermal and groundwater development, and the environmental problem. Since the resolving power and the signal-to-noise ratio depend on the electrode configuration mainly, it should be carefully selected according to the field situation. Because the target structure underlies the river at this area, electrodes were installed on the riverbed to maximize the sensitivity to the structure. Unlike the land survey, the underwater survey suffers from the low signal level and the conventional electrode configurations are inappropriate. We adopted modified dipole-dipole and modified pole-pole arrays (Kim, et al., 1999) to enhance the signal-to-noise ratio in the field measurement as well as the resolution of the inverted subsurface image. Measured data were inverted to the true resistivity image by the 2.5 dimensional inversion that is based on the finite element modeling and the active constraint balancing (Yi and Kim, 1988). The inversion algorithm could incorporate the riverbed topography from GPR survey and the electrical conductivity of river water into the inverse process.

3 Seismic reflection survey Seismic reflection method investigates subsurface by analyzing reflected waves from layer boundaries at depth. The airgun was used to generate seismic wave and the reflected waves were measured using streamer cable. The data were acquired along the prescribed route with the aid of DGPS system and Hypack navigation system. High-resolution seismic sections were obtained from the measured data after data processing including gain recovery, deconvolution, band-pass filtering, and airgun delay correction. Results of geophysical surveys Underwater electrical resistivity survey Electrical resistivity sections showed the thin sediment layer composed of sand and gravel over the basement having resistivities ranging from 400 to 1,000 ohm-m. In particular, the vertical low resistivity anomalies were clearly imaged in the true resistivity sections. Fig. 2 shows the locations of the six anomalous zones identified on the resistivity sections. The anomalous zone A, B, and C are the most evident and extend to the depth of more than 50m vertically, which were interpreted as the fault fractures associated with the Kyeong-Gang fault system. Especially, the zone C was verified to be a fault filled with fault gauge by the boring result of the well BB-59. The anomalous zone D and E having relatively higher resistivity were interpreted as minor faults. Figure 2. Low resistivity anomalous zones identified by electrical resistivity survey.

4 Seismic reflection survey Through the interpretation of seismic reflection sections, it was found that the depth of basement ranges from 6.5 to 14m and three basement uplifts are developing in the survey area. At the right half of the river, the amplitude of bedrock reflections becomes weaker that implies more weathering or fracturing of the bedrock. Figure 3 shows the basement relief structure identified by seismic reflection method. The bent trend of uplift zones at the right half of the river imply the possibility of more fracturing at this part of the river. Figure 3. Basement relief obtained by seismic reflection method. Determination of foundation locations The geophysical results showed that the weak zone of the main fault would elongate more than 50m vertically. Assumed that the foundation of pier would be constructed on the main fault, FLAC modeling predicted the subsidence of 66 mm, which exceeds the allowable limit. Based on the geophysical survey results and considering the maximum possible pier interval, we selected three locations for the additional core drillings to confirm that the bedrock would be suitable for the pier foundations. Fault gauge or clay was not found at the additional boring sites and the bedrocks at those sites were suitable for constructing foundations. Using these results, the new locations of pier foundations were determined to depart from the main faults. Although a part of one foundation of pier was rested on a minor fault but it was proved stable by FLAC modeling.

5 Conclusion The results discussed so far led us to the conclusion that the improved geophysical methods over the water-covered area and the interpretation technique could provide the reasonable images of the weak zone of subsurface beneath river water. The interpretation results matched with the drilling ones very well. Based on the geophysical and drilling results, the bridge was designed to avoid the weak zone by adjusting pier spacing without ground reinforcement. Reference Kim, J.H., Yi, M.J., and Chung, S.H., 1999, A comparison of electrode arrays in two-dimensional resistivity survey, The 73rd Symposium of the Korean Institute of Mineral and Energy Resources Engineering, Expanded Abstract, pp Yi, M.J., and Kim, J.H.(1998), Enhancing the resolving power of the least-squares inversion with active constraint balancing, 68 th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, pp