表面波震測法應用於堤壩檢測之可適性探討

Abstract

堤壩之安全性評估必須考慮築壩材料的強度與動態性質，使用非破壞性表面波震測法，可經濟且快速的量測壩體剪力波速。當應用表面波震測法於壩體檢測時，無論在壩頂或是壩面施測皆有其與一維地層之基本假設不符之處，有進行適用性探討之必要。本研究以三維數值模型進行規劃，並以波譜元素法模擬表面波傳遞行為，由地表地形變化以及材料側向變化兩種三維效應之影響著手，探討使施測結果產生誤差之主要影響因子以及其影響程度，並尋求可能降低影響或避免影響之方案。 研究成果顯示，在壩頂平行壩軸施測時，上下游壩面之坡度所造成的影響極小，可忽略不計；但心層與殼層之材料差異所造成的側向變化影響顯著，且影響程度隨波長的增加而越顯嚴重，因此建議於分區碾壓壩之壩頂施作，其有效頻散曲線之範圍可採用之波長最大值為測線與邊緣距離之1.8倍；若為壩面上之施測，在垂直壩軸施作之狀況下，由於表面波傳遞的自由表面非水平，因此需注意採用斜距作為分析使用的接收器間距，以獲得正確之頻散曲線；但在平行壩軸施作之狀況下，為避免受到拉夫波分量的干擾，於現地應將受波器以垂直坡面之方向插入，而非採重力方向。而由此兩例可知，於壩面施作所獲得之剪力波速剖面其代表方向應為垂直壩面之方向，而非重力方向。 本研究成果顯示，雖然壩體幾何形狀與構造狀況與表面波震測之基本假設不符，但在本研究所提出之施測建議下，其仍具適用性。後續可進行現場資料收錄以作為本研究之驗證。

Safety assessment of embankment dams must evaluate the strength and the dynamic property of the construction material as a satisfactory criterion. By using non-destructive surface wave method, the dam shear wave velocity can be quickly measured economically. However, in the application of surface wave method on dam detection, this approach does not satisfy the basic assumption of one-dimensional ground layer, no matter the survey is carried out on crest or shell. Hence the necessity to investigate the viability of this approach arises. This study utilized three-dimensional (3D) numerical models and simulated surface wave transmission behavior using spectral element method. Initiating from 3D effects from surface topography variation and lateral variation of dam material, the main impact factors and their influencing degree are investigated and corresponding countermeasures to reduce their effect on survey result is proposed. Results showed that in the case of survey line parallel to dam crest axis, the impact from the slope of dam shell is minimal and negligible. However, the lateral material variation due to the material difference in dam core and shell affected the survey results significantly, and the influencing degree increases as the wavelength increases. Therefore, it is suggested that the maximum workable wavelength in effective dispersion curve as 1.8 times the distance between the surveying line and the edge of crest for seismic survey on the crest of zoned earth dam. However, if the survey is conducted on the dam shell and perpendicularly to the crest axis, receiver spacing adopted in dispersion analysis should be taken as the oblique spacing instead of horizontal spacing in order to generate correct dispersion curves, since free surface of the surface wave transmission is not horizontal. In the case of survey line parallel to dam axis, receivers should be inserted perpendicularly to the slope face instead of along the gravitational direction, in order to avoid the interference from Love wave components. From these cases, it is noted that the shear wave velocity profile generated from dam shell surface seismic survey represented the velocity profile perpendicular to the slope surface, rather than the gravitational direction. Although the geometry and internal structure of dam is inconsistent with the basic assumption of surface wave method, this study found that this approach can still remain applicable by adopting the suggested surveying recommendations. Seismic data from field survey is recommended to further verify the surveying recommendations in this study.