- 學位論文
土壤液化潛勢地區之近地表P波與S波波速變化研究
Variation of the Near Surface of P-Wave and S-Wave Velocities in a Soil Liquefaction Potential Region
摘要
土壤液化一般發生在近地表為鬆散砂質土壤且地下水位甚高之沖積地，為地震發生時不可忽視的二次災害。土體液化可能導致地面上之結構物沉陷、傾斜或倒塌，淺埋之輕型結構物與維生管線上浮或斷裂，或土壤噴出地表而堆積在房屋內，以及農田因為噴砂開裂而影響農作物。本研究利用P波折射以及多波道主動式震源表面波波速分析(MASW)法於新化北勢液化範圍內外佈設了七條測線，藉以探討近地表P波與S波波速變化與土壤液化之相關性。研究結果顯示，當地之P波與S波波速變化與土壤液化範圍有直接的相關，在未發生液化之區域，P波與S波橫向上之速度變化相差甚小，而液化區域內，S波速度明顯偏低但P波速度卻相反偏高。此外，液化發生區域之地表下5公尺範圍內，P波與S波速度並沒有明顯的變化，而主要的S波波速變化出現於地下5公尺至18公尺之間。液化發生區域內之P波速度主要的波速變化出現在地下5公尺至8公尺之間，於深度8公尺以下趨於平緩，這可能因為飽和含水狀態影響。對於地表並未發現任何液化現象之區域，深度8公尺以下之區域，整體橫向的S波波速與液化發生區域內之波速一致，因此推測該區域之地表下應有液化發生。雖然土壤液化的潛勢評估常會以工程的方式來進行，本研究認為P波與S波波速變化亦可以有效的做為土壤液化可能性評估的一種依據，並可更有效率的進行大範圍的調查。
並列摘要
Soil liquefaction is a natural hazard that we can never ignore during a large earthquake. Soil liquefaction is normally occurred at somewhere near the surface, while the sandy soil is loose and having higher level of ground water. Soil liquefaction can potentially cause the surface constructions subsiding, inclining or collapsing, and damage the shallow buried light structures and lifelines. Soil spurts to the surface will affect crops in the farmland as well. This study utilized the P-wave refraction method and the multichannel analysis of surface waves (MASW) method to obtain the variation of the near surface P-wave and S-wave velocities in a particular site in southern Taiwan, where liquefaction occurred in 1946 and again in 2010. The purpose of this study is to investigate the correlation between the velocities variations of the seismic waves near surface and the liquefaction within and outside the liquefaction area. Totally, seven survey lines were deployed; results of the study show that velocities of the P-wave and S-wave are directly related to the area with soil liquefaction phenomena. For the areas not having liquefaction, velocities of the P-wave and S-wave are almost the same in the horizontal direction, but varied vertically from the ground surface down to the depth of 18 m. However, within the liquefaction area, the velocities of P-wave and S-wave in the horizontal directions are not varied significantly within 5 meters beneath the surface; major variances of the P-wave and S-wave velocities appear at between 5 and 18 meters below the ground surface. Within the soil liquefaction area, the velocity of S-wave is low; however, the P-wave velocity is higher in contrast. In addition, because of the saturated water content, variations of the P-wave velocity in the horizontal direction remain the same below the depth of 8 m. For the area not having liquefaction seen on the surface, velocity of the S-wave below the depth of 8 m is similar to that regions having liquefaction in the horizontal direction, which inferred that the region would have liquefaction under the surface within the entire area of all liquefied positions. Although evaluating the potential of occurring soil liquefaction in a large earthquake was commonly conducted by using an engineering method. In this paper, we show that the potential of soil liquefaction can be evaluated by using the velocities variations of seismic waves as well. In addition, the seismic method can be applied more efficiently to a broader region.