台灣西南部粉土質細砂CRR與qc關係之標定

Abstract

土壤液化潛能評估常用之簡易法（Simplified Procedure）以乾淨砂做為評估基準，依照細粒料（通過200號篩之材料）含量做修正。對於台灣西南部具有高細料含量 (Fines Content, FC =細粒料重量佔砂土總重量之百分比值) 之砂土在進行簡易法土壤液化潛能評估分析時，進行細料含量之修正便顯得非常重要。簡易法土壤液化潛能評估之細料含量修正的方法是由人為判斷依經驗進行修正，少有其他直接試驗證據或理論根據來佐證。近來的研究指出CPT-qc做液化潛能分析細料含量修正只有在影響到CPT之排水性時才會有明顯之需要。因此本研究嘗試使用土壤之排水特性來取代細粒含量修正，以一系列室內試驗與現地驗證建立台灣西南部排水行為下粉土質砂土CRR與qc之土壤液化臨界曲線。作者於員林與高雄兩試驗站進行一系列CPT試驗，收集員林試驗站粉土質砂土中Laval sampler試體之動態三軸試驗結果，並於高雄試驗站粉土質砂土中進行Gel Push取樣。現地試驗與低擾動試體試驗結果指出，CPT-qc關係會隨細料含量而向左移動，但細料含量對CPT-qc關係之靈敏度遠不如現有方法所建議。此一現象之主要原因可能是因為天然沉積粉土質砂土內常夾雜有高透水性之粗砂層，以至於即使在高細料含量砂土層中之CPT也接近於排水行為。

The simplified procedure commonly used for liquefaction potential assessment starts by considering the granular material as clean sand. For sand with fines (particles passing #200 sieve), an adjustment for the fines content (FC) is required. This adjustment becomes significant as the sands in South Western Taiwan usually contain high fines contents. The available schemes for fines content adjustment are highly empirical and may lead to different conclusions depending on the method chosen. Recent stduies have indicated that in using CPT for liquefaction potrential assessment, the fines content adjustment becomes important only when the fines start affecting the drainage condition in CPT. The objective of this research is to conduct a systematic evaluation on the CRR-qc releationship using available data from previous studies and from additional field tests as well as laboratory experiments on high quality samples taken from test sites. The author performed a series of CPT at test sites in Yuan Lin and Kao Hsiung. High quality silty sand samples were taken with a Gel push sampler at the Kao Hsiung test site. Results showed that there is indeed a general trend in CRR-qc releationship that for a given CRR the qc decreases with FC as suggusted by most of the existing simplified procedures. The CRR-qc releationship was however, much less sensitive to the fines content as what has been suggested. For CPT in natural alluvial soil, the drainage conditions are strongly influenced by the possible existence of thin layers of clean sand. It is suspected that these thin permeable layers caused the much reduced fines content effects on CPT in the silty sand mass.