台灣山高坡陡水資源保存不易，自1960年代起，因超抽地下水導致地層下陷。因此，本研究選定近年來台灣地層下陷較嚴重之濁水溪沖積扇平原，藉由分析地下水位與地層下陷量之時間序列資料，描述地層壓縮之力學特性。由於沖積扇地層複雜，本研究將地層視為粗顆粒及細顆粒組成之均質模式，假設地層總應力為定值，將地下水位數值轉換為有效應力數值並與地層壓縮量進行繪圖，針對黏、彈、塑性三者力學特性探討。繪圖結果顯示研究區域地層同時擁有黏、彈、塑特性之混合行為。地層變形量由隨著地下水位變化呈現暫時性的黏彈性變形，及長時間持續壓縮的永久黏塑性變形共同組成。本研究改良傳統模式，將黏彈塑性模式分為黏彈性以及黏塑性部分，分別給予不同黏滯係數進行數值模擬。並以地下水位及分層監測井共站之箔子站為例，進行本模式及前人模式之比較，結果顯示本研究模式在第一、第二及第三含水層之壓縮量和現地資料比較之相對誤差分別為9.7%，1.6%，4.8%，相較其它模式準確。最後，將此研究應用在扇央之田洋站以及扇尾之虎尾站進行濁水溪沖積扇地質參數之推估及力學特性之探討。研究結果指出，在相同測站中，彈性模數和塑性所需壓縮時間隨著地層深度增加，但塑形模數則隨地層深度下降。以空間分布分析，虎尾站彈性模數最大，箔子站彈性模數最小，顯示靠近扇頂之含水層較扇尾含水層堅硬。但以黏滯係數進行比較，虎尾站最小，箔子站最大，顯示虎尾站之地層壓縮在未來持續時間為最久。各測站彈性黏滯係數皆遠大於塑性黏滯係數，黏彈性的變形反應因水位變化產生的壓縮震盪，在箔子第三含水層及田洋第二及第四含水層，水位變化及地層彈性壓縮的為即時反應，其延遲可以忽略，黏彈性變形可簡化為彈性變形。

In last few decades, water demands rapidly increases in central Taiwan. During 2014-2015, the over extraction of groundwater causes 309.1 km2 significant subsidence (subsidence rate > 3 cm/year) in Choushui River alluvial fan.
This study collect the groundwater level data and multi-layer compaction data to analyze the mechanism in Choushui River alluvial fan. The stratum is assumed to a homogeneous mixture of coarse-grains and fine-grains. Using the concept of effective stress to find the appropriate mechanical model to represent the complicated stratum. The result shows that the elastic, plastic and viscosity behaviors appear in the stratum. The compaction is composed by periodically visco-elastic compaction and long-term visco-plastic compaction. The study propose the visco-elasto-plastic model composed by a visco-elastic part and visco-plastic part in series. The elastic part and plastic part have different viscosity to represent two trends of compaction. The ration of Young’s modulus to viscosity is defined as the response retarded (RRF) factor. Boltz station located at distal of alluvial fan is chosen to compare the effective of various model. The average absolute error of model proposed in this study is 9.7%, 1.6% and 4.8% for aquifer 1, 2 and 3, respectively. It indicates the applicability and versatile of the model. Finally, the proposed model is applied to the Boltz, Tianyang and Huwei stations to investigate the mechanical characteristics in Choushui River alluvial fan. The result shows that the Young’s modulus of elastic part increases with depth. However, the Young’s modulus and RRF of plastic part decreases with depth. The result that the Young’s modulus of elastic in Huwei is maximum and Boltz is minimum indicates the stratum is solid in proximal fan. However, the RRF of plastic in Huwei is minimum and Boltz is maximum. It indicates the Huwei needs more long time to approach the compaction asymptotic value. RRF of elastic is much larger than RRF of plastic in all stations. The visco-elastic compaction represents the short-term periodical compaction and the visco-plastic compaction represents the long-term permanent compaction. The visco-elastic can be simplified to the elastic model at some sites because the time lag between variation of groundwater level and compaction can be ignored.

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