The Permeability of Weak Sandstone and The Microstructure Change due to Seepage
|Abstract:||軟弱岩石的透水係數除了與顆粒粒徑分佈、孔隙大小、泥質含量及膠結物種類等因素有關，且深受試體飽和度之影響，如欲以傳統之定水頭或變水頭滲透實驗方法正確決定材料滲透性常有其困難。為求正確決定材料滲透性，本研究乃研製一微流量幫浦，藉由伺服馬達強制控制流經試體之微流量，以控制固定流量之方式來控制滲透試驗之進行，經量測試體兩端之水壓差即可正確決定試體之滲透性。本儀器可控制圍壓與反水壓，因此可藉由檢核B值來確認飽和程度。本研究由台中大坑及新竹寶山分別取回黃色砂岩之岩樣，進行一系列由室內定流量三軸透水試驗。所求得的滲透係數整理如下：台中大坑黃色砂岩介於2.9 ×10-5~8.3 ×10-5cm/s；新竹寶山黃色砂岩介於為2.7 ×10-4~3.2 ×10-4cm/s，灰色粉砂岩則介於為8.6 ×10-6~9.1 ×10-6cm/s。若試體材料性質相同（譬如：無明顯相通不連續面或無明顯差異極大之互層），現場的透水性質與室內透水試驗結果相近，對本批岩樣而言，室內透水試驗結果可做為描述現場岩體透水性質之依據。
The permeability of weak rock depends on the grain characteristics, pores, cementation, and saturation of material. In this study, a device of constant-flow permeability test was built up for correctly determining the true permeability of weak rock specimen. In the test, a constant-flow pump is developed to control a constant flow through a specimen in a triaxial cell. The head difference between two ends of the specimen is measured. Then, the permeability of the specimen is calculated on the basis of the Darcy's law. The developed device is able to control the confining pressure and the back-pressure of a specimen; hence, the saturation of specimen can be examined by checking the B value. A series of constant-flow permeability tests on samples of weak sandstone from Ta-kang, Taichung and from Bau-san, Hsinchu were carried out and compared with the in-situ permeability test. The permeability of Ta-kang sandstone lies within 2.9 ×10-5~8.3 ×10-5cm/s. The permeability of Bau-san sandstone lies within 2.7 ×10-4~3.2 ×10-4cm/s, which is found closely consistent with the result from the in-situ lugeon test. Weak sandstone usually will degrade when it is exposed to unfavorable environmental conditions. The degradation of weak sandstone may results in the reduction in its strength and deformability. This study also attempts to investigate the microstructure change of weak sandstone after the degradation due to seepage through the material. By comparing the microstructure of sample before and after the degradation due to seepage, it was found that the void of sandstone increases significantly after degradation. On the basis of microscopic observation, the reason is possibly due to an increase of void near the grain-matrix interface in the material and/or the dissolution of the cementation/matrix.
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