核廢料地質處置工程圍堵系統中緩衝材之要求重點在利用皂土遇水膨脹自癒特性，但也應具有高導熱性，因此常於皂土中添加顆粒性底材。一般建議使用：50%皂土添加50%石英砂。緩衝材會因衰變熱的高溫作用，導致皂土中水分蒸散使含水量下降而乾縮甚或出現裂縫，導致減低緩衝材的完整性。另一方面，鑑於當緩衝材受地下水入侵後卻可產生吸水膨脹而自癒填塞裂縫。本文試驗主要以50%石英砂混合50%皂土之緩衝材，在吸水七天後進行加熱乾裂與再吸水回脹癒合試驗，以探討添加石英砂對緩衝材之乾縮破裂與吸水癒合行為之影響。 主要獲得以下研究結論： (1) 緩衝材之乾縮裂隙發展程度與其內部含水量蒸散損失的多寡有關，含水量之損失量愈多產生的乾裂面積愈大、表面破裂孔隙率愈大。而且，緩衝材表面破裂孔隙率與其含水量損失量為一非線性關係。(2) 皂土在添加石英砂後之最終乾裂面積、破裂孔隙率均低於純皂土，顯見石英砂在皂土質緩衝材中具導熱效益。(3) 純皂土在乾裂與癒合行為中，裂縫的成長與癒合速率皆較快於皂土－級配砂，顯示裂縫之生成或癒合速率與皂土之含量相關。而添加石英砂雖可抑制裂縫的發展，相對也會阻礙皂土吸水回脹時填充裂縫的自癒能力。(4) 方格維度Db確可用以描述裂縫分佈之演進，純皂土裂縫分佈之Db值為1.43，而添加優良級配砂皂土之Db為1.14，顯示純皂土裂縫分佈較隨機分佈於二維平面上，而添加石英砂後裂縫則被侷限於沿某單一方向分佈。 In deep geological disposal for the radioactive waste repository, after the emplacement of a container, the gap between the container and the rock wall of borehole would be filled with a buffer material. Buffer material is expected as a barrier with groundwater movement and self-sealing ability. The buffer is proposed to consist of a mixture of 50% bentonite 50% quartz sand. This study aims to investigate the shrinkage behaviors of 50% bentonite - 50% quartz-sand mixtures with various sand gradations by experimental tests. The factors of sand gradation, initial density and initial water content are considered in this experimental program. In this test, the compacted buffer material is first swelling by absorbing water in seven days and then cracking by heating shrinkage (60oC, 80oC, 100oC) in an oven. Finally, the damaged bentonite-based material is re-saturated to recover the water content and seal cracking. The shrinkage and sealing behaviors are discussed. Some conclusions are drawn as following: (1) Shrinkage cracks development of buffers materials is related to the loss of water content. The surface crack area or crack porosity is increased with the water decreasing. (2) The surface crack area of sand-bentonite mixture is less than bentonite. This implies the quartz sand within bentonite has the heat conduction effect. (3) The growth of cracking and self-healing in bentonite are faster than those in sand-bentonite mixture. This indicates the creaking and sealing behaviors are correlated to the bentonite content in buffer materials. The quartz sand can suppress the crack developing, but also hinders the bentonite swelling to seal the cracks. (4) The box dimension, Db is capable of describing the complexity of cracking progress. In this study, the value of Db for bentonite is 1.43, and 1.14 for well-graded sand-bentonite mixture. This indicates shrinkage crack distribution of bentonite is more randomly distributed than the quartz sand- bentonite mixture.
