鑽探擾動性對軟岩力學性質之影響

Abstract

軟弱岩石由於具有固結差，膠結不良，強度不高的特性，因此，欲瞭解軟岩的材料力學性質，鑽探取樣以進行室內試驗為必須之步驟；然而目前的鑽探取樣方法常造成岩樣的擾動，而影響軟岩力學性質與行為。本研究嘗試利用改良之鑽探取樣方法(以壓克力管取代伸縮式三層岩心筒之岩心片)於新竹寶山地區實施現地鑽探取樣，期能取得不擾動之岩樣供室內試驗之用。另外，本研究亦利用現地透水試驗、現地孔內震波試驗之結果與其他室內之試驗結果比較，以探討鑽探取樣擾動性對軟岩性質之影響。經由本研究改良方法實施現地鑽探取樣結果顯示，岩心提取率較傳統的方法為佳，且於岩心的搬運及試體的製作時可減少其擾動。而現地透水試驗之結果顯示，新竹寶山地區之頭嵙山層香山相岩層，其透水性主要與距地表之距離有關，愈接近地表的Lugeon值愈高，主要原因為地表解壓，岩體滲透性變大造成。另由現地透水試驗與室內透水試驗之比較結果顯示，兩者的透水係數很接近，因此，本研究所改良之鑽探取樣方法，對鑽探所取之岩心其擾動之影響甚小，且室內透水試驗結果可做為描述現場岩體透水性質之依據。現地孔內震波試驗以國內首度引進之懸垂式P-S波探測儀進行，此種試驗方式主要優點為僅需一鑽孔、且可快速量測及可以量測較深之地層、試驗時僅需要小區域的操作空間。經由本研究試驗結果顯示S波波速從180 m/s至556 m/s間，而P波波速從1667 m/s至2083 m/s間。

Due to the poor lithification of soft rocks, the drilling cores obtained from rock triple tubes are easily disturbed. The mechanical properties determined from the disturbed specimens are incorrect and can not be adopted for engineering design. In order to drill reasonable undisturbed cores and investigate the effect of sampling disturbance on the mechanical properties of poorly cemented sandstone, a modified core tube is developed and a series of in situ tests is conducted in this thesis.The inner tube (two pieces of thin steel half-hollow cylinders) of an extensible triple core tube is replaced by a thin plastic hollow cylinder. The modified tube was adopted for sampling at the site of the second reservoir of Bao-San, Hsinchu. The strata are composed of sandstone, mudstone, shale, and their interbeds. The sandstone is poorly cemented. Using the modified tube, the quality of drilling core are improved and the average core recovery is higher than 90 %.In situ tests includes Lugeon tests using a single packer and P and S wave velocity measurements using a suspension PS logging system. Lugeon tests aim to determine the field hydraulic conductivity of rocks. The suspension PS system is adopted for measuring the P and S wave velocity in a borehole. To evaluate the effect of sampling disturbance on the mechanical properties of the sandstone, both of the test results are employed for comparing with the results determined from core specimens in the NCTU laboratory. In general, the value of hydraulic conductivity determined from laboratory is approximately equal to that from fields where the discontinuities (bedding planes and joints) in test sections are not clear. However, for the rock at test section with clear discontinuities, the field conductivity is larger than the laboratory conductivity. This evidence indicates that the hydraulic conductivity is strongly influenced by the existence of discontinuities and the core specimens are undisturbed using the modified tube. The similar trend is obtained from the results of measured velocity from field and laboratory.