Title: 軟岩用高壓傍壓儀研製
Development of a High-stress Pressuremeter for Weak Rocks
Authors: 鄭孟雄
Meng-Hsiung Cheng
Jyh Jong Liao
Keywords: 傍壓儀;pressuremeter
Issue Date: 1998
Abstract: 台灣中北部麓山帶地區出露之極軟弱年輕之砂岩層,普遍具有固結差,膠結不良之特性。岩層取樣過程對岩樣本身之組織構造可能已造成破壞。岩石力學實驗室得到之強度試驗結果一般均甚為散亂。傍壓儀試驗是一種現地試驗的方法,因為無需取樣,可以避免許多與取樣相關之困難,而得到軟岩之力學性質。 傳統之傍壓儀所能承受之最大壓力大多在5MPa以下,而軟弱岩石中做傍壓儀其極限壓力常在10MPa以上,因此若要符合試驗所需,傍壓儀必須能夠承受足夠之以上之高壓。在軟弱岩石中進行傍壓儀試驗之另一個目的是量測岩石之應力應變關係,為達到此一目的,傍壓儀必須有能夠量測微小之徑向應變之能力。但傳統傍壓儀以體積變化來推算徑向應變之方法不能滿足此一要求。 為符合在軟弱岩石中做試驗之需求,本研究已研製一能夠承受25MPa及量測微小變形之傍壓儀。本傍壓儀之擴張可用高壓空氣或油壓做應力或應變之控制。傍壓儀之徑向應變使用4套裝設應變片之簧片在傍壓儀中央以90度之間隔,量測4個方向之應變量。此量測方法,除可精確量測微小徑向變形外,並可瞭解鑽孔之初始形狀及岩體本身受力時之差異性。本系統全部所量測之類比訊號在傍壓儀內部轉換成數值訊號然後傳遞到地表之資料擷取系統,因此可以減低訊號傳遞時之雜訊。
The outcrops in the central and northern regions of Western Taiwan often contain very weak and young formations. They are often poorly cemented. The rock sampling process is likely to damage the structure that originally existed in the rock masses. The strengths obtained in laboratory experiments are often scattered. Pressuremeter test is an in-situ testing method that requires no sampling. It has the potential of avoiding some of the problems that due to sample disturbance. The maximum applied stress of pressuremeter for soil tests is often below 5 MPa. A pressuremeter test in weak rocks usually needs high-pressure beyond 10 MPa to reach limit pressure. The other goal for pressuremeter tests is to determine the stress-train relations of rocks. It means that the test needs to do precise strain measurements during testing. However, using conventional pressuremeters, the volumetric strain of rocks is usually measured for calculating the radial strain, which is not accurate enough for weak rocks. To overcome the drawbacks, the author has successfully developed a new pressuremeter for testing of weak rocks. The developed pressuremeter is allowed for measuring a pressure up to 25 MPa. The pressure is induced by the expansion of the probe of pressuremeter. Either compressed air or hydraulic oil can be adopted as the expansion media. In cooperating with proportional valves and a data acquisition system, pressuremeter tests can be controlled either by stress or strain rate. Four sets of deformation gauge at the middle of the probe of the new pressuremeter are made for measuring the radial strains. The deformation gauge is composed of thin metal with four pieces of strain gages on its both surfaces. The interval between the deformation gauge is 90o. Then, using this device, the four radial deformations at the middle of the probe can be directly measured. Based on the measured radial strain, one can additionally estimate the initial shape of borehole and evaluate the homogeneity of rock mass. To avoid the effect of electric noise, all of the analogic signal outputs from measuring devices are transformed to digital signal in probe. This thesis presents the design of the high-stress pressuremeter and the calibration result of the system.
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