軟弱岩石 之三軸試驗

Abstract

極軟弱至軟弱岩石之力學特性介於土壤與岩石間，傳統土壤或岩石力學之實驗設備無法有效的用來決定軟弱岩石之力學特性。本研究針對軟弱岩石特性設計之三軸試驗系統，已完成測試。本儀器系統採用局部應變量測之設計，可對試體施加最高20MPa之圍壓。先前學者指出軟岩在應變小於0.01%時，變形性是線性、彈性、與應變速率無關，只與壓密應力有關，此範圍內之楊氏模數（Young's modulus）稱為Emax。但是軟岩取樣時試體會產生細微裂縫，所以進行室內試驗必須回復現地應力，才有意義。本研究採用台中大坑所取得之岩塊進行一系列之壓密不排水三軸試驗。結果顯示在3MPa圍壓下裂縫已閉合，Emax約為4000MPa、Emax/σc值約為1500、Emax/qmax值約為320。軟岩之彈性模數隨應變量增大而明顯減小，表示變形性為非線性。在相同有效圍壓下，孔隙率高之試體應力應變屬於延性行為，孔隙率低屬於脆性行為且剪力強度較高。若試體存在細微裂縫則在低圍壓下初始應力應變曲線會有凹口向上的情形。在低圍壓下軟弱砂岩之尖峰強度與殘餘強度隨圍壓上昇而呈線性增加。

The mechanical properties of very soft to soft rock are between soil and rock. It is not easy to determine these properties using the conventional soil or rock testing equipment. A new triaxial testing system specifically developed for soft rocks has been developed. The new system is capable of applying up to 20 MPa confining stress to the specimen and measure local strains. Earlier reports have indicated that the deformability of soft rock at strains less than about 0.01% appears to be linear, elastic, and strain rate-independent. Under very small strains, the Young's modulus referred to as Emax, is mainly dependent on the consolidation pressure. The rock coring process is likely to cause fractures in the rock samples due to the loss of confining stress. Thus, it is advisable to re-apply the in situ stress upon the specimen in the triaxial tests. A series of consolidated undrained triaxial tests were performed on specimens cored from rock blocks obtained in Dakeng, Taichung. Results indicate that closure of cracks originally existed in the Dakeng rock block occurred when the confining stress exceeded 3MPa. Under a consolidation stress of 3MPa the Emax was approximately 4000MPa, was 1500, and was about 320. As strain increases, the Young's modulus decreases significantly, indicating a non-linear deformability. Under the same confining stress, high porosity soft rock specimens with a higher porosity had a more ductile behavior and lower shear strength. The stress-strain curve had an upward concave shape if the soft rock specimen was tested under low confining stress and cracks were present within the specimen. Under low confining stress, the peak and residual strength of the soft rock increased linearly with the confining stress.