軟弱岩石之應力應變與屈服行為

Abstract

極軟弱岩石其強度與勁度都介於一般硬岩與土壤之間，此大地工程材料無論在試驗觀察或行為之預估上，常因為此一特性而增加其困難度。文獻中對於此類軟弱岩石之相關敘述也有限。為了能夠進一步瞭解台灣中北部 (桃園至台中) 麓山帶地區出露之『極軟弱至軟弱』岩石之力學行為，本研究研發改良完成適用於此類材料之三軸試驗裝置。此裝置之試驗能量在一般硬岩與土壤三軸試驗儀器之間，以充分滿足軟弱岩石試驗之需求。試體之變形採用三軸室內局部應變以及傳統三軸室外之變形量測，應變量測精度達10-4%以下，可以量得微應變下試體之線彈性乃至於大應變下材料之屈服行為。三軸室壓與軸向壓力可以用電腦做伺服控制，以進行應力歷史控制之試驗。本研究使用此一試驗裝置對新竹地區所取得之軟弱砂岩試體進行了一系列之壓密不排水，應力歷史控制三軸試驗。 本研究顯示，由兩個試驗地區所取得之試體均較無一般硬岩之脆性行為，試體受剪時易產生高額孔隙水壓。三軸試驗中在小應變下確實呈現線彈性之行為，而隨應變量增大，則顯現非線性變形，其楊氏模數會有遞減情形發生。而當應變量在0.1％時，試體裂隙閉合使楊氏模數增高。此外，所得到之室內三軸試驗確實可與室外之PMT與P-S波量測作一明顯比對，三軸試驗所得之楊氏模數Emax，極為接近由PMT試驗所得之楊氏模數Eur，而和經由P-S波量測所得之楊氏模氏Ef，則相差在10倍的範圍以內。

The strength and stiffness of soft rocks lie between hard rocks and soils. These characteristics are often the causes for difficulties in observing or predicting the behavior for this type of geotechnical engineering materials. Reports on issues related to soft rocks have been limited. In order to enhance our understanding of the outcrops of very weak and young rock formations in the central and northern regions of Western Taiwan (Tao Yuan to Tai Chung), the objective of this research projectis to develop a triaxial device specifically for soft rocks. To fully fulfill its requirements, the capacity of this soft rock triaxial device lies between those designed for hard rocks and soils. The triaxial cell was equipped with local strain measurement devices and traditional external strain measurement devices capable of monitoring strains less than 10-4%, so that the behavior of soft rock from small strain, linear elastic state to yielding could all be observed. The axial and cell pressure could be servo-controlled independently to enable stress path controlled tests to be performed. A series of consolidated undrained, stress path controlled triaxial tests have been conducted using this test device.eir results. The test results indicted that the speciments (samples) from two test sites in Hsinchu have less brittle behavior than typical hard rock. When sheared, positive pore water pressure was generated in most cases. The results also illustrated the linear elastic behavior of the soft rock during the small strain triaxial tests. As strain increaes, the Young’s module decreases, and when the axial strain reached 0.1% the cracking in the speciments would close hence the Young’s Module increase. It also shows the results from this research can be compaired with the insitu PMT and P-S logging Tests. The Young’s module from triaxial test (Emax) is very close to the result from PMT test (Eur), but in comparison with the result from P-S logging (Ef) the difference between two moduls is less than 10 time degrees. Keywords: Weak Rock, Triaxial Test, Stress-strain relationships, yield behavior