膠結不良砂岩之殘餘強度試驗與解析

Abstract

台灣中北部麓山帶地質常見膠結不良軟弱砂岩，此等岩性之邊坡往往易發生漸進破壞，常於邊坡滑動面產生非常大之變形前，破壞面某些局部區域因材料應變軟化之力學性質，剪應力達尖峰強度後隨之衰減至終極之殘餘強度。軟弱岩石之殘餘強度性質，即為本研究探討之主題。 理論上，環剪試驗為決定大地材料殘餘強度的最好方法。因此，殘餘強度試驗方法中以Bromhead環剪儀最具代表性，然而該儀器卻因試體破壞面受到限制，而未能適合於軟弱砂岩試體。本研究承襲前人改良之環剪試驗儀及中空試體製作方法，嘗試改善試體製作方式與儀器系統穩定，以求得軟弱岩石正確之殘餘強度，期能制定ㄧ套可靠試驗流程，而反應材料真實行為。 本論文所進行之殘餘強度試驗種類包括固定正向力試驗及固定體積試驗，本研究中亦提出合理與簡化之力學模式用以解析試驗結果。為進ㄧ步探討大剪應變下裂面形成之過程與殘餘強度行為，本研究並進行固定正向力之反覆環剪試驗，以了解殘餘強度與裂面於反覆環剪下之發展，有助於進一步了解軟弱岩石殘餘強度之性質。

Progressive failure may occur in a poorly cemented soft-rock slope. Very often, stress concentration may cause localized failure in the slope prior to the development of a full sliding surface, followed by the final large sliding deformation along the sliding surface. For strain-softening geomaterial such as a poorly cemented rock, the post-peak shear strength usually will drop to the residual shear strength and may result in stress-redistribution and progressive failure of the slope. This thesis aims to study the residual strength of poorly cemented rock using ring-shear test which is the best method for obtaining the residual strength of geomaterial. The Bromhead ring shear apparatus has often been used for determining the residual strength of soil. However, this type of apparatus is not suitable for soft rock. A device appropriate for testing the residual strength of soft rock with hollow sample was developed by Liu (2004). In this work, the designs of the ring-shear device, the approaches for preparing hollow specimen, and the procedures for assembling the device set were improved to overcome problems encountered using the previous version of the device. In addition to the improvement of the device and test procedures, a simplified strain-softening model was developed. An approach for data interpretation using the suggested simplified model was also proposed. Various types of ring shear tests, including constant-normal-force test, constant-volume test, and repeated shear test, were carried out. Stress-strain curve and strength properties were back calculated from the curve of torque against rotational angle obtained from ring shear tests. This study helps to understand the evolution of the failure surface and the strain-softening behavior of poorly cemented rock under large shear strain.