砂岩之峰後力學特性研究

A Study of Post-peak Behavior of Sandstone

楊逸賢(Yi-Hsien Yang)

指導教授 ： 鄭富書

國立臺灣大學/工學院/土木工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.00171

摘要

台灣西部麓山帶之岩層以未變質的第三、四紀砂岩為主，由於成岩作用時間短，岩石之膠結及壓密程度不佳，加上地體構造活動頻繁，潮濕多雨的氣候環境，形成許多不利於大地工程之自然因素；而此地區的軟弱岩體在隧道開挖後，隧道易產生大規模的變形及坍塌，並導致隧道支撐結構的破壞，嚴重地影響到施工安全及建设工期。
隧道開挖後，其周圍岩體易發生破壞，雖然這些破壞後的圍岩尚具有一定的承載力，但仍需要進行結構支撐，因此，瞭解並釐清破壞後(峰後)之岩體在受壓狀態下的變形特性，對於維護隧道支撐結構的穩定性顯得相當重要。
本研究以北投地區之木山層砂岩為研究對象進行傳統三軸試驗，試體狀態分為乾燥與含水試體兩類；藉由一系列三軸試驗之結果，獲得不同圍壓下之完整應力-應變曲線，並探討其在峰後狀態的力學特性及變形行為，結果顯示：
(1) 在砂岩強度方面：
達殘餘強度之ϕ值較達尖峰強度之ϕ值略低，而c值則會大量下降，故推測砂岩試體峰後強度下降主要與膠結力有關。砂岩試體遇水後，單壓強度下降46 %，傳統三軸試驗之尖峰強度下降約30 ~43 %，而殘餘強度下降約16 ~30 %。
(2)在砂岩峰後變形性方面：
(a)剪應力階段引致之體積應變：峰後之體積變形主要受側向變形之影響，隨圍壓之增加，其峰後的體積膨脹量會減少。
(b)剪應力階段引致之剪應變：峰後之剪變形主要受軸向變形之影響，隨圍壓之增加，峰後之剪變形會愈趨向延性變形行為。
(c)彈性變形方面：峰前及峰後之彈性變形均為壓縮之趨勢；彈性體積變形發展由峰前至峰後，其變化量會逐漸減少，而彈性剪變形之變化量則會逐漸增加。
(d)塑性變形方面：對於塑性應變增量部分，峰值過後，塑性應變增量開始漸減，在剪應力驟降的過程中達到最低點，之後又開始增加，當達殘餘強度後，塑性增量隨時間增加，呈緩慢地增加。對於塑流角部分，可發現塑性流方向均與尖峰及殘餘破壞包絡線成正交。
(3)砂岩之組成模式方面：
本研究之彈性模式採用高階格林彈性模式，而塑性模式係以廣義塑性模式為基礎作修改而得，模式遵守諧和流動準則。針對不同圍壓應力下之木山層砂岩試驗結果及國外砂岩之試驗結果進行模擬，其模擬結果皆與實驗資料趨勢接近，表示此組成模式可模擬不同地區的砂岩變形特性。

關鍵字

木山層砂岩 ； 完整應力-應變曲線 ； 峰後 ； 組成模式 ； 廣義塑性

並列摘要

The Tertiary sandstone is widely distributed in foothill area of western Taiwan. Due to this relatively young rock-geneses period, weathering and other factors, these sedimentary strata are mostly weak rocks. In the past, these weak rocks have caused several engineering difficulties such as squeezing of the tunnel under construction due to shear-induced and creeping deformations.
Failure of surrounding rock is easily occurred during tunnel excavation in weak rock. Therefore, it is needed to understand the post-peak behavior of the sandstone under triaxial stress and to develop a constitutive model that can properly describe these mechanical characteristics.
In this research, it focuses on the post-peak behavior of Mushan sandstone, sampled from Beitou area, Taipei. The specimens are prepared in dry and wet conditions. Then, a series of conventional triaxial tests were conducted to obtain the complete stress-strain curves under various confining pressures. The experimental results indicate that: (1) the friction angle at the residual strength state is slightly lower that at the state of peak strength, and the cohesion will decreases apparently. When samples were wet, the strengths decrease about 46% in uniaxial compressive tests. The peak strengths decrease about 30-43 %, and the residual strengths decrease about 16-30% in triaxial compressive test; (2) the post-peak volumetric strain mainly controlled by lateral deformation, and it will decreases with increasing of confining pressure. However, the post-peak shear strain mainly controlled by axial deformation and it will increases with increasing of confining pressure; (3) The variation of volumetric elastic deformation decreases gradually, but the variation of shear elastic deformation increases gradually when the state converts from pre-peak to post-peak. In the post-peak stage, the increment of plastic strain first begin to decrease, and then it reaches the lowest value due to the softening; next, it will increases until the state of stress at residual strengths. The directions of plastic flow angle are orthogonal with peak and residual failure envelopes.
Based on the experimental results, a constitutive model is proposed. The constitutive model is based on the Green elastic model and generalized plasticity model, and the plasticity model follow associated flow rule for post-peak simulation. The results of simulation based on the proposed model are consistent with the experimental results under triaxial compression for Mushan sandstone and other sandstones.

並列關鍵字

Mushan sandstone ； complete stress-strain curve ； post-peak ； constitutive model ； generalized plasticity

參考文獻

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