中北部地區極軟弱砂岩之物理與力學性質之初步探討

Abstract

本研究提出台中大坑地區之卓蘭層，與新竹地區頭嵙山層香山相之軟弱砂岩之基本物 理性質及材料力學性質之初步研究成果。其中研究工作含鑽探取樣與室內物性、單壓、三 軸...等工作。其中鑽探取樣工作乃嘗試採用"氣體-泡沫(Air Foam)"為介質，探討是否適 用於此等極軟弱砂岩之取樣，期能取得完整岩樣供室內試驗之用，並做為未來極軟弱岩石 取樣之參考。 經以改良方法實施現地鑽探之結果，雖然取樣率並非十分理想，但是岩心品質已較傳 統方法為佳，因此若可再進一步改良岩心筒及鑽頭，並配合純熟之鑽探技術，應可進一步 提昇於軟弱地層之鑽探。 由頭嵙山層香山相及卓蘭層之粒徑分析可知，兩者砂顆粒粒徑約介於2~0.075mm，屬 中至細砂，其中以頭嵙山層香山相之砂顆粒分佈較均勻，而卓蘭層之泥質(粉土+黏土)含 量比較高。而在相同試驗控制模式下，泥質(粉土+黏土)含量較低及砂顆粒之均勻性較低 者，力學強度有增高趨勢。 對於本次極軟弱砂岩之力學行為研究中，取自台中大坑卓蘭層之軟弱砂岩之單壓強度 約介於7~13 MPa。至於三軸壓縮部份，隨著圍壓之增加，體積壓縮量增大而切線模數有減 緩趨勢。整理卓蘭層砂岩之應力-應變曲線並由現今經驗破壞準則得知，於尖峰點之主應 力狀態所構成之破壞包絡線，經由Hoek-Brown,Bieniawski與Johnston之經驗準則得知， 大致呈非線性、凹口向下之曲線，然Bieniawski無法描述張力階段之應力狀態。至於殘餘 段之主應力關係，亦可由Hoek-Brown,Bieniawski,Johnston之經驗準則得知，大致呈由原 點向外延伸且凹口向下之非線性曲線。 This thesis presents the preliminary results of physical and mechanical properties of the Cholan Formation in Takeng and the Toukoshan Formation in Hsinchu. Tasks in this research include (i) drilling and sampling in very weak sandstone and siltstone, (ii) laboratory determination of physical properties, (iii)uniaxial and triaxial compression tests. Drilling and sampling in field by air-foam medium was attempted to retrieve good quality samples in order to investigate the real behavior and properties of the very soft rocks in laboratory subsequently. The improved procedure of sampling,although still not perfect, enables a better recovery of sample quality than the conventional methods in which uses water as the drilling medium. With properly modified core-barrel and drilling -pit, sample quality can be further improved. Skillful practice of drilling can also help to obtain better samples of very soft rocks. From the grain-size analyses, the grain sizes of the sandstone in the Cholan and the Toukoshan Formations lie within 0.075~2mm, which belong to the range of fine to medium sand. The grain size distribution of the Toukoshan Formation is more uniform than the one of the Cholan Formation. While the sandstone in the Cholan Formation contains higher clay contents than those of the Toukoshan Formation. Under same experimental conditions, a sandstone with lower clay content and uniformity will have a higher strength. The mechanical properties of the tested very weak sandstones are summaried as follows. For the uniaxial compression tests on samples obtained from the Cholan Formation in Takeng, the uniaxial strength of the sandstone lies within 7~13 MPa. For the triaxial compression tests on the same samples, the confining pressure affects the volume-change tendency and the tangent modulus. As the confining pressure increases, the volume-change tends to increase and the tangent modulus tends to decrease. The failure envelope appears nonlinear and concave downward which agrees with existing empirical criterion (Hoek- Brown, Bieniawski, Johnston)of peak and residual strength.