邊界元素法控制之標度槽測試

Abstract

由於砂土缺乏凝聚力，所以砂土的不擾動試樣很難取得。為量測砂土的工 程性質，現地試驗是常用的方法。圓錐貫入試驗是一種廣泛使用的現場試 驗方法。然而圓錐貫入試驗的結果多靠經驗公式來分析，經驗公式大多經 由試驗室中的標度試驗而來。標度試驗的重要缺陷是它的邊界效應。常用 的方法是將標度試體，如三軸試驗一樣的裝在單一的橡皮膜中。根據經驗 標度槽橫向邊界對圓錐貫入試驗影響最大。傳統標度槽之橫向邊界可以施 加固定應力或零應變，這兩種邊界都無法模擬介於兩者之間的真正現地情 況。有人提出以修正係數的方法來抵消邊界效應，但是修正係數方法的正 確性與邊界影響之機制至今都尚未得到證實。前期的研究已發展出使用邊 界元素法控制之標度槽系統。為測試此一新標度槽系統，已完成了一系列 應力和邊界元素法控制的CPT標度槽試驗。論文敘述新標度槽系統之試驗 程序和現有的試驗結果。 Due to the lack of cohesion, it is difficult to obtain undisturbed samples of sand. In-situ testing is often used to determine the engineering properties for sands. The cone penetration test (CPT) is a popular in-situ testing method. However, as for many other in-situ testing methods, we rely on empirical rules to interpret CPT data. A significant part of these empirical rules came from laboratory chamber calibration tests. An important drawback of the calibration chamber is its boundary effects. The sand specimen is typically encased in a rubber membrane as in the case of a triaxial test. The lateral boundary of the specimen in a calibration chamber is either stress controlled or rigid. Neither of these two boundary conditions can truly duplicate the field conditions which lies somewhere in between. Correction factors have been proposed to account for these boundary effects. However, the validity of the use of correction factors and the mechanisms of boundary effects have not been independently verified. The main objetive of this research project is develop a chamber system that is capable of simulating field conditions. Instead of using a single membrane to encase the specimen, twenty inflatable rings are placed around the specimen. The boundary movement induced by the cone penetration is measured at each ring level by an extensometer. The displacement readings are then fetched into a Boundary Element Method (BEM) program where the sand extending out from the lateral boundary to infinity is modeled as a linear elastic material. The BEM computes the corresponding pressure at every ring level for the given displacement readings. Major components of the chamber system include a sand pluviator, chamber cell (the rings), pneumatic and hydraulic control systems. A series of preliminary cone penetration tests have been performed in the new chamber under stress controlled and BEM simulated lateral boundary conditions. This thesis describes details of the new chamber system, and presents available test results.