表面波震測於含改良柱體地層之改良成效評估

Abstract

軟弱地盤經過地盤改良後可達增加土壤剪力強度、承載力等目的，但在檢核地盤改良成效時，於地中形成改良柱體之場址會突顯既有檢核方法如標準貫入試驗、鑽探取樣與室內試驗等取樣範圍小、費時且昂貴又不具代表性的缺點，因此尋求可反應整體改良效果之檢核方法有其必要性。本研究嘗試以表面波震測法取樣範圍大、快速且經濟的優點作為檢核方法，以輔助並提供更具代表性的檢核資訊。 表面波震測法取得剪力波速剖面，透過剪力波速能確實的反應土體勁度及強度的特性，本研究探討表面波震測在異質性地層之均質化反應，並透過通過與未通過改良柱體之測線，探討表面波震測之側向取樣空間；在不同改良率之工程案例，比較改良前後表面波震測剪力波速之提升率與場址處之工程性質(如改良率、SPT-N值、複合剪力強度等)，以期找出此提升率所能提供之工程訊息。

Soft ground can be improved to increase the shear strength and bearing capacity by methods such as jet grouting and stone column which may result in heterogeneous ground with improved columns. Experimental methods (Standard Penetration Test, Sampling, Laboratory testing, etc.) used to assess such ground improvement are subjected to several limitations such as small sampling volume, time-consuming, and cost ineffectiveness. Current homogenization equations based on improvement ratio may not be representative. The objective of this study is to investigate the seismic surface wave method as a non-invasive and direct method for assessing the overall quality of the ground improvement, which is characterized by large sampling volume, efficiency, and cost effectiveness. The surface wave test is can be used to determine the shear-wave velocity depth profile which reflects the stiffness and strength of the geotechnical materials. This study investigated the homogenization of surface wave testing in heterogeneous improved ground with improved columns. The lateral sampling space of the surface wave testing was also investigated by field testings of different survey line locations relative to the improved columns. From case studies with different improvement ratio, the difference in shear wave velocity before and after ground improvement is compared with various site characteristics such as improvement rate, SPT-N, and the complex shear strength. The engineering information that can be extracted from the improvement rate of shear wave velocity obtained by surface wave testing is discussed