應用三維波傳模擬表面波震測於複合土體有效剪力波速量測之探討

Abstract

為了提昇軟弱地盤之強度與勁度，大地工程師們常用高壓噴射灌漿或是擠壓砂樁等地盤改良工法以達到上述目的，由於改良後會在地中形成固結物，使得土壤為高度非均質性。現有一課題值得探討：如何決定改良成效？傳統檢核方法如鑽孔取樣、標準貫入試驗因取樣空間小、耗時且成本高等限制，想要取得此地盤整體平均的改良率有相當的難度，相反的表面波震測具有大範圍取樣，非破壞性及快速施作等優點，其反算得到的剪力波速剖面實質上反映了複合土體之彈性行為，但表面波方法有一基本假設是測線下方為水平一維層狀地層，使得非均質土壤在剪力波速量測上卻有均質化的特性，為此文獻進行了二維數值模擬探討，評估地盤改良率與剪力波速提昇率之關係，但二維模擬建立的地層模型無法反映實際地盤改良場址，加上表面波震測會因側向地層變化造成訊號收錄有一定程度之影響，進行三維數值模擬有其必要性。 本研究採用三維波譜元素法進行探討，研究成果顯示，表面波於改良後場址之行為，若是簡化牆狀地層模型，其剪力波速之均質化會遵守一維等效波速理論之下限；若於柱狀地層模型，剪力波速之均質化不再遵守一維等效波速理論之下限，而是在等效模數理論之上下限之間，可根據模擬資料提出均質化等效速度與改良率之關係。表面波的側向影響範圍約為波長的1.3倍，在同樣範圍內，測線外異質土壤與測線內原土壤的速度比越大，量測速度受到的影響越大；當速度比相同的情況下，原土壤速度越低，量測速度受到的影響也越大。

Soft ground is often improved to increase strength and stiffness by methods such as jet grouting and stone column which result in heterogeneous ground with improved columns. Conventional methods used to assess such ground improvement are subjected to several limitations such as small sampling volume, time-consuming, and cost ineffectiveness. It’s difficult to assess the average property of the improved ground and the actual replacement ratio of ground improvement. The use of seismic surface wave method (i.e. multi-station analysis of surface wave, MASW) for such a purpose seems to be a good candidate. But the surface wave method is essentially a 1-D method assuming horizontally-layered medium. What MASW measures in the highly heterogeneous improved ground remains to be investigated. The feasibility of MASW in characterizing the heterogeneous ground with improved columns has been demonstrated and the relationship between replacement ratio and velocity increase has been studied by 2D model. This study aims to investigate the homogenization of shear wave velocity measured by MASW in real 3D condition. Testing results may also be affected by the survey line location relative to the improved columns. The lateral sampling space of the surface wave testing was also investigated by 3D modeling. 3D spectral element method was utilized for the numerical investigation. Numerical results show that the effective shear velocity can be derived based on lower bound of equivalent velocity model when the heterogeneous ground is a 2D wall-like structure. However, in real 3D column-embedded structure, the relation between the effective shear wave velocity and replacement ratio falls between upper bound and lower bound of the equivalent modulus model. A predictive model is derived based on the numerical simulations. The lateral sampling range is about 1.3 wavelength. In the same spatial range, the measured velocity is affected more by offline heterogeneous material for higher velocity contrast. At constant velocity contrast, lower velocity material along the survey line is affected more by offline heterogeneity.