高壓噴射灌漿柱徑檢測之電探技術發展

Abstract

高壓噴射灌漿工法為目前土壤改良常用之技術，土壤改良之成效與改良柱體之完整性(包含柱徑與連續性)有緊密相關，因此對於改良柱體完整性檢核是必要的。地電阻影像法具有探測或監測地層2D或3D電阻率空間分佈的能力，基於準確性、經濟性與工作性等考量，故應用極具潛力之單孔地電阻影像探測法研究對改良柱體完整性之定性與定量檢測方法。本研究將改良柱體以3D數值模擬方法建構地電阻施測數值模型，分析不同條件下之改良柱體軸對稱2D電阻率影像，且為了準確判定改良柱徑之變化，而建立量化判釋方法並評析量化判釋方法之準確性與限制。研究結果顯示，若改良柱體具嚴重的非對稱性，則僅能定性判斷，尚無法準確量化其有效柱徑；但若改良柱體在軸對稱時之完整性檢核已足夠準確而具有可行性；若為空心之改良柱體，即電極測線附近待測物之電阻率差異過大，會造成判釋結果為非幾何尺寸之平均而無法準確量化，但由電阻率影像可發現電阻率之平均現象。為驗證此方法之可行性與數值模擬之結果，進行室內砂箱試驗與現地試驗，在接近軸對稱的情況下，由結果可驗證單孔地電阻影像探測法在柱體完整性檢測具有可行性，不僅可完整描繪出改良體的位置與形貌，更可量化檢視其柱徑。此外，為因應目前市面尚無孔內地電阻現地試驗專用之電極纜線，故本研究針對耐久性、功能性、方便性與通用性等需求研發專用之模組化移動式孔內電極，並經測試證實其設計理念之可行性。

Jet grouting method is currently among the common ground improvement technique, which performance is closely related to the integrity of improved columns (diameter and continuity). Therefore, column integrity assessment is necessary for quality control. Electrical Resistivity Tomography (ERT) imaging method is capable of detecting or monitoring spatial distribution of resistivity in 2D or 3D stratum. Considering detection accuracy, economy, workability and other aspects, in-hole ERT which has high application potential, was selected as integrity assessment of the improved column in terms of qualitative and quantitative evaluation. In this research, a 3D numerical model was constructed to simulate ERT measurement profile for improved columns. Axial-symmetrical 2D resistivity imaging profile of the improved columns under different circumstances was analyzed. In order to accurately identify the variation of column diameter, diameter quantification method was proposed and its accuracy and restriction was appraised. Results showed that in case the improved column is highly asymmetrical, only qualitative analysis is reliable and effective column diameter cannot be measured precisely. For those axial symmetrical columns, column integrity can be examined accurately and is shown feasible. However, for hollow improved columns, the resistivity variation of the material under test around the electrodes is too large, leading to quantification misjudgment as the effective resistivity is not of geometrical average, yet ERT can still realize this averaged resistivity phenomenon. To verify the feasibility of this approach and the result of numerical simulations, both laboratory sandbox experiment and field test were conducted. Both experimental result verified that column integrity assessment using in-hole ERT is feasible for columns that are near axial symmetrical. The position and appearance of the improved columns were clearly detected by this approach and the column diameters were also accurately quantified. Currently, there is lack of commercial electrode cable designated for in-situ borehole ERT. Therefore, this study aimed at developing a modularized portable electrode cable for in-situ borehole ERT, which would meet the requirements of durability, functionality, convenience, versatility and others. The feasibility of this cable design concept is tested feasible through a series of experiment in this study.