從力學特性與微觀組構看EICP改良砂土成效

Abstract

近年來，酶誘導碳酸鹽沉澱（EICP）作為一種仿生工程技術，受到土木工程界的廣泛關注。透過EICP進行的生物膠結可通過堵塞空隙和利用碳酸鈣將土壤顆粒粘結來增加土壤的強度、剛度和抗液化性。該方法涉及使用脲酶與尿素和氯化鈣相結合以誘導碳酸鈣沉澱。EICP處理的有效性受化學成分濃度和脲酶影響。初步研究旨在確定尿素、氯化鈣與脲酶的最佳濃度及EICP處理樣品的抗滲深度。透過Falcon試管試驗，尿素濃度範圍為0.25M至1M，比例分別為（CaCl2：尿素）1:1、1:1.2、1:1.5和1:1.75，脲酶濃度分別為1 g/l、3 g/l、5 g/l和6g/l。研究使用Falcon試管測試顯示，當尿素濃度為1 M，氯化鈣濃度為0.67 M，脲酶濃度為3 g/l時，獲得了較為高的碳酸鈣和沉澱率。本研究採用了由1M尿素、0.67M氯化鈣二水合物和3g/l脲酶組成的最佳第一階段EICP膠結溶液配方。隨後，對四種不同組合進行了落錐試驗，測量了不同養護時間（3天和7天）下單次處理週期（N-1）和第7天的兩次處理週期（N-2）中的滲透深度。值得注意的是，兩次處理週期（N-2）中的滲透深度明顯低於第3天和第7天的數值。進行落錐試驗後，通過酸消化法測量碳酸鈣沉澱量以分析樣品的均勻性。酸消化試驗結果表明，較低濃度的尿素和氯化鈣產生的樣品更為均勻，而較高濃度則導致樣品頂部碳酸鈣含量增加，底部碳酸鈣含量減少。這種差異歸因於脲酶與膠結溶液的快速反應，使樣品底部不透水。隨後，使用已確定的最佳EICP配方來研究實驗測試前樣品中碳酸鈣沉澱的均勻分佈。在排水和不排水條件下，研究了三軸試樣中碳酸鈣的分佈。結果顯示，在不排水條件下進行的EICP處理獲得了更為均勻的碳酸鈣沉澱。EICP溶液滯留在紙模中，從而有助於產生更均勻的沉澱。該研究的目的是通過在兩種不同的脲酶條件下進行單調固結排水三軸試驗和不排水循環三軸試驗，來評估EICP處理砂的力學行為。對純砂和EICP處理砂進行了7天和14天養護下的單調三軸試驗，分別在50、100和200 kPa的圍壓下進行。EICP膠結顯著提高了單次處理週期內的偏應力、膨脹性和剪切強度參數。在所有養護期間，EICP處理砂均顯示出明顯的有效內聚力。生物膠結的增加提高了峰值和殘餘狀態下的有效內聚力，而有效摩擦角則保持不變。研究還對純砂和EICP處理樣品在7天養護下100 kPa圍壓的抗液化循環反應進行了研究。循環抗液化性通過減少壓縮應變和延長達到液化的週期數而得到改善。為進一步理解碳酸鈣沉澱引起的微觀結構變化，進行了顯微結構分析，包括場發射掃描電子顯微鏡 (FE-SEM) 分析、能量色散光譜 (EDS) 元素映射和X射線衍射 (XRD) 研究，其樣本來自Falcon試管試驗的純砂和EICP處理樣品。FE-SEM和EDS分析結果顯示，EICP處理增強了顆粒間接觸和顆粒與方解石沉澱的接觸強度。XRD結果則確認通過EICP處理形成的晶體主要由方解石組成。最後，三軸試驗後的酸消化顯示在飽和、固結和剪切過程中，處理樣品的碳酸鈣損失量較小。由於本研究討論了單次處理週期，更多的處理週期和較高的膠結濃度可以使樣品在飽和和剪切過程中更加耐受。

In recent years, Enzyme-induced carbonate precipitation (EICP) has emerged as a bio-inspired and innovative technique that has captured the attention of geotechnical engineers specializing in soil stabilization. Bio-cementation via EICP increases the strength, stiffness, and soil liquefaction resistance by clogging the voids and binding the soil particles with calcium carbonate. This method involves the utilization of urease enzymes combined with urea and calcium chloride to induce calcium carbonate precipitation. The effectiveness of the EICP treatment is influenced by the concentration of the chemical components and the urease enzyme. The preliminary research aimed to identify the optimal concentrations of urea and calcium chloride with the urease enzyme and penetration depth resistance in EICP-treated specimens. Falcon tube tests were conducted with urea concentration (0.25M to 1M) in various ratios (CaCl2: Urea) – 1:1, 1:1.2, 1:1.5, and 1:1.75 with different proportions of urease enzymes (1 g/l, 3 g/l, 5 g/l, and 6g/l). The research employed falcon tube tests, revealing that a combination of 1 M urea, 0.67 M CaCl2, and 3 g/l urease enzyme resulted in a higher CaCO3 and precipitation ratio. This study adopted an optimal formulation of a one-phase EICP cementation solution consisting of 1M Urea, 0.67M Calcium chloride dihydrate, and 3g/l urease enzyme. Subsequent fall cone tests were conducted on four different combinations, measuring penetration depth at various curing times (3 and 7 days) in a single treatment cycle (N-1) and two treatment cycles (N-2) on day 7. Notably, the penetration depth at two treatment cycles (N-2) significantly decreased compared to values observed on days 3 and 7. Following the fall cone tests, specimen uniformity was analyzed by assessing CaCO3 precipitation using the acid digestion. The acid digestion test results indicated that lower concentrations of urea and calcium chloride yielded more uniform specimens. In contrast, higher concentrations led to non-uniform specimens with elevated CaCO3 at the top and reduced CaCO3 at the bottom. This disparity was attributed to the rapid reactivity of the urease enzyme with the cementation solution, rendering the bottom portion of the specimen impermeable. The identified optimal EICP formulation was then employed to investigate the uniform distribution of calcium carbonate precipitation in the prepared specimens before subjecting them to experimental testing. The distribution of calcium carbonate in the triaxial specimen was studied in both drained and undrained conditions. The EICP treatment under undrained conditions obtained a more uniform calcium carbonate precipitation. The EICP solution gets retained in the paper mold, which helps produce a more uniform precipitation. The objective of the study involves assessing the mechanical behavior of the EICP-treated sand by performing monotonic consolidated drained triaxial and undrained cyclic triaxial tests in two different urease enzymes. The monotonic triaxial tests on pure sand and EICP-treated sand with 3,7 and 14 curing days were carried out at 50, 100, and 200 kPa confining pressures, respectively. Bio-cementation through EICP significantly enhanced the deviatoric stress, dilatancy, and shear strength parameters in one treatment cycle. A noticeable effective cohesion was observed for EICP-treated sand for all curing durations. The increase in bio-cementation increased the effective cohesion at both peak and residual state while the effective friction angle remained constant. The cyclic response under an effective confining pressure of 100 kPa in the pure sand and EICP-treated specimen was studied in 7 days of curing time. The cyclic resistance to liquefaction improved by reducing compression strain and prolonged cycles to attain liquefaction. Microscopic analyses were performed to comprehend further the microstructural transformations resulting from calcium carbonate precipitation. These include field-emission scanning electron microscopy (FE-SEM) analysis, Energy Dispersive Spectroscopy (EDS) elemental mapping, and X-ray Diffraction (XRD) studies on calcium carbonate precipitate from falcon tube test, pure sand, and EICP-treated sample. The FE-SEM and EDS analysis revealed augmented strength particle-to-particle contact and particle-to-calcite precipitation due to EICP treatment. Notably, XRD results confirm that the crystals formed through EICP treatment primarily comprise calcite. Finally, acid digestion after the triaxial test revealed a minor amount of loss of CaCO3 in the treated specimen during the saturation, consolidation, and shearing process. As the study discussed the one cycle of treatment, multiple treatment cycles with higher cementation makes the specimen resistant under saturation and shearing process.