Unconfined compressive strength of MICP and EICP treated sands subjected to cycles of wetting-drying,freezing-thawing and elevated temperature:Experimental and EPR modelling

Microbial-induced carbonate precipitation(MICP)and enzyme-induced carbonate precipitation(EICP)are two bio-cementation techniques,which are relatively new methods of ground improvement.While both techniques share some similarities,they can exhibit different overall behaviours due to the differences in urease enzyme sources and treatment methods.This paper presented 40 unconfined compressive strength(UCS)tests of MICP and EICP treated sand specimens with similar average calcium carbonate(CaCO3)content subjected to cycles of wetting-drying(WD),freezing-thawing(FT)and elevated temperature(fire resistance test e FR and thermogravimetric analysis e TG).The average CaCO3 content after a certain number of WD or FT cycles(ACn)and their corresponding UCS(qn)reduced while the mass loss increased.The EICP treated sand specimens appeared to exhibit a lower resistance to WD and FT cycles than MICP treated specimens possibly due to the presence of unbonded or loosely bonded CaCO3 within the soil matrix,which was subsequently removed during the wetting(during WD)or thawing(during FT)process.FR test and TG analysis showed a significant loss of mass and reduction in CaCO3 content with increased temperatures,possibly due to the thermal decomposition of CaCO3.A complete deterioration of the MICP and EICP treated sand specimens was observed for temperatures above 600C.The observed behaviours are complex and theoretical understanding is far behind to develop a constitutive model to predict qn.Therefore,a multi-objective evolutionary genetic algorithm(GA)that deals with pseudo-polynomial structures,known as evolutionary polynomial regression(EPR),was used to seek three choices from millions of polynomial models.The best EPR model produced an excellent prediction of qn with a minimum sum of squares error(SSE)of 2.392,mean squared error(MSE)of 0.075,root mean square error(RMSE)of 0.273 and a maximum coefficient of determination of 0.939.

酶诱导碳酸钙沉淀技术加固TBM壁后吹填豆砾石最优配比试验及机制研究

TBM(tunnel boring machine,简称TBM)隧道中豆砾石作为管片与围岩填充层对管片承载及抗渗具有重要意义,水泥浆液流动性差,无法充满回填层进而导致壁后空洞、灌浆不密实等缺陷,酶诱导碳酸钙沉淀(enzyme-induced calcium carbonate precipitation,简称EICP)技术是作为一种环保高效的加固方法,灌浆材料均为液体具有很好的流动性和扩散性,用于豆砾石回填层灌浆施工有望解决上述问题。为实现吹填豆粒石EICP灌浆最优效果,尝试将标准砂与豆砾石共同作为回填骨料,为定量化分析二者的最优配合比,开展了不同豆砾石与砂的配合比(0.5、0.75、1.0、1.25、1.5)和不同灌浆次数(9、12、15次)的砂柱固化试验。通过无侧限抗压测试、渗透性测试、碳酸钙含量测定、超声波速测定、扫描电镜(scanning electron microscope,简称SEM)试验,从宏观和微观角度分析不同石砂配合比对EICP固化效果的影响。结果表明:EICP加固豆砾石混合砂的最优配比为1:1.5,经15次灌浆加固后,试样单轴抗压强度最高可达4.55 MPa,渗透系数达1.72×10^(-5)m/s;对于含砂量较高的试样,颗粒间孔隙易被碳酸钙晶体填充密实,碳酸钙晶体有效胶结比例较高,结石体无侧限抗压强度较高。研究成果可为EICP技术加固隧道掘进机壁后吹填豆粒石的工程应用提供理论依据。

木钙源EICP溶液固化路基黄土性能研究

黄土地区有砟货用铁路路基遇水强度下降,严重影响线路的正常运营。针对该工程问题,采用植物源脲酶诱导碳酸钙沉积技术固化路基黄土,在胶凝液中开展了以环保型木钙代替传统钙源进行技术改进的系统研究。结果表明:首先,木钙源EICP溶液能同时显著提高黄土的无侧限抗压强度和抗剪强度。其中,无侧限抗压强度提高至0.232 MPa,提高了20.8%;抗剪强度指标C值提高至69.06 kPa,提高了31.2%;Φ值提高至26.8°,提高了52.3%。其次,木钙源EICP溶液的抗剪切效果明显优于传统氯化钙源和乙酸钙源的EICP溶液,并且各组分存在最优浓度。其中,1000 U/g活性脲酶的最优浓度为3 g/L,尿素的最优浓度为1 mol/L,木钙的最优浓度为1 mol/L。随着各组分掺量的增加,黄土的抗剪强度呈现先增大后减小的规律。最后,通过XRD、SEM等微观试验结果分析得出木钙源EICP溶液改良黄土强度的作用机理,发现其中脲酶诱导产生的碳酸钙晶体既胶结了黄土骨架颗粒,也填充了粒间孔隙,同时提高了土颗粒间的摩擦强度和粘结强度,最终改善土体的力学性能。